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America's Oldest Rock - Part 1 · PlanetGeo: The Geology Podcast

• 0:00
  SPEAKER_00: If you want to find old rocks, if you want to stand on the oldest rocks in in North America, well not North America, why would you in the United States, you've got to be kind of in the north-central part of the region?
• 0:14
  SPEAKER_00: Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, why it.
• 0:22
  SPEAKER_00: Chris.
• 0:23
  SPEAKER_00: Yeah.
• 0:23
  SPEAKER_00: Hey.
• 0:24
  SPEAKER_00: How are we doing, Jess?
• 0:25
  SPEAKER_00: You got your weedy, your weeds jacket, your your weed stomping boots on today.
• 0:29
  SPEAKER_02: I have two questions for you.
• 0:30
  SPEAKER_02: One.
• 0:31
  SPEAKER_02: Okay, good.
• 0:32
  SPEAKER_02: Do you have an idea as to where we're going today?
• 0:35
  SPEAKER_00: Uh yes.
• 0:36
  SPEAKER_00: America's oldest rocks, part one.
• 0:39
  SPEAKER_00: Part one of two.
• 0:41
  SPEAKER_00: Part one of two.
• 0:41
  SPEAKER_00: Part one of two so far.
• 0:43
  SPEAKER_02: We don't know where this is going to go entirely.
• 0:45
  SPEAKER_00: I mean, no, we we know where we're going today, but where we're going in general.
• 0:48
  SPEAKER_02: Uh I mean, you know, think about this.
• 0:50
  SPEAKER_02: We were doing a series on Earth's oldest rocks, and then this came across our radar, and we're like, oh, well, we'll we'll do a little mini-series on the United States or America's oldest rocks.
• 1:01
  SPEAKER_00: Yeah, that's right.
• 1:02
  SPEAKER_00: And Chris, there were we're basing so this came out recently.
• 1:05
  SPEAKER_00: We're recording this in uh summer of 2025.
• 1:08
  SPEAKER_00: There's a paper that came out um in Hold on.
• 1:11
  SPEAKER_02: I didn't get to ask my other question, Jesse.
• 1:13
  SPEAKER_02: Okay, jumping right in.
• 1:14
  SPEAKER_02: Okay.
• 1:15
  SPEAKER_02: Which leads in nice, nice segue, Jesse, into my second question, which is Am I gonna get any airtime here?
• 1:23
  SPEAKER_02: I would are you gonna let me join in on the conversation.
• 1:26
  SPEAKER_00: I would not bet on it if I was you.
• 1:28
  SPEAKER_00: So okay.
• 1:30
  SPEAKER_00: Get your drink ready.
• 1:31
  SPEAKER_00: Uh oh my gosh.
• 1:33
  SPEAKER_00: No, of course, Chris, of course.
• 1:35
  SPEAKER_00: You'll you'll get uh at least 5% of the airtime.
• 1:38
  SPEAKER_00: Um let me just we've got to give credit where credit's due here.
• 1:41
  SPEAKER_00: None of this the science is uh original.
• 1:44
  SPEAKER_00: We're summarizing a bunch of stuff, and there was actually a really recent article written in GSA Today, the Geological Society of America, um, kind of by month, every other month, I think they published this.
• 1:54
  SPEAKER_00: And there's a nice article, kind of review paper written by Carol Frost and others on kind of uh highlighted this.
• 2:00
  SPEAKER_00: And this got picked up in the science news, and this came across my desk like oh.
• 2:04
  SPEAKER_02: How many times did it get sent to you by your friends and people that you know and so on?
• 2:08
  SPEAKER_02: Because it got sent to me about a hundred times.
• 2:10
  SPEAKER_00: Okay, all right, that's way more than me.
• 2:12
  SPEAKER_00: I did not that's that's interesting.
• 2:14
  SPEAKER_00: I did not get it sent to me.
• 2:15
  SPEAKER_00: Two people pinged me.
• 2:16
  SPEAKER_00: Um and but it's a really nice kind of summary um and kind of poses the question.
• 2:21
  SPEAKER_00: And the title is USA's oldest rock question mark, a simple question with a complex answer, which is really the interesting part, and we're gonna dive into this because the complexity, I think, is where it's interesting, right?
• 2:33
  SPEAKER_02: Let's double-click on that that quote because that's a really good quote, and it pretty much sums up the field of geoscience.
• 2:39
  SPEAKER_00: That's especially the early earth.
• 2:41
  SPEAKER_00: Yes, USA's oldest rock question mark, a simple question with a complex answer.
• 2:45
  SPEAKER_00: That's the title of this paper written by Carol Frost, Paul Muirler, Marion Bickford, and and Robert Stern.
• 2:51
  SPEAKER_00: It summarizes a whole bunch of research.
• 2:53
  SPEAKER_00: And I've Chris, I've been interested, you know, I this is my home turf.
• 2:56
  SPEAKER_00: I work on old rocks, I'm deeply familiar with the Acosta Nice complex, worked there for a long time.
• 3:01
  SPEAKER_00: Other old rock locations I'm really familiar with.
• 3:04
  SPEAKER_00: I was not intimately familiar before sitting down and really diving into the weeds here with these areas.
• 3:10
  SPEAKER_00: I kind of knew there's like hints of old crust here, but there's a lot of places with hints of old crust, and so it gets really complicated when teasing these apart.
• 3:18
  SPEAKER_00: And I never really sat down and thought about what are the oldest rocks in the United States, and you know what are the debates on either side of all of these locations.
• 3:27
  SPEAKER_00: So well, you're right.
• 3:29
  SPEAKER_02: That's yeah, go ahead.
• 3:30
  SPEAKER_00: Why did why did you get a hundred people pinging you this?
• 3:32
  SPEAKER_00: What did they say?
• 3:33
  SPEAKER_00: And what was your instinct?
• 3:36
  SPEAKER_02: My well, you know my instinct because I started getting this, and you know, after a few days, I'm like, okay, Jesse, we have to do an episode on this.
• 3:43
  SPEAKER_02: And I sent you the link, and you're like, Look, you idiot, I already sent you this like two weeks ago.
• 3:51
  SPEAKER_00: I had I just forget.
• 3:53
  SPEAKER_00: I yeah, I sent you an email.
• 3:55
  SPEAKER_00: You were uh getting on a plane somewhere for a weekend, and I sent it to you and and hoped to catch you before you got on the plane.
• 4:00
  SPEAKER_00: And I was like reading material for the plane, dot dot dot.
• 4:03
  SPEAKER_02: Yeah, did you know now it didn't happen.
• 4:05
  SPEAKER_02: I spent too much time watching reruns of the office.
• 4:08
  SPEAKER_02: So yeah, there you go.
• 4:10
  SPEAKER_02: That took precedent over whatever you were sending me.
• 4:12
  SPEAKER_02: So it goes to show you know the level that I hold you.
• 4:16
  SPEAKER_02: I you know, you send stuff and it just kind of goes over the head or or by my ears, whatever.
• 4:21
  SPEAKER_02: I just let it rip and let it go.
• 4:22
  SPEAKER_02: But when other people start sending me stuff, I start to take notice that.
• 4:26
  SPEAKER_00: Then you pay attention.
• 4:27
  SPEAKER_00: Well, this is a this really is a simple question with a really complicated answer.
• 4:32
  SPEAKER_00: And I I have some thoughts on on the answer, but the answer is perhaps surprising.
• 4:38
  SPEAKER_02: Well, let's save that.
• 4:39
  SPEAKER_02: Let's save that.
• 4:40
  SPEAKER_02: Okay.
• 4:40
  SPEAKER_02: First of all, Jesse, let's set the stage.
• 4:42
  SPEAKER_02: What part of the country are we talking about?
• 4:46
  SPEAKER_02: And and I'm really kind of alluding to the image that you have here up on the screen.
• 4:50
  SPEAKER_02: So what are we talking about here?
• 4:53
  SPEAKER_00: Yeah, let's turn to this image number one.
• 4:54
  SPEAKER_00: And basically, the point is we're looking at the United States, the lower 48 here.
• 4:58
  SPEAKER_00: If you want to find old rocks, if you want to stand on the oldest rocks in in North America, well, not North America, why would you in the United States, you got to be kind of in the north-central part of the region.
• 5:08
  SPEAKER_00: The little bit of Michigan, there's a little bit of Michigan, the upper peninsula, the western part of the upper peninsula, Michigan.
• 5:12
  SPEAKER_00: If you kind of start there, head down through Wisconsin, Minnesota, we'll touch on that, down into a little bit of Colorado, Wyoming, the Tetons have some Archean crust exposed.
• 5:23
  SPEAKER_00: So this is a good thing.
• 5:27
  SPEAKER_00: Exactly.
• 5:28
  SPEAKER_00: Down that far south, and then kind of loops back up, and you kind of go through a little bit of Idaho, and you you basically we're we're talking about those upper kind of central states, is where there's old crust, at least old Archean, greater than 2.5 billion year old crust, is under your feet.
• 5:42
  SPEAKER_00: There's a lot of sediments on top of it.
• 5:44
  SPEAKER_00: Some of these areas, like the Wyoming province, the Wyoming craton here, has little hints of exposure, but not it's not everywhere.
• 5:53
  SPEAKER_00: Like there's a lot of sediments on top of these things, but they're underneath down below the basement.
• 5:57
  SPEAKER_00: What we're talking about, the basement is actually really old.
• 6:00
  SPEAKER_00: So my question for you, Chris, is what are the oldest rocks you've stood on in the United States, do you think?
• 6:06
  SPEAKER_02: Ooh.
• 6:07
  SPEAKER_02: That's a really good question.
• 6:09
  SPEAKER_02: Um is it the Tetons?
• 6:11
  SPEAKER_02: Do you like the the No, I don't think so.
• 6:14
  SPEAKER_00: Um Beartooth.
• 6:15
  SPEAKER_00: You've done hiking around the bear tooth?
• 6:17
  SPEAKER_00: Yeah, I have.
• 6:17
  SPEAKER_00: Yep, yep, yep.
• 6:18
  SPEAKER_00: Okay, so you've you've made some of those nices, I'm sure.
• 6:21
  SPEAKER_02: But I've spent Jesse a fair amount of time in the Western UP, and uh the Water Smeat gneiss is it's not yeah, it's there, you know, but it's not all over the place.
• 6:32
  SPEAKER_02: You you have to like kind of do your research and and to go find it.
• 6:36
  SPEAKER_02: But I think it's a beautiful rock.
• 6:38
  SPEAKER_00: Oh, it's spectacular.
• 6:39
  SPEAKER_02: These these are and we'll we'll get into that in a little bit more here in a um shortly.
• 6:44
  SPEAKER_02: But one thing, Jesse, that uh just to bring up, it it you said that the Wyoming is really hit and miss.
• 6:50
  SPEAKER_02: And this map is a little bit misleading then in that respect, because it looks like it's it's really uh prolific in certain parts of the of that area, but it's really not.
• 6:59
  SPEAKER_02: It's very, very hit and miss.
• 7:01
  SPEAKER_00: So, Chris, the way this is a confusing thing for uh that us uh uh you know early earth geologists or continental scale geologists kind of deal with, is we often ignore we ignore what we call the Paleozoic cover.
• 7:16
  SPEAKER_00: Or the Phanerozoic cover.
• 7:18
  SPEAKER_00: We we just ignore kitty litter, it's just kidding stuff on the surface.
• 7:22
  SPEAKER_00: Yeah, yeah.
• 7:22
  SPEAKER_00: So often what you'll see in these maps is a map that is it'll be called a basement map or a geological basement map of North America.
• 7:32
  SPEAKER_02: And what that means that's a really important point, Jesse.
• 7:35
  SPEAKER_02: Really important right now.
• 7:36
  SPEAKER_00: Yeah, what that means is there's all sorts of sediment on top of the basement.
• 7:39
  SPEAKER_00: In Michigan, we got the Michigan basement.
• 7:41
  SPEAKER_00: Yeah, the Michigan basin, right?
• 7:42
  SPEAKER_00: And there's kilometers, kilometers, and kilometers of sediment.
• 7:45
  SPEAKER_00: But if you drill down, you will hit basement.
• 7:48
  SPEAKER_00: And the basement is the igneous and metamorphic, what we call the crystalline core of the continent.
• 7:53
  SPEAKER_00: That's the stuff that's fully crystalline, uh, it's really resilient rock, and the sediments are just sitting on top of that.
• 8:00
  SPEAKER_00: Uh so that's an unconformulaire.
• 8:02
  SPEAKER_00: And the the maps that that uh those of us who study the early earth and those of us who work in these areas consider is we kind of ignore that sedimentary cover, and we we're looking at when did the continent get accreted?
• 8:13
  SPEAKER_00: When did the continent, the roots of the continent form, not when did the sediments right beneath our feet get deposited?
• 8:19
  SPEAKER_00: That's not the question.
• 8:20
  SPEAKER_00: The question is when did the core of the continent get aggregated together?
• 8:23
  SPEAKER_00: And here you can see the oldest parts are up in this north central part, which begs the question what's the rest of it?
• 8:29
  SPEAKER_00: Well, the rest of it is younger, the rest of it is 1.9 billion year old.
• 8:33
  SPEAKER_00: There's a bunch of 1.9 billion year old terrain.
• 8:36
  SPEAKER_02: Oh, just 1.9, no, no big deal.
• 8:38
  SPEAKER_02: You know, that's still really, really old stuff.
• 8:40
  SPEAKER_02: That's right.
• 8:41
  SPEAKER_02: So, Jesse, um, kind of like where we're going today, because you asked me when when we first started talking about doing this little miniseries, you said, I don't know.
• 8:51
  SPEAKER_02: Is it is it two in the weeds?
• 8:52
  SPEAKER_02: And and um, I'm like, no, I don't think it is, but but it's complicated.
• 8:58
  SPEAKER_00: You're you know, Chris, Chris, your your response was I don't really care about that.
• 9:03
  SPEAKER_02: It was.
• 9:03
  SPEAKER_02: It was like I don't really care about that.
• 9:05
  SPEAKER_00: It's interesting, so who cares?
• 9:06
  SPEAKER_00: Yeah, that's right.
• 9:07
  SPEAKER_02: We both love this stuff, and so but I want to just touch on why is you know, it's a simple question with a complicated answer.
• 9:13
  SPEAKER_02: Why so complicated?
• 9:15
  SPEAKER_02: Well, I guess the answer lies in that, you know, when you when you deal with old rocks, you're talking about rocks then that have had many, many things happen to them.
• 9:24
  SPEAKER_02: They've had a rough history, right?
• 9:26
  SPEAKER_02: I mean, they've they've had a rough life.
• 9:28
  SPEAKER_02: Many things have happened, they've been heated, they've been squeezed, they've been reworked in several different ways.
• 9:34
  SPEAKER_02: And and so it's not a very straightforward look.
• 9:40
  SPEAKER_02: You gotta see through all that stuff to get at the origin, but then you also you want to see what events happened along the way.
• 9:49
  SPEAKER_02: What were the big, you know, as this rock grew up, you know, what happened to it?
• 9:54
  SPEAKER_02: What were these traumatic events that kind of that happened as this rock was brought up?
• 9:59
  SPEAKER_00: So Chris, I think that that's a really that's a really great point because we need the whole toolbox, we need all of the geologist toolkit to study these rocks.
• 10:08
  SPEAKER_00: What we're gonna do today in this particular episode, part one, we're gonna focus on the primary technique that we use to get at the first age of the rock or the main age of the rock.
• 10:21
  SPEAKER_00: And what do we mean by the main age?
• 10:23
  SPEAKER_00: The main age in our head is not not what are the age of all the minerals in the rock, because most of these rocks are really metamorphic, and most of the minerals are recrystallized during some metamorphic event.
• 10:35
  SPEAKER_00: Most of these things are what we would call ortho gnyces.
• 10:38
  SPEAKER_00: And ortho gnyses we generally think of as these are metamorphosed igneous rocks, and so we're trying to get at the igneous age, not the oldest igneous igneous age, but the dominant igneous age.
• 10:50
  SPEAKER_00: So, when was this rock last a magma?
• 10:52
  SPEAKER_00: And then it's been metamorphosed multiple times, and all of these rocks that we'll talk about, all three of them, are really complicated.
• 10:58
  SPEAKER_00: So, first of all, what we're gonna do here, Chris, is we're just gonna set the stage.
• 11:01
  SPEAKER_02: What are the three locations that are kind of in just to interrupt you but before we get into that?
• 11:07
  SPEAKER_02: I I just want to touch on something that you just said that I think is really important for understanding this episode.
• 11:15
  SPEAKER_02: And you hit on recrystallization and and what is you know, what is recrystallization?
• 11:21
  SPEAKER_02: It's a this is a very important metamorphic term.
• 11:24
  SPEAKER_02: It's kind of basic, but not everybody will be familiar with what this means.
• 11:28
  SPEAKER_02: So we're talking about minerals that dissolve, all these minerals dissolve at different temperatures or or you know, they become they melt at certain temperatures, and then they will precipitate in new locations, which result then in often newer and larger crystals that existed before.
• 11:47
  SPEAKER_02: And and that's so we're talking about you know new mineral growth, and we're talking about mineral growth that happens on top of this kind of core, the the protolith, if you will.
• 11:58
  SPEAKER_00: That's exactly right.
• 11:59
  SPEAKER_00: So you'd imagine, okay, take a granite.
• 12:01
  SPEAKER_00: The granite has granodired tonalite or something.
• 12:04
  SPEAKER_00: It's got quartz, feldspar, um, two feldspars, probably.
• 12:08
  SPEAKER_00: It's got some biotite in it, maybe a little bit of amphibole, perhaps some muscovite, and then a whole bunch of accessory minerals.
• 12:14
  SPEAKER_00: It's got some zircon, it's got some titanite, maybe some apatite in there.
• 12:19
  SPEAKER_00: That's the dominant rock.
• 12:20
  SPEAKER_00: Now you heat that thing up, and what happens?
• 12:22
  SPEAKER_00: Well, you heat it up, and a lot of this stuff will happen in the solid state.
• 12:25
  SPEAKER_00: So not necessarily all melted.
• 12:27
  SPEAKER_00: The amphibole will break down, the muscovite starts to break down, loot releases some water, maybe a little melt is produced.
• 12:34
  SPEAKER_00: Those atoms go somewhere else.
• 12:37
  SPEAKER_00: And uh then you you kind of because it's all getting squeezed too.
• 12:40
  SPEAKER_02: So now that you've got this hydrothermal fluid, now it's on the move.
• 12:44
  SPEAKER_00: Now it's on the move, and and the minerals are solid state recrystallization.
• 12:48
  SPEAKER_00: So when that happens, they they reset their geochronometers, many of them, the radio uh radioactive geochronometers that we use.
• 12:55
  SPEAKER_00: And so this is complicated.
• 12:58
  SPEAKER_00: These rocks are incredibly complicated.
• 13:00
  SPEAKER_00: If you look at one mineral with one isotopic system, you're gonna get one age.
• 13:04
  SPEAKER_00: If you look at a different mineral with a different isotopic system, you're gonna get a different age.
• 13:08
  SPEAKER_00: And so we're gonna focus here on this episode of going through really the details, some of the weedy parts of zircon uranium led geochronology, because that mineral in that particular isotope system we've talked a lot about before on this podcast, but that's one that's really good at tracing the oldest igneous events.
• 13:27
  SPEAKER_00: It's not perfect though, as we'll highlight here.
• 13:30
  SPEAKER_00: It is absolutely not perfect, and it's not clean and clear, and it's definitely not clean and clear in these three rock units that we're gonna talk about here.
• 13:38
  SPEAKER_00: So, Chris, that that's kind of the and we're gonna stop there.
• 13:40
  SPEAKER_00: We're not gonna give you an answer to what the oldest rock in the United States is today, in part because the answer is uh it's a little complicated.
• 13:47
  SPEAKER_00: It's not a clean, cut and dry answer.
• 13:48
  SPEAKER_00: But we'll that'll come in at least part two.
• 13:51
  SPEAKER_00: So, should we go through the candidate areas, Chris?
• 13:54
  SPEAKER_00: Like the the locations that there is really old stuff.
• 13:58
  SPEAKER_00: Like this map shows 2.5 and older crust, basement, and rocks.
• 14:03
  SPEAKER_00: And one question you might ask before we jump to that is what's in between these blocks?
• 14:07
  SPEAKER_00: Basically, you think of these as Archean, really old 2.5 billion year old blocks, sort of microcontinents.
• 14:13
  SPEAKER_00: You can kind of think of them like that.
• 14:14
  SPEAKER_00: And they were all aggregated together.
• 14:16
  SPEAKER_00: Most of them have kind of sutures in between them that are like 1.9, 1.7 billion years, some of them 1.1 billion years old.
• 14:23
  SPEAKER_00: So they're kind of they've been amalgamated over time, little micro plates, micro continents that were aggregated together to form what is now the um Canadian Shield, basically.
• 14:33
  SPEAKER_00: So the locations we're going to talk about, there's two in the superior province, the Minnesota River Valley areas.
• 14:40
  SPEAKER_00: There's there's two down there in this bottom southern part of the superior province, and one in the Wyoming Craton, the Wyoming province, that are fairly well studied.
• 14:49
  SPEAKER_00: Here, Chris, what's called the Minnesota River Valley.
• 14:53
  SPEAKER_00: This is the Morton Gneiss.
• 14:54
  SPEAKER_02: Yeah, and I think by countertops.
• 14:58
  SPEAKER_00: Yes, and it's a it's a really big quarried stone.
• 15:01
  SPEAKER_00: I mean, it's a it's a building stone.
• 15:03
  SPEAKER_00: Uh it's a beautiful, beautiful rock.
• 15:05
  SPEAKER_00: But I think this image in particular shows, first of all, the beauty of these things.
• 15:08
  SPEAKER_00: This is why gneisses are so cool.
• 15:10
  SPEAKER_00: They're really pretty rocks, but the complexity.
• 15:13
  SPEAKER_00: I mean, you just look at that, Chris.
• 15:14
  SPEAKER_00: Does that scream complicated history to you?
• 15:17
  SPEAKER_00: I know it does to you, but do you think it screams tortured?
• 15:20
  SPEAKER_02: Um and yeah, very complicated.
• 15:22
  SPEAKER_02: Absolutely.
• 15:23
  SPEAKER_02: There's a lot.
• 15:24
  SPEAKER_02: This is complicated.
• 15:26
  SPEAKER_02: There's a lot that happened to this rock to you go back to what what it originally was to get it to this, uh, it's a complicated history for sure.
• 15:34
  SPEAKER_00: And what do we mean by complicated history?
• 15:36
  SPEAKER_00: Chris, like what what are uh what are things that you can you paint us a picture for events that could reset a rock like this or alter or deform?
• 15:45
  SPEAKER_00: Like what is the torture part?
• 15:47
  SPEAKER_00: What do the torture events look like to a rock like this in your mind?
• 15:50
  SPEAKER_02: Uh well I think most of it, you know, it all comes back to plate tectonics and and so varying uh degrees or varying recipes of uh temperature, pressure, uh, and those chemically active fluids that you were alluding to before, which which lead to recrystallization.
• 16:06
  SPEAKER_02: So uh Jesse, in a you take a intro-level geology class and and you you know show a picture of a granite and then you subject it in this kind of directional stress and you heat it up a lot and it turns into a gneiss.
• 16:20
  SPEAKER_02: It's that really it's this really simple way of showing how you can take a uh you know a normal-looking granite and turn it into this beautiful looking gneiss.
• 16:28
  SPEAKER_02: But in reality, this may have happened in multiple steps to make this rock that we're looking at right here look the way it does now.
• 16:36
  SPEAKER_02: This was uh probably a much more complicated multi-stage event thing that happened as this rock got matured.
• 16:45
  SPEAKER_00: Yeah, so getting caught up in a plate collision, right?
• 16:48
  SPEAKER_00: We'll heat this rock up, recrystallize it.
• 16:50
  SPEAKER_00: There's some fluids around, those fluids will go through and they'll reset a bunch of the minerals in this rock.
• 16:56
  SPEAKER_00: If it gets heated up enough, it could be partially melted, which will do certain things to the minerals and it'll remove some fraction of the mass of the rock.
• 17:05
  SPEAKER_00: Even just sitting in a hydrothermal environment, if it gets heated up, fractured, broken apart, and fluids pass through there, that will affect the minerals in this rock a lot too, and reset the ages.
• 17:17
  SPEAKER_00: So, so there's multiple, multiple, multiple events that can affect the ages of this rock and make it look this complicated.
• 17:24
  SPEAKER_00: It's not like, oh, take a granite, metamorphose it, boom, you get this.
• 17:27
  SPEAKER_00: That is true, but you could metamorphose it six times and it would still look like a nice like this.
• 17:31
  SPEAKER_00: So so it's it's complicated to tease this stuff apart.
• 17:35
  SPEAKER_02: Well, so Jesse, um, this was widely, at least for a time, and we're not gonna give our thoughts on this yet, but this was widely thought to be the oldest rock in the United States.
• 17:45
  SPEAKER_02: In fact, let me skip back.
• 17:47
  SPEAKER_02: There's a sign.
• 17:48
  SPEAKER_02: There's a sign here.
• 17:49
  SPEAKER_02: Check it out.
• 17:50
  SPEAKER_00: This is the the sign that I think still exists in the Minnesota River Valley area that says world's oldest rock, some of the oldest exposed rocks in the world here.
• 17:59
  SPEAKER_00: J'allists estimate this grenadic gnice was formed 3.8 billion years ago.
• 18:03
  SPEAKER_00: Um, let's give away a little bit.
• 18:05
  SPEAKER_00: That's not the it's not the oldest rock in the world.
• 18:08
  SPEAKER_00: We've talked about this before.
• 18:09
  SPEAKER_02: Okay, that's yeah, we can say that.
• 18:11
  SPEAKER_00: We we can say that.
• 18:12
  SPEAKER_00: This is not the world's oldest rock, which gives you an age of the sign.
• 18:16
  SPEAKER_00: So this sign was at least older than 1989 when the Acosta Nice complex was kind of discovered as to be a four billion-year-old rock, right?
• 18:24
  SPEAKER_00: So this is a the discovery of old, of potentially really old parentage of this particular rock in the Minnesota River Valley terrain goes back to the 1970s, really, 1960s, 1970s, which is Chris, the early days of geochronology.
• 18:39
  SPEAKER_00: Like the 1960s were the early days of geochronology, and geologists by that point, as soon as they started able to make you know isotopic measurements of rocks and do and do some geochronology, focused in on this area and said there's some really old hints of stuff here.
• 18:54
  SPEAKER_00: And uh a lot of work.
• 18:55
  SPEAKER_00: I have a lot of respect for that.
• 18:57
  SPEAKER_02: A lot of mad respect for the work that was done and how good it was at such an early stage.
• 19:04
  SPEAKER_00: It's an unbelievable, it's an unbelievable story.
• 19:07
  SPEAKER_00: Just that again, that geologist's intuition of like, hey, we make some measurements, early days rubidium strontium was used.
• 19:13
  SPEAKER_00: And oh wait, there's hints of old stuff here.
• 19:15
  SPEAKER_00: They look in more detail, do some early zircon work.
• 19:17
  SPEAKER_00: And Chris, these were the days when in order to get a zircon uranium-led measurement.
• 19:22
  SPEAKER_00: Now we put tiny, tiny, tiny microscopic pinpricks with our laser system into a grain and get an age from a 10 micrometer little outer layer of a grain.
• 19:35
  SPEAKER_00: Back then, they had to take a whole bucket full, five-gallon bucket of rock, crush that up to get zircons out.
• 19:41
  SPEAKER_00: Then they took a bunch of zircons, like think of um a little vial and fill up a bunch of that with pure zircon and then dissolve all of that to get one measurement because their instruments were not that sensitive.
• 19:53
  SPEAKER_00: So it's amazing they could even think that these rocks were that old with that data.
• 19:58
  SPEAKER_00: I mean, the data is unbelievable.
• 20:05
  SPEAKER_00: So one of one of my PhD supervisors, Larry Heeman, is is um, or he, you know, he's retired now, but he was of that generation, the generation after that, right?
• 20:13
  SPEAKER_00: So he's kind of doing this his degree um in the the 80s, 70s kind of time.
• 20:17
  SPEAKER_00: And literally his entire master's degree was here's an instrument, get it set up so that you can make like three measurements, right?
• 20:24
  SPEAKER_00: That's the entire master, that's two years of work, master's the it's just the data was so much harder to collect.
• 20:29
  SPEAKER_00: And I think now, like the modern generation, we often forget that because I have students who can collect a thousand data points in a day in my lab.
• 20:37
  SPEAKER_00: Um we forget how valuable data was back then.
• 20:41
  SPEAKER_00: Anyway, that's right.
• 20:41
  SPEAKER_00: We do it so Minnesota River Valley, this is in southwestern Minnesota, and these are the Morton Gneiss, which you said, which is the building stone quarry, and then there's another block, crustal block, that is separated by a shear zone in the middle that uh separates the Morton Gneiss from the Montevideo block of gneisses, which is a little bit further to the east.
• 21:02
  SPEAKER_00: And there's hints of old components all throughout there.
• 21:06
  SPEAKER_00: So that's like the number one candidate number one.
• 21:09
  SPEAKER_02: Candidate number one.
• 21:10
  SPEAKER_02: That's right.
• 21:10
  SPEAKER_02: I'm jumping at the bit.
• 21:11
  SPEAKER_02: So let's get to candidate number two, Jesse.
• 21:14
  SPEAKER_02: So can you go to that a second?
• 21:16
  SPEAKER_00: Yes.
• 21:16
  SPEAKER_00: This is a watersmeat.
• 21:18
  SPEAKER_00: The waters meet nice, near and dear to your heart.
• 21:20
  SPEAKER_00: Or you've spent a lot of things.
• 21:21
  SPEAKER_00: It is a beautiful rock.
• 21:22
  SPEAKER_02: Look at this rock.
• 21:23
  SPEAKER_02: It's it's gorgeous.
• 21:24
  SPEAKER_00: Very beautiful.
• 21:25
  SPEAKER_00: Not nearly as as uh complicated, this particular sample, as the uh the photo of the the um Morton nice, but potentially old.
• 21:34
  SPEAKER_00: Well, it is old, very old.
• 21:36
  SPEAKER_00: Uh potentially the oldest rock in America.
• 21:38
  SPEAKER_02: And I'm a little offended that you said it's not nearly as complicated, actually.
• 21:42
  SPEAKER_00: Well, it is as complicated, it does not look as complicated.
• 21:45
  SPEAKER_00: You don't see the nice banding patterns, right?
• 21:48
  SPEAKER_00: Like this looks like a little bit of a simpler rock.
• 21:49
  SPEAKER_02: Yeah, you're right.
• 21:50
  SPEAKER_02: With the Morton, with the Morton complex or the Morton ice, you can clearly see fluids were moving through this, and you can see the direction, you know, that the fluids were moving through.
• 22:00
  SPEAKER_02: This that's that's much more obvious in in that rock for sure.
• 22:04
  SPEAKER_02: That's exactly right.
• 22:05
  SPEAKER_02: And this is again uh in the western upper peninsula, you know, the porcupine region.
• 22:10
  SPEAKER_00: Uh so upper peninsula, Michigan, south southern, southwestern part there, where this is in the southern, the superior province or the superior craton is this huge Archean block that starts.
• 22:22
  SPEAKER_00: We're at the southern bottom of this.
• 22:24
  SPEAKER_00: The Minnesota River Valley is kind of the southern bottom extent of that.
• 22:27
  SPEAKER_00: And this Waters Meet and Ice province in Michigan is also part of that southern superior province.
• 22:32
  SPEAKER_00: And the superior province extends all the way up into the Hudson Bay, all the way up through Quebec, and actually contains some really old rocks up there around that are like 3.8 billion years old.
• 22:42
  SPEAKER_00: Some of them we've talked about before the New Vagatuck Greenstone Belt, maybe potentially it's 4.2 billion years old, all part of the superior craton, superior province, which is one of the bigger uh provinces in the Canadian Shield, or it is the biggest by far.
• 22:55
  SPEAKER_02: So and then lastly, but certainly not least, the third candidate, the Wyoming Cratonic Rocks.
• 23:04
  SPEAKER_00: The Wyoming Cratonic Rocks.
• 23:05
  SPEAKER_00: And we'll we'll get into this a little bit more, but this is the Sakawee, I think it's pronounced Sakawee block, part of the Wyoming province out here, and it's a couple samples in the granite mountains in central Wyoming, basically.
• 23:17
  SPEAKER_00: So it's it's part of there's little hints of exposure if you look at a geological map, hints of exposed Wyoming uh basement rocks from the Wyoming Craton.
• 23:26
  SPEAKER_00: The Tetons are built of Wyoming province rocks, the the bottom of the Tetons there, and a la lot of that area is exposed Archean rocks, but it's little hints of it, and so the Sakuae block is a particularly old block within there.
• 23:41
  SPEAKER_02: Alright, I have a question, Jesse.
• 23:42
  SPEAKER_02: Sorry I interrupted you.
• 23:43
  SPEAKER_02: I know I threw you off.
• 23:45
  SPEAKER_02: I just got excited and I can't help myself, so I just jumped right in.
• 23:47
  SPEAKER_02: But you've been to a place in Wyoming called Vadavu.
• 23:53
  SPEAKER_02: You remember that?
• 23:54
  SPEAKER_00: Uh-huh.
• 23:54
  SPEAKER_02: You remember that?
• 23:55
  SPEAKER_02: Okay.
• 23:56
  SPEAKER_02: Uh, is that a part of this craton that were the really old rocks that we're talking about or not?
• 24:02
  SPEAKER_02: I maybe you don't know the answer to this.
• 24:04
  SPEAKER_02: I'm not sure.
• 24:04
  SPEAKER_02: But like by the rattlesnake mountains.
• 24:07
  SPEAKER_02: Um you know, I don't know.
• 24:09
  SPEAKER_00: I'd have looked that up.
• 24:10
  SPEAKER_00: Um okay, all right.
• 24:13
  SPEAKER_00: I I I don't know.
• 24:14
  SPEAKER_00: I don't know off the top of my head what that is.
• 24:16
  SPEAKER_00: Okay.
• 24:16
  SPEAKER_00: Um if it's if it's like Archean gice.
• 24:18
  SPEAKER_00: That was an unfair question.
• 24:19
  SPEAKER_00: That was no, no, no, it's fair.
• 24:20
  SPEAKER_00: Totally fair.
• 24:21
  SPEAKER_00: You know, whether it's it's Archean gneisses there or not.
• 24:24
  SPEAKER_00: Uh I would suspect so if it's like you know, granitoid basement rocks, then it's probably is, but I'm not sure.
• 24:31
  SPEAKER_00: Okay.
• 24:32
  SPEAKER_02: Yeah, it's more I don't think it is then.
• 24:33
  SPEAKER_02: I don't think it is.
• 24:34
  SPEAKER_00: Okay.
• 24:35
  SPEAKER_00: Well, we'll look it up.
• 24:36
  SPEAKER_02: It's not granitoid.
• 24:37
  SPEAKER_02: All right, yeah, yeah, yeah.
• 24:37
  SPEAKER_02: We'll find out all right.
• 24:38
  SPEAKER_00: We'll put it in part two.
• 24:39
  SPEAKER_02: Okay, let's move on.
• 24:40
  SPEAKER_02: So let's get into now, and this is where I'm gonna have to fight, like I'm I'm treading water here right now, and I'm I'm trying to keep my head above water, but um to get a word in edgewise, but let's get into the uranium to lead, and we'll touch upon some of the other things, but the focus is gonna be on uranium to lead, and we're gonna touch upon other things to make a point, and I think it's a really important point.
• 25:05
  SPEAKER_02: But oh definitely so let's go ahead, Jesse, and I'm just kind of maybe I should do it, maybe we should do it this way, Jesse.
• 25:13
  SPEAKER_02: Yeah, but how about if I just ask you a bunch of questions, right?
• 25:15
  SPEAKER_00: Okay, perfect.
• 25:16
  SPEAKER_02: Um, so let me let's lead off with this.
• 25:19
  SPEAKER_00: Um and you just want to know when you want me to look at the the images here and go through some of these images.
• 25:24
  SPEAKER_00: Yeah, gotcha.
• 25:25
  SPEAKER_02: Yeah, yeah, yeah.
• 25:26
  SPEAKER_02: So we're as you said before, and you've said this many times in our previous episodes, when we talk about urine to lead, the gold standard is the mineral zircon.
• 25:35
  SPEAKER_02: Okay.
• 25:36
  SPEAKER_02: Why zircons, and uh, I'll take this one, then I'll throw it to you.
• 25:41
  SPEAKER_02: The zircons because it's got a high uh uranium content and the crystal structure excludes original daughter lead for the most part.
• 25:55
  SPEAKER_02: There's there's very little initial daughter isotope present when the zircons form.
• 26:01
  SPEAKER_02: So when you then analyze it after however old the mineral may be, the daughter isotope, the vast majority of that daughter isotope came from decay of uranium.
• 26:12
  SPEAKER_00: That's exactly right.
• 26:13
  SPEAKER_00: Well, yes, there's two two reasons.
• 26:15
  SPEAKER_00: First of all, it's not there.
• 26:17
  SPEAKER_00: So if you think of what do we do?
• 26:19
  SPEAKER_00: What do we measure today?
• 26:20
  SPEAKER_00: We measure uranium in a zircon grain and we measure lead.
• 26:23
  SPEAKER_00: And the decay equation says that ratio, uranium to lead ratio, or actually the lead in that we measure in that zircon, the total lead of a particular isotope, that equals the original lead in the grain plus the lead that's there from radioactive decay.
• 26:40
  SPEAKER_00: We can figure out the lead that was there from radioactive decay by using the uranium decay constant, the uranium concentration and decay constant, but we have to figure out that initial daughter isotope term.
• 26:51
  SPEAKER_00: And zircon is really great because, as you said, it excludes lead.
• 26:55
  SPEAKER_00: The other benefit is that the lead isotope system allows us to trace what we call common lead or that initial lead, external lead, not radiogenic lead, not radioactively produced lead, really well just because the isotope systematics.
• 27:08
  SPEAKER_00: There's there's a couple different isotopes that are necessary.
• 27:10
  SPEAKER_02: Well, yeah, because you're using uranium-238 and uranium-235, they have very, very different half-lives.
• 27:18
  SPEAKER_00: And and lead has another isotope, 204 lead, which is not produced by radioactive decay.
• 27:23
  SPEAKER_00: So if there's any 204 lead in the zircon grain in the measurement, we know.
• 27:28
  SPEAKER_02: Exactly.
• 27:29
  SPEAKER_00: We have to do some corrections there and we have to worry about that.
• 27:31
  SPEAKER_00: But if there's no 204 lead, then we don't have to.
• 27:34
  SPEAKER_02: Can we make a point here about that?
• 27:36
  SPEAKER_02: You know, you've you've talked about lead 204, lead 206, lead 207.
• 27:40
  SPEAKER_02: Um, those are such similar isotopes that when zircon is growing, it doesn't distinguish between either one of them.
• 27:51
  SPEAKER_02: So that's why the point that you made about lead 204, if that's there, that means that there was also lead 206 and lead 207 there originally.
• 27:59
  SPEAKER_02: It doesn't care.
• 28:00
  SPEAKER_02: Like I I've often used the analogy of this is like building a brick wall.
• 28:05
  SPEAKER_02: Is if you're building a brick wall, that's how you crystals grow, right?
• 28:09
  SPEAKER_02: The a brick will fit in that wall as long as it fits in the wall, it has to be the right size and the right shape.
• 28:16
  SPEAKER_02: And with ions and isotopes, they have to be the right size, the right shape, the right charge.
• 28:20
  SPEAKER_02: There's virtually no difference between lead 204, 206, and 207.
• 28:24
  SPEAKER_00: You can fit these in any spot, but all the same.
• 28:27
  SPEAKER_00: The same, just changing the color of the brick, basically, like you said.
• 28:29
  SPEAKER_00: So that's that's a really nice analogy, and and it allows us to do this.
• 28:32
  SPEAKER_00: And so, Chris, I'm gonna flip ahead to to this image here, image number four that or image number five that we're looking at here.
• 28:38
  SPEAKER_00: This is one version of Zircon Uranium LED your chronology, the plot that we use.
• 28:44
  SPEAKER_00: And this is called a Weatherhill plot.
• 28:46
  SPEAKER_00: It's named that after George Weatherhill, who kind of created the plot in the 50s.
• 28:51
  SPEAKER_00: What we're showing here is the dual decay systems of uranium to lead.
• 28:57
  SPEAKER_00: 238 uranium decays to 206 lead.
• 29:00
  SPEAKER_00: We're showing that on the y-axis, that ratio of 206 lead divided by 238 on the y-axis.
• 29:06
  SPEAKER_00: On the x-axis, we're using the other decay system.
• 29:08
  SPEAKER_00: 235 uranium decays to 207 lead.
• 29:11
  SPEAKER_00: That one has a shorter half-life.
• 29:13
  SPEAKER_00: The half-life is like 700 and some million years, it's almost extinct.
• 29:17
  SPEAKER_00: The ratios on that plot, basically the x-axis ratio and the y-axis ratio, each give us an age.
• 29:25
  SPEAKER_00: Each of them is kind of an independent age.
• 29:28
  SPEAKER_00: There's a line on this plot.
• 29:31
  SPEAKER_00: The line we call quote unquote Concordia.
• 29:34
  SPEAKER_00: The line tells us where those two ratios, the x ratio and the y ratio, give us the exact same age.
• 29:41
  SPEAKER_00: The decay rates, the half-lives of uranium-235, uranium-238 differ.
• 29:45
  SPEAKER_00: So this line is a curve in space because of the different half-lives here.
• 29:51
  SPEAKER_00: And we've talked about this before, I know on the podcast, but it's complicated, so it's good to refresh this.
• 29:56
  SPEAKER_00: So y-axis ratio or y-axis value is an age, x-axis value is an age.
• 30:02
  SPEAKER_00: Where those two different ages agree with one another, we plot a line through that curve, and each there's points on that line, each point on that line corresponds to an age.
• 30:13
  SPEAKER_00: Does that make am I framing that pretty well, Chris?
• 30:15
  SPEAKER_00: Does that make sense so far?
• 30:17
  SPEAKER_02: Yeah, it's a good thing.
• 30:20
  SPEAKER_02: Well, I think so.
• 30:21
  SPEAKER_02: I don't know.
• 30:21
  SPEAKER_02: Basically, you're saying that we're talking about unaltered grains right now, right?
• 30:27
  SPEAKER_00: Yes, these are okay, that's an important point.
• 30:30
  SPEAKER_00: Really good point.
• 30:31
  SPEAKER_00: We're talking about like primary, reliable age, reliable.
• 30:36
  SPEAKER_02: No recrystallization has happened.
• 30:37
  SPEAKER_00: It's unaltered in an ideal world.
• 30:39
  SPEAKER_02: It's it was born and nothing happened to it, and and then Jesse comes along and tests it in the lab building.
• 30:45
  SPEAKER_00: If I find a perfect, undisturbed 3.5 in orange here, I've got a 3.5 billion year old grain.
• 30:52
  SPEAKER_00: If I measure that grain in my lab, I'm gonna get a little circle that is the uncertainty.
• 30:58
  SPEAKER_00: It's the data point plus the uncertainty is the circle.
• 31:01
  SPEAKER_00: We just plot a 95% confidence interval around it, and that's the that's the uncertainty.
• 31:05
  SPEAKER_00: We'll call two standard error uncertainty.
• 31:07
  SPEAKER_00: It's gonna plot on the line, on the curve, on this Concordia curve at 3.5 billion years old, and both ages agree.
• 31:14
  SPEAKER_00: This gives us a really good check.
• 31:16
  SPEAKER_00: Both ages agree.
• 31:17
  SPEAKER_02: What you mean both ages, lead or uh uranium-235 and uranium-238?
• 31:22
  SPEAKER_02: Both of those ages agree that different different half-lives decayed out to different daughters and so on.
• 31:27
  SPEAKER_02: Just trying to clarify.
• 31:29
  SPEAKER_02: Now, nope, okay, so that's an important point with what you just explained.
• 31:32
  SPEAKER_02: Now, let's talk about Jesse.
• 31:35
  SPEAKER_02: Well, first of all, how often do you get a rock like that?
• 31:38
  SPEAKER_02: Well, that old, pretty rare.
• 31:41
  SPEAKER_02: Okay, right, right.
• 31:42
  SPEAKER_02: Okay, so let's go to reality now.
• 31:44
  SPEAKER_00: Well, okay, so let's let me just yeah, let me just finish, let me just finish this image real quick, Chris, and say that I tried to show on this image we have different um different data points that are different ages.
• 31:56
  SPEAKER_00: So if I found a 2.8 billion year old grain, that's the orange curve.
• 32:00
  SPEAKER_00: There's slightly less lead in that analysis than there is uranium because it's younger.
• 32:05
  SPEAKER_00: There's less lead because it's younger.
• 32:07
  SPEAKER_00: If I find a 1.8 billion year old grain, that's the yellow one.
• 32:10
  SPEAKER_00: It's plotting further down on this curve, less lead.
• 32:14
  SPEAKER_00: There's still a bunch of uranium in there.
• 32:16
  SPEAKER_00: And then if I find a really young grain, a 500 million-year-old grain, uh, there's gonna be hardly any lead in that thing, right?
• 32:22
  SPEAKER_00: We're just not having much detectable lead.
• 32:24
  SPEAKER_00: So the origin that y is zero and x-axis is zero and y-axis is zero.
• 32:29
  SPEAKER_00: If a grain formed yesterday, that's where it's gonna plot.
• 32:32
  SPEAKER_00: There's gonna be no lead.
• 32:33
  SPEAKER_00: There's gonna be a bunch of uranium, no lead, which because we made these particular ratios, pulls us to zero here.
• 32:39
  SPEAKER_00: So high ratios on the x-axis and the y-axis mean old grains, basically.
• 32:45
  SPEAKER_00: So that's in a perfect world, if we have grains of different ages, they're gonna plot at different points along this Concordia curve.
• 32:51
  SPEAKER_00: It's a curve in space, not a straight line.
• 32:53
  SPEAKER_02: So now, here's the thing, Jesse, is that the rocks that we're talking about, the potential candidates for the oldest rocks in the US, they're not this.
• 33:05
  SPEAKER_02: These are all without exception, yeah.
• 33:08
  SPEAKER_02: Yeah, right.
• 33:09
  SPEAKER_02: These are rocks that have had a very hard life.
• 33:13
  SPEAKER_02: A lot of trauma happened to them as as they came of age.
• 33:17
  SPEAKER_02: And so let's talk about that.
• 33:20
  SPEAKER_02: So what happens when they get altered since their formation?
• 33:23
  SPEAKER_00: Yeah, you said you asked a good really insightful question there.
• 33:26
  SPEAKER_00: Like, how common is it that we get a 3.5 billion year old rock that is is really reliable age information?
• 33:32
  SPEAKER_00: And the answer is there's basically none.
• 33:35
  SPEAKER_00: There's one, and it's a standard, it's a rock that if you pick the zircons out of there, almost all of the zircons are going to be perfectly what we call concordant.
• 33:45
  SPEAKER_00: The data falls right on this concordial line.
• 33:47
  SPEAKER_00: They're per both isotope systems are perfectly aligned in all the zircons in that whole rock.
• 33:52
  SPEAKER_00: It's a sample called OG1.
• 33:54
  SPEAKER_00: It's from Western Australia.
• 33:55
  SPEAKER_00: We use it as a standard in our lab.
• 33:57
  SPEAKER_00: There's like there's one of them, one of them from all these old rock locations.
• 34:00
  SPEAKER_00: The vast majority are not that.
• 34:02
  SPEAKER_00: Some parts of the zircons are overprinted, are reset, and we're gonna then now what you're you're talking you're referring to is moving towards thinking about how does resetting happen.
• 34:11
  SPEAKER_00: And this is where it gets or combined.
• 34:13
  SPEAKER_00: Hold on, hold on.
• 34:14
  SPEAKER_02: Uh it's not yeah, it's not, I don't know if I like that.
• 34:19
  SPEAKER_02: It's not just resetting.
• 34:21
  SPEAKER_02: We're talking about a it's a partial reset.
• 34:24
  SPEAKER_00: Yes, great point.
• 34:25
  SPEAKER_00: Okay, great.
• 34:26
  SPEAKER_00: Great, great, great point, great point.
• 34:28
  SPEAKER_00: Yep.
• 34:28
  SPEAKER_02: Okay, okay, okay.
• 34:29
  SPEAKER_02: So you mean by partial reset?
• 34:31
  SPEAKER_02: Let's be deliberate about that, right?
• 34:33
  SPEAKER_02: Okay, so Jesse, if a grain formed 3.5 billion years ago, the original grain did.
• 34:39
  SPEAKER_02: Okay, so this is old, old stuff, and then it gets partially reset.
• 34:45
  SPEAKER_02: What happens?
• 34:46
  SPEAKER_02: What does it look like on this plot?
• 34:48
  SPEAKER_00: Yes, and I this is something, Chris, I do and do this exercise with my my graduate students for their like candidacy exams.
• 34:54
  SPEAKER_00: It's say, okay, how does this work?
• 34:56
  SPEAKER_00: Do you fundamentally understand what happens here?
• 34:58
  SPEAKER_00: So let's say, Chris, we've got this perfect 3.5 billion.
• 35:01
  SPEAKER_00: Hey, hold on, hold on.
• 35:01
  SPEAKER_02: Do they fundamentally understand what happens here?
• 35:04
  SPEAKER_00: Yeah, yeah, they do.
• 35:05
  SPEAKER_00: Most my graduate students are great.
• 35:07
  SPEAKER_00: So they all they all kind of you know intuitively get this at some level, but it's not easy.
• 35:12
  SPEAKER_00: And I didn't get it either.
• 35:13
  SPEAKER_00: I this took me a while for me to understand and visualize and understand how this happened.
• 35:17
  SPEAKER_02: But it's so this is uh this gets me.
• 35:19
  SPEAKER_02: I love this.
• 35:20
  SPEAKER_02: Um I don't know.
• 35:22
  SPEAKER_02: Um maybe it's the influence that you've had on me over these years.
• 35:26
  SPEAKER_02: I don't know, but I just get uh um my juices are flowing.
• 35:29
  SPEAKER_02: I'm I'm ready.
• 35:30
  SPEAKER_02: I'm excited.
• 35:30
  SPEAKER_00: It's exciting stuff.
• 35:32
  SPEAKER_00: So let's take this beautiful, pristine 3.5 billion year old sample uh from Western Australia that all the zircons are perfectly 3.5 billion years old.
• 35:40
  SPEAKER_00: They plop perfectly on the Concordia line when we analyze them.
• 35:43
  SPEAKER_00: Now let's alter those things.
• 35:45
  SPEAKER_00: Let's partially alter them.
• 35:46
  SPEAKER_00: Let's say we do an experiment, let's say we take those grains and we put them in some hydrothermal water chamber under pressure and we alter those grains.
• 35:55
  SPEAKER_00: We kind of let the water acid kind of eat away at those grains.
• 35:58
  SPEAKER_00: What's gonna happen to that?
• 36:00
  SPEAKER_00: Well, the uranium that's locked in the grain still that has not decayed is still perfectly happy in the crystal lattice.
• 36:09
  SPEAKER_00: Totally just sitting there nice and happy.
• 36:12
  SPEAKER_00: The lead that's in that grain has formed by radioactive decay from uranium, but uranium doesn't just go right to lead.
• 36:21
  SPEAKER_00: There's like 10 or 13 steps in the decay from uranium to lead.
• 36:25
  SPEAKER_02: Depends on which isotope we're talking about.
• 36:27
  SPEAKER_02: Yep.
• 36:27
  SPEAKER_00: There's a couple models out there that that kind of show what happens to a crystal lattice when uranium decays, and it's kind of like a shotgun kick kicking back into your shoulder.
• 36:37
  SPEAKER_00: The helium nucleus goes out the front end, and there's this recoil.
• 36:41
  SPEAKER_00: The grain that the atom that's left behind recoils and it blows apart the crystal lattice, and that happens over and over and over.
• 36:48
  SPEAKER_00: So the lead is sitting in kind of a damaged crystal lattice, and the lead doesn't fundamentally like to be in zircon, it's just too big of an ion, it's a two-plus charge, it doesn't fit very well.
• 36:59
  SPEAKER_00: So, what happens if we start to push some water into that?
• 37:03
  SPEAKER_02: Can I just make a simple point here again?
• 37:06
  SPEAKER_02: Lead doesn't like to be there for the very same reason why zircon excludes lead to begin with.
• 37:13
  SPEAKER_02: Exactly.
• 37:14
  SPEAKER_02: Same exact reason.
• 37:15
  SPEAKER_00: Yes, okay, all right.
• 37:16
  SPEAKER_00: Exactly.
• 37:17
  SPEAKER_00: So what's gonna happen now?
• 37:18
  SPEAKER_00: Go back to this this concordia plot of showing the the the sixth image here, where we're showing a resetting event.
• 37:25
  SPEAKER_00: Basically, what happens to the data to the grains as we reset this stuff?
• 37:29
  SPEAKER_00: And you have to think in uranium-lead ratio.
• 37:31
  SPEAKER_00: So, Chris, this ratio, this is where people get confused.
• 37:34
  SPEAKER_00: These ratios I have on my x-axis, I have 207 lead divided by 235 uranium.
• 37:40
  SPEAKER_00: If I You're normalizing it, yep.
• 37:42
  SPEAKER_00: What's that ratio gonna do if I alter this grain?
• 37:46
  SPEAKER_00: The uranium's gonna stay there, it's not gonna get overprinted.
• 37:49
  SPEAKER_00: The lead's gonna be.
• 37:50
  SPEAKER_02: So the ratio gets smaller.
• 37:51
  SPEAKER_00: The ratio gets smaller, perfect.
• 37:53
  SPEAKER_00: Okay.
• 37:53
  SPEAKER_00: Now what about the y-axis, which is 206 lead divided by 238 uranium?
• 37:58
  SPEAKER_00: What's gonna happen to that ratio?
• 38:00
  SPEAKER_02: It's gonna drag it down towards the towards the y-intercept.
• 38:05
  SPEAKER_00: Exactly.
• 38:05
  SPEAKER_00: Okay, perfect.
• 38:06
  SPEAKER_00: So it's not what it's not gonna do is it's not gonna drag it along the Concordia line.
• 38:11
  SPEAKER_00: Along, it's not gonna do it this curvature in space.
• 38:14
  SPEAKER_00: It's gonna go straight line from wherever it started towards 0, 0, towards the origin of this plot, towards 0y, 0x.
• 38:22
  SPEAKER_00: This is the power of zircon.
• 38:24
  SPEAKER_00: This is really actually this is another reason why zircon is great, because this resetting or alteration or partial resetting creates lines in this space where real crystallization events create curvature.
• 38:39
  SPEAKER_00: So when those grains, what that means is if a grain experiences lead loss or what we call discordance, the the analysis falls off of that Concordia line, we call that discordance.
• 38:50
  SPEAKER_00: If there's discordance, it's easy to tell it because the grain falls off the line.
• 38:55
  SPEAKER_00: We can just look at it one data point and say, ah, that doesn't fall on the line.
• 38:59
  SPEAKER_00: It the ages do not agree with one another.
• 39:01
  SPEAKER_00: This grain has probably lost lead in its past.
• 39:04
  SPEAKER_02: So are you telling me, Jesse, that when you get a data point that it doesn't fit, that you don't just toss it out?
• 39:11
  SPEAKER_02: Is that really what you're saying?
• 39:13
  SPEAKER_02: Yes.
• 39:13
  SPEAKER_02: Shocking.
• 39:14
  SPEAKER_02: We don't just throw it away.
• 39:16
  SPEAKER_00: Don't you like, I don't like that.
• 39:18
  SPEAKER_00: It's gone.
• 39:19
  SPEAKER_00: It's gone.
• 39:19
  SPEAKER_02: No, that's exactly how I thought that's what you all did.
• 39:22
  SPEAKER_02: Okay.
• 39:22
  SPEAKER_02: All right.
• 39:23
  SPEAKER_02: I've learned something too.
• 39:24
  SPEAKER_00: Only occasionally.
• 39:25
  SPEAKER_00: Uh so no, we don't throw it away.
• 39:28
  SPEAKER_00: And here's the it's because it's really powerful.
• 39:31
  SPEAKER_00: It's important.
• 39:31
  SPEAKER_00: And this highlights the value.
• 39:33
  SPEAKER_00: Why is that important?
• 39:35
  SPEAKER_00: That's equal.
• 39:36
  SPEAKER_00: Yeah, go ahead.
• 39:37
  SPEAKER_00: Yeah.
• 39:37
  SPEAKER_00: Here's why, especially if we have multiple zircon grains, let's say, that have experienced different amounts of alteration.
• 39:45
  SPEAKER_00: Some are perfectly pristine, some are totally altered, and there's a whole bunch in between, they'll form what we call a line, a Discordia line in space.
• 39:53
  SPEAKER_00: And that line is a super powerful interpretive tool.
• 39:57
  SPEAKER_00: We can fit a line to that.
• 39:58
  SPEAKER_00: In this case, the uncertainties, the the circles that represent the uncertainty are way too big on this plot.
• 40:04
  SPEAKER_00: I made them big so you could see them on this graph.
• 40:06
  SPEAKER_00: But usually they get smaller as we get closer towards the zero, towards the origin.
• 40:10
  SPEAKER_00: But if we have a linear array of data like this, we can say, oh, look, there was a primary grain that's around 3.5 billion years old.
• 40:18
  SPEAKER_00: Then there's a linear array, there's some minor resetting, major resetting, and almost partial, almost completely reset stuff down near the origin.
• 40:25
  SPEAKER_00: We plot a line through that, and where that regression line, line through the data, intercepts the Concordia curve, gives us an estimate for the crystallization age, the upper one, the old one's crystallization.
• 40:38
  SPEAKER_00: The young one is alteration or metamorphism or disturbance of the grain.
• 40:42
  SPEAKER_00: And that's usually a geological event.
• 40:44
  SPEAKER_02: Which is super important.
• 40:45
  SPEAKER_02: We need to know that super important.
• 40:46
  SPEAKER_00: Yeah.
• 40:48
  SPEAKER_02: So all right.
• 40:48
  SPEAKER_02: Question for you, or kind of a question, a statement and a question all rolled into things.
• 40:55
  SPEAKER_02: So um, can we talk about a term that I'm not sure because you and I didn't really talk about this.
• 41:01
  SPEAKER_02: Do you use the term closure temperature?
• 41:04
  SPEAKER_02: Do you use that?
• 41:05
  SPEAKER_00: I yes.
• 41:07
  SPEAKER_00: Um yes.
• 41:08
  SPEAKER_00: And okay it's all right, an interesting one.
• 41:11
  SPEAKER_00: Yeah.
• 41:12
  SPEAKER_02: Can we can we talk about that?
• 41:13
  SPEAKER_02: I think it's relevant, right?
• 41:16
  SPEAKER_02: All right, so the closure temperature is basically you take a mineral and uh you let's say the closure temperature for zircon is 900 degrees Celsius.
• 41:26
  SPEAKER_02: What that means is that any temperature, if that mineral, zircon is subjected to temperatures below 900 degrees, it retains the radiogenic daughter isotope.
• 41:36
  SPEAKER_02: It's gonna retain the lead.
• 41:38
  SPEAKER_02: Okay, if you go above 900, you're gonna have lead loss.
• 41:42
  SPEAKER_02: You're gonna lose some of the daughter isotope, which you're gonna drag it down and uh again along that straight line that you're talking about.
• 41:49
  SPEAKER_02: So I have a a question that I want to continue this point.
• 41:52
  SPEAKER_02: Okay, okay.
• 41:53
  SPEAKER_00: Can I yes, go ahead with the question?
• 41:55
  SPEAKER_00: Yeah.
• 41:56
  SPEAKER_02: Okay, so is there a point where this gets I I know the answer to this, but I want you to explain it.
• 42:02
  SPEAKER_02: Is there a point where this gets gray?
• 42:05
  SPEAKER_00: The closure temperature.
• 42:06
  SPEAKER_02: Yeah, yeah, yeah.
• 42:07
  SPEAKER_02: So the the the this kind of idea of like partial resetting.
• 42:11
  SPEAKER_00: Yeah, okay.
• 42:12
  SPEAKER_00: Yes, absolutely.
• 42:13
  SPEAKER_00: The universally.
• 42:16
  SPEAKER_00: So the way to think about this, the way to think about closure temperature in an ideal mineral, in an ideal system, is exactly as you described it.
• 42:23
  SPEAKER_00: Like there's a temperature where the the atoms are vibrating fast enough that the daughter isotope, the product isotope can escape the lattice and get out of the lattice, right?
• 42:32
  SPEAKER_00: Um, and so there are certain systems where closure temperature is really useful, um, mostly with like the noble gas ones, potassium argon or things like that.
• 42:42
  SPEAKER_00: Uranium-led zircon, closure temperature doesn't mean much for this one.
• 42:47
  SPEAKER_00: I'll come back to why that is.
• 42:48
  SPEAKER_00: But but closure temperature is useful to think about for a bunch of different grains.
• 42:56
  SPEAKER_00: A bunch of different minerals and a bunch of different isotope systems.
• 42:58
  SPEAKER_00: Um, and so e there's different minerals that will have different quote unquote closure temperatures.
• 43:02
  SPEAKER_02: Yeah, but can I can I jump in on your a second?
• 43:05
  SPEAKER_00: Well, let me let me answer a question first about why it gets gray.
• 43:08
  SPEAKER_00: Okay, yeah.
• 43:08
  SPEAKER_00: Because I didn't really answer it.
• 43:10
  SPEAKER_00: It it it that you you attacked me instead.
• 43:12
  SPEAKER_02: You said closure temperature is not important, and I don't appreciate that.
• 43:15
  SPEAKER_02: I want to respond to that.
• 43:17
  SPEAKER_00: Okay, it's it's not important for the for anyway.
• 43:19
  SPEAKER_00: For zircide uranium.
• 43:21
  SPEAKER_00: Well, I disagree.
• 43:22
  SPEAKER_00: It is important.
• 43:23
  SPEAKER_00: Full on attack you.
• 43:24
  SPEAKER_00: It's full frontal attack here.
• 43:26
  SPEAKER_00: We're just storming storming the hill.
• 43:28
  SPEAKER_00: Um, closure temperature.
• 43:30
  SPEAKER_00: When uh when elements are, you have to think about how does an element get out of a mineral grain?
• 43:35
  SPEAKER_00: How does a uh a product isotope or a daughter isotope get out?
• 43:39
  SPEAKER_00: And it it's diffusive.
• 43:40
  SPEAKER_00: It's gonna have a harder time getting out of the core of the grain than the rim.
• 43:44
  SPEAKER_00: If it's sitting right at the rim, it can get out really easily.
• 43:46
  SPEAKER_00: So it's a diffusion process.
• 43:48
  SPEAKER_00: So it's both time and temperature sensitive, even in a perfect scenario.
• 43:53
  SPEAKER_00: So the closure temperature is like kind of an approximate, it's a temperature window, really, that it's sensitive.
• 43:58
  SPEAKER_00: With that reset, it's really
• 44:00
  SPEAKER_02: Really important point, Jesse, because it speaks to this early time of geochronology to begin with.
• 44:06
  SPEAKER_02: That they didn't have the ability to look at an individual grain and look at the history of that grain.
• 44:12
  SPEAKER_02: Oh, yeah, what happened resolving to the cloud?
• 44:15
  SPEAKER_00: To make one measurement, you lose all that information.
• 44:18
  SPEAKER_00: Like you lose core rim relationships, you lose the character of the grain.
• 44:22
  SPEAKER_00: Absolutely.
• 44:23
  SPEAKER_00: That's a really good point.
• 44:24
  SPEAKER_02: So, all right.
• 44:25
  SPEAKER_02: Um, I'm gonna argue, I'm gonna push back a second.
• 44:27
  SPEAKER_02: So the reason why I say that closure temperature is important with a zircon is because it's got a very high closure temperature, 900 degrees.
• 44:36
  SPEAKER_02: But you're not just gonna use that.
• 44:39
  SPEAKER_02: Biotite has a closure temperature that's much lower, 300 degrees Celsius.
• 44:43
  SPEAKER_02: Monazite is what 725 degrees.
• 44:47
  SPEAKER_02: So you're gonna use you know, as a rock gets tortured, as it has its hard life as it's being brought up and coming of age, it's gonna be subjected to varying degrees of metamorphism, maybe lower heat and then higher heat, and then back down.
• 45:01
  SPEAKER_02: So you're gonna have by testing out different minerals, you're gonna be able to zero in on these events that's and what the conditions were.
• 45:11
  SPEAKER_00: So that's why I say that so in that in that context, absolutely the closure temperature matters.
• 45:16
  SPEAKER_00: And the the value of zircon is that it is insensitive to re or it is it is much uh more resilient than the other minerals that you just mentioned.
• 45:25
  SPEAKER_00: So that's absolutely true.
• 45:26
  SPEAKER_00: This is a very resilient mineral.
• 45:28
  SPEAKER_00: However, the closure temperature of 900 refers to a perfectly crystalline zircon.
• 45:35
  SPEAKER_00: But as we just talked about, zircon self-radiates.
• 45:38
  SPEAKER_00: The uranium blows the crystal structure apart over time.
• 45:41
  SPEAKER_00: And so actually, you can get lead, you can remove lead from a zircon mineral at like 200 degrees if it's really radiation damage, really blown apart, that lead kind of is easy to get out, and then the closure temperature of the pure crystal in zircon doesn't really apply to that system anymore.
• 45:59
  SPEAKER_00: So it's really complicated, but this makes Zircon additionally powerful because Zircon can record, as we'll talk about next chapter, or actually in the next couple images.
• 46:06
  SPEAKER_00: Well, I think yeah, go to the next image, Jesse.
• 46:09
  SPEAKER_00: I think Okay, each Zircon can record multiple events, and the zircons in all of these terrains, the three we mentioned, they all record multiple events.
• 46:18
  SPEAKER_00: Each grain, each individual zircon grain records more than two events typically.
• 46:23
  SPEAKER_00: And here's where it gets sticky, and here's where it gets debatable, and here's where the the we're gonna focus kind of next episode on this is get into the details.
• 46:31
  SPEAKER_00: But here's this image, Chris, is is again we're talking about Concordia plot.
• 46:36
  SPEAKER_00: We went through what happens when we take an old grain, 3.5 billion year old grain, reset it today, partially reset some of it, totally reset other parts of it.
• 46:46
  SPEAKER_00: We create this linear array where the line intersects the Concordia curve at zero and at 3.5 billion years old.
• 46:53
  SPEAKER_00: What happens if we do that resetting at an old time period?
• 46:56
  SPEAKER_00: Let's say we have a two billion year old metamorphic event.
• 47:00
  SPEAKER_00: Well, it pulls that metamorphic event two billion years ago, pulls the zircon analyses down towards.
• 47:06
  SPEAKER_00: I always kind of describe it as the origin, what used to be zero.
• 47:10
  SPEAKER_00: Two billion years ago, are the zero zero point on this plot used to be much higher up, and it has decayed away.
• 47:18
  SPEAKER_00: The plot has kind of expanded as time has has gone on forward.
• 47:22
  SPEAKER_00: So it will have pulled it down to zero.
• 47:25
  SPEAKER_00: It will have totally removed all the lead two billion years ago.
• 47:28
  SPEAKER_00: But then in the last two billion years, since that resetting event, a totally reset grain will have more lead, will have grown in over the last two billion years.
• 47:37
  SPEAKER_02: So it's only going to show that it's a 1.5 billion years old instead of two or instead of 3.5.
• 47:43
  SPEAKER_00: Exactly.
• 47:44
  SPEAKER_00: It'll be, yeah, it'll pull a perfectly reset.
• 47:46
  SPEAKER_00: So if you take a 3.5 billion year old grain and you reset it, totally reset it, it'll get pulled down towards 2 billion years on this Concordia plot, and both ages will agree because that grain got totally reset two billion years ago, and then since then uranium has happily decayed to lead and no more resetting has happened.
• 48:04
  SPEAKER_00: So we would have a perfectly concordant analysis two billion years ago in a metamorphic rim area, let's say.
• 48:11
  SPEAKER_02: Okay.
• 48:12
  SPEAKER_02: So you're able to see through it, right?
• 48:15
  SPEAKER_00: You're able to see through it.
• 48:16
  SPEAKER_00: And you're we're actually able to resolve an igneous event, 3.5 billion years.
• 48:21
  SPEAKER_00: And when the event happened, yep.
• 48:22
  SPEAKER_00: Two billion year metamorphic event.
• 48:23
  SPEAKER_00: Okay.
• 48:24
  SPEAKER_00: We can tell that.
• 48:25
  SPEAKER_02: But what if it's not totally reset?
• 48:27
  SPEAKER_02: Because I don't that doesn't happen very often, right?
• 48:30
  SPEAKER_02: I mean, it's not a total reset.
• 48:32
  SPEAKER_02: Because after all, it has a this is why I think it again it's important is that the closure temperature is 900 degrees.
• 48:39
  SPEAKER_02: You have to exceed that to get to a total reset.
• 48:43
  SPEAKER_00: Yes, or you have to totally recrystallize the grain with some fluids, which can happen at lower temperature.
• 48:47
  SPEAKER_02: Okay, but then you're gonna be able to see.
• 48:49
  SPEAKER_02: Okay, but if it's a recrystallized grain, you're gonna be able to see rim to core.
• 48:53
  SPEAKER_02: You're gonna be able to work your way down into the grain.
• 48:55
  SPEAKER_00: That's exactly it.
• 48:55
  SPEAKER_00: And that's why the early data, as impressive as it was to get data from these rocks in the early days, it's a coarse data set.
• 49:03
  SPEAKER_00: You're we're doing surgery with a with a rock hammer at that point because you're you're analyzing hundreds of zircon grains to get one data point.
• 49:11
  SPEAKER_00: When actually nowadays we can see, oh, this is the core, this is the rim, this is an altered portion of the grain.
• 49:16
  SPEAKER_00: We can look at these things with images that we'll show in the next episode that we talk about this in part two.
• 49:22
  SPEAKER_00: Here, we'll get into some of the details here, but we can really tease apart a detailed history.
• 49:26
  SPEAKER_00: But even then, it's still complicated.
• 49:27
  SPEAKER_00: Like Chris, at the end of this, I show some possible data, and this is just made-up data.
• 49:32
  SPEAKER_00: But we could have a 3.5 billion year old crystallization event, a 2 billion year resetting event or partial resetting event, and then all that stuff could be partially reset recently.
• 49:41
  SPEAKER_00: These rocks are sitting on the surface with water flowing through them.
• 49:44
  SPEAKER_00: There's often a little bit of modern lead loss that pulls it down towards zero.
• 49:48
  SPEAKER_00: So it can get really complicated.
• 49:50
  SPEAKER_00: You can have a smearing of data across here that is complicated to interpret, I guess.
• 49:56
  SPEAKER_02: I mean, Jesse, I have an idea in my head and I can't hold it any longer.
• 49:59
  SPEAKER_02: So I'm gonna interrupt you because otherwise I forget.
• 50:03
  SPEAKER_02: So I have a couple things swirling around in my head as we as we begin to wrap this up.
• 50:07
  SPEAKER_02: Get them out.
• 50:08
  SPEAKER_02: We don't want you stressed out over there, Chris.
• 50:09
  SPEAKER_02: So I am stressed.
• 50:10
  SPEAKER_02: So, analogy.
• 50:12
  SPEAKER_02: Um, if you're gonna build a deck in your backyard, you can't just build the deck with a hammer.
• 50:16
  SPEAKER_02: You need a whole tool bag, you know, you need a tape measure, you need a square, you need you need a plumb bob, you need a chalk line, you need pencil, you need all this stuff, you need a saw.
• 50:24
  SPEAKER_00: You need a tractor, probably too.
• 50:26
  SPEAKER_00: Exactly.
• 50:27
  SPEAKER_02: Yes, yes.
• 50:28
  SPEAKER_02: So just like that, you don't just use uranium to lead and all of what you described beautifully here today as your tool.
• 50:40
  SPEAKER_02: Okay, you am I right in this?
• 50:43
  SPEAKER_02: Like you also use things like samarium to neodymium, you use letetium to hafnium, right?
• 50:49
  SPEAKER_02: Because these these decay processes rely heavily on other minerals entirely, not zircons, they rely on garnets, and they have they're useful because garnet has a high closure temperature, and it also has these isotopes have long half-lives, which make it appropriate for dating older.
• 51:05
  SPEAKER_00: And also, Chris, garnet is a mineral that grows mostly during metamorphosis.
• 51:09
  SPEAKER_02: Metamorphic.
• 51:10
  SPEAKER_02: That's right.
• 51:10
  SPEAKER_02: That's right.
• 51:11
  SPEAKER_00: So we can say, oh, look, there's garnets in this, this Morton gice has garnets in it.
• 51:14
  SPEAKER_00: And so we can date the garnets and clearly does time.
• 51:17
  SPEAKER_00: That gives us time for metamorphism, which helps us interpret the zircon data as well and say, okay, do these lower, does this lower intercept age, this lower regression line, does that fit the garnet ceremonyum age that we get?
• 51:28
  SPEAKER_00: Or so I just appetite but that's the tightenite uranium lead age.
• 51:31
  SPEAKER_00: So that's a really good idea.
• 51:33
  SPEAKER_02: I think that's an important point, is that we're not just using this tool, even though we're really, really good at it.
• 51:39
  SPEAKER_02: We're we're gonna use these other tools as well, because we have a big, we have a big ass toolbox, Jesse, and we're gonna use it, right?
• 51:46
  SPEAKER_02: And and so there's that.
• 51:49
  SPEAKER_02: My final thought that I don't didn't want to lose.
• 51:51
  SPEAKER_02: I was worried I was gonna lose it, but I didn't.
• 51:53
  SPEAKER_02: I got it.
• 51:53
  SPEAKER_00: We're always worried that you're gonna lose stuff out of that leaky brain here.
• 51:57
  SPEAKER_00: My goodness leaks like a Civ these days.
• 52:00
  SPEAKER_00: It's unbelievable.
• 52:02
  SPEAKER_02: All right.
• 52:02
  SPEAKER_02: Um here's my thought then because you and I, I think, walked through how good we are at doing this.
• 52:10
  SPEAKER_02: Okay.
• 52:11
  SPEAKER_02: We, you know, we're we're really good at being petrochronologists.
• 52:16
  SPEAKER_02: And when I say we, I uh that does not include me.
• 52:20
  SPEAKER_02: I'm talking about you know, you're not gonna be able to do the scientific community, yes.
• 52:22
  SPEAKER_02: Yes, yes, yeah.
• 52:24
  SPEAKER_02: So um then why is this a complicated answer if we're so good at this?
• 52:31
  SPEAKER_00: Because there's so many events.
• 52:33
  SPEAKER_00: The answer is because there's so many events, and because even despite all of our tools, these rocks are so tortured that it's hard to still see through it.
• 52:45
  SPEAKER_00: We even with all the tools we have, because again, we know that there's a bunch of metamorphic events, we can date a bunch of these metamorphic events really reliably using Garnet, Monazite, Apotite, different isotope systems and those different minerals.
• 52:56
  SPEAKER_00: We can date the metamorphism.
• 52:58
  SPEAKER_00: There's good consensus on that.
• 53:00
  SPEAKER_00: It's when we get back to what is the original, what is the primary age of this rock, that's when it gets a little sticky.
• 53:06
  SPEAKER_00: Because again, we're we only when we get to that question, we really only have zircon uranium lead.
• 53:12
  SPEAKER_00: And there are, as we've gone through, a whole bunch of ways to reset or partially reset the uranium lead and make it complicated to interpret.
• 53:20
  SPEAKER_00: Now, there is some clarity, you know, zircon uranium lead provides some clarity, but what happens if you have a mixed rock?
• 53:27
  SPEAKER_00: I'm gonna go back to this image of the Morton Knights.
• 53:30
  SPEAKER_00: This is a mixed rock.
• 53:32
  SPEAKER_00: There are a bunch of different ages in here.
• 53:33
  SPEAKER_00: So if you crush up this sample that we're looking at with pink veins and dark bands and a bunch of gray bands, they're all gonna have different histories, they're all gonna have different initial ages.
• 53:44
  SPEAKER_00: So you're gonna get a bunch of different populations of zircons that look a little bit different, have different ages coming out of them, and the data is gonna be messy.
• 53:51
  SPEAKER_00: And like this is what I did my PhD on is looking at complicated rocks like this, and it's you can get really messy data where you're just like, My goodness, I don't know how to interpret this.
• 54:00
  SPEAKER_00: I don't think I can reliably estimate the age of this rock, and that's just the way it is, unless you want to spend years on it, then you could probably could.
• 54:08
  SPEAKER_02: Okay, well, that leads me to my next thought, if you will, and and I want you to chime in on it.
• 54:13
  SPEAKER_02: But is maybe part of it that this has been studied a long time ago, and so it's not as attractive to a PhD candidate as oh, I want to solve this problem because it's already been looked at so much early on.
• 54:29
  SPEAKER_02: Is that that's gotta be a part of it, right?
• 54:31
  SPEAKER_02: I mean, I think there's these these people are looking for fresh things, fresher anyway.
• 54:36
  SPEAKER_00: I think that's probably partially true, and it's also partially true that a little bit of the you know, the techniques that provide a lot of insight, like the ion microprobe shrimp analyses, we talked about that in some of our early chapters in Ursulus Rock's um visual podcast series here, those techniques have been around for a while.
• 54:56
  SPEAKER_00: So we need new techniques.
• 54:57
  SPEAKER_00: As soon as we get a new technique, we can throw the book at it again.
• 55:00
  SPEAKER_00: We can revisit these areas and and gain more information.
• 55:04
  SPEAKER_00: And for me, just reading about this study, reading the summary, you know, I've got some ideas about how we could address this.
• 55:10
  SPEAKER_00: I think there's some new techniques we've been working on that we could apply to some of these grains and actually gain more information, perhaps.
• 55:16
  SPEAKER_00: So, you know, there's definitely more than a lot of people.
• 55:17
  SPEAKER_02: It makes me want to be a student again, Jesse.
• 55:19
  SPEAKER_02: It makes me want to like seriously, like here's what you should do.
• 55:22
  SPEAKER_00: I've told you this before, but when I was a PhD student, I got this uh TA award for our department.
• 55:27
  SPEAKER_00: So I went to the faculty of science, like all the TAs who won this teaching award, you know, had a dinner with the dean and stuff like this at the at the faculty club.
• 55:34
  SPEAKER_00: Anyway, one of the teeth one of the people who won the teaching award was a retired math teacher, retired public school teacher who was getting his PhD.
• 55:42
  SPEAKER_00: And he was like, it's the best.
• 55:44
  SPEAKER_00: I just was in retirement, just decided I'm gonna go get my PhD.
• 55:47
  SPEAKER_00: And you know what?
• 55:48
  SPEAKER_00: He's like, I'm it's super fun.
• 55:49
  SPEAKER_00: I'm a graduate student with a bunch of my former high school students who are getting their PhD in math too.
• 55:54
  SPEAKER_00: He's like, it's super crazy, super fun.
• 55:57
  SPEAKER_00: And so, you know what?
• 55:58
  SPEAKER_00: I think you should come to Penn State.
• 55:59
  SPEAKER_00: Okay.
• 56:00
  SPEAKER_00: All right, come come come get a graduate at Penn State.
• 56:02
  SPEAKER_02: It's it's in the back of the head, Jesse.
• 56:04
  SPEAKER_02: I'm telling you.
• 56:04
  SPEAKER_02: The seed has been planted, it's adequately been planted.
• 56:07
  SPEAKER_00: We'll come and uh we'll do some analytical work, it'll be great.
• 56:10
  SPEAKER_02: I had another thought, but I lost it because I had a different thought that I went jumped on first.
• 56:16
  SPEAKER_02: Really, my my sieve of a brain is not serving me well today.
• 56:21
  SPEAKER_02: Short marketing over there.
• 56:22
  SPEAKER_02: I am, I am.
• 56:23
  SPEAKER_02: But uh, what a fun talk though, Jesse.
• 56:26
  SPEAKER_02: Um oh, I know what my thought was.
• 56:29
  SPEAKER_02: We're not done with this because we uh we have another we're gonna talk about what we think is the oldest rock in the in the US uh with our next episode.
• 56:40
  SPEAKER_02: But we also need to do this uh continent stabilization thing, you know.
• 56:46
  SPEAKER_02: We've talked about this before.
• 56:47
  SPEAKER_02: You throw it around all the time, and I'm like, okay, well, we can't just do that, we can't throw around continent stabilization because Joyce has no idea what you're talking about.
• 56:55
  SPEAKER_02: Yeah, so we have to go into this, we have to explain what that means, and that's a part of this.
• 57:00
  SPEAKER_02: Like, this is uh it's a tack onto this series.
• 57:03
  SPEAKER_00: Totally.
• 57:03
  SPEAKER_00: I mean, it's an interesting question.
• 57:05
  SPEAKER_00: Why are all the old rocks at the edges of these cratonic areas, right?
• 57:08
  SPEAKER_00: Like it's a I don't know the answer to that, but it's an interesting observation.
• 57:12
  SPEAKER_00: And so, yeah, all right, totally.
• 57:14
  SPEAKER_00: We're not done.
• 57:14
  SPEAKER_00: We we gotta link all these things together.
• 57:16
  SPEAKER_00: How the heck do the continents form and when?
• 57:18
  SPEAKER_00: Such a good question, such an important question.
• 57:20
  SPEAKER_02: And then we're then then we'll do, I think maybe after that, then we'll talk about the Canadian Shield specifically.
• 57:26
  SPEAKER_02: There we go.
• 57:26
  SPEAKER_02: Perfect.
• 57:27
  SPEAKER_02: A little love that way.
• 57:28
  SPEAKER_00: Yeah, yeah, absolutely.
• 57:29
  SPEAKER_00: Super exciting stuff.
• 57:30
  SPEAKER_02: Hey, we got a plan.
• 57:30
  SPEAKER_00: We got a plan.
• 57:31
  SPEAKER_00: We got a plan.
• 57:32
  SPEAKER_00: There we go, man.
• 57:33
  SPEAKER_00: So much stuff to do, so much stuff in the works.
• 57:35
  SPEAKER_00: I love it.
• 57:35
  SPEAKER_00: We got a lot of stuff in the works.
• 57:37
  SPEAKER_00: Um, well, I think that's a wrap, probably, probably, Chris, for part one here on the United States oldest rocks.
• 57:43
  SPEAKER_00: Summary, zircons, even as good as is, can be complicated.
• 57:47
  SPEAKER_00: If you like this stuff, you can find more in our Earth's Oldest Rocks visual podcast series on our Camp Geo Mobile app.
• 57:52
  SPEAKER_00: First link in your show notes.
• 57:53
  SPEAKER_00: Check it out there.
• 57:53
  SPEAKER_00: We've got all the images in our our patented approach to providing you kind of these visual podcast series where you get a little bit of video, a little bit of hints of images, you get these nice gifts we've made, but you can scroll through them without skipping through the audio.
• 58:06
  SPEAKER_02: You know, if you're on YouTube or but or Spotify, yeah, because when Jesse gets windy, then you can just start scrolling right on through and it's no problem.
• 58:13
  SPEAKER_02: Exactly.
• 58:14
  SPEAKER_00: Exactly.
• 58:14
  SPEAKER_00: So it's really, I think, a nice approach that we've got there.
• 58:17
  SPEAKER_00: So Camp Geo Mobile app, first link in your show notes, tons of other content there if you like this episode, and we'll also post part two as well there.
• 58:24
  SPEAKER_00: Uh you can get early access to that.
• 58:26
  SPEAKER_00: So go there.
• 58:27
  SPEAKER_00: You can send us an email, a contact us link on our website, planetgeopast.com.
• 58:30
  SPEAKER_00: Follow us on social media at Planet GeoCast.
• 58:33
  SPEAKER_00: Cheers.