| 00:00 | begin. Okay, alright, so thing to discuss is more complicated. |
|
| 00:09 | the ability of argon data to assess disturbances to the system, um disturbances |
|
| 00:18 | can come about either by an episode reheating or by very slow cooling. |
|
| 00:26 | First let's just take the more simple of episodic loss, which we've already |
|
| 00:30 | about the episode of the Duluth for example. This is another |
|
| 00:37 | a similar sort of thing. As recall, these are samples someplace in |
|
| 00:42 | that had the same sort of a Luton intruding some other rocks. |
|
| 00:47 | are a variety of horn blends as recall, which are at various distances |
|
| 00:54 | the content. And again, we see that the closest to the contact |
|
| 00:59 | the most profound amount of loss. can see that some of these samples |
|
| 01:03 | up to, you know, Pennsylvanian here. But this one the maximum |
|
| 01:09 | it gets up to about 180. you go farther away from the uh |
|
| 01:16 | the disturbance, these will all be in these agents. But closer in |
|
| 01:23 | get this. So an episode of is pretty easy to come up just |
|
| 01:29 | understand, I think. But we to understand that similar age spectra in |
|
| 01:36 | in fell spars can look a little like they came either from slow cooling |
|
| 01:41 | reheating. You get an age spectrum this that goes from some value here |
|
| 01:46 | slowly goes down to some lower How did that happen. Well, |
|
| 01:51 | it, was it as we looked ? Some some age of 180 that |
|
| 01:55 | disturbed 100 million years ago maybe. quite often it is from another |
|
| 02:02 | It's from and and I alluded to last time and I'll get two more |
|
| 02:08 | a minute. But if we we that fell. Spars might not be |
|
| 02:14 | simple case of a single thing, they all have different sizes, which |
|
| 02:18 | the different domain sizes will lead to closure temperatures within the same crystal. |
|
| 02:24 | so if you cool slowly enough, one crystal Will record many different temperatures |
|
| 02:32 | a long period of time. It be that there are parts of the |
|
| 02:36 | that are retaining, there are gone 180 million years ago. That might |
|
| 02:40 | the temperature of say 300°. But other of the same crystal do not begin |
|
| 02:46 | retain their are gone until 100 million ago. That might be a temperature |
|
| 02:51 | like 180°. So it doesn't have to a single event that jams it out |
|
| 02:58 | gives us a situation like this. still look like this. There'll still |
|
| 03:04 | more gas in the middle than on outside. Well, that's the single |
|
| 03:13 | model. What we really have are domains. Some of these little domains |
|
| 03:17 | not be holding onto any gas until get cold enough and they're all mixing |
|
| 03:22 | . Um And let me come back that. We're gonna, we're gonna |
|
| 03:27 | and talk about this feldspar stuff straight and we'll come back. So I'm |
|
| 03:31 | ahead a few slides, I showed this this, this diagram yesterday where |
|
| 03:36 | have an Iranian diagram that originally kind goes nice and straight looks fine, |
|
| 03:41 | then falls away and and and we said that it might be because there's |
|
| 03:47 | variation in domain size, it's not single crystal that has just one size |
|
| 03:54 | we can we can vary, we we can model this strictly as a |
|
| 04:00 | of domains that have different sizes. don't have to change any of the |
|
| 04:05 | variables. We can just say there's big ones and some little ones and |
|
| 04:08 | we gotta do is vary the relative of the big ones and little ones |
|
| 04:13 | the relative volume fraction of all the sizes. Obviously if we have one |
|
| 04:19 | one, but it only makes up of the volume, that's different than |
|
| 04:22 | it made up something else. those are two sort of parameters we've |
|
| 04:26 | in our modeling opportunities, We've got domain sizes and basically, if we |
|
| 04:33 | that the small ones are one, know, the big ones are the |
|
| 04:36 | ones, 100 or 1000 or and then we have to vary the |
|
| 04:42 | fraction and I'm not, and that's a complicated endeavor. But this, |
|
| 04:49 | shows the results of and what what is is that, remember when we |
|
| 04:54 | a step heating experiment, we're heating different temperatures and going through there and |
|
| 05:02 | we're doing it as a diffusion well, we can. The nice |
|
| 05:05 | about feld's bars is that we get age experiment and a diffusion experiment at |
|
| 05:10 | same time. And this is because Argon 39 is uniformly distributed throughout the |
|
| 05:17 | because we just made it yesterday, artificially put in there. So it |
|
| 05:22 | a great condition to do a diffusion . We know the initial value and |
|
| 05:27 | can then as we heat up the , we can watch how much argon |
|
| 05:31 | out each one. Eventually we can know what F was. And that |
|
| 05:35 | us to calculate D not on a we get a diagram like this and |
|
| 05:41 | that We can make a mathematical And this mathematical model is based entirely |
|
| 05:47 | the diffusion of Argon 39. Argon is what we made in the laboratory |
|
| 05:56 | . From this, we build a model and we say, what |
|
| 06:01 | what would the various domains? Big , little ones. All luminous |
|
| 06:06 | unimportant ones. What would those various have to be in order for this |
|
| 06:11 | to look the way it is. the time and temperature heating schedule that |
|
| 06:15 | use, this is a this is result we not. We need to |
|
| 06:21 | up with a model that produced And remember this is produced in the |
|
| 06:26 | over the over the course of a days, perhaps this is a laboratory |
|
| 06:33 | . And so we say, we know how much argon came out |
|
| 06:37 | temperature at this time and so In the context of this multi diffusion |
|
| 06:44 | modeling. Can we match it? yeah, we can computer does some |
|
| 06:50 | and we end up showing that. , we think there are four different |
|
| 06:53 | there, of these different sizes of different volume fractions. And if we |
|
| 06:57 | that model and we apply the the heating schedule that was done in |
|
| 07:01 | laboratory, we can reproduce those And so now we have a model |
|
| 07:05 | the mineral for the mineralogy of the . Now we can take that and |
|
| 07:11 | and and we can then take a history, take some cooling history and |
|
| 07:19 | what that would produce over millions of . Because if we've got a mineralogical |
|
| 07:26 | that implies things about various closure The little domains are this big. |
|
| 07:31 | they retain gas at this temperature. big domains are this big and so |
|
| 07:36 | . You following me, that makes . So, if we have a |
|
| 07:40 | model that's based on our lab data , we can then try out some |
|
| 07:46 | histories such as this one here and and see what our mineralogical model will |
|
| 07:53 | for our age model, This age , remember not was produced as essentially |
|
| 08:01 | consequence of the cooling over millions of . So we've taken our mineralogical model |
|
| 08:07 | was produced because of heating over days using that to help us predict what |
|
| 08:14 | long term model, long term data look like if we applied various thermal |
|
| 08:20 | . And this shows that this This model works very well. This |
|
| 08:24 | the model in through here for this bar, and the red is the |
|
| 08:29 | , and the black is the is actual data. So we can say |
|
| 08:33 | from this single crystal or group of , this single sample, this single |
|
| 08:41 | , we can produce a model which that we can understand its continuous thermal |
|
| 08:47 | history from temperatures, it looks like 2 90 down to about 1 |
|
| 08:54 | That's pretty great. That comes from one sample. A lot of modeling |
|
| 08:58 | into that, but just one Um And so you see that that's |
|
| 09:04 | that's not an episodic event, That's a boom, hit it with a |
|
| 09:07 | of heat and make it go out . That's just cooling slowly. That's |
|
| 09:11 | you see that we start and you can see variations in the |
|
| 09:15 | We can see that prior to about million years ago. This thing wasn't |
|
| 09:21 | and hardly off half a degree C a billion years, then then for |
|
| 09:25 | while it cools. And then from on it pulled out about four degrees |
|
| 09:30 | . And so um this this is from some basement rock. This is |
|
| 09:35 | granite or a nice or something like because otherwise it wouldn't cool this |
|
| 09:39 | But if you're trying to understand the range from when this from which this |
|
| 09:44 | um you're gonna be looking for events 68 million. You may also wish |
|
| 09:51 | if you're if you're doing a whole study, it may look they used |
|
| 09:54 | so why did it start to pull rapid? I do rock school, |
|
| 10:06 | went through this yesterday. Why do rocks cool? Small problem. Big |
|
| 10:18 | . Well whether you know, maybe to the surface, right erosion. |
|
| 10:24 | is trivial erosion is a big And understand the difference between weathering and |
|
| 10:30 | . No weathering is taking a feldspar make it into a play, erosion |
|
| 10:37 | taking the feldspar from colorado. And it in the gulf of Mexico, |
|
| 10:44 | corrosion is moving. Weathering is We're talking about erosion. And so |
|
| 10:52 | did, why did this sample start more rapidly, yep, it just |
|
| 11:02 | to the surface. How did it that? No, no, |
|
| 11:07 | This is a granite. You just erosion a minute ago. They were |
|
| 11:14 | were right on the answer and then ran away from it erosion. |
|
| 11:19 | why did it start to cool even rapidly because the erosion increased. Maybe |
|
| 11:30 | had a Techtronic event, Maybe the got higher or maybe that's a climate |
|
| 11:35 | thing. Something caused the erosion to . That's the reason why these things |
|
| 11:41 | their cooling rate 68 million years this rock was just sitting there. |
|
| 11:46 | wasn't moving. That's very very slow . Right? But then something happened |
|
| 11:51 | it starts to go trust the cool eight times faster than it was a |
|
| 11:54 | back, something happened. It's the above. It is now being pulled |
|
| 12:01 | so fast that this rock is fooling if you wanted to, if you |
|
| 12:08 | sure about this geochemical nonsense here, might you look for in the regional |
|
| 12:13 | to to cement that interpretation? If true, If there's increased erosion above |
|
| 12:19 | mountain in this mountain range, what should we expect to find? Where |
|
| 12:29 | this eroded? What happens to this means we're coming close to the |
|
| 12:34 | Right. So and here we're going 280° to let's just say, we |
|
| 12:42 | down 100 degrees in 15, let's say 100, Yeah. In the |
|
| 12:54 | millionaire, How much material is a When you, when a rock cools |
|
| 13:04 | How much closer is it to the ? About four kilometers. That's a |
|
| 13:12 | of stuff. Where does it go speaking, it goes into some basin |
|
| 13:28 | right now, we don't know if knew if this was the Himalayas, |
|
| 13:33 | might look for a basin in Northern or may have been the basin all |
|
| 13:36 | way out in the in the bay Bengal. If this was the Rocky |
|
| 13:40 | , we'd look for maybe some, know, we look for basis |
|
| 13:43 | The basins ultimately could be in the Mountains. It could be the gulf |
|
| 13:46 | Mexico. That's kind of hard to together. But sometimes you would want |
|
| 13:50 | least first look if there's a base nearby, what time period would you |
|
| 13:55 | rapid sedimentation to take place in that ? No, no, not |
|
| 14:07 | what time, when, when would ? This basin is right next to |
|
| 14:13 | mountain ring. If this mountain range being uplifted this basin, what's happening |
|
| 14:22 | ? What where does, where where does this sediment in the basin |
|
| 14:28 | from? Mountains maybe. So this us that this mountain is being |
|
| 14:37 | erupted, eroded kind of fast during interview here. This this pale Eocene |
|
| 14:46 | . We would expect to see some in strata with an equally impressive rate |
|
| 14:52 | change here. And if we could we could link the two together and |
|
| 14:55 | , aha, this is the We've got the source area, we've |
|
| 14:58 | the sink. This tells us what period to look for in the |
|
| 15:04 | This area was being eroded Before Not at all. From 68 to |
|
| 15:10 | quite rapidly. And that can come this modeling of these felt sparks kind |
|
| 15:16 | cool. Um what else do we to say? Well, let me |
|
| 15:25 | , okay, we're gonna do Do I want to do? I'm |
|
| 15:29 | , I'm gonna skip that. Let's go to a couple more examples of |
|
| 15:33 | we can use this in. Uh , we'll just use these two examples |
|
| 15:38 | then we'll be done how we can this in understanding the tectonics and the |
|
| 15:44 | area. No mountain building and you know, when and how much |
|
| 15:50 | a result from southern Tibet And what have here is a mountain and the |
|
| 15:58 | here doesn't really show it very but that's a big mountain. That's |
|
| 16:02 | that's an elevation gain of 1000, , 1000 m. Okay, Bottom |
|
| 16:09 | that hill is at 3600 m. top is at 40 600 m above |
|
| 16:14 | level. So What you see there five sample locations Each about 250 m |
|
| 16:26 | . And those sample locations are where , this is, this is all |
|
| 16:30 | big granite, the big granite here a grand jury. And we have |
|
| 16:36 | that go down here and these numbers represent the Argon 40, 39 bio |
|
| 16:42 | ages for this Pluto. They all from the same flu time. So |
|
| 16:49 | probably all similar. We can assume these bio types all have the same |
|
| 16:54 | temperature and that's all we really need interpret them. I don't really need |
|
| 17:00 | know what closure temperature it is. the same now in a minute. |
|
| 17:05 | pay attention to exactly what the closure is. But for the moment, |
|
| 17:08 | just say they're the same. What you notice about these numbers? They're |
|
| 17:19 | as you go? Well increasing as go up or decreasing as you go |
|
| 17:22 | . Yeah. Anything else? The the the distance that the time |
|
| 17:32 | between the two gets quite small as get to the bottom first, we |
|
| 17:35 | a distance of 3.5 million. And the last two have only 600,000. |
|
| 17:42 | these this is, let me tell another piece of information about this |
|
| 17:47 | This granite has been dated by uranium Zircon ages And that result was 42 |
|
| 17:59 | . This this granite has been dated uranium lead on Zircon And the result |
|
| 18:04 | that that that method um brought back 42 million. These are all younger |
|
| 18:11 | that. Right? Remember what is what how do we interpret uranium lead |
|
| 18:17 | data on on a grant for granted happened 42 million years ago? I |
|
| 18:30 | hear you younger then. We got number 42. What does it |
|
| 18:43 | We spent a whole lot of time lunch talking about that that that that |
|
| 18:47 | , what the Zircon data needs? the closure temperature of lead in the |
|
| 18:58 | ? Something like that. Okay, when we date as your car with |
|
| 19:03 | closure temperature of 800°. How can we its geologic significance? What happened |
|
| 19:10 | That's the crystallization age? I spent minutes for lunch talking about the difference |
|
| 19:16 | the uranium lead Zircon age and the and the Argon age in the same |
|
| 19:21 | . They differ by 3000 years. ? Because one has a lower closure |
|
| 19:27 | . This is the same problem only lots slower. When we get a |
|
| 19:35 | million year old zircon out of this , what do we say happened 42 |
|
| 19:40 | years ago? That's the age of granite crystals form. What does it |
|
| 19:46 | when we data bio tight out of same granite? Thanks same as |
|
| 20:03 | No, no, you don't. in general. What does it mean |
|
| 20:06 | we data bio type by the argon , does it mean the age of |
|
| 20:11 | ? Like the other thing we just , what's the opposite of crystallization? |
|
| 20:33 | , all of this always goes back knowing the closure temperature. That's why |
|
| 20:40 | put that slide up first thing in class. That's everything resolves around |
|
| 20:44 | What's the closure temperature of argon and type? Yeah, about three. |
|
| 20:54 | look at this, we have a pretty big mountain 1000 m takes all |
|
| 21:01 | to climb up there. Top of mountain has a via tight age of |
|
| 21:09 | . The bottom of the mountain has by tight age of 18. How |
|
| 21:14 | we explain that? If the if uranium lead Zircon ages 42, what's |
|
| 21:23 | closure temperature of argon and bio Okay, that's very important. How |
|
| 21:29 | we use that to interpret the geologic of this granite? What happened to |
|
| 21:37 | sample? 26.8 million years ago? . What happened to this sample 23.3 |
|
| 21:46 | years ago. Same temperature. These different elevation by 250 m. Does |
|
| 21:58 | mean this is older, younger, , younger, younger. How did |
|
| 22:06 | , what's going on? You just what these things mean individually? What |
|
| 22:16 | that mean in concert? Where where is 300° down there? How |
|
| 22:29 | down is 300°. You got how what the geothermal gradient? You got to |
|
| 22:35 | that too? 25 into 300 is , right, that's all you gotta |
|
| 22:43 | . There's about 12 km down. we can say that that's that when |
|
| 22:51 | You rightly pointed out that 26.8 is time when that rock got to 300°. |
|
| 22:57 | would also say this time when that got to 12 km. Because I |
|
| 23:02 | , if we if we assume that a simple relationship of 25° Super |
|
| 23:08 | So Then what happened to the other of 23 million years? That would |
|
| 23:16 | the time when it got to 12 . This sample was 12 km below |
|
| 23:21 | surface. How far is this sample the surface? 12 km -250 |
|
| 23:32 | So what do we have here? 12 km depth Is always 12 km |
|
| 23:39 | us. Now, let's assume that Earth's surface is this sort of |
|
| 23:44 | We are watching the passage of this of rock passed. This place is |
|
| 23:50 | , this this place where it's always and I saw a therm If you |
|
| 23:56 | here, if we got we got kilometer a rock down here. This |
|
| 23:59 | is now, if this thing is 12 km, this thing's at 13 |
|
| 24:03 | , it's too hot to retain its down here. This one just get |
|
| 24:09 | that right temperature, starting to hold it a little time goes by now |
|
| 24:14 | guy is holding on to his heart then in seven in 8,000,007 million |
|
| 24:21 | whatever that is, nine million This sample gets to that place. |
|
| 24:27 | . You see that got it So we can see not only that |
|
| 24:33 | thing moved up, which of course did because Grant has always start down |
|
| 24:37 | at the surface today, we know happened, but with this we can |
|
| 24:43 | when and how fast And in we can say that this was an |
|
| 24:48 | uplift from 23 to 17. It getting faster. This took three million |
|
| 24:52 | . And you know, these samples strategically located to have 250 m between |
|
| 24:57 | all safe That 250 m took three years. That took three took to |
|
| 25:06 | one. We are seeing the acceleration tectonics in this region. Such that |
|
| 25:12 | really going crazy at about 18. a tectonic story. Just by seeing |
|
| 25:21 | thermal history off one side of one . And we could, we could |
|
| 25:28 | combine that, that these are the types, bio types with elevation. |
|
| 25:33 | also did the case bars and the fishing track data. And they have |
|
| 25:38 | closure temperatures. And you see that case bars are pretty much all the |
|
| 25:43 | . The case cars haven't closed your less than the five types. And |
|
| 25:48 | By the time we accelerated to go a temperature of 200°, we were going |
|
| 25:53 | fast at the top of the bottom about the same age. And then |
|
| 25:59 | , so we can put this all in terms of a exhumation ring, |
|
| 26:04 | rate of erosion Back here before 20 years ago. And we can |
|
| 26:09 | we can take this back all the to 42 million. And this thing |
|
| 26:13 | formed. Not much happened, but about 20 million years ago, this |
|
| 26:19 | starts rocketing up. It's going at a year for a very brief period |
|
| 26:23 | time, Then cooled off again. caught it just in the act of |
|
| 26:28 | it made most of the most of transit. This rock made from, |
|
| 26:32 | from it's uh formation, 13 km the surface to the surface. It |
|
| 26:38 | most of that Right here in this or 2 million year interval? That's |
|
| 26:45 | news. I mean, you so that's a story about the uplift |
|
| 26:50 | history of this for this area. can put all of that tectonic story |
|
| 26:54 | . And then, as I if you're in a region where you |
|
| 26:57 | link the provenance to the sink, you'll be looking for The sedimentary deposits |
|
| 27:04 | that age because that material had to . So we're talking about uplifting 10 |
|
| 27:09 | of stock in a really short period time. You wouldn't think that's, |
|
| 27:15 | know, just looking at that all you can say is, |
|
| 27:17 | that's a that's a granite started down now. It's up here with this |
|
| 27:21 | chronology stuff. We can put timings . That's a tectonic story that will |
|
| 27:26 | be related to everything around us, the base information. Now, let's |
|
| 27:34 | this one to another granite just around corner. This is another granite nearby |
|
| 27:39 | 50 kilometers or less away. And is the loss of granite and the |
|
| 27:45 | dark. But anyway, it's it's another big mountain and this time |
|
| 27:50 | have one and a quarter kilometers of here, But in this case the |
|
| 27:55 | tight ages are exactly the same 62.8 62.5 with uncertainties. We can't tell |
|
| 28:03 | difference between these two. How must interpret that? Is this another example |
|
| 28:14 | really rapid erosion or is it could if it is, I mean that |
|
| 28:23 | that is a one and a quarter kilometers in, Well, a very |
|
| 28:28 | amount in a 10th of a million . Let's say That is 12 mm |
|
| 28:35 | year. One millimeter per year is extremely fast. I mean, and |
|
| 28:44 | and that's the case for sedimentation or because I mean the erosion erosion is |
|
| 28:49 | you know, has to go And if, you know, if |
|
| 28:52 | ever sit down and look at basin , a basin that's filling up at |
|
| 28:57 | a millimeter per year is a really up basin. And if something is |
|
| 29:01 | up at half a millimeter somewhere else being eroded and half a million. |
|
| 29:06 | insisted on this being only because of . We would have to be talking |
|
| 29:14 | 15 mm a year, 20 per . If you go to a |
|
| 29:20 | S. A. Or an A . U. Meeting and you give |
|
| 29:22 | talk about erosion in a mountain belt you say, I think this means |
|
| 29:26 | there is a time of two millimeters year erosion. A lot of |
|
| 29:31 | you know, they will stop, know looking at their program or imagining |
|
| 29:34 | they're going to go out to lunch they'll stop and say what two |
|
| 29:38 | I gotta pay attention to this. is either nonsense or extremely interesting, |
|
| 29:45 | ? But if they were to say mm per year, you're an idiot |
|
| 29:51 | that isn't happening. That's really very high. But is there an |
|
| 29:58 | ? How can we how can we these ages to be the same |
|
| 30:03 | 1200 m cliff space? Not really cliff, but a mountain Over 1200 |
|
| 30:11 | . How can we get those two tight ages to be essentially the |
|
| 30:16 | What what geologic story could we make would allow that? Mhm. It |
|
| 30:24 | with the closure temperature. What is closure temperature? Okay, This says |
|
| 30:30 | the top and the bottom of that , we're at 300 at the same |
|
| 30:37 | , we discussed the erosion possibility that thing, here's your 300 degree ISIS |
|
| 30:43 | and this this pile of of granite through here so fast that we can't |
|
| 30:48 | him. That's one. Is there else? Where do gran its form |
|
| 31:02 | there? Exactly how far down Are they all the same? What |
|
| 31:11 | it was shallow? Let me go to this one. At what depth |
|
| 31:22 | this granite form? What constraints can put on it? We know that's |
|
| 31:33 | million years old. We know that tight at the top. There is |
|
| 31:37 | million years old. What was the of the rocks surrounding this granite when |
|
| 31:45 | was formed? It was above 300°, . It wasn't above 300°. We would |
|
| 31:52 | cooled down to the surrounding temperature pretty . And in that case there by |
|
| 31:58 | age would be pretty much the same the zircon age. Right. This |
|
| 32:02 | us that this rock was intruded at depth, fairly exceeding 300 degrees. |
|
| 32:09 | we wouldn't have this much variation. waited around this was 42 million years |
|
| 32:13 | eventually this thing got up to 300 and then we saw the accelerating movement |
|
| 32:18 | 300 degrees but 300 degrees here. was put down somewhere deeper. Is |
|
| 32:26 | the only option Down here? 300 here? It's another option. Is |
|
| 32:35 | possible? No, can't intruder granted less than 300°. How do we |
|
| 32:42 | How do we get volcanoes? Mount Helens. Where is that magma coming |
|
| 32:49 | ? What? Well under there How far down 12 kilometers? The |
|
| 32:57 | chamber for Mount ST Helens coming up kilometers in one burke. No Magma |
|
| 33:07 | Beneath Volcanoes might be only one or km down there. I mean the |
|
| 33:12 | chamber beneath Iceland is right right Right. And what if that? |
|
| 33:21 | what about this then? What if had a bunch of magma And here's |
|
| 33:27 | here's 300 again. What if we all this magma this? 700 800° |
|
| 33:33 | and we push it up to here at once. 300. It'll cool |
|
| 33:41 | below 300 pretty fast, no matter we are. And so that's what |
|
| 33:46 | think this one's telling us. It's , this is not an outrageously fast |
|
| 33:52 | of erosion. This is a shallower of intrusion. This is not a |
|
| 33:59 | tectonic story. This is a boring story. Pluto intruded relatively shallow, |
|
| 34:07 | all the time. And when it , the bio types are not interesting |
|
| 34:13 | a subsequent tectonic perspective because they're already in and telling us that that |
|
| 34:19 | it was cold when we started. may learn more information by going to |
|
| 34:22 | things that have a lower closure the helium, the fishing track, |
|
| 34:26 | we'll get to just now. But but this rock pool below 300°. Pretty |
|
| 34:31 | everywhere. Pretty much from the very , we don't have an app a |
|
| 34:36 | lead zircon image from this blue, we didn't think it was necessary. |
|
| 34:43 | if we did get it, what you predict it to be? |
|
| 35:07 | What number? Give me a What was the age you would predict |
|
| 35:11 | a uranium lead zircon anywhere in this . Okay. How much older? |
|
| 35:22 | 80 million or 63 million. We've that this thing is Being intruded up |
|
| 35:39 | above 300°. Right? And then So means that this is not only that |
|
| 35:45 | is the cooling age. This is this rock got to 300°. But if |
|
| 35:50 | had to jam it up here above What condition was it is when you |
|
| 35:54 | it up here, it was at partially melted. Right? Solid things |
|
| 35:58 | pass up through the crust that So we had to get up here |
|
| 36:03 | . But then we had to cool quickly. This is the crystallization. |
|
| 36:07 | . So what would the zircon hb here? Hmm. That's the closure |
|
| 36:20 | . What age if you yes, that closure temperature. And given that |
|
| 36:25 | two things are 300, what would expect for any zircon here? He's |
|
| 36:31 | to be 63 because this is telling that the whole thing crystal cooled, |
|
| 36:36 | and then cooled. Mhm. The is below 300. Such that we |
|
| 36:43 | tell the difference. Remember, we're saying that this is a fantastic erosion |
|
| 36:48 | . We're saying this is just cooling the big bunch of stuff because we |
|
| 36:52 | it in a freezer Freezer being rocks are only nine km below the |
|
| 37:03 | So 63 for the Zircons, that sense. Neither one of your getting |
|
| 37:11 | . Okay, so, we've got closure temperature of these things are |
|
| 37:16 | Right, and go back to this , Same closure temperature. Why are |
|
| 37:23 | are these ages so different? Because took us a long time. |
|
| 37:26 | here's our special zone. Right? here we got 26 then 18. |
|
| 37:32 | took passed through here and we know passed here because we know it started |
|
| 37:36 | here, 42 million. That's the age of the rock. This rock |
|
| 37:41 | for 18 million, 18 million years this bio type cooled off sufficient to |
|
| 37:48 | onto the Argo. But we don't that's the case here here. What |
|
| 37:54 | know is that these the by tight the top and the bottom of this |
|
| 37:58 | big hill, give the same And the simplest way to do that |
|
| 38:02 | to take this granite and intruded at very shallow depth. Take some |
|
| 38:08 | make it down here. It's magma up here, becomes cold so cold |
|
| 38:14 | the bite types cooled so rapidly. these guys are cooling that rapidly, |
|
| 38:20 | everything else that has a higher closure must have also cooled rapid. That |
|
| 38:27 | leave any time for anything else. the other, all the other ages |
|
| 38:30 | we might get that have a higher temperature. They should also be |
|
| 38:35 | I would think 66 would be a big number for a situation like |
|
| 38:41 | It was the other way. Um was like, I didn't have files |
|
| 38:51 | wouldn't tell you anything. That would tell you that the crystallization age was |
|
| 38:57 | . What happened afterwards is still an question because we don't have those |
|
| 39:02 | You'd have, you know. But if you got a bio type, |
|
| 39:05 | was 63, then you say, , we cooled from 900 to 300 |
|
| 39:10 | quarter million years. And that's either of cooling directly after emplacement or because |
|
| 39:16 | erosion. You have to decide which which in this case it would be |
|
| 39:20 | unbelievably fast. But let's go with intrusion. And particularly if we, |
|
| 39:27 | know, if we date something else here, like let's do appetite fishing |
|
| 39:32 | that has a cool at a temperature 100 degrees. If we, you |
|
| 39:36 | , we could get like 20 million there and they said, well, |
|
| 39:39 | cooled really fast to 300 degrees, then it took another 40 million years |
|
| 39:43 | cool to 100 degrees. That's the part, that's the erosion part. |
|
| 39:47 | is just the emplacement part. There's cooling because granite school down when they |
|
| 39:55 | , Where they stop, depends on fast they cool initially. If |
|
| 39:59 | if you intruder granite at 20 you know, it's still hot for |
|
| 40:03 | long time. But you can some are intruded at five km and that's |
|
| 40:08 | , that takes all those high temperature kilometers and starts taking them off. |
|
| 40:11 | get the economy and the corn blend the bio type. Maybe not the |
|
| 40:16 | spot, not the fishing tracks. get to those later. That's the |
|
| 40:21 | the erosion part, that's the tectonic . So we've got cooling because of |
|
| 40:27 | . We've got cooling because of erosion . And it's only the erosion tectonics |
|
| 40:31 | that's gonna show up in the I would think everywhere it's gonna be |
|
| 40:41 | , at the bottom and 63 at top. This is saying that it |
|
| 40:46 | mean I collected some samples here, we didn't bother to analyze them because |
|
| 40:50 | all we have no expectation of anything than 62. Okay, what else |
|
| 40:59 | got? Um Well let's just look this one. Then. Here's another |
|
| 41:10 | . And this gets back to our in central texas. We've seen rubidium |
|
| 41:13 | for these rocks. We've seen uranium for these rocks. Now let's look |
|
| 41:16 | our country, the same rocks. got an an football of 1076 plus |
|
| 41:21 | -8. You gotta buy tide of plus or -8. What does this |
|
| 41:26 | us about this? Granite? If recall the Zircon age was 1084 and |
|
| 41:33 | Rubidium strontium age was 1081. But have relatively high uncertainties plus or |
|
| 41:42 | What's the closure temperature of of argon Amfa Bill 500. What's the closure |
|
| 41:51 | in biotech. That's sweet. So are the same. These about the |
|
| 41:57 | . Right? What? Well, they're they're not just close. They're |
|
| 42:03 | . It's impossible to tell the Look at the look at the |
|
| 42:07 | They overlap so close might not be enough here. They are indistinguishable |
|
| 42:15 | that doesn't mean they're not truly It's just that our uncertainties are too |
|
| 42:18 | better. And you know this this analysis was done in 1999 if we |
|
| 42:23 | one of those fancy mass spectrometers, showed you before lunch where they |
|
| 42:27 | Nowadays, years later, these You know, you could get better |
|
| 42:32 | . But these these data are are for this for the educational value of |
|
| 42:37 | plot. What does this tell us the intrusion depth of this grant? |
|
| 42:54 | got 500 and 300. Right. yet the two systems can't tell the |
|
| 43:03 | . How is that possible? Same sarah. We have to have rapid |
|
| 43:09 | . It's unlikely that that rapid cooling because of tectonics or climate. An |
|
| 43:14 | way is to just have this granite at a depth less than what? |
|
| 43:33 | than 12 or 13 km. This 300 is the lowest one here, |
|
| 43:39 | ? 300 divided by 25 is add a few degrees for the surface |
|
| 43:45 | are we're at so we're at 12 12 inch kilometers. Let's say this |
|
| 43:49 | was intruded less than 12. That's hard to imagine lots of 12 kilometers |
|
| 43:54 | way down. So that's what this us. And some from that. |
|
| 43:59 | know, if we see a lot proteins that are intruded low. You |
|
| 44:02 | , if you see evidence for lots photons being intruded high up in the |
|
| 44:07 | that tells you something about the general structure. Maybe that'll tell you about |
|
| 44:11 | pattern. You know, it's just story that you put together big. |
|
| 44:14 | this tells us that this was not Luton that started way, way down |
|
| 44:21 | . Um All right now, let's it, but in uh you people |
|
| 44:36 | drill down and measure the temperature down . Well, 25°/km goes into, |
|
| 44:48 | into 300°12 times 25° means every 100 is four km. Follow me 25 |
|
| 45:01 | four is 100. So that means go down 100° every four km. |
|
| 45:07 | 300° is 12:00. That's all there to it. Every upset again. |
|
| 45:22 | , every volcano is Okay, I think we're gonna move on. |
|
| 45:32 | are going to stop sharing so that can get out of that. And |
|
| 45:42 | gonna open up another, what are going to? We're going to fish |
|
| 45:47 | tracks. Okay, new system, our chart of the Duke Clyde's all |
|
| 46:09 | potential atoms that we know about. proton number here, neutron number |
|
| 46:15 | They go up through here. You , here's here's height, then helium |
|
| 46:20 | here. Here's, here's uranium up . The colors here, black are |
|
| 46:28 | stable ones, those ones kind of the middle here. Um red and |
|
| 46:33 | are different kinds of decay, yellow and yellows and other kinds of |
|
| 46:38 | And then green ones. These green up here. These high ones here |
|
| 46:41 | new slides that have significant contributions of fission. Remember that's when something big |
|
| 46:50 | uranium 238 spontaneously breaks into two pieces are relatively large. Um you |
|
| 46:58 | here's the decay, we talked about before, a bunch of alpha decays |
|
| 47:02 | some beta decays. This is small compared to this. This is the |
|
| 47:07 | way in which something happens, uranium into two things. And this curve |
|
| 47:12 | here called the camel curve for obvious , shows and this is a note |
|
| 47:18 | this is a logarithmic scale over but it shows the yield in percent |
|
| 47:23 | the mass numbers that are produced by . So it's about 9% of the |
|
| 47:30 | have a mass of about 90 and 9% have a mass of about |
|
| 47:36 | And then the then the percentages drop as you go away from us. |
|
| 47:40 | you know, still you have, you have a 60 that means you |
|
| 47:44 | have a 1 60 or 100 that you have 100 and 30. They |
|
| 47:49 | to add up to 38 two And most often you get a piece |
|
| 47:55 | about 90 and a piece at about 40 for that would be 1 51 |
|
| 48:02 | . Okay, it's two pieces. break into two pieces and they go |
|
| 48:07 | fishing and there's energy associated with that that they do go in some physical |
|
| 48:15 | has shown there. And these things they are not just alpha particles, |
|
| 48:20 | are, you know 40, 100 times bigger. Oh 2030 times |
|
| 48:25 | than an alpha particle. Their passage the crystal causes changes in the |
|
| 48:31 | significant changes in crystal um Before but we get to that, let's talk |
|
| 48:37 | the rate at which this happens the of alpha decay in uranium 2 38 |
|
| 48:43 | has a half life of 4.5 billion . So we're making we're making lead |
|
| 48:49 | alpha decay at that rate. For probability, the rate of spontaneous fishing |
|
| 48:56 | a lot slower. Not 10 to ninth, it's 10 to 15 |
|
| 49:01 | It's a two million times slower. that that happened very often. And |
|
| 49:08 | you know, that's an issue. that in this example here, you |
|
| 49:12 | , I showed you another example yesterday I was introducing the concept of spontaneous |
|
| 49:17 | . I think it was strontium and . I showed you here in this |
|
| 49:21 | , it's barium and krypton can be of different things, you know that |
|
| 49:25 | not this is this curve just shows possibility. Sometimes it's this and |
|
| 49:33 | You know, sometimes it's not important it is because we're not measuring those |
|
| 49:39 | ever again. We're not, we're we're not tallying up the barium or |
|
| 49:43 | and that's particularly because that's a big . They're not always the same. |
|
| 49:47 | know, we have to we have find all of these different things and |
|
| 49:50 | them up. It's impossible, but is always the same is these two |
|
| 49:54 | go like this and they change the . Um The minerals that we use |
|
| 50:00 | fishing track are the same minerals we for uranium lead, they are |
|
| 50:05 | they are zircon. They're sweet. Those are the only minerals that are |
|
| 50:10 | enough in many rocks and have enough to be worth doing. And they |
|
| 50:15 | have closure temperatures that are convenient for of geologic applications. So also seen |
|
| 50:24 | muscovite, maybe some glass. But all these cases, here's what's going |
|
| 50:29 | . This is a this is just model. But we have our original |
|
| 50:35 | would be sitting here sitting in some crystal lattice. Then this thing shoots |
|
| 50:42 | in two directions and the crystal lattice perturbed because of that, the distance |
|
| 50:48 | that would go is about 15 So it's a it's an observable distance |
|
| 50:54 | the medium, you know, the microscope. So here's another model |
|
| 50:58 | We've got this this uranium 238 and shoots that crystal for that new ion |
|
| 51:05 | the thing. And as it does it displaces displaces some of the um |
|
| 51:11 | of the lattice. There may be relaxation after this is over. But |
|
| 51:15 | still clearly a zone of of damage change. And that's a damage zone |
|
| 51:22 | we can observe. So every time occurs we make one of those that's |
|
| 51:29 | daughter product is the is the physical . The scratch if you will, |
|
| 51:36 | not gonna be measuring like before we some chemical thing, measure some |
|
| 51:41 | We measure some argon, measure some , we measure some astonishing both. |
|
| 51:45 | sort of geochemical thing. Right we don't do that. We observed |
|
| 51:51 | physical changes in the mineral with a . That's how we figure out |
|
| 51:55 | The daughter product is not a chemical , it is a lattice dislocation. |
|
| 52:06 | things to know about these tracks is they they are cylindrical as along the |
|
| 52:13 | section or their circular along that cross . And they um maybe that's all |
|
| 52:21 | need to say right now. Um they are made they might be um |
|
| 52:30 | says mm. That's wrong. That 10-20 mm. No, 10-20 microns |
|
| 52:37 | should be in. Let me just that. Where did it go |
|
| 52:50 | That's totally wrong. There we There are 6 to 10 nanometers in |
|
| 53:01 | . They're 10 to 20 micrometers in . Um Okay, Now being only |
|
| 53:14 | nm wide, they're pretty hard to with a microscope. But we can |
|
| 53:21 | that. We take these samples if take a sample like an appetite or |
|
| 53:27 | and we can enhance the visibility of things by polishing the sample. Getting |
|
| 53:35 | . Getting it nice and flat and putting some acid on that surface because |
|
| 53:40 | damage zones are are damaged zones. acid will attack them preferentially opening up |
|
| 53:46 | very thin lines to be lines that can now see with a microscope. |
|
| 53:50 | you can't do this without preparing the . But once you do, then |
|
| 53:55 | fishing tracks are observable. And and here's an example of one. Look |
|
| 53:59 | the scale bar down there, 2020 . So this is only like a |
|
| 54:04 | across here. No, this is half a millimeter across 10th of a |
|
| 54:08 | is a small little chip. But can see all these tracks all over |
|
| 54:13 | place. Each one of those tracks one of these fishes that took place |
|
| 54:18 | caused this to happen. Those are daughter products. Okay, um how |
|
| 54:27 | we use that then to date Well, the number of fishing tracks |
|
| 54:31 | going to be a function of And the uranium concentration. And we |
|
| 54:37 | then um this this fission tracks would density efficient tracks would be a |
|
| 54:44 | The time. Here we have we to keep track of the fact that |
|
| 54:48 | losing most of our uranium 2 38 alpha decay. But sometimes we do |
|
| 54:52 | like this. Okay, so that's like in that other instance where we |
|
| 54:55 | to decay constants. We have to at that ratio here is the amount |
|
| 54:59 | uranium and then here's the decay Um They are the number of fishing |
|
| 55:07 | is going to be a function of of uranium concentration. And one more |
|
| 55:13 | . Uh Well, we get to in a minute. And one more |
|
| 55:16 | , which is temperature because one thing haven't mentioned yet is what's very interesting |
|
| 55:22 | these fish and tracks is in all these minerals, you heat the mineral |
|
| 55:28 | the crystal, relax and goes back the way it used to be, |
|
| 55:32 | fishing tracks will disappear. And that's a kneeling and kneeling and they're |
|
| 55:42 | And the nice thing about this is the temperature at which that healing takes |
|
| 55:47 | is quite a low temperature In an . It's around 100°. What temperature do |
|
| 55:55 | make oil at? It's about 100°. why this this technique is known as |
|
| 56:07 | pretty well in oil companies because this you about whether a sand, you |
|
| 56:12 | , if you take a sandstone that's a bunch of appetite in it. |
|
| 56:16 | it's got lots and lots of fishing in it, you know, maybe |
|
| 56:19 | gives you a parent fishing tracks Let's say it's a say we got |
|
| 56:25 | Eocene sandstone. Well, let's say sandstone. What if all of the |
|
| 56:31 | track ages from the minerals in that stone give ages that are Kirby, |
|
| 56:39 | means the sandstone has never been above because none of the none of the |
|
| 56:45 | are younger than the deposition allayed. if a sample is a cretaceous sandstone |
|
| 56:53 | permian zircons in it. And then zircons get heated up enough. They |
|
| 56:59 | Maya scenes are calm. And the that you need to heat them too |
|
| 57:05 | about the same temperature that you're looking in a petroleum base. So that's |
|
| 57:11 | that they correspond because there's very little to decide other than, you |
|
| 57:14 | look at a look at a you get hot. You know, |
|
| 57:17 | can't tell by looking at the Sandstone 100°. It's not enough to start doing |
|
| 57:21 | lot of die genetic stuff. But know, if you look at the |
|
| 57:25 | tracks, they'll say, well, lot of these fission tracks have been |
|
| 57:29 | since deposition. That means the sample to get hot enough to enable those |
|
| 57:34 | . What temperature was it at about degrees. I'm getting ahead of myself |
|
| 57:38 | little bit. Um, well, come back again because the more efficient |
|
| 57:47 | more efficient will occur. But but those are brand new tracks. We |
|
| 57:51 | want it. This is not, not a, it's not a resurrection |
|
| 57:54 | the previous one. Um, You what, we're a little bit behind |
|
| 58:00 | and it really not too important how do this. I'm not gonna talk |
|
| 58:04 | the method because it's kind of We're just going to talk about the |
|
| 58:11 | . Let's go to interpretation. Um , let's just go here to the |
|
| 58:22 | basin. Well now let me let me just go back to |
|
| 58:30 | Let's just take this as a this is data, you need to |
|
| 58:35 | the closure temperature for various systems We've talked, we've got some |
|
| 58:40 | we've already talked about by a type argon case bar, potassium argon, |
|
| 58:46 | we haven't talked about yet, We're not talking about healing yet, |
|
| 58:49 | this shows the broad closure temperature for fishing track systems for fishing track, |
|
| 58:57 | on the cooling rate, you've got closure temperature. And let's just take |
|
| 59:01 | cooler. Right here, we have about 100 degrees For for Zircon. |
|
| 59:07 | more like 200°. That's good enough for . The temperature at which Fishing tracks |
|
| 59:16 | sensitive. Intercon about 200 In fishing . It's about 100. So, |
|
| 59:28 | us then use that to talk What's this. Uh Talk about the |
|
| 59:33 | basin basin is found in the south of Australia near uh near Melbourne. |
|
| 59:44 | is a series of samples that were in the drill hole. This is |
|
| 59:49 | that was collected. I think it's . I don't think it was cuttings |
|
| 59:53 | sure its core. And so we we're not talking about depth here |
|
| 60:00 | We're talking about temperature, which they in the well as well. And |
|
| 60:05 | can then plot um two things we talk about that. Okay, what's |
|
| 60:11 | ? Um we can plot two things versus temperature in this basin up here |
|
| 60:18 | the top of the well where the is Surface temperatures, the fishing track |
|
| 60:24 | are all about 120 million. But you get down to a temperature of |
|
| 60:30 | 50 degrees, 60 degrees. You , you see a remarkably quick dimunition |
|
| 60:36 | the age of the fishing tracks. that when we get down to 100 |
|
| 60:40 | 20 degrees, the apparent fishing track is zero. There are no fishing |
|
| 60:47 | in this at the bottom of Well, The sample was collected, |
|
| 60:51 | core was collected at a depth of at the depth associated with 120° that's |
|
| 60:58 | , pretty deep. Well, That's gonna be what, 4, |
|
| 61:02 | km down there. But they, know, they make wells like that |
|
| 61:08 | the bottom of this. Well, got no fishing tracks and we pass |
|
| 61:13 | through here, they get older, their oldest most this zone right in |
|
| 61:23 | between about 80 and 100 degrees. known as the partial and healing zone |
|
| 61:30 | we've partially in yield or we've gotten of some of the fishing tracks, |
|
| 61:34 | not all of them. And this us to this diagram over here, |
|
| 61:38 | haven't talked much about. But in , another exciting piece of information we |
|
| 61:42 | get from fishing tracks is not only number of the fish tracks, but |
|
| 61:46 | length. Because a fishing track. it starts fishing takes place, a |
|
| 61:52 | track in an appetite is almost uniformly 15, 16 microns long. And |
|
| 61:59 | this heated up, a very nice happens is that they get smaller, |
|
| 62:03 | just in their length. They don't small. They're this big. They |
|
| 62:07 | this big throughout their whole thing. don't squeeze down like this. So |
|
| 62:12 | don't have to worry about something that's to go. You only see it |
|
| 62:15 | like this. And so over time track that's this big. If you |
|
| 62:20 | it up, will eventually that And it does that. And what if |
|
| 62:27 | , you know, and and if stops, if you take a thing |
|
| 62:30 | and you keep, it at say it will, you know, kind |
|
| 62:35 | go then if you take that thing you cool it off, you will |
|
| 62:40 | made a short shorts. That That fishing track is shorter forever. |
|
| 62:44 | only gets long once it can get again and again. So, if |
|
| 62:50 | look at the fishing track length of samples, the average track length can |
|
| 62:56 | safe samples stay up at the top the well here, it's 15 |
|
| 63:00 | That's what we expect it to That's a natural that sufficient track. |
|
| 63:05 | . But as we go down in Well, by the time we get |
|
| 63:08 | not much data here, but by time we get down to 08, |
|
| 63:11 | have zero length right here, we've down about nine. This is just |
|
| 63:16 | average. We'll see that's even more than the average is the highest a |
|
| 63:19 | , showing the whole distribution of lengths are very important. Let's see do |
|
| 63:25 | have something? Okay we'll get two come on now. Yeah. All |
|
| 63:29 | . So let's that well is great . Let's just consider something a little |
|
| 63:34 | more generic. Let's talk now about about a well but about a |
|
| 63:38 | Same idea though. We are looking differences in depths or differences in |
|
| 63:44 | And so what we have for any track situation is we'll have up at |
|
| 63:49 | top where it's cold. Any fishing that is born up there will be |
|
| 63:54 | big long and stay long. Any track that's bored down here will be |
|
| 64:02 | um erased too hot. These guys the middle start out big and then |
|
| 64:10 | kind of do this because it's it's cold to immediately get rid of |
|
| 64:14 | But it's too hot for this thing stay forever. So the longer you |
|
| 64:18 | at I mean if you stood at you stayed at say 90° eventually it |
|
| 64:24 | disappear. But I have to do slow. If you were if you're |
|
| 64:27 | 200° like that you're at 20° not . And so that's called the partial |
|
| 64:34 | healing zone or the P. Z. Let's imagine now a couple |
|
| 64:41 | scenarios about cooling through the partial healing and we're going to talk about cooling |
|
| 64:47 | hot to cold as an uplift. from the mountain belt later on. |
|
| 64:52 | can turn this around and imagine a rock going back down through the |
|
| 64:57 | But let's the simplest case would be down there. And imagine we have |
|
| 65:02 | simple cooling history in which we cool a linear rate through there were just |
|
| 65:09 | every million years we get a little . Well following this curve here, |
|
| 65:16 | fishing track that is formed here will look like this. It's gone now |
|
| 65:22 | because too hot. But once we into the P. A. |
|
| 65:25 | This crystal here is formed. It's big. But today it looks like |
|
| 65:31 | because it had to spend all of time passing through the partial and healing |
|
| 65:35 | and all that time through the partial healing zone. It's being shortened. |
|
| 65:40 | it gets up above the P. . Z. Then it stopped being |
|
| 65:43 | . But because this one was born here and spent all this time going |
|
| 65:47 | here, it's almost gone. And of these lesser. So this one |
|
| 65:51 | started here, it spent less So it's a little longer. This |
|
| 65:54 | a little longer still. And all these. Now, all of these |
|
| 65:58 | here are the maximum length. And if we were to plot up a |
|
| 66:04 | a gram of a bunch of fishing lengths, This kind of curve would |
|
| 66:10 | this sort of of distribution some short tracks forming those are the ones that |
|
| 66:17 | it down here and took a long to pass through um some full |
|
| 66:22 | Those are these ones and these ones the middle are all the ones that |
|
| 66:26 | over this. So this distribution indicates cooling through the partial and healing zone |
|
| 66:36 | the length of the distribution of tracks quite wide. We've got long |
|
| 66:40 | we've got short ones. You only that by going through the paling, |
|
| 66:44 | kneeling zone over a long period of . Um Here's a couple of other |
|
| 66:53 | . Let's let's start with number That's the simplest one. We basically |
|
| 66:57 | did number two. Let's go to one. Imagine we have some lickety |
|
| 67:01 | uplift goes through the pressure and healing . We would see track length distribution |
|
| 67:06 | looks like that number one. Nothing long tracks some 15, some some |
|
| 67:14 | not very many elevens because the passage here to there. I mean this |
|
| 67:19 | this one here, this one that up at 11, that was that |
|
| 67:23 | that was formed here. But compare with # two. A efficient track |
|
| 67:29 | was formed at this moment has all this time to be shortened and it |
|
| 67:34 | up looking like that. And so distribution is much wider out here because |
|
| 67:40 | district because the path is longer So not only can we tell the |
|
| 67:46 | of cooling, but by looking at distribution of the track lengths, we |
|
| 67:50 | talk about the rate of poof. thing. Number one attack distribution cooled |
|
| 67:56 | with the partial and healing zone. wide distribution pulled slowly. Imagine a |
|
| 68:04 | complicated scenario. No three, we up pretty fast, not as fast |
|
| 68:09 | one but slower but faster than But then we go back down and |
|
| 68:16 | we have a bi modal distribution because these things have to be shortened as |
|
| 68:22 | get pushed down the second time. that one that one's almost gone because |
|
| 68:27 | had all this time to be shortened then all this time to be |
|
| 68:30 | We don't have to very many of guys because this this back up |
|
| 68:35 | It's not enough to all these, of these ones that were produced on |
|
| 68:38 | period here didn't have enough time to short. So this this bi modal |
|
| 68:44 | considers is from an up and down of way. What do we got |
|
| 68:50 | ? Another example um this these are now of of starting from the surface |
|
| 68:58 | going down this would be the basin example. What if we had a |
|
| 69:03 | we and we're going to imagine that of these, all of these rocks |
|
| 69:08 | we're burying our simple, let's say are volcanic rocks. So they start |
|
| 69:12 | this remarkably tight distribution and they're continuing grow, You know, they're they're |
|
| 69:20 | fishing is always happening, But until get down to the partial and healing |
|
| 69:25 | , nothing happens to the fish and . And so a path down here |
|
| 69:29 | a that only gets down to about is gonna have a track length |
|
| 69:34 | That's really hard to tell the difference that and a real life because it |
|
| 69:39 | got to be hot enough, Good, not good. What does |
|
| 69:48 | look like? The track length distribution the highlight would be what why do |
|
| 70:00 | , why do tracks get smaller? get smaller because they're because they're they're |
|
| 70:10 | some condition that's hot enough to make go down. And that condition doesn't |
|
| 70:14 | until you get to about 75°. So you look at the highlight that never |
|
| 70:19 | buried, every single fishing track in real light would be as big as |
|
| 70:23 | ever got. You'd have a very distribution. They'd all be 14 or |
|
| 70:27 | microns law. Let's take that sample bury it. But let's only bury |
|
| 70:33 | to 70°. Nothing's going to happen to . It will be a highlight that |
|
| 70:41 | spent its entire life at the surface a highlight that has been buried to |
|
| 70:48 | C will be indistinguishable from their efficient length distribution Because nothing happens until you |
|
| 70:57 | down to a certain temperature, 68° not hotter. So that's the sensitivity |
|
| 71:02 | the system above 68°. Everything's the But between about 70 and about 1 |
|
| 71:11 | we have these characteristics. Look look at B. Now if we |
|
| 71:14 | B. And we put it right in the middle of the P. |
|
| 71:18 | . Z. And let's say we it down there in a drill |
|
| 71:23 | we'll bring it up, it's going have this very broad distribution because all |
|
| 71:27 | the fish and tracks are being actively down at that temperature. If we |
|
| 71:32 | it down even further, where it's , it's up to 125 degrees. |
|
| 71:36 | really knocking these guys down. There that many fishing tracks, and some |
|
| 71:40 | them are long because they were created , but many of them were almost |
|
| 71:44 | . Two or three microns long. one's 15, this was four. |
|
| 71:48 | we got a very broad distribution. got to we've got three, we've |
|
| 71:52 | we've got 14, 15, because are pushing it down into high |
|
| 71:57 | If we were to take another path went down like this here, there |
|
| 72:02 | be no official trust whatsoever. let's look at these other ones, |
|
| 72:10 | little more complicated. D we go and back up. That's when we |
|
| 72:14 | the bimodal distribution. E we just , let's see e o if we |
|
| 72:20 | down further and are about the how they differ, depends on the |
|
| 72:25 | at which we do this, this going down faster. This is taking |
|
| 72:29 | , but they're pretty close, That would be, and in |
|
| 72:32 | the the track length distributions are not unique solution. Sometimes there's a lot |
|
| 72:38 | different, different thermal histories can give a very similar one. So it's |
|
| 72:42 | always a a real precise deal, F. And D. Have |
|
| 72:48 | you know, they're they're going down about the same temperature. E on |
|
| 72:52 | other hand, is going up and have this sort of tail here. |
|
| 72:57 | this is a cooling history and all these old these long ones here are |
|
| 73:02 | this part here. So the rate cooling through the partial and healing zone |
|
| 73:11 | the distribution of track lengths. And course it'll affect the number of tracks |
|
| 73:16 | have, the longer it's been since passed through the partial and healing |
|
| 73:20 | the more tracks you have. But you deal, if you if you |
|
| 73:23 | at it more closely, you'll see the distribution of their links tells you |
|
| 73:27 | lot about how you pass through this temperature zone. What else we |
|
| 73:34 | Um Okay then there's modeling, there's program that's called hefty, which is |
|
| 73:41 | to look at both helium and fishing , that's where the H. |
|
| 73:44 | F. T. Comes from. uh this is a program that will |
|
| 73:48 | some inputs, you know, your number, your track length distribution, |
|
| 73:53 | helium data, your fish and track . And it will then do monte |
|
| 73:58 | simulations are just randomly throwing thermal histories at it. It's not a very |
|
| 74:03 | program but it just does over and . What about this? What about |
|
| 74:06 | ? And then it it takes all all the thermal histories that matched the |
|
| 74:11 | close enough and plots it up like . So you can see these are |
|
| 74:15 | some examples of from one paper, can't remember where I got this, |
|
| 74:19 | it shows that the modeling bob, a paper model for example, that |
|
| 74:25 | all the way back to the but only part of the model that's really |
|
| 74:32 | is the stuff after 100 and 20 above 120 degrees. Well, it's |
|
| 74:36 | all hot. Right? So it have been could have been 200. |
|
| 74:39 | could have been at zero. But know that by about 100 and 20 |
|
| 74:44 | years ago we got 200 degrees below degrees. We're fine above 100 |
|
| 74:50 | All this all this stuff we don't . So it's and that's that should |
|
| 74:57 | not surprising if the closure temperature is degrees. We're not going to be |
|
| 75:01 | about what this system won't tell us what happened at 200 degrees. We |
|
| 75:06 | a different system for that. Just more examples of the modeling and then |
|
| 75:16 | why of course, you know, oil companies are keen to do this |
|
| 75:20 | because the The graph of oil hydrocarbon , you know, sort of maxes |
|
| 75:27 | at about three km depth or about And this is the partial and healing |
|
| 75:35 | here. This is the track length . Track fishing track age decreasing across |
|
| 75:42 | zone here. The zone in which tracks in appetite are most sensitive, |
|
| 75:48 | actively uh very closely corresponds to the in which china carbons are made most |
|
| 75:55 | efficient. So there's a good correspondence um We did that already. Um |
|
| 76:05 | again, here's the Otway basin. see the strata graphic age of the |
|
| 76:10 | . We go down in the basin once we get to a temperature of |
|
| 76:13 | 60 degrees, everything starts to The ages go down to zero and |
|
| 76:19 | fish and track lengths go down to . And here's the distributions here. |
|
| 76:24 | the track length distribution, nice and and narrow, widening, widening, |
|
| 76:30 | widening. Next one, they're all . Um here's another example that's um |
|
| 76:40 | of a famous example that is used understand the structural history of a |
|
| 76:44 | This is near Red Lodge Montana, is where U. of H. |
|
| 76:48 | its field camp. Now, not that's important. And along this is |
|
| 76:52 | highway that takes this route us to which goes from pretty low elevation here |
|
| 76:58 | to about three km above sea up here in the near near |
|
| 77:04 | And so there are samples along this and then there are other samples in |
|
| 77:09 | subsurface. This Amoco bear tooth went one well. And so when we |
|
| 77:14 | at the data here, these are , the elevation goes from kilometers above |
|
| 77:18 | level zero up to almost three. samples here were sampled along the |
|
| 77:26 | And these samples here were sampled in , in the well. And what |
|
| 77:31 | have is an elevation, a really elevation because we've got, we've got |
|
| 77:35 | big cliff here on in the mountains then we're right next to quite a |
|
| 77:39 | well. And what we have here the appetite fishing track age plotted on |
|
| 77:45 | line. And the elevation here and associated with these graphs are the fish |
|
| 77:50 | track length distribution. And so what we got here is above three km |
|
| 77:56 | sea level. We have this very slight shallow slope in which we |
|
| 78:01 | from ages about 40 million pages. 280 million. But look what happens |
|
| 78:08 | we get below that elevation, we a whole bunch of agents that are |
|
| 78:12 | exactly the same from about 45 to million years old. Notice also that |
|
| 78:19 | we look at the ages that are , look at their track length |
|
| 78:24 | They're very wide. Sometimes they are model. Look at these track contributions |
|
| 78:29 | you go down the well, they're very narrow Averages of 30, |
|
| 78:35 | 30, 30 12, 12, . All the ages are the |
|
| 78:42 | All the track links distributions are What does that tell us about the |
|
| 78:48 | of this place. And remember what's temperature at which these systems are most |
|
| 78:58 | . Well, the single temperature in middle of all, that would be |
|
| 79:01 | . The range from 70 to 1 something like that. So, let's |
|
| 79:05 | say 100 for now, How How did this happen? What? |
|
| 79:14 | too cold to talk about intrusions We don't have intrusions that that that |
|
| 79:18 | have intrusions that go down to 100°. furthermore, these rocks are are key |
|
| 79:28 | . I didn't tell you that of , but I mean, the thing |
|
| 79:31 | need to know, these are, are nice is these rocks are extremely |
|
| 79:41 | , yup. How'd that happen? is probably an example of remarkable |
|
| 79:55 | Now there's a big uh there's some thrust faults in here. Okay, |
|
| 80:02 | those. And that's how you can something cool rapidly by shoving up a |
|
| 80:07 | , right? And then have erosion top of that. The bare truth |
|
| 80:13 | system throughout here is real close And this is suggesting that there was |
|
| 80:20 | lot of activity on that fault system at 40 million years. So, |
|
| 80:25 | to push this up high mountain up enough and then that and deal with |
|
| 80:30 | and faulting is to you for a fault. You cool the rocks off |
|
| 80:38 | because you're bringing material off in a fault. You don't do it immediately |
|
| 80:43 | the rocks are still covered by whatever were covered by. But if you |
|
| 80:46 | a tall mountain that wasn't there before rains more on the mountain and you |
|
| 80:50 | more erosion and you cool them off afterwards, and that's probably what's happening |
|
| 80:55 | . You got a thrust fault takes flat surface and makes it into a |
|
| 81:00 | Rains more on the mountain, erosion over Whammo, we get erosion, |
|
| 81:04 | really we get, we get three of erosion here in a very short |
|
| 81:12 | of time. And so this is of those and this is a famous |
|
| 81:16 | . This example is famous because of the distance that we're talking about several |
|
| 81:22 | . That that that that thing I you from my PhD where we had |
|
| 81:27 | kilometer climb to the top and looked the by types. Well, |
|
| 81:30 | you know, that's a big This is three kilometers, but of |
|
| 81:34 | we have a drill hole to help out here, But because we have |
|
| 81:37 | of this information up and down three and they're all basically the same. |
|
| 81:41 | a pretty remarkable tectonic event and it's tectonic event. It's not an igneous |
|
| 81:46 | because we know these rocks are to , these closure temperatures are too slow |
|
| 81:50 | represent intrusion. We can't intrude a rock like this to one km, |
|
| 81:55 | moreover, these are mostly not igneous are mostly better for for rocks. |
|
| 82:05 | I look at this age elevation I've shown you two examples of it |
|
| 82:09 | is a classic way to understand the of uplift of places like this by |
|
| 82:15 | for the system. Now, note that this is what we see in |
|
| 82:21 | fishing track, David, what if were to look at the same |
|
| 82:26 | we get the same samples. And now do um the the argon bio |
|
| 82:33 | agents in these are key in We're not gonna see this this remarkable |
|
| 82:42 | , right? Because the only time rocks were cooling lickety split is when |
|
| 82:47 | were going through the temperature of 100 . Now, here's 100. So |
|
| 82:53 | 100°. The only time you can record rapid nous is when they're passed. |
|
| 83:06 | you're looking at thermal chronometer that are in that sensitive zone bio tights closing |
|
| 83:11 | here. And so, you whatever is going on here, you |
|
| 83:14 | , it was the rapid nous was . So in that example from |
|
| 83:19 | it was the biotechs that found our uplift because that that occurred when the |
|
| 83:23 | moved from 3 52-50 really rapidly. the other changes that those rocks took |
|
| 83:30 | were more normal here. The odd is when these rocks are moving from |
|
| 83:35 | 50 to 50 all the other changes back into normal. This is the |
|
| 83:41 | . You know, it's like if were trying to, I don't |
|
| 83:46 | trying to run home instead of walk , there's, you know, you |
|
| 83:51 | will be times when you could manage run real fast. But most of |
|
| 83:53 | time you're just gonna walk. And if the camera was on you when |
|
| 83:57 | were running, we find it. if it was something, it was |
|
| 83:59 | somewhere else, we'd only see you slowly. Most of the time, |
|
| 84:02 | have to have to be looking at right time, You have to be |
|
| 84:05 | at the right temperature, thermo thermo . This one's 100. That one |
|
| 84:12 | 300. Other ones are different This is why you gotta know what |
|
| 84:15 | temperatures are. And then once you what they are, you can interpret |
|
| 84:19 | correctly or you can know which one apply to an unknown situation. Here's |
|
| 84:29 | mean track length of the same true thrust. And you see that |
|
| 84:33 | we go down the track length, out at 14 and they go down |
|
| 84:36 | 11 in this very normal way, that the temperature is kneeling them. |
|
| 84:45 | , here's another example of the same of thing. This comes from Mount |
|
| 84:50 | in uh, in Alaska. And , because it's a big, big |
|
| 84:54 | , we have a lot of this all this is not a drill |
|
| 84:57 | this is, you know, this Mount Denali, one of the tallest |
|
| 85:01 | in North America. And we go 1500 m 6500 m. Some mountain |
|
| 85:06 | got these samples for the geologists And here again, we get this |
|
| 85:12 | zone, this shallow zone and then this and this cape in this here |
|
| 85:18 | us when something happens. And this here sometimes referred to as an Exume |
|
| 85:25 | and healing zone because I think back the to the basin where you have |
|
| 85:30 | part of the, part of the core, where the ages are kind |
|
| 85:33 | flat along their there, they're the at the top and then go down |
|
| 85:36 | zero over here and then make this . And that was in a modern |
|
| 85:40 | hole. Well, if we were take that that basin and lift it |
|
| 85:44 | , we would just add a certain of ages to everything. And that |
|
| 85:47 | range across here would be there. that's what we're seeing here in the |
|
| 85:52 | . This is these rocks were sitting 100 degrees until four million years |
|
| 85:58 | And the whole thing popped up, are all safe because they went through |
|
| 86:02 | partially kneeling zone rapidly. These quite them because they were sitting in that |
|
| 86:07 | healing zone for a long time and that shape. Yeah, okay. |
|
| 86:18 | . And so again, now, you were putting together a full regional |
|
| 86:21 | of this region, you'd say, , there's a basin somewhere that's filling |
|
| 86:26 | the sediment about four million years ago we we've pulled off 2.5 kilometers of |
|
| 86:33 | in about four million years. That's lot that went somewhere. Now, |
|
| 86:39 | course it doesn't have to have been right at the bottom of the mountain |
|
| 86:42 | it could be rivers could take it long distance. But, uh, |
|
| 86:46 | still some compliments somewhere where that material now. Um, yeah, this |
|
| 86:54 | the last one we'll do in fishing . Um, this guy, Glenn |
|
| 86:59 | , Andyg Leto was like the Pioneer fishing tracks in the 1980s and, |
|
| 87:07 | then afterwards. And this is, is the basis by the way, |
|
| 87:11 | here. And Andy Glencoe lived right in Melbourne. Um, and they |
|
| 87:17 | samples from all over Australia. You , Just because they lived in |
|
| 87:23 | And this is an interesting map, published a while back in 2002, |
|
| 87:28 | looking at the surface of the fishing age of rocks, at the surface |
|
| 87:33 | Australia. And you'll notice that a of them give ages between about 1 |
|
| 87:40 | 50 indicating that Australia has been a flat place for a long time. |
|
| 87:48 | not a lot of variation throughout all this area. You've got, you've |
|
| 87:51 | very little variation in the time since This is telling us that Australia at |
|
| 87:59 | aside from this part here, where get the young agents, aside from |
|
| 88:03 | most, the eastern alps of this tells us that Australia has been |
|
| 88:09 | tectonic lee quiet for hundreds of millions years because otherwise we see variations in |
|
| 88:16 | . We see faults and breaks in continuity, but pretty, you |
|
| 88:21 | There's a, there's 1000 km, km, 500 km diameter circle in |
|
| 88:30 | the vision track surface ages are all saved 500 km. That's uh, |
|
| 88:37 | that? In In American 500 km 100 km is 60 miles. |
|
| 88:48 | 600 Last. Can you do Thank you. 500 km is 300 |
|
| 89:02 | . That's about the distance from here Dallas or Oklahoma, the sides of |
|
| 89:08 | size of east texas. That has without variation for 250 million years. |
|
| 89:18 | a that's a bit of information that from this low temperature thermal chronology. |
|
| 89:23 | when we start looking at things of surface, we need really low closure |
|
| 89:28 | because the surface is up here Anyway, if we're looking at variations |
|
| 89:32 | the surface, we can't be, can't be enquiring about what the rocks |
|
| 89:36 | doing when they were down here. once you start talking about surface |
|
| 89:43 | you know, when the mountains were , what the rivers were doing, |
|
| 89:46 | they were cutting into the mountains. sort of tectonic geo morphology, you |
|
| 89:51 | really low closure temperatures because you're not to learn that by measuring the density |
|
| 89:56 | the helium a or the argon age the horn blend. That's gonna tell |
|
| 90:00 | when it was down here at 500 . And that's an important bit of |
|
| 90:04 | . But if you want to relate to what sort of landforms were going |
|
| 90:09 | that. Not that I won't tell anything but a variation of 100 |
|
| 90:14 | That's beginning to say, look, rocks have all but been no deeper |
|
| 90:19 | three kilometers for 200 million years Three kilometers still a lot. But |
|
| 90:24 | mean the width of the variation is . Okay, that's the end of |
|
| 90:32 | Time, is it? It is 2.30. All right. Yes. |
|
| 90:40 | want to take extremely short break? . five minutes. Just in and |
|
| 90:49 | . Yeah. Okay. Our last that we're going to talk about takes |
|
| 90:54 | back to uranium. Actually started fishing . This is our third way of |
|
| 90:58 | about how uranium decays to make something using in a datable system. And |
|
| 91:04 | gonna call this the uranium thorium. marium helium dating system. Sometimes people |
|
| 91:10 | call it uranium helium. Um It indeed the oldest geo chronological system because |
|
| 91:18 | they first they first recognized that there things that were radioactive uranium decays to |
|
| 91:24 | plus helium, they said, let's take something, we'll use |
|
| 91:29 | And this was this was not long uh Kelvin had estimated the age of |
|
| 91:39 | earth based on the cooling, Did I tell you this story already |
|
| 91:43 | ? Yes. You know, So said the earth was 6 30 million |
|
| 91:47 | old and they went to africa. they used the principal position. They |
|
| 91:51 | some old rocks, they dated they got 30 million. They |
|
| 91:56 | oh no, you know, because the geologists thought the earth should be |
|
| 91:59 | than 30 million. The biologists thought earth would be older than 30 |
|
| 92:02 | The first, the first actual date was ever calculated from isotopic geology, |
|
| 92:08 | 30 million. And they were all . I said, oh no, |
|
| 92:12 | gotta revise everything, we know, kelp in was right. But then |
|
| 92:15 | used the same rocks and they dated by uranium lead And they got 2000 |
|
| 92:21 | . And everybody's happy. Why did get such different numbers? Because of |
|
| 92:25 | closure temperature, closure temperature of helium is the lowest of anything we're going |
|
| 92:30 | talk about again, we can talk the decay of uranium 2 38 decays |
|
| 92:38 | a bunch of alpha particles, uranium 35 also alpha particles, thorium 2 |
|
| 92:43 | bunch of alpha particles. And even 1 47 has one alpha particle. |
|
| 92:52 | Add up all these things. The of all these, you know, |
|
| 92:55 | got and here's 876, 1 depending the relative concentrations of these guys and |
|
| 93:03 | time you're gonna get a bunch of guys. And so that is then |
|
| 93:09 | to be a a another way to . We can then uh put all |
|
| 93:16 | things together and say the helium concentration going to be equal to all of |
|
| 93:22 | . Right, You've got the decay and the and the seven and the |
|
| 93:26 | and the six and the one depending how many you get in each one |
|
| 93:29 | these chains. Now obviously you can't that for tea because tea is in |
|
| 93:34 | single exponent there. Um But we that for times much less than the |
|
| 93:41 | constant of uranium 2 35 that is times of less than about 100 million |
|
| 93:47 | . The time can be estimated as helium concentration divided by the production |
|
| 93:53 | And the production rate can then be calculated uh here based on how much |
|
| 93:59 | makes the concentration of uranium thorium. Miriam that you have. So you |
|
| 94:05 | , you know, you measure that in some chemical way. Um And |
|
| 94:14 | for and you have to keep in of course that uh the half life |
|
| 94:19 | thorium and samaria were kind of long to the other. And sometimes it's |
|
| 94:25 | cumbersome to talk about um the combined of uranium thorium and sumerian. So |
|
| 94:34 | came up with this smart idea to about what they call the effective uranium |
|
| 94:41 | . Effective uranium concentration takes all of uranium and 23% of your thorium and |
|
| 94:48 | a percent of your. So marium that all up and says that's we're |
|
| 94:52 | going to say that's a number that be if we could have what we |
|
| 94:57 | in the sample or we could have this much if it was all your |
|
| 95:01 | , we're just taking this much thorium Samarian and pretending like it's uranium and |
|
| 95:06 | doing these by adding these factors here we've got to worry about the difference |
|
| 95:11 | care a and the difference in alpha . This even sit down. So |
|
| 95:17 | just talk about eu effective uranium rather having to deal with all this other |
|
| 95:24 | . So again, the minerals are same as before, appetites are common |
|
| 95:29 | or the Big East. There's you know, there's a bunch of |
|
| 95:32 | minerals that people have tried to work . I've worked on calcite, people |
|
| 95:36 | worked on go fight and Garnet and just going to pay attention to the |
|
| 95:41 | East right here and The alpha, amount of uranium and thorium in these |
|
| 95:49 | in appetite and blue dots. Generally got alpha per year from milligram as |
|
| 95:57 | as 10,000, as much as as million. So we got lots of |
|
| 96:02 | produced in these samples. Um and like all these other things, the |
|
| 96:10 | of the daughter products, which in case are these alpha particles is dependent |
|
| 96:15 | the uranium uranium and thorium concentration or effective uranium concentration and the thermal history |
|
| 96:23 | year Because helium is small. It leave the system even at low |
|
| 96:29 | the closure temperature for these various systems be as high and something like garnet |
|
| 96:34 | maybe 300° in appetite in calcite and other things, it's as low as |
|
| 96:40 | the lowest one we got. Um , unfortunately, we have a little |
|
| 96:48 | of a problem here in that because we have to measure uranium thorium |
|
| 96:53 | solarium and helium. We're back to problem of the parents and daughters being |
|
| 96:59 | different geo chemically, we cannot measure on the same machine. However, |
|
| 97:04 | do not have the same. We have as bad of a problem as |
|
| 97:08 | had with the potassium argon because in argon, in order to get all |
|
| 97:12 | the argon out of the sample, had to melt it and to destroy |
|
| 97:16 | sample for helium, we don't have melt it so we can get all |
|
| 97:22 | helium out of the sample without destroying . Then take that piece that we've |
|
| 97:27 | up and go measure the uranium thorium and the scenario. So it still |
|
| 97:33 | two machines, but it doesn't require split the sample into different pieces. |
|
| 97:41 | . Now, one thing that we to worry about in this system that |
|
| 97:46 | really don't have to worry about in , at least it's a much bigger |
|
| 97:49 | here is something called recoil or alpha , and that is because when just |
|
| 98:00 | with uh just as with uh fishing decay, when alpha decay happens, |
|
| 98:10 | alpha particle is thrown out some Perhaps as much as 20 Mike |
|
| 98:17 | It's now in a new place from it was These, these these two |
|
| 98:20 | and two neutrons. They were a of their uranium family. And now |
|
| 98:25 | over here And that's 20 microns. , why that's important is that what |
|
| 98:31 | the sample was only 17 microns from edge of the crystal? Sometimes not |
|
| 98:38 | time, but sometimes it here's the you're 17 microns from the crystal, |
|
| 98:45 | be this circle, you know, our sample will be a circle through |
|
| 98:49 | this is being ejected. If you the right direction, you will be |
|
| 98:53 | 20 microns and out. And so that that helium that was due to |
|
| 99:00 | decay, it's not a part of crystal anymore. So we will we |
|
| 99:04 | sometimes have a distribution curve. Suppose a very small uh crystal, it's |
|
| 99:11 | 100 microns across the alpha retention At edge of that crystal is gonna be |
|
| 99:21 | because a uranium Iranian Iranian pay to curse here, can go any |
|
| 99:29 | Half of those directions are within the , half of those directions take us |
|
| 99:32 | of the Christian and then that goes to about 90,% once we get a |
|
| 99:38 | 20 microns away. But in in this 20 micron zone, we |
|
| 99:43 | are expecting through, not through thermal . You know, we talked about |
|
| 99:48 | before with the argon, we only that sort of thing because of some |
|
| 99:53 | perturb UNt's This is not thermal, is pure physics. This is just |
|
| 99:58 | alpha particles? Do not go there . No, we don't have to |
|
| 100:03 | with this in healing a potassium decays argon. This is our gun. |
|
| 100:09 | Well the whole thing changes right in . This is a little piece that |
|
| 100:13 | thrown away. So this is a that we can fix by noticing that |
|
| 100:21 | effective uh well sort of a fudge , the F. T. Value |
|
| 100:28 | is for a sphere, if we a spherical character of a sample, |
|
| 100:35 | bigger the sample, the less this an issue, right? Because the |
|
| 100:39 | the volume, the more material we away from the edge. And so |
|
| 100:44 | a sample of Radius 250 microns, can we can we can retain about |
|
| 100:52 | of our alien. Which means that we Analyzed sample that's 250 microns |
|
| 101:00 | We have to increase the age that get by about two or three because |
|
| 101:08 | want to understand the thermal significance of age and without, without the the |
|
| 101:15 | between This and 100 it's not It's just so we had that. |
|
| 101:24 | , if the crystal gets small only 50 microns across, we've got |
|
| 101:28 | much bigger uh correction factor. And course this is a model for a |
|
| 101:34 | with a pure sample isn't actually spherical have. How appropriate is this |
|
| 101:40 | So the lesson here is bigger crystals better. The bigger we get, |
|
| 101:46 | less we have to worry about are , did we measure the size |
|
| 101:50 | Are we applying the appropriate size So the teenager you get here, |
|
| 101:56 | mean, and of course the smaller you get the smaller closure temperature you |
|
| 101:59 | . So that's another issue. But if it had a big closure |
|
| 102:04 | you're gonna lose it all because of . And that's the problem. So |
|
| 102:08 | a thing bigger is better. That's that's the cylinder approach. That's so |
|
| 102:15 | just another way of looking at So shoot me. That was the |
|
| 102:19 | . That's effect. Number one is effect number two. I didn't really |
|
| 102:24 | . Number one is that can buy our helium measurement is so nation, |
|
| 102:30 | we assume that everywhere in the crystal the same concentration, if this is |
|
| 102:35 | map of concentration, this means 10 . This 50 PPm crystal like that |
|
| 102:42 | 50 PPm. If we account for project, we account for recoil, |
|
| 102:52 | gonna get a crystal that really looks that. Or a crystal that looks |
|
| 102:56 | that because this is the recoil that lost out here. Right. And |
|
| 103:01 | can account for that by measuring the and adding it back in this stuff |
|
| 103:05 | not lost because of some thermal history . See, let me explain it |
|
| 103:10 | way, if we had these crystals the right light and we measured this |
|
| 103:16 | and the amount of uranium let's say got an age of 98 million years |
|
| 103:23 | . That's not when the thing was Because we're losing 2% of our of |
|
| 103:28 | helium. Not because it's been, know, anything's happened to it. |
|
| 103:31 | because it's just naturally goes away because was here. So we have to |
|
| 103:35 | that 98 million year old age to million. That's when the eruption |
|
| 103:40 | That's the geologically significant thing. And we do that every for every crystal |
|
| 103:45 | gets below about, you know, a millimeter. Once you get up |
|
| 103:49 | 500 bronze, this isn't such a deal. But for small crystals, |
|
| 103:53 | is essential. Essential. But not big deal. If you if you |
|
| 103:59 | imagine that the concentration of uranium is the same. But if the concentration |
|
| 104:04 | uranium is more complicated and that's that's fine. But what if it's |
|
| 104:08 | complicated like that? What if you an appetite that grows with lots of |
|
| 104:13 | in the middle and not so much the outside? Well, you're gonna |
|
| 104:17 | ejecting only from the outside, but from the inside and you're going to |
|
| 104:23 | the age because you're going to have distribution that's different from what you |
|
| 104:28 | Or I suppose you have a really uh uranium rim, you're gonna overestimate |
|
| 104:33 | cage because you're gonna, you're gonna some here, but there's still gonna |
|
| 104:37 | a lot more out here than So we would like to try and |
|
| 104:44 | crystals that aren't zoned, but this an issue. Um and so with |
|
| 104:50 | concerns that I have broadly outlined here uh through experiments that I've described |
|
| 104:56 | we can come up with the broad temperatures of these things here, appetite |
|
| 105:03 | 70 Zircon and seen about 180. so now we have these in our |
|
| 105:13 | . You're interested in closure temperature and interested in geologic events where the, |
|
| 105:17 | the temperatures are. These, these excellent choices. But now we've looked |
|
| 105:23 | systems that go as high as 800 as low as 70 and we've got |
|
| 105:29 | lot of choices in between, depending what what geologic problem you're looking at |
|
| 105:34 | the right system. Um Those are , here are some other helium |
|
| 105:42 | This is from compilation by Alexis Hall put this all together. The closure |
|
| 105:48 | of helium in the deli. I very high here is regarded floor right |
|
| 105:53 | is the other, there's appetite And so even in the, even |
|
| 106:00 | the helium system all by itself, got quite a range. Some of |
|
| 106:05 | are known better than others. The that are known really well, our |
|
| 106:10 | and, and sites not very many tight night. Um let's see now |
|
| 106:20 | thing I want to point out is there's this variation in appetite and here |
|
| 106:27 | . This big range and it's pretty understood. So I want to go |
|
| 106:30 | that in at least a little Um So let's go to here. |
|
| 106:34 | we have the clothes for for We have a graph of closure temperature |
|
| 106:42 | the the log of the concentration of four. The concentration of Helium four |
|
| 106:49 | a kind of proxy for how long system has been there. It's kind |
|
| 106:55 | a proxy for radiation jam. The helium four you have, the more |
|
| 107:00 | has been going on that, you , and you'll notice that the higher |
|
| 107:07 | concentration of helium, the higher the temperature, there's actually the longer this |
|
| 107:14 | sits around and gets helium. Um a crystal damage Proceeded to go up |
|
| 107:24 | temperatures as low as 50 or The temperatures as high as 120. |
|
| 107:29 | is that? Well, they think because and you can see it in |
|
| 107:34 | of changes in activation energy or do Well, just look at the |
|
| 107:38 | at the closure temperature broadly goes up here's why they think that happens. |
|
| 107:44 | you have a pristine appetite crystals, the activation energy of moving me liam |
|
| 107:51 | that thing should be always the same . But if you produce some radiation |
|
| 107:59 | , you produce basically what they think as kind of a well of activation |
|
| 108:03 | that it's easy to fall into this but that the activation I need to |
|
| 108:07 | out of this zone is really hot so you're producing kind of ins and |
|
| 108:12 | in terms of activation energy in the that that put little ramps figuratively |
|
| 108:20 | trying to get out. It's not to say there are places where because |
|
| 108:24 | transition from a damaged zone to a damage zone, it becomes harder. |
|
| 108:30 | in appetite, the more damage you , the higher the closure temperature And |
|
| 108:36 | a bunch of these little damage spaces funds these little places and each one |
|
| 108:40 | harder to get out of and the closure temperature goes up as much as |
|
| 108:45 | doubles from from 60 to 120. so that's something you need to pay |
|
| 108:51 | to and you can see it a of times. Uh what that means |
|
| 108:55 | the temperature of the helium. The temperature of helium in appetite will evolve |
|
| 108:59 | time. You can start out crystal have a close temperature of 70, |
|
| 109:04 | years later. Have closed temperature of and two samples from Detroit. All |
|
| 109:11 | from, from sand stones will have range of closure temperatures because they came |
|
| 109:16 | different places and have different concentrations. actually gonna be a good thing. |
|
| 109:22 | have a Sandstone which has all these kinds of appetites in them. That's |
|
| 109:27 | a bunch of different thermo chronometer. one tells you about 60. This |
|
| 109:30 | tells about 70. This one tells about 90. And so when you |
|
| 109:34 | a sandstone and you see a distribution ages that plots very closely with the |
|
| 109:40 | uranium. You can say something about history. Um I'm gonna skip |
|
| 109:48 | So that's all I want to say that basically the closure temperature of, |
|
| 109:54 | helium in appetite Can be as low 60° for a pristine appetite. But |
|
| 110:02 | the appetite damage, as the radiation increases, that closure temperature goes |
|
| 110:07 | Now, I should point out that we're gonna talk about Zircons. This |
|
| 110:11 | here is all about zircons. And thing to note about the difference between |
|
| 110:16 | and zircons is that Zircons have a more uranium and thorium in. You |
|
| 110:22 | , let me, let me go . I don't have to go back |
|
| 110:24 | far to this picture here. The blue dots, Alright, what do |
|
| 110:34 | do? The light blue dots are . And the dark blue dots are |
|
| 110:42 | . So you see that some appetite has have only one or two ppm |
|
| 110:47 | and similar amounts of thorium. But crystals can have 1000 5000 ppm uranium |
|
| 110:54 | an equal amount of thorium. you know, 10:00 200 times 1000 |
|
| 111:03 | more. That's going to be an . And indeed what we're gonna see |
|
| 111:11 | uh radiation damage in zircon is that radiation damage will produce a situation that |
|
| 111:23 | like this. Is that here we , let's just go straight to closure |
|
| 111:26 | here's closure temperature versus something that they alpha dose in this case they tried |
|
| 111:34 | they tried to be more sophisticated in case rather than just saying the total |
|
| 111:37 | of uranium, they say, alpha dose is the amount of alpha |
|
| 111:44 | that have occurred since this. Since sample was at a certain temperature. |
|
| 111:50 | basically taking since the, since the track age of this sample fission |
|
| 111:56 | Once fission tracks start to feel the damage is growing at a hotter temperature |
|
| 112:03 | radiation goes away. So they take fishing track age in many of these |
|
| 112:08 | and they say how many alpha particles been produced since then. And what |
|
| 112:13 | see is that initially just like in just like an appetite. The closure |
|
| 112:20 | appears to go up from a low about 100 and 50 degrees to up |
|
| 112:24 | close to 200 degrees. But once get above an alpha dose of of |
|
| 112:30 | times 10 to the 16th alphas per . Look at what happens to the |
|
| 112:35 | tip, it starts to really fall . They're actually predicting closure temperature for |
|
| 112:40 | thing less than zero. And what saying is that radiation damage just tears |
|
| 112:48 | crystals apart. And at that level radiation damage, helium is just streaming |
|
| 112:53 | all the time. It's easy. you just you just give a stern |
|
| 112:58 | to the zircon and the helium Um this is a graph showing um |
|
| 113:05 | damage and and uranium thorium age I forget where this is from. |
|
| 113:11 | what we have is is here we've 2000 parts per million and we've got |
|
| 113:17 | very low age here. We've got parts per million and it's much |
|
| 113:21 | So once you get to some damage , you start losing helium like |
|
| 113:28 | Um and so that's that, you , zircons and appetites are a lot |
|
| 113:34 | complicated than we first thought. And should point out that this is one |
|
| 113:37 | the newest of these techniques. Um think it was at the 1995. |
|
| 113:44 | might seem a long time ago to . But at the 1995 Gs A |
|
| 113:48 | I went to in New Orleans. remember seeing these guys from caltech give |
|
| 113:53 | talk about Iranian helium dating the first that everybody, I've seen a big |
|
| 113:58 | was like, it's fantastic because they now showing about stuff that was low |
|
| 114:02 | stuff 9. 1995. There was paper in 1987 And then nothing. |
|
| 114:10 | had been 100 had been 80 years they had tried to date those rocks |
|
| 114:14 | Iranian helium. And they got, know, 30 million. And they |
|
| 114:17 | , Oh no, what are we ? The data to buy uranium |
|
| 114:20 | They got an old answer and then helium thing was just sets off until |
|
| 114:24 | had the better technology to assess closure . Now, we can say with |
|
| 114:29 | clarity That these things represent temperatures in range from 200 to 100 and it's |
|
| 114:35 | it's even and and the temperatures tend change with radiation damage. Um |
|
| 114:42 | I'm not gonna go through that. , I'm not gonna, I'm gonna |
|
| 114:46 | the calcite stuff too. Yeah. , I'll just show this this this |
|
| 114:57 | this is the last one or almost last one. Yeah, let's just |
|
| 115:01 | about this this paper here. Uh , that's not the paper. This |
|
| 115:06 | this is the results of the paper by house at all. This was |
|
| 115:10 | in nature and I think like 2000 like that. And what they were |
|
| 115:15 | is showing because of this low closure . What you can do in terms |
|
| 115:20 | landscape evolution, imagine you have some that have some periodic topography, eyes |
|
| 115:30 | lows. And the, the, model here is, suppose we're looking |
|
| 115:35 | the samples are going to come from sierra Nevada, in California, big |
|
| 115:39 | mountain range. And if you look the length of the mountain range. |
|
| 115:43 | way, rivers are coming out of mountain range at some level here. |
|
| 115:47 | so when the rivers come out, mountains are lower and on the ridges |
|
| 115:51 | the sides of the rivers, the are higher. Now, if we |
|
| 115:54 | at the mountain this way it it looks like this. But if |
|
| 115:57 | look at the mountain this way, looks like that. And that's what |
|
| 116:01 | saying here is to say, if we have these rivers cutting on |
|
| 116:05 | side of the mountain, they would valleys and ridges and that would produce |
|
| 116:10 | variation in the partial retention for for . We call it partial retention because |
|
| 116:16 | not a Neil, they're just Okay, it's the same concept top |
|
| 116:21 | bottom, just like the partial and zone for fishing trucks. So the |
|
| 116:26 | pretension zone will mimic uh photography because clothes because the temperature is so close |
|
| 116:34 | the actual surface, the partial attention for argon, it's also a |
|
| 116:40 | but it's way down here and it's going to mimic the topography unless you |
|
| 116:44 | mountains that are 10 kilometers high. just for these normal mountain size is |
|
| 116:49 | partial retention zone of appetite will in mimic the topography anything further down. |
|
| 116:56 | , no, But because it's so , we can start looking at bedrock |
|
| 117:02 | and tell us about the paleo elevation the surface topography because it's such a |
|
| 117:08 | top closure temperature. And so what did was imagine, you know where |
|
| 117:13 | , where the samples are below the . They should be older because it |
|
| 117:17 | longer to get out of here. below the valleys should be younger. |
|
| 117:22 | so then they did that they went down the sierra Nevada and they collected |
|
| 117:27 | from here, The red line shows elevation Going from 1 to 3 km |
|
| 117:33 | the blue line shows the way the think that's the running average of the |
|
| 117:40 | that are shown in the dots and the old samples are down here and |
|
| 117:46 | samples are down here, where the are near the ridges, the ages |
|
| 117:51 | younger, where the samples are near valleys, the ages are older and |
|
| 117:55 | what we predict from this year. what the show that look at these |
|
| 118:01 | . These agents are 70 million to million. This suggests that these |
|
| 118:08 | here's this is one river, this another river, another river coming out |
|
| 118:13 | us. And the pattern of these tell us that the these rivers have |
|
| 118:20 | there for tens of millions of digging down, making these patterns like |
|
| 118:25 | . So the western edge of North has had this edge on it. |
|
| 118:31 | sierra Nevada has been there for a long time, which has implications for |
|
| 118:36 | sorts of tectonics and geo morphology and picture stuff. Um And this can |
|
| 118:42 | done because the closure ship is so , Closing temperature of helium in appetite |
|
| 118:51 | somewhere between 70 and 120, depending radiation. The closure temperature of helium |
|
| 118:58 | zircon is somewhere between 200 and The planning on radiation. And that's |
|
| 119:10 | of that. So what time is ? 3:00. Okay, I want |
|
| 119:16 | move on to something else. At we can start this and we'll finish |
|
| 119:25 | next week. Okay, so the thing we're gonna do is not going |
|
| 119:45 | . Last thing we can do is doing geo chronology in sand stones. |
|
| 119:54 | we've been applying most of this to rocks, right? And and as |
|
| 120:01 | said, the best way to date sedimentary rock is to date an igneous |
|
| 120:06 | . But in many cases we don't that option. Uh Oh, I |
|
| 120:14 | one. Maybe I should, I think skipped one seven. This is |
|
| 120:28 | short one. Let's do this We're gonna go to thermo chronology and |
|
| 120:35 | talked about this some a little We can go through it pretty |
|
| 120:40 | Uh and then, and then we'll about sand stones. What? |
|
| 120:46 | I'm not sharing. Thank you. not. Okay. Uh hmm. |
|
| 121:17 | should I do that? Yeah, is thermo chronology and faulting. You |
|
| 122:14 | see it? Maybe I Oh maybe not. Let me open edit display |
|
| 122:48 | should be there. Uh Here, . What's what's your email there? |
|
| 123:14 | is. Okay. And yours. what way? Go ahead. Oh |
|
| 123:28 | , send it to Andy here looks man in the song. Yeah. |
|
| 124:05 | . Okay, I just email them you. Alright, so very |
|
| 124:33 | how can we use Thermo chronology to us about the timing of faulty. |
|
| 124:38 | mentioned this earlier that, you we've got ball thing is going to |
|
| 124:44 | topography and this is particularly, then gonna be able to use this to |
|
| 124:49 | us about vaulting, it's the lower closure temperature usually the better. |
|
| 124:55 | we already saw these things where we the elevation profiles and we go up |
|
| 125:00 | whether we see a change in slope those age versus elevation profiles, the |
|
| 125:05 | the plate, the point at that is the time in which tectonics change |
|
| 125:09 | rates. You change the rate of for some reason, that would |
|
| 125:14 | you know, something along here. , and in general, we don't |
|
| 125:20 | of the depth of closure to be this. Well, it depends on |
|
| 125:24 | we're talking about, but generally we're imagine the closure temperature will mimic the |
|
| 125:29 | for these low temperature things. And know, we can just start with |
|
| 125:34 | simple situation here, we have some erosion rate. DZ DT, We |
|
| 125:40 | look at an elevation profile like we've before and if it's a nice straight |
|
| 125:45 | , it's a simple erosion rate going same throughout and, and whatever this |
|
| 125:52 | temperature is, We passed through We've shown examples of this in two |
|
| 125:56 | examples already. Um, and the for fishing tracks, same thing. |
|
| 126:04 | know, if we pass through a A Z will get certain characteristics. |
|
| 126:10 | all of this was just sort of erosion. You can do the same |
|
| 126:14 | for a short meeting event. Something . But what about faulting in |
|
| 126:22 | If we have a situation like Normal faulting, if we have |
|
| 126:26 | what some people call tectonic degradation, this off, then samples up here |
|
| 126:34 | even this sample, but certainly samples here would immediately be cooled off. |
|
| 126:39 | when we look at a uh, we look at a sample and we |
|
| 126:43 | and we see that, you we've done a bunch of thermo chronology |
|
| 126:46 | we say it's hot, hot, and pulled off rapidly. Like we |
|
| 126:50 | when that feldspar changed. It's great it's in the it's in the foot |
|
| 126:57 | of a normal fall, that would a reason for it to cool off |
|
| 127:01 | when the false moves fault moves, sample gets cold immediately. And so |
|
| 127:09 | , you expect for fishing tracks, example, you'd expect them to be |
|
| 127:13 | cooling. A lot of narrow track . Conversely for us all things a |
|
| 127:18 | more complicated. You take a you trust it. You make the |
|
| 127:23 | are higher now, but immediately after rusty, you know, these happens |
|
| 127:28 | higher. The sample is still the distance it was from the surface, |
|
| 127:33 | that won't last very long because when make a mountain, you're just asking |
|
| 127:37 | more rain, right? Mountains are going to be eroded faster than than |
|
| 127:42 | they were not mounted. So this takes place pretty fast after you make |
|
| 127:47 | , you can't make a mountain forever . And so you make erosion here |
|
| 127:53 | the sink cools off. It cools probably slower than this. Um, |
|
| 127:59 | it could cool off pretty soon And so when we think of trying |
|
| 128:05 | understand faulting simply by changes in cooling , we will look for normal faults |
|
| 128:12 | cool immediately thrust samples in the foot of normal faults to cool immediately samples |
|
| 128:20 | the hanging wall of thrust faults to soon after. And this doesn't involve |
|
| 128:27 | of the crosscutting relationship kind of You know, we suppose we have |
|
| 128:30 | dike that crosscut default. Then we the dike by non thermal. Then |
|
| 128:35 | take the dike by the highest closure system we have available because we don't |
|
| 128:41 | to worry about thermal perturbations afterwards. want to know the crystallization age of |
|
| 128:46 | dyke. If there's a, that tells us the age of the ball |
|
| 128:50 | than this stuff. But we don't to date that fault with alien dating |
|
| 128:55 | that could be way too much Um, burial. Um, You |
|
| 129:04 | , was is much the same. have a sample, you bury |
|
| 129:08 | then you erode it back for the fishing track agent. You're gonna expect |
|
| 129:12 | to go back and forth like this you may have those sort of by |
|
| 129:17 | fishing track distribution. We talked I think that's all. One |
|
| 129:23 | Okay, well we've already talked about way, but this is the Taranaki |
|
| 129:27 | very similar to the Otway basin. think I'll skip that and new |
|
| 129:33 | New Zealand. Yeah, we can that part. So that's all I |
|
| 129:37 | to say about vaulting. Is that , thermal chronology by itself is good |
|
| 129:43 | the foot walls of normal falls. hanging walls of those forms. If |
|
| 129:51 | had more time we'd spend more time that. Okay, so now we |
|
| 130:03 | move on to looking at sedimentary rocks we will not finish this today, |
|
| 130:10 | we will start it and finish up rest on friday and then move on |
|
| 130:14 | some case studies. So, this is a talk I gave at |
|
| 130:24 | . S. A. And I've it to some other things and, |
|
| 130:27 | these are some of my former students postdocs that helped out with it. |
|
| 130:34 | , a couple of things we have worry about when we do sedimentary |
|
| 130:37 | The first is sampling bias. Are collecting samples from our sandstone and we're |
|
| 130:42 | to talk always about sand stones, you could do a shale but it's |
|
| 130:46 | fine. Great Sand stones. We to, are we doing a good |
|
| 130:51 | of sampling? I show this Uh, it's a famous thing from |
|
| 130:56 | presidential election of 1948. Uh, President Truman but, but, but |
|
| 131:03 | Chicago, daily tribune the night before said, well we know Dewey's gonna |
|
| 131:07 | . So just print up the Dewey defeats Truman the next day. |
|
| 131:11 | was clear that Dewey had not defeated . And the reason that the Chicago |
|
| 131:16 | got it wrong is they only called that had phones And in 1948, |
|
| 131:25 | was a very different group of people had phones than who didn't have |
|
| 131:29 | This was a phone survey. And learned that the people who had phones |
|
| 131:34 | going to vote for, do the people who didn't have phones were |
|
| 131:38 | to vote for Truman. And this a famously wrong headline. You |
|
| 131:42 | that's the winner. That's Dewey. mean, that's the Truman holding up |
|
| 131:45 | thing says, ha, ha, got it. Well, the reason |
|
| 131:48 | got it wrong is because they didn't , you know, it was, |
|
| 131:51 | know, telephones were a new thing 1948 and they said, well, |
|
| 131:55 | just call people up and see who gonna vote for. And most of |
|
| 131:59 | people who had telephone said, I like that. Do we got |
|
| 132:02 | was a sampling bias and we have worry about that when we consider this |
|
| 132:07 | geology. Um, how, what of sampling biases are we going to |
|
| 132:11 | in there? Well, we could provenance biases because the material that is |
|
| 132:16 | to our basin is going to be by the tectonics of the various |
|
| 132:21 | by the irritability. Are we eroding shales or gran, it's, it's |
|
| 132:26 | be by the climate over here, rains more or whatever, it |
|
| 132:29 | so that's gonna affect the grains that to us in our basis. So |
|
| 132:35 | gotta worry about that. We got worry about the as these grains are |
|
| 132:39 | liberated, how they come to They will be sorted by hydraulic |
|
| 132:43 | They may get broken up during sedimentary that's gonna, if they get broken |
|
| 132:47 | too small, we can't use Um The grain composition will matter. |
|
| 132:53 | talked about how radiation damage can change , uh whether they have the shape |
|
| 132:58 | matter. Um Then once you get just geologic considerations, then we get |
|
| 133:04 | into our laboratory, we've got to mineral separation. That that involves all |
|
| 133:09 | of choices we make in the Are we gonna do, we have |
|
| 133:13 | hand pick them? That means we this when we don't like this |
|
| 133:16 | we're gonna have to pick sizes and these things and then when we get |
|
| 133:19 | to the other laboratory were actually analyzing here I'm talking about Iranian lead analysis |
|
| 133:25 | it could be other things. We to, we are going to be |
|
| 133:29 | by how many grains we picked Where if we're doing these laser stuff |
|
| 133:33 | we put the shot point, is on the corners on the middle. |
|
| 133:37 | We are then maybe going to decide to throw away data and when to |
|
| 133:41 | it, How big was the spot ? Where was the spot size? |
|
| 133:45 | are all they're all akin to calling , only houses with telephone or maybe |
|
| 133:52 | doesn't, you know? Or maybe are not. We just got to |
|
| 133:56 | so of, of, of, all of this sort of related to |
|
| 134:00 | of this. And the first question really need to ask is how |
|
| 134:05 | how many grains is enough? We to recognize that we are dealing with |
|
| 134:11 | is guaranteed, well, almost guaranteed be a heterogeneous population. Right? |
|
| 134:16 | is a sandstone. And unless we're with remarkably narrow provenance, we're going |
|
| 134:22 | be dealing with sand stones that have source areas or the source the |
|
| 134:28 | The single sand deposit may have come lots of different places. And so |
|
| 134:35 | could be a problem depending on what doing this. And so how many |
|
| 134:42 | is enough? We gotta sandstone, a bunch of grain sand there. |
|
| 134:46 | many is enough? You have any of how many would be enough? |
|
| 134:50 | want to, let's say, let's restrict our right. Right now |
|
| 134:54 | gonna be talking about the grains that date. So we're talking about only |
|
| 134:59 | grains. We'll talk about, say , maybe we'll talk about sanity, |
|
| 135:05 | let's just consider those two possibilities a or zircon there. They exist in |
|
| 135:13 | sand or sandstone. Uh, but a in a rock this big, |
|
| 135:18 | know, there's probably a lot of cons, you know, could be |
|
| 135:23 | of them. How many do we to analyze to get a sense that |
|
| 135:28 | done a good job. Just what's gut feeling? What do you |
|
| 135:36 | That's enough? What do you Well, let's start with 20 more |
|
| 135:50 | less than 20. How Many More Enough? That's plenty. Okay. |
|
| 136:00 | , it probably isn't 20 twenties, way, not enough. 100 is |
|
| 136:05 | close. Um but before we get those details, we have to consider |
|
| 136:11 | enough to do. What do we to just have one representative of every |
|
| 136:17 | ? Suppose we're just looking at, know, we want to suppose that |
|
| 136:20 | got a we've got a cretaceous source a pre Cambrian source and a Jurassic |
|
| 136:27 | . What if all we need to is recognize that by saying, we |
|
| 136:30 | one of those, we found one those who found one of those, |
|
| 136:33 | a different question than we want to . A perfectly faithfully reproduce the distribution |
|
| 136:39 | Cameron's are much more important than the and then the cretaceous are a little |
|
| 136:43 | important. No, that's a that's curve rather than just, yep, |
|
| 136:46 | , yep. You know, if , if I want to know how |
|
| 136:49 | people at the University of Houston, , you know, uh were born |
|
| 136:54 | India as opposed to are there any born in India here at the University |
|
| 137:00 | Houston. Well, you know, told me you're from India. So |
|
| 137:02 | I know that at least one is group is taken care of, that's |
|
| 137:06 | way to look at or another way look at it is well, what's |
|
| 137:10 | , what's the proportion are you the one? Are there, are there |
|
| 137:14 | to answer that question? You clearly more more, you know? So |
|
| 137:21 | on what you want, depend depending where you're going, depends on how |
|
| 137:24 | grains is enough. But if we back to this first question of sampling |
|
| 137:30 | subgroup, we just basically want to , you know, in that |
|
| 137:35 | you know, here I would I would have taken care of the |
|
| 137:37 | of India and Louisiana just with you would be enough if those were the |
|
| 137:43 | two subgroups I was interested in, got lucky the first two I analyzed |
|
| 137:47 | those out. That's enough. That's though, but let's consider sampling the |
|
| 137:53 | first and then we'll talk about faithfully the entire distribution. These are two |
|
| 137:59 | that are fairly uh fairly similar. Vermes paper gets a lot more attention |
|
| 138:07 | because it was published 1st but what two guys did was go through some |
|
| 138:12 | of assumptions of this or that and and show a few things And |
|
| 138:18 | Anderson did in 2005, he Well, if I have a subgroup |
|
| 138:28 | represents a certain percentage like ST 2% will be the chances that I never |
|
| 138:34 | that group based on how many I And so he says that if you |
|
| 138:39 | a subgroup that represents 2% of your and you analyze 20 grains, you |
|
| 138:47 | a failure rate of about 65%. only find that 2% thing about one |
|
| 138:52 | three times. If you have ever 100 grains, your failure rate will |
|
| 138:58 | be 18% and if you analyze 300 , your failure rate will be essentially |
|
| 139:05 | . Now of course, if, it's too, if it's rather a |
|
| 139:09 | representation, then you only need to about 20 grains to have a failure |
|
| 139:15 | of of one or 2%. That's way to look at it. Um |
|
| 139:21 | never mind. Oh, so if wanted to have a failure rate of |
|
| 139:25 | 20% of a 2% population, you to analyze at least 80 creeks. |
|
| 139:34 | here's a different way to look at . And both of these guys did |
|
| 139:36 | calculation, they said, well what the detection limit? If you have |
|
| 139:45 | 90? If you want a 95% , how many grains do you need |
|
| 139:50 | analyze and what, what, what the one that's most famous is |
|
| 139:55 | here for Mish Mish said that If want a detection limit of five, |
|
| 140:02 | means that your subgroup represents 5% of total And you want to have a |
|
| 140:08 | confidence that you will find that you need to analyze 117 grains. |
|
| 140:16 | that's, that's the most famous number came out of this paper. People |
|
| 140:20 | about 117. It's like a magic I've talked with for me. He's |
|
| 140:25 | . That's all they got out of paper was the one number. But |
|
| 140:29 | a place to start. Anderson has has a much more optimistic view. |
|
| 140:34 | see, he uh, he would that you don't need 117. You |
|
| 140:39 | need 80. So these are these are different assumptions they make about |
|
| 140:44 | , but whether it's 60 or 100 17 depends. But a lot of |
|
| 140:51 | say that well you really should go 100. And the good news is |
|
| 140:55 | with all of these systems are You put all these zircons are all |
|
| 140:58 | valves bars into the system and you say analyze these guys and you can |
|
| 141:03 | it pretty quickly, you know, in, in, in the silicon |
|
| 141:07 | they can do 100 grains. So can do, they can do maybe |
|
| 141:11 | grains in a day. So you put it in there, you turn |
|
| 141:15 | on tomorrow morning, you've got 200 50 points on your graph. |
|
| 141:23 | now to reproduce the entire distribution. know, to really have a sense |
|
| 141:28 | what we're showing here is the same . That's a much more difficult thing |
|
| 141:34 | on what statistical criteria you choose. can get very, you know, |
|
| 141:41 | by, by some of these, you basically need to analyze all of |
|
| 141:44 | . The the end necessary is But even with the different um with |
|
| 141:51 | uh choices, You still need somewhere at least 300 to have a sense |
|
| 142:00 | Analyzing the total population. Not just we there are some people from Louisiana |
|
| 142:05 | they're in this group, it's about . That's a much more difficult thing |
|
| 142:10 | do. So that's 300 just to sure that you represented all the people |
|
| 142:15 | represent less 5% or more. That's about 100. Okay, so the |
|
| 142:21 | thing we can talk about when we about the triangle dating is the way |
|
| 142:26 | which the data are presented here is way in which you you plot the |
|
| 142:31 | versus the strata. Graphic age in case it's a cooling age because we're |
|
| 142:36 | about felt Spars but you could be about any age. You just plot |
|
| 142:41 | versus strata. Graphic age. And is, this is valuable when you're |
|
| 142:45 | at the variation over time. This the disadvantage of not showing the uncertainty |
|
| 142:50 | the individual points. Um but that's a huge deal. Sometimes the other |
|
| 142:57 | is to plot these things called probability plots which takes the individual Gaussian distributions |
|
| 143:06 | the samples. You know, this 100 and 20 plus or minus one |
|
| 143:09 | so forth. You take all those and you add them up in one |
|
| 143:13 | curve, that's what's called the probability . It's a kind of instagram but |
|
| 143:19 | better than a hissed a gram because includes uncertain. And so the samples |
|
| 143:23 | have a very narrow uncertainty will will up on the graph very high and |
|
| 143:27 | ones with the horror uncertainty will be . But you add them all up |
|
| 143:30 | you get some sort of cumulative look and you can say that you know |
|
| 143:34 | individuals all add upsets that there's a about 110, another peak at about |
|
| 143:40 | and so forth. So you can take that diagram and just add it |
|
| 143:46 | take the same data. These data on this diagram looks like this is |
|
| 143:51 | cumulative probability in which you say that oldest, the range of ages here |
|
| 143:57 | from 72 to 1 30 99% of the data are younger than 1 29 |
|
| 144:04 | of all the data are younger than 10 and so forth. And we |
|
| 144:09 | see all three of these types of presentations made um in various situations. |
|
| 144:18 | more thing is this thing called the density estimate which tends to spread out |
|
| 144:25 | data and oh gosh I'm gonna have the k. Is some some value |
|
| 144:31 | the Colonel. And the h is thing called bandwidth. And what it |
|
| 144:37 | is it takes the individual data points it really ignores their individual uncertainty. |
|
| 144:43 | just says that they should be treated the same. And if you have |
|
| 144:47 | data that's okay. And so the line is the kernel density plot of |
|
| 144:52 | blue line which gives you a very sense. But some people are some |
|
| 144:56 | are keen on doing it this Um The problem people sometimes don't like |
|
| 145:02 | the probability distribution plots is they get spiky because the uncertainties can be so |
|
| 145:07 | . If you transform A if this the true distribution And you sample it |
|
| 145:17 | times there's that number again um You get a probability distribution that might look |
|
| 145:23 | this and but the kernel distribution it make it look real more like the |
|
| 145:30 | data. And if you analyze 10,000 then you don't really have to worry |
|
| 145:36 | . But for analyzing only 117 with these good uncertainties it looks fight here |
|
| 145:42 | it really is. This is the distribution. This is the sample |
|
| 145:46 | But if we use this kernel we up looking like that that looks more |
|
| 145:50 | that. But if we we we the luxury of analyzing it 10,000 times |
|
| 145:56 | they all look the same. Um other way that's often done is either |
|
| 146:05 | the kernel density or the probability density you will show different samples and along |
|
| 146:12 | line and then you'll just knock them one on top of each other and |
|
| 146:16 | maybe strata graphically from lowest to something like this. And so you |
|
| 146:20 | , you can visually inspect the differences these sand stones as you look up |
|
| 146:25 | strategic city like that. Um that's . Okay, so why to try |
|
| 146:35 | thermo chronology? There's four reasons why might go to a sandstone and interrogate |
|
| 146:41 | grains with ice topic investigation such as the first and one that gets a |
|
| 146:48 | of play these days, especially with zircons is for understanding of paleo geography |
|
| 146:55 | we're going to describe the provenance in good way here, we're going to |
|
| 146:59 | these zircons, let's say and say well the terrain that were being eroded |
|
| 147:04 | this base and included zircons of these ages and we know provenance is up |
|
| 147:11 | that have these ages. So we say, oh these grains came from |
|
| 147:14 | and these grains came from someplace Another reason to do, to try |
|
| 147:19 | thermo chronology is to understand the tectonics the provenance. This is more done |
|
| 147:26 | sort of argon dating because the argon , you know, if you're looking |
|
| 147:30 | the mountain range up there, you , the grains came from that mountain |
|
| 147:35 | as they come up through the mountain , they will go through their closure |
|
| 147:38 | and then they'll come out here into basin, the difference in time between |
|
| 147:42 | they were in their closure age, they're in the base and tells you |
|
| 147:45 | about the vigorous nous of tectonics, talk about these things in in detail |
|
| 147:51 | , the third thing to talk about that this maximum deposition all age, |
|
| 147:55 | we don't have any clue about how the sandstone is. Of course we |
|
| 147:58 | to know uh we can just measure bunch of grains and then the youngest |
|
| 148:03 | is the oldest that the sandstone could be. That's another thing. And |
|
| 148:09 | the fourth thing is to understand the de positional thermal history of a |
|
| 148:15 | And we're interested in the basin sandstone is deposited, it gets buried |
|
| 148:19 | conceded up. How did it change we did that? Now you can |
|
| 148:25 | do if you're doing one of the three, you're not gonna have much |
|
| 148:30 | about the fourth one. And if doing the fourth one, you're not |
|
| 148:35 | to learn much about the first Because if you're if you've gotten to |
|
| 148:38 | point where post deposition, all thermal is something you learn that means you're |
|
| 148:44 | information from the provenance. But if got information from the provenance, you've |
|
| 148:49 | haven't got hot enough to tell us deposition, all post deposition, all |
|
| 148:53 | history. So They're sort of you're doing one of these first three |
|
| 148:58 | the 4th 1. But you can't all four at the same time. |
|
| 149:04 | , good. Okay. Once we've got these minerals to choose from |
|
| 149:09 | same group of minerals. We've got high temperature ones. Medium temperature low |
|
| 149:12 | ones. The ones that are used often are the highest ones and the |
|
| 149:16 | use uranium Lanzer Con because it doesn't changed and tells us about the |
|
| 149:22 | And then the other ones down you look at argon in the cell |
|
| 149:27 | , fishing, tracking the appetites and and everything. So you're either interested |
|
| 149:37 | the when you when you're doing the temperature stuff like Zircon and oh, |
|
| 149:44 | will also be used a lot and that will be used a lot and |
|
| 149:46 | that will be used a lot and maybe that those are the ones that |
|
| 149:49 | most of. Um When you're interested in paleo geography, we're gonna be |
|
| 149:56 | to hire closure temperature one. When interested in maximum deposition allayed, you |
|
| 150:01 | do almost anyone except the really, low ones When you're interested in tectonics |
|
| 150:06 | the provenance, you want to stay from the really high temperature ones. |
|
| 150:10 | when you're interested in post definitional thermal , you're definitely down on the low |
|
| 150:15 | in. Okay, we did this . So I'm gonna skip that. |
|
| 150:31 | And here's a bunch of Sir cons their and their measurement pits. But |
|
| 150:35 | kind of talked about that too So now let's move into talking about |
|
| 150:42 | use of the tribal thermo chronology to us about paleo geography description of the |
|
| 150:49 | . Um Now the first thing you , well we could just investigate the |
|
| 150:53 | area directly. Wouldn't that be easier trying to understand the source area |
|
| 150:58 | from some sandstone? And the answer that is well, um you make |
|
| 151:04 | problem with, with investigating the it'll minerals, the disadvantages the spatial |
|
| 151:11 | . You got a sandstone. All can say is this Circon came from |
|
| 151:16 | up there. Right. If you to look at the paleo geography, |
|
| 151:21 | know, if you went and analyze mountain range, this mountain, |
|
| 151:23 | You know more details about the paleo , but you have to go to |
|
| 151:27 | mountain range and figure it out so the disadvantage of spatial evolution, The |
|
| 151:33 | of course is much faster than visiting mountain range. Um It's also the |
|
| 151:38 | that maybe that mountain range doesn't exist . If you're looking at a |
|
| 151:42 | the provenance may have been eroded You know, the Himalayas will be |
|
| 151:46 | long before the Bengal fan is We will know, we know about |
|
| 151:51 | , there's a place called the ancestral mountains. You're familiar with this |
|
| 151:56 | It's a bunch of mountains that that were found in colorado. They're |
|
| 151:59 | the Central Rockies because they're in the place as today's rockies, but they |
|
| 152:03 | there in the permian at the pennsylvania we know about them in large |
|
| 152:10 | Not entirely, but in large part looking at this place called the Anadarko |
|
| 152:15 | which is in Oklahoma and Kansas and and colorado. It's a basin full |
|
| 152:23 | sand that came from that way. know that there was a place called |
|
| 152:29 | the Iraqi ancestral Rocky mountains were There had to have been a mountain |
|
| 152:34 | to produce this all this pile of . That's the Anadarko basin. That |
|
| 152:39 | be the geologist of 100 million years now will have the same problem when |
|
| 152:43 | look at the Bengal fan, where the big mountain range? The Himalayas |
|
| 152:48 | be gone by then, but the fan may not. So that's an |
|
| 152:53 | in that if the source doesn't exist , we're just left with looking at |
|
| 152:57 | sand and still we can learn a that way. Um here's an example |
|
| 153:04 | looking at samples from western United States the uh in uh Mesozoic. And |
|
| 153:14 | , yeah, so you can look samples from the mesozoic and we know |
|
| 153:19 | rivers in this time when all the from the Western Appalachians all the way |
|
| 153:24 | the Western coast of North America. know that because when you look at |
|
| 153:27 | sand stones, places like Arizona and , they have all of these 400 |
|
| 153:33 | year olds are constant, which is dominant signal you expect from the |
|
| 153:39 | They do not have a lot of that come from up here which would |
|
| 153:42 | like two billion years. So we track the, the fact there must |
|
| 153:47 | been rivers going across the whole Um, here's another example, maybe |
|
| 153:53 | little more detail. This goes for rocks. We've got paleo seen rocks |
|
| 153:58 | with the orange outcrops and ceremony in in in green. And these are |
|
| 154:05 | where there are sand stones and people taken to try to cons and looked |
|
| 154:09 | them and based on the here's the we got cretaceous rocks, the center |
|
| 154:15 | and police seen rocks. We can them in the cretaceous based on these |
|
| 154:20 | . We've got samples here along the coast based on these samples in the |
|
| 154:26 | coast. Rivers were going north because don't have any of those are key |
|
| 154:31 | grains that coming from Wyoming in the main ian rocks, they don't |
|
| 154:37 | We only get grains here from the Appalachian rocks. But when, by |
|
| 154:42 | time you get to the paley So this is santa mania. That's |
|
| 154:45 | million paley scene. That's 50 million the 40 million years that this this |
|
| 154:51 | tracks, we switch and now we evidence for these river systems going up |
|
| 154:56 | Wyoming and up into all of all of these protozoa rocks of West |
|
| 155:00 | States. Now these zircons in in the orange outcrops not far away |
|
| 155:06 | the green outcrops give a very different because we look at the distribution of |
|
| 155:13 | and we say, aha, there's , there's a representative of the arche |
|
| 155:17 | that came from Wyoming, here's a of the proto resort that came from |
|
| 155:21 | . Now we don't, excuse Arizona, it could have come from |
|
| 155:25 | lot of places, but it came Arizona or New Mexico or some, |
|
| 155:28 | know along there, it did not from the Appalachians. And when we |
|
| 155:34 | these samples down here in the all they got Is this 400 million |
|
| 155:38 | old grains. They are not the billion year old grades you get Wyoming |
|
| 155:42 | 1700 million grains you get from They don't exist. And so by |
|
| 155:48 | analyzing some sand stones here in we can understand broadly speaking the paleo |
|
| 155:55 | of north America at two important So that's the thing to do. |
|
| 156:02 | here's, here's a here's a similar on a much smaller scale. This |
|
| 156:06 | some work that my student Marie de Santos did for her undergraduate thesis a |
|
| 156:13 | years ago and we were looking at place in New Mexico where we had |
|
| 156:17 | out to outcrops to two sections of of this rock that was called the |
|
| 156:23 | formation in the florida mountains, that thinner, it was finer grained and |
|
| 156:29 | interpreted that as lancastrian or alluvial sedimentation 40 km away in the Victorian |
|
| 156:37 | it was thicker, it was we interpret that as alluvial fan sedimentation |
|
| 156:42 | when we put that together it was , well they're both been called the |
|
| 156:46 | formation, their flu viel and alluvial . Alluvial fan over here flew viel |
|
| 156:52 | , moving into the rivers, moving the swamps and lakes here. And |
|
| 156:56 | we said this is all part of big basin, this is the bottom |
|
| 156:59 | the base and this is the side a perfectly good um sediment, a |
|
| 157:05 | interpretation. Then we looked at the zircons we took we took sand stones |
|
| 157:11 | each one of these sections and analyzed bunch of their construct and here they |
|
| 157:16 | in red and the blue. They're anything like each other. The Victorian |
|
| 157:22 | has this huge spike at 1400 The Florida Mountains have a very big |
|
| 157:27 | at 1000 million. And you know we've analyzed almost 100 grains in |
|
| 157:33 | That's getting close to a good Does this strategy, does this does |
|
| 157:39 | paleo geographic model make sense now? , probably not right because this suggests |
|
| 157:47 | all of this material, this is a course version coming off some mountain |
|
| 157:52 | . This is the fourth version. is the fine version. But the |
|
| 157:57 | that the ages and their cons shouldn't so different. They're all coming more |
|
| 158:01 | less from the same place and all seeing is variation in grain size. |
|
| 158:07 | we have to to reject that So either they were either these two |
|
| 158:14 | which now exist 44 km apart from other or never a part of the |
|
| 158:18 | basin or maybe you could you could away with it by saying that this |
|
| 158:23 | of this basin was receiving receiving sediment the axis of the base may be |
|
| 158:29 | up and down here. Whereas these are coming off the side, this |
|
| 158:32 | the middle of the basin was coming and down. You know, that |
|
| 158:36 | that works. But just the simple of this is the course side and |
|
| 158:39 | is the fine side and they're all of one big deal doesn't work. |
|
| 158:44 | was a total surprise, you good thing we bothered here that that |
|
| 158:48 | is wrong or at least it needs significant tweaking. So whether it's whether |
|
| 158:54 | the whole continent or just a little part of new Mexico were able to |
|
| 159:01 | potential drainage basins by saying these are . And of course this is the |
|
| 159:07 | of stuff you can can't do with photography. You know, you've looked |
|
| 159:13 | sand stones or you have that Maybe you didn't learn about the history |
|
| 159:18 | looking at the courts, feldspar, fragments and it tells you tells you |
|
| 159:23 | about de positional environment or provenance or , but all courts kind of looks |
|
| 159:30 | same. And so all failed sparks of look the same and that you |
|
| 159:33 | plot them on diagrams like that. they can sell plot in the same |
|
| 159:37 | and still have very different. This an example of that. These these |
|
| 159:42 | these these these sequences of sand stones silt stones, people called them the |
|
| 159:48 | formations global formation and we thought they made sense. But the say |
|
| 159:55 | Um So you can you can you also if you have some known source |
|
| 160:04 | you have a zircon sample already have tribal sample. If you have a |
|
| 160:09 | you have a no if you have measured here and you have some known |
|
| 160:14 | , maybe you you know you can , you know there are ways in |
|
| 160:18 | you can you can mix up them you can say well this this black |
|
| 160:23 | is the result of 10% of And 80% of b. And 10% |
|
| 160:26 | c. And that's a nice that's paleo geography. Now that still may |
|
| 160:33 | the result of some of those biases talked about where it's raining more on |
|
| 160:37 | source seed than it is on A but it still tells you how |
|
| 160:40 | were being delivered to the base. And you can do it strata |
|
| 160:47 | there's a study in in Peru where looked at all of these places which |
|
| 160:52 | strata graphically going from 16 to 10 . And they looked at these different |
|
| 160:57 | in here and they were able to at these all these potential sources and |
|
| 161:01 | that for this sample. You know were the relative contributions. This this |
|
| 161:05 | became really important here at 30 million important here. So this is giving |
|
| 161:09 | again a uh understanding of the relative of places in the paleo geography. |
|
| 161:18 | that's um that's paleo geography, whether a big picture or a little picture |
|
| 161:26 | looking at the variation comparing them. they different? How are they mixing |
|
| 161:30 | ? Um That's a very helpful Um We'll try one more and then |
|
| 161:39 | stop tectonics and the provenance. Another to do this, as I |
|
| 161:46 | you can look at the courts feldspar diagram and it'll tell you some things |
|
| 161:51 | there are tectonic signals you can get of this, but you get even |
|
| 161:57 | if you can understand the ages of things and again, we could just |
|
| 162:02 | the source directly. Why don't we with that? Well, again, |
|
| 162:06 | , their spatial resolution. Um this faster both in the field and the |
|
| 162:10 | . We even if, you even if we were going to |
|
| 162:14 | we can analyze all these parts of mountains or we can just look at |
|
| 162:18 | sand in the river. Now, in the river has the low spatial |
|
| 162:22 | . It's a lot easier to scoop the sand and go home and just |
|
| 162:26 | it there in the lab and not in the case of the |
|
| 162:30 | but again, the same thing goes the source isn't there anymore. Now |
|
| 162:36 | tectonics and the provenance, we're gonna on material with lower closure temperatures. |
|
| 162:42 | uranium lead. Zircon is not very here because it doesn't have a very |
|
| 162:46 | tectonic signal. But nothing like argon and feldspar or helium and |
|
| 162:53 | They would do because look at this here, we've got, if we |
|
| 162:58 | a situation here, we've got a down there and it's it's below its |
|
| 163:02 | temperature. It's hot, let's just it and when it's down there, |
|
| 163:06 | isotopes are diffused out of that surrounding . But as we get up |
|
| 163:11 | you know, these isotopes. Once get to, once we get to |
|
| 163:14 | about this point, the daughters start be retained, their carried along with |
|
| 163:19 | and then eventually the erosion goes and it up there. And now this |
|
| 163:24 | , The amount of time the material below the closure temperature changes sides top |
|
| 163:29 | . And so it's better if this temperature is not very high, it's |
|
| 163:33 | down here. Then passage through this may have nothing to do with the |
|
| 163:38 | when we finally get up here. we want to get this pretty |
|
| 163:41 | low closure temperatures better. And so we're then going to see is since |
|
| 163:48 | talking about the triple dating, we're be looking at these samples that are |
|
| 163:52 | here in the basin. And what going to be comparing in many cases |
|
| 163:56 | the age of cooling to the age deposition. And the cooling occurs as |
|
| 164:04 | move through the mountain range and then is out here in the basin. |
|
| 164:08 | the difference between those two times is the lag time. That's a tectonic |
|
| 164:14 | . The shorter the lag time, more vigorous the tectonics. Um an |
|
| 164:22 | of this um comes from the Bengal where samples were collected in the ocean |
|
| 164:31 | program down here leg 1 16 and , you know, they drilled down |
|
| 164:37 | deep. They had to go all way out here, 2000 km away |
|
| 164:42 | the mountains in order to get Enough . If they drilled up there by |
|
| 164:49 | , you'd only get very young rocks , because it's the sedimentation rates too |
|
| 164:53 | . They can't drill down 15 So they go out to the edge |
|
| 164:57 | the, of the Bengal fan. they get more more age age. |
|
| 165:02 | they and what we have here are that went all the way down in |
|
| 165:05 | lower mice. If they had done up here, then it just got |
|
| 165:09 | last million years, let's just say . Alright, so we plot this |
|
| 165:18 | from my PhD and we plotted um mineral, this is for fell spars |
|
| 165:27 | the Bengal famine Argon 40 39 ages felts bars. And we actually did |
|
| 165:33 | heating on these with the laser just of course, turn it up, |
|
| 165:36 | up, turn it up and what plotting here is the minimum age, |
|
| 165:40 | know, we may have, we sometimes you only did three or four |
|
| 165:44 | . We didn't have enough gas in in the thing to measure more than |
|
| 165:47 | or four steps. And it was crude. But sometimes we would get |
|
| 165:50 | young of those three or four steps plotting the youngest one here because the |
|
| 165:55 | one is the most important. And what do we got here? |
|
| 166:00 | this, this is a log this is a linear scale. So |
|
| 166:04 | red line is the 1-1 line. what you'll find is that every place |
|
| 166:10 | look going from the ages of about million to about 16 million. We |
|
| 166:15 | able to find grains that essentially plotted that 1-1 line. That's the lag |
|
| 166:21 | . We're talking about lag time when go back back here to lag |
|
| 166:25 | when we say cooling age to deposition . Well, I guess you can |
|
| 166:28 | that for every single grain. But most important lag time is for the |
|
| 166:33 | grain because it is. And so this case the lag time throughout the |
|
| 166:41 | that's available to us here is always . What does zero lag time |
|
| 166:47 | That's really something because this is, is felt spars, these are coming |
|
| 166:52 | the Himalayas. We're going to say closure temperature of argon and feldspar is |
|
| 166:58 | degrees. That's a pretty good guess . Where is 200° down there? |
|
| 167:14 | , something like that. I mean could be six, Not less than |
|
| 167:20 | , several kilometers. Right. And . And these sedimentation ages come from |
|
| 167:28 | . Okay. So there you there's some uncertainty there too. But |
|
| 167:32 | the uncertainty of the of the Argon and certainty of the fossils, in |
|
| 167:37 | case we found ages that we really tell the difference between the two of |
|
| 167:42 | . What do we got then? got ages that telling us that this |
|
| 167:46 | , this this sample here Came from km below the surface, up to |
|
| 167:52 | surface and out 2000 km to the of the Bengal fan. Such that |
|
| 167:57 | can't tell the difference, Meaning that probably occurred within a quarter of a |
|
| 168:02 | years. That whole thing six km the top out here Couple 100,000 |
|
| 168:09 | That's really something yeah, we know really something tectonic because Himalayas don't have |
|
| 168:23 | . If there were volcanoes around, always gonna be some some ages that |
|
| 168:27 | the same age as the as the . Right. If we were to |
|
| 168:31 | this exercise off the coast of Costa , we find the same thing, |
|
| 168:38 | always find some grains that were the age as sedimentation because they were thrown |
|
| 168:42 | into the air by a volcano and down at the bar. That's not |
|
| 168:46 | lee interesting. But there are no in the Himalayas because it's a different |
|
| 168:50 | setting. So that means this is of this is tectonic. And so |
|
| 168:57 | we've got here and and but but only do we have done grades, |
|
| 169:00 | we have all grades in all of other places. Just as a little |
|
| 169:04 | when we were doing this, when was doing this for my PhD, |
|
| 169:07 | one was the first grade we analyzed it was like, you know, |
|
| 169:12 | is what we were wondering, can find zero age grains found this |
|
| 169:16 | the first one, wow, this the second one he analyzed and then |
|
| 169:23 | the others are in between. This the second one is the first |
|
| 169:25 | Well that's not important. It's just . Um but the fact that we |
|
| 169:30 | , we've got this long range We got big variation here. What's |
|
| 169:34 | variation? Tell us? This tells that there's a slot coming up like |
|
| 169:40 | , right? But the variation and continued variation tells us that it's not |
|
| 169:45 | whole mountain range coming up like crazy if it was the whole matching range |
|
| 169:50 | up at three or four a we'd eventually get rid of all our |
|
| 169:54 | samples. There'd be no old stuff . We still are getting old grains |
|
| 169:59 | into the Bay of Bengal at three years. Same as when we were |
|
| 170:03 | in 16 million. So what this telling us is that although there are |
|
| 170:07 | in the Himalayas that have come up a rocket during this whole time, |
|
| 170:11 | must be very small place because if was the whole mountain range we'd eventually |
|
| 170:16 | away. All the old stuff would be pulling up young stuff, the |
|
| 170:20 | thing would be brand new. So can say that it comes up like |
|
| 170:24 | rocket but only in little bitty So again, where are those little |
|
| 170:29 | spots are? Well we recognize one them in that place. In in |
|
| 170:34 | to bet that was one place, are other places that people are showing |
|
| 170:39 | you know how many there are where are that's going to be a lot |
|
| 170:42 | work. But we were able to this by just analyzing a few dust |
|
| 170:46 | By the way. I should point this was done a long time ago |
|
| 170:50 | this was considered a lot of data then. Remember I said you should |
|
| 170:54 | do 11717, even close to I think the one place we got |
|
| 171:00 | was maybe 20. But the good is is that because these these young |
|
| 171:07 | seem to be as prevalent as they , we didn't have, we didn't |
|
| 171:10 | to analyze 1000 grains to get We found what we were looking for |
|
| 171:14 | single time. If we could find zero age grain by only doing 20 |
|
| 171:20 | , it must be, it must be 1% of the total. It |
|
| 171:23 | be a lot of the total. nowadays this would be considered a remarkably |
|
| 171:29 | amount of data. Um And so mentioned, so we've got this problem |
|
| 171:38 | volcano. The lag time is proportional tectonics. The smaller the lag |
|
| 171:42 | the more the exceptional the tectonic But of course this is only for |
|
| 171:47 | volcanic grains. As I mentioned, are we going to deal with the |
|
| 171:51 | of volcanic grains being in our system looking exciting. But being boring, |
|
| 171:56 | really boring. It's just a But if you don't know, it's |
|
| 172:00 | now you sound like that. Like study in the Himalayas where they're going |
|
| 172:03 | like crazy, how can we tell difference? Well, first option would |
|
| 172:08 | to work in the Himalayas. you know, but if you |
|
| 172:11 | But what if you what if you're something in the Rocky Mountains or you're |
|
| 172:15 | in an active zone which is also a subduction zone? Well you do |
|
| 172:19 | thing called double dating. Here's an from uh the alps where we've got |
|
| 172:26 | bunch of fishing track ages from the basin and the fishing track ages are |
|
| 172:34 | pretty near the age of deposition. What is that? So, but |
|
| 172:41 | we can double date something that means date the same sample by both Iranian |
|
| 172:46 | and fishing track. Then the samples have the same value for those two |
|
| 172:51 | are volcanic, right? Because volcanic cool rapidly. And if you get |
|
| 172:57 | same answer for efficient track and as from the same zircon that has to |
|
| 173:03 | a volcanic zircon, see that if different, then you can ignore the |
|
| 173:12 | age and look at the second age a cooling age. But if they're |
|
| 173:15 | same, they must be volcanic and volcanic is not what we're interested |
|
| 173:21 | So if you have different systems and same age it's volcanic. Um And |
|
| 173:27 | here's some examples of some zircons. see what's that doing in there I |
|
| 173:34 | . Yeah, let's just look at . Um These were this is a |
|
| 173:40 | from Western United States where they looked all these samples. Here's the cooling |
|
| 173:44 | positional age. When you start talking lag times and stuff. But this |
|
| 173:48 | a bunch of data. This is the appetites. These are the non |
|
| 173:53 | genic psychogenic appetites. We had to some of these appetites because they had |
|
| 173:57 | same age as their fishing track and ages fishing track and their lead agent |
|
| 174:03 | the state. So once we get of that, then we can start |
|
| 174:07 | about tectonics lag time, all that . Um I think I'm gonna stop |
|
| 174:15 | and we'll pick up the rest. Starting on friday we'll finish this uh |
|
| 174:21 | the first part of friday. And I'll go through some exercises that which |
|
| 174:26 | basically be practice test will just give a bunch of scenarios and ask you |
|
| 174:32 | to sort the amount, The key answering all these questions is to know |
|
| 174:37 | the closure temperature of every system And once you know the closure temperature |
|
| 174:43 | know have to start thinking about why why do rocks change temperature change temperature |
|
| 174:48 | they will be very cooling off because some kind of erosion, whether it's |
|
| 174:52 | know, normal rain erosion or maybe tectonic erosion. They're heating up because |
|
| 174:58 | they've been next to a futon that nearby or because they're being buried. |
|
| 175:02 | can be buried because of sedimentation or of thrust faulting. Uh they can |
|
| 175:09 | off if they're in an igneous, they're in a granite, they can |
|
| 175:12 | off because they have been eroded away because they've been intruded into rocks that |
|
| 175:17 | much colder than them. Ah metamorphic don't move around like that. So |
|
| 175:24 | a metamorphic basin and they're cooling it's just because of erosion, um |
|
| 175:31 | rocks cool off very fast and we always decide if a rock is volcanic |
|
| 175:38 | how fast it's eroding, you so when we see a or or |
|
| 175:44 | eroding how fast it's cooling And we apply that same understanding of fast cooling |
|
| 175:51 | um active tectonic regions where you have big fault moving really rapidly and cooling |
|
| 175:56 | a whole bunch like when we saw in Denali that mountain range goes up |
|
| 176:00 | 20,000 ft and a lot of, all the same age. So it's |
|
| 176:04 | surprising that one of the tallest mountains the world has experienced a lot of |
|
| 176:09 | since four million years ago. so those are all the different ways |
|
| 176:17 | which rocks cool off. And all these then are valuable in assessing the |
|
| 176:25 | history, the detailed structural history, subsequent erosion and deposition, all history |
|
| 176:33 | various places. And by choosing the thermal chronometer, we can work on |
|
| 176:41 | problems. If you want to know this sandstone came from, uranium lead |
|
| 176:45 | , because that's gonna tell us about ultimate age of these places where it |
|
| 176:49 | from. But if you don't care where it came from, you just |
|
| 176:53 | to know what happened since it's been , You probably want to look at |
|
| 176:57 | in appetite. That's the 70°. So on your question, depends on your |
|
| 177:07 | . So have a review of all things. Um because we can go |
|
| 177:12 | them on next friday, we'll have sense of how to use them. |
|
| 177:17 | you know, come come with a , you know, giving you let's |
|
| 177:21 | go back to uh is it, it in this diagram? I |
|
| 177:27 | No, it's not here. But , I've given you that diagram several |
|
| 177:31 | where you've got the whole range of chronometer uranium lead Argon fishing track |
|
| 177:39 | you've got a list of maybe, don't know, 15 of them. |
|
| 177:45 | your temperature of argon and close your of fission tracks and appetite. Close |
|
| 177:50 | temperature of lead in zircon. These all things you need to know. |
|
| 177:56 | gone through how it is, we these things and some of the particulars |
|
| 178:00 | how we make these measurements. Some the particulars of how we need to |
|
| 178:03 | about system. Any questions Okay? have a question during the week. |
|
| 178:15 | me an email. Um, otherwise can pack it up for today and |
|
| 178:23 | , start again on friday with the of this and then the exercises. |
|
| 178:29 | . |
|
