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America's Oldest Rocks - Part 2 - The Unveil · PlanetGeo: The Geology Podcast

• 0:00
  SPEAKER_04: Where you're gonna you're gonna state at the end of this episode what the oldest rock in the United States is.
• 0:06
  SPEAKER_04: You usually don't do that.
• 0:07
  SPEAKER_04: No.
• 0:11
  SPEAKER_03: Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it worked, and why it mattered.
• 0:23
  SPEAKER_03: So, Chris.
• 0:24
  SPEAKER_03: You got an intro?
• 0:26
  SPEAKER_03: I got this one.
• 0:27
  SPEAKER_03: I got the lead-in.
• 0:28
  SPEAKER_03: That always makes me a little nervous.
• 0:30
  SPEAKER_03: Because uh this episode, Chris, it's very good.
• 0:33
  SPEAKER_03: We have a hard out.
• 0:34
  SPEAKER_03: We have another meeting that we have to take in 55 minutes.
• 0:38
  SPEAKER_03: And so we have a hard out which is gonna help you keep me out of the weeds, I think, on this one.
• 0:45
  SPEAKER_03: Yeah, all right.
• 0:46
  SPEAKER_04: I'll I'll do my best.
• 0:47
  SPEAKER_04: Holy cow.
• 0:48
  SPEAKER_03: This is uh hey, we're we're gonna get some answers though.
• 0:50
  SPEAKER_03: This is an exciting episode because this is part two of America's oldest rocks, the United States, the oldest rocks in the United States.
• 0:56
  SPEAKER_03: We're gonna give a partial answer to the question.
• 1:00
  SPEAKER_03: Uh or as good as we know answer to the question today.
• 1:04
  SPEAKER_03: So this and all the other Earth's oldest rocks audiobook chapters are available on our CampGo mobile app right now.
• 1:09
  SPEAKER_03: You can go there, first link in the show notes.
• 1:10
  SPEAKER_03: But Chris, maybe we could start out from where we left off last time.
• 1:14
  SPEAKER_03: Like what set the stage for us and let's have a high-level conversation about what old rocks are.
• 1:19
  SPEAKER_04: Because I think you had some questions from the the very detailed script I put together that was like, uh, yeah, I read through this and I'm I'm I'm thinking, wait a second, Jesse, we we try to keep our episodes uh like under 45 minutes, not not two hours.
• 1:32
  SPEAKER_04: This is ridiculous.
• 1:34
  SPEAKER_04: Um, so we really last time we left off setting the stage with the three candidates for the oldest rocks, and but then we spent a lot of time talking about you know how we do this.
• 1:47
  SPEAKER_04: How how do geochronologists like yourself do this, what tools are used, and we we got into some of the the technical aspects of geochronology, right?
• 1:57
  SPEAKER_04: Today we're going to write it.
• 2:00
  SPEAKER_04: We're gonna get into this and that's right, we are we are definitely in your wheelhouse.
• 2:05
  SPEAKER_04: But today that we're gonna actually and this is funny to me because you usually back away from this where you're gonna you're gonna state at the end of this episode what the oldest rock in the United States is.
• 2:18
  SPEAKER_04: You usually don't do that.
• 2:19
  SPEAKER_04: No, so I found that to be really funny.
• 2:21
  SPEAKER_03: Uh uh, you mean make a uh a definitive statement about what I'm saying?
• 2:26
  SPEAKER_03: I know.
• 2:26
  SPEAKER_04: I was I was impressed.
• 2:28
  SPEAKER_04: I was impressed.
• 2:28
  SPEAKER_04: But Jesse, we need to begin um before we get into the specifics of what are the oldest rocks in the US and how do we know this?
• 2:38
  SPEAKER_04: We need to answer some questions, I think.
• 2:40
  SPEAKER_03: Um what were the questions that came to your mind when you went through this?
• 2:44
  SPEAKER_04: Like what were the big Here's the first question.
• 2:47
  SPEAKER_04: All three candidates are metamorphic.
• 2:50
  SPEAKER_04: Metamorphic rocks.
• 2:51
  SPEAKER_03: Can I can I interrupt real quick, Chris, and just it list the candidates?
• 2:54
  SPEAKER_03: We we just to refresh, we had the Minnesota River Valley, which are these ones up in Minnesota along the river valley, as the name would imply.
• 3:01
  SPEAKER_03: We have some Wyoming Kraton rocks out in Wyoming, and then we have the Waters Meet Ice, which is up in the northern uh upper peninsula of Michigan, well, the western upper peninsula of Michigan.
• 3:09
  SPEAKER_03: So so two rocks from this kind of upper Michigan, Minnesota area, and then one from Wyoming.
• 3:14
  SPEAKER_03: So those are candidates.
• 3:15
  SPEAKER_03: So sorry, go ahead.
• 3:15
  SPEAKER_03: Okay.
• 3:16
  SPEAKER_04: So all three of those are metamorphic rocks, in fact, like really metamorphic rocks.
• 3:21
  SPEAKER_04: But in fact, all old rocks, you study the Acosta, all old rocks are metamorphic.
• 3:28
  SPEAKER_04: So why is that the case?
• 3:31
  SPEAKER_04: Why are all of the Earth's oldest rocks going to be metamorphic?
• 3:36
  SPEAKER_03: Well, oh, okay.
• 3:37
  SPEAKER_03: So just why are they metamorphic rocks?
• 3:39
  SPEAKER_03: Well, like why are they not igneous rocks?
• 3:41
  SPEAKER_03: Okay.
• 3:42
  SPEAKER_03: I think the answer's kind of simple.
• 3:43
  SPEAKER_03: They've had a hard life, they've been on the earth for a long time, and plate tectonics has been going on for at least two and a half billion years by most people's estimates.
• 3:51
  SPEAKER_03: So they've been reworked, they've been metamorphosed, they've experienced a hard life.
• 3:56
  SPEAKER_03: My PhD supervisor is very funny.
• 3:57
  SPEAKER_03: He, whenever we see these zircons, we talk about zircon grands a lot.
• 4:00
  SPEAKER_03: Whenever we see the old ones, the ones that have had a rough life, they're radiation damaged.
• 4:05
  SPEAKER_03: We'd see these in the imaging techniques that we use.
• 4:07
  SPEAKER_03: You always go, ooh, it's had a hard life.
• 4:10
  SPEAKER_03: Ooh, it's had a hard life.
• 4:12
  SPEAKER_03: And that is uh that's applicable to the many of these old rocks.
• 4:15
  SPEAKER_03: Now, there are some that are better preserved, like there are some 3.5 billion year old rocks that are really well preserved.
• 4:22
  SPEAKER_03: They still have experience, though, low-grade metamorphism, like Green Shit's faces metamorphosis or something.
• 4:27
  SPEAKER_03: So it's it's really common.
• 4:28
  SPEAKER_03: They've just been in the crust for a long time and not always at the surface, you know, living a nice happy life.
• 4:35
  SPEAKER_04: All right, that leads me to the next question then.
• 4:38
  SPEAKER_04: Well, first of all, hold on.
• 4:39
  SPEAKER_04: Side, side sub question.
• 4:41
  SPEAKER_04: So when you look at me, do you say the same thing when you turn it when I when I jump on our link here and we're looking face to face, do you look at me and be like, ooh, no, that guy's had a hard life.
• 4:52
  SPEAKER_01: No comment.
• 4:54
  SPEAKER_01: No comment.
• 4:55
  SPEAKER_03: Uh all right.
• 4:56
  SPEAKER_03: I'll I'll leave that one to you know the imagination of the listener to figure out what we think about when you jump on Zoom.
• 5:02
  SPEAKER_03: It depends if you're coming with a furrowed brow, a cranky Chris or not.
• 5:06
  SPEAKER_04: Uh that's I never oh, please.
• 5:08
  SPEAKER_04: We we had a discussion about you this morning.
• 5:10
  SPEAKER_04: Your hair was tied back a little too tight.
• 5:12
  SPEAKER_04: Um, get to your second question there, Mr.
• 5:16
  SPEAKER_03: Bull.
• 5:16
  SPEAKER_04: Yeah, next next question.
• 5:18
  SPEAKER_04: So when we talk about the ages of a metamorphic rock, and I think this is really important.
• 5:23
  SPEAKER_04: What are we talking about?
• 5:24
  SPEAKER_04: Oh are we talking about the age of the rock before metamorphism, or are we talking about the metamorphic event?
• 5:32
  SPEAKER_03: This is such a great question.
• 5:33
  SPEAKER_03: That um so when we are talking about the oldest rocks, Earth's oldest rocks, earth's oldest mineral grains, etc., we are usually we're seeing through metamorphism.
• 5:44
  SPEAKER_03: So for the acostonice complex, which we've got a couple chapters on the acostinites complex in detail on our audiobook there, that rock, those rocks have all been metamorphosed at 1.9 billion years ago.
• 5:55
  SPEAKER_03: And they've been metamorphosed at 2.8 billion years ago, at 3.0 billion years ago, at 3.3 billion years ago, 3.6 billion years ago.
• 6:03
  SPEAKER_03: So they've seen a lot of metamorphism.
• 6:05
  SPEAKER_03: But when we, as old rocks people, as people who study old rocks, we're usually the vast majority of the time trying to see through that metamorphism.
• 6:13
  SPEAKER_03: And so we're talking about the primary formation age when the sediment was deposited or when the igneous rock cooled and crystallized from a magma first, the first time.
• 6:23
  SPEAKER_03: But you're totally right.
• 6:25
  SPEAKER_03: Most of these rocks, most of the minerals in most of these rocks are all metamorphic and much younger.
• 6:30
  SPEAKER_03: Uh, however, the atoms in the rock were aggregated together as a rock, you know, at some time at that initial at that initial age, yeah.
• 6:40
  SPEAKER_04: Okay.
• 6:41
  SPEAKER_04: Um oh shoot, I had a question that you that just popped into my head and then it just popped out right back.
• 6:46
  SPEAKER_03: Yeah, this leaky faucet of a brand of your is really throwing a wrench into our episode already.
• 6:54
  SPEAKER_04: Um, so I would have I so I guess I expected you to answer the question differently.
• 6:58
  SPEAKER_04: Oh, right.
• 6:59
  SPEAKER_04: You would answer it and say, Well, Chris, both.
• 7:02
  SPEAKER_04: We're after both.
• 7:03
  SPEAKER_04: Well, that's we're after the protolith, and we're after all of the torturings that happened along the way.
• 7:10
  SPEAKER_04: That's true.
• 7:10
  SPEAKER_03: I mean, but you know, the torturings, if I want to go, let's say, to the acostodiscoptics, or let's go to some of these rocks we're looking at, the torturing events are two billion years ago.
• 7:18
  SPEAKER_03: And the torturing event, the metamorphic event, tells us about the earth two billion years ago because that's when the event occurred.
• 7:24
  SPEAKER_03: It'll tell us about metamorphic conditions, plate tectonics two billion years ago, whereas the original age will give us some information about what the earth was like, let's say 3.6 billion years ago, in the case of the United States' oldest rock.
• 7:36
  SPEAKER_03: So it's two different questions.
• 7:38
  SPEAKER_03: So let me ask you.
• 7:39
  SPEAKER_04: We would be after both.
• 7:40
  SPEAKER_04: Yeah.
• 7:41
  SPEAKER_04: Why is it important to see through?
• 7:45
  SPEAKER_04: Because you that was your answer.
• 7:46
  SPEAKER_04: We want to see the age of the rock before it was tortured and metamorphosed.
• 7:52
  SPEAKER_04: Why is this an important question to answer?
• 7:56
  SPEAKER_04: I mean, like, what are what are what are researchers really after with this?
• 8:01
  SPEAKER_03: When we're talking about these old rocks, it's well, there's simple questions like how much continental crust was there back in time?
• 8:08
  SPEAKER_03: When did continents grow?
• 8:10
  SPEAKER_03: When did the continents on our earth form?
• 8:12
  SPEAKER_03: Uh, we look at Venus and we see that there's some things that kind of maybe look like continents, but it has a very that's a very different planet, uh the Earth's toxic twin.
• 8:21
  SPEAKER_03: Why are they so different and when did they become different?
• 8:23
  SPEAKER_03: That's a a question that can be answered potentially by looking at old rocks on Earth.
• 8:28
  SPEAKER_03: And so, you know, questions like were there oceans?
• 8:32
  SPEAKER_03: We talked about that um from the Jack Hill Zircons.
• 8:34
  SPEAKER_03: Now new, well, 20-year-old data now has told us that there were oceans four billion years ago, which is a new finding.
• 8:40
  SPEAKER_03: We just had zero information about the early earth.
• 8:43
  SPEAKER_03: So we're really telling the full life history of our planet when we go back and look at these fragments of old rocks to paint a picture for what the Earth looked like uh 3.6 billion years ago.
• 8:54
  unknown: Good.
• 8:54
  SPEAKER_04: Good answer.
• 8:55
  SPEAKER_04: That's a good answer, Jesse.
• 8:56
  SPEAKER_04: That's a good answer.
• 8:57
  SPEAKER_04: I'll take it.
• 8:58
  SPEAKER_04: I'll take it.
• 8:58
  SPEAKER_04: It was a good answer.
• 8:59
  SPEAKER_04: Plus, you know, um understanding things like this, uh, understanding where we came from, you know, we've we've done this whole series on it's a good time to be a geoscientist, and and we've said several times that uh, you know, uh the problems that we are facing today are surficial problems, but all of the answers are deep inside.
• 9:19
  SPEAKER_04: The answers to those problems, the solutions to those problems are deep in the earth.
• 9:23
  SPEAKER_04: And and this gets to that, right?
• 9:26
  SPEAKER_04: Answering these kinds of questions, yeah.
• 9:28
  SPEAKER_04: Yes.
• 9:28
  SPEAKER_03: And the earth has changed dramatically throughout its life, and you know, understanding why and how it changed and what happened after the feedback loops during those change periods is really important.
• 9:37
  SPEAKER_03: So, yes, I I hope that's a valuable valuable answer uh that that people buy.
• 9:42
  SPEAKER_03: Um, because I it's true.
• 9:44
  SPEAKER_04: Well, well, we hey, you know what?
• 9:45
  SPEAKER_04: It's our podcast and we buy it.
• 9:47
  SPEAKER_04: So okay, there we go.
• 9:48
  SPEAKER_04: All right, I explain.
• 9:50
  SPEAKER_04: All right, Jesse.
• 9:51
  SPEAKER_04: Uh, we need to move on then, right?
• 9:52
  SPEAKER_04: Because we actually have to get into the the stuff.
• 9:55
  SPEAKER_04: The timer is kicking down, yes, of course.
• 9:59
  SPEAKER_04: So, yeah, we really spent some time in our last episode talking about this these Concordia plots and this discordance of the Concordia plots, um, and and how ages can be reset.
• 10:13
  SPEAKER_04: So this happens all the time, and you have an uh uh kind of an analogy with this with the Acosta Nice.
• 10:20
  SPEAKER_04: The because this is your wheelhouse, right here.
• 10:22
  SPEAKER_03: That's right.
• 10:22
  SPEAKER_03: Sorry.
• 10:23
  SPEAKER_03: So just to the if you want the primer, uh go back a little story part one on this because we'll give a we give a pretty, I think a pretty decent intro into uranium lead geochronology with as it applies to zircon, which is the mineral we're gonna focus on a lot today.
• 10:35
  SPEAKER_03: I'm gonna turn to image number one here in a minute, Chris, because this image shows some real data.
• 10:40
  SPEAKER_03: Again, we have these two axes.
• 10:42
  SPEAKER_03: We're plotting lead over uranium, the product lead that was formed from the decay of the parent uranium.
• 10:47
  SPEAKER_03: There's two different styles of that, and those are on the X and Y axis independently.
• 10:52
  SPEAKER_03: Where those two ages, those two sand dials, give us the same age, we plot a curve in space that's called Concordia.
• 10:59
  SPEAKER_03: And any data point that falls on that is called concordant, and we kind of trust that that age is right because there's two independent uh measurements giving us the same age.
• 11:07
  SPEAKER_03: This data, as you can see, much of it does not fall on that line, it falls along an array.
• 11:12
  SPEAKER_03: And this is real data from the Acosta Nice complex.
• 11:15
  SPEAKER_03: So what we do as geochronologists when we get an array of data like this is we we do some we just do a statistical test.
• 11:22
  SPEAKER_03: It's a it's it's a little bit more complicated complicated than a linear regression because it takes into account uncertainty and scatter, but we do effectively an advanced linear regression through this.
• 11:31
  SPEAKER_03: So plot a line through the data, and then where that line, that regression line, intersects the curve, the upper and lower parts of the curve, then we think those are interpretable ages.
• 11:41
  SPEAKER_03: The upper one would be a crystallization age, the lower one would be a metamorphic resetting alteration age.
• 11:48
  SPEAKER_03: Does that make sense?
• 11:48
  SPEAKER_03: So when we get a linear array of data like this, does we get upper and lower intercepts basically?
• 11:53
  SPEAKER_03: Why are you smirking at me?
• 11:55
  SPEAKER_04: What this is hilarious.
• 11:57
  SPEAKER_04: Why?
• 11:58
  SPEAKER_03: Because you are you are hilarious because I'm talking passionately about my you know my area of expertise, and you're giving me a hard time for it.
• 12:05
  SPEAKER_03: It's really hard to focus.
• 12:07
  SPEAKER_03: It's like when I'm up there lecturing and I've got the T I've got the I've got the bros in the back row snickering to each other.
• 12:12
  SPEAKER_03: That's what you're doing right now, and you're throwing me off my game.
• 12:17
  SPEAKER_04: Well, I I just I'm sorry, I couldn't help myself because you were you were like you're like a little puppy here.
• 12:25
  SPEAKER_04: Um you are so excited.
• 12:27
  SPEAKER_04: You just put a little bone in front of me.
• 12:30
  SPEAKER_04: To talk about your little Concordia ply here.
• 12:32
  SPEAKER_04: You're so excited that you started talking faster because you knew that I'm gonna limit you to time because we have to.
• 12:38
  SPEAKER_04: We gotta move through.
• 12:39
  SPEAKER_04: So you just started talking so much faster.
• 12:42
  SPEAKER_03: I can see Chris looking at his watch over there and be like, hey, he's got a minute and a half, and then I'm interrupting.
• 12:49
  SPEAKER_04: I'm sorry.
• 12:49
  SPEAKER_04: That was no, it's it is good information.
• 12:52
  SPEAKER_04: I I use these concordia plots too when I talk about geochronology.
• 12:56
  SPEAKER_03: Um for working on, especially old rocks like this, where Zircon is our main tool in our toolbox.
• 13:05
  SPEAKER_03: These are these are what we deal with, these types of Concordia plots.
• 13:08
  SPEAKER_03: So anyway, okay.
• 13:10
  SPEAKER_03: That's just a case study, an example of that.
• 13:13
  SPEAKER_03: But it's hard, it's hard to know what's what are the like we're just all we're doing is interpreting these intercepts, the where the line intercepts the Concordia curve.
• 13:21
  SPEAKER_03: We don't know exactly what those events necessarily are, that upper event and the lower event.
• 13:26
  SPEAKER_03: We can know the numbers, but we don't know what they are.
• 13:29
  SPEAKER_04: Are these resetting events?
• 13:32
  SPEAKER_04: Yes, they are.
• 13:34
  SPEAKER_04: Well, okay, the and or partial resetting events.
• 13:37
  SPEAKER_03: Yeah, exactly.
• 13:37
  SPEAKER_03: Partial because a full resetting would move the grain, the a data point all the way down to zero.
• 13:42
  SPEAKER_03: Exactly.
• 13:42
  SPEAKER_03: That's a great, great point, Chris.
• 13:44
  SPEAKER_03: Good leading question.
• 13:45
  SPEAKER_04: Okay, so in and resetting, we're talking about resetting the clock or partially resetting the clock.
• 13:53
  SPEAKER_04: That can be from metamorphism, that can be fluid alteration, you know, fluids moving through salty fluids, hot salty fluids, and and then a change in the whole geochemistry.
• 14:06
  SPEAKER_04: Um, or it could be a thermal event, or really a combination of all three of them.
• 14:13
  SPEAKER_04: That's right.
• 14:14
  SPEAKER_04: Yeah, you know, it it it can be that.
• 14:16
  SPEAKER_04: Um so Jesse, can we move into a second?
• 14:21
  SPEAKER_04: We've never really done this.
• 14:22
  SPEAKER_04: This actually occurred to me yesterday.
• 14:25
  SPEAKER_04: We've talked a lot about uranium to lead and the importance of zircon.
• 14:28
  SPEAKER_04: I mean, zircon is your is your thing.
• 14:31
  SPEAKER_04: We've never, I don't think anyway, really talked about what zircon is.
• 14:36
  SPEAKER_03: Oh, yeah, okay.
• 14:38
  SPEAKER_04: And we need to do that, and then then I'll let you launch into this, you know, zircon.
• 14:43
  SPEAKER_04: What happens during metamorphism to zircon?
• 14:45
  SPEAKER_04: Okay, can so can we do that first?
• 14:47
  SPEAKER_04: Absolutely.
• 14:47
  SPEAKER_04: No, it's a really good point.
• 14:48
  SPEAKER_03: Okay, it's a it's an absolutely excellent point.
• 14:50
  SPEAKER_03: Uh I'll maybe just while we're going through this, I'll move to image number two here, which is just a little gif that's showing this is an electron image of a zircon grain that I've kind of color-coded some of the growth zones.
• 15:01
  SPEAKER_03: So you can see growth zoning here in these two different images, and then uh it's it flashes between a couple different styles of imaging, but this grain is the width of a hair.
• 15:10
  SPEAKER_03: And so this is a tiny sand grain that we're imaging in detail, and there's complicated growth zoning.
• 15:16
  SPEAKER_03: So, anyways, sorry, Chris, go ahead.
• 15:17
  SPEAKER_04: Now that could be the width of a crisple hair on his head, which is a whole different thing.
• 15:23
  SPEAKER_04: Okay, not a crisple.
• 15:24
  SPEAKER_04: A normal hair.
• 15:26
  SPEAKER_04: Okay.
• 15:26
  SPEAKER_04: Not the little not the little fuzz that really really tries its hardest.
• 15:31
  SPEAKER_04: So, okay, zircon.
• 15:33
  SPEAKER_04: Zircon is ZRSIO4.
• 15:36
  SPEAKER_04: So it's a lot like olivine.
• 15:38
  SPEAKER_04: If you think about it, it's a single tetrahedron that forms this pyramid shape, right?
• 15:42
  SPEAKER_04: So that's what zircon is.
• 15:44
  SPEAKER_04: But the question is, how does uranium get into it then?
• 15:48
  SPEAKER_04: Oh yeah.
• 15:48
  SPEAKER_04: You know, because you're talking about a zircon grain, which is actually a mineral that has zirconium in it, but how does uranium get into it?
• 15:55
  SPEAKER_04: Um and and it really the the zircon grain prefers to have zirconium in it, but as uh uranium concentrates in the magma, ionic substitution can take place, which is to say that the zircon grain is growing and it's got mostly zircon in it, but then as uranium and sometimes even thorium concentrate in the melt, then they'll go in and they'll kick the zirc the zirconium out and replace it.
• 16:24
  SPEAKER_04: Right.
• 16:24
  SPEAKER_04: Ionic substitution, right?
• 16:26
  SPEAKER_04: Exactly.
• 16:26
  SPEAKER_04: And and but that and that can happen because the ionic radii of thorium, of uranium is very similar to zirconium, and the charges are very similar.
• 16:38
  SPEAKER_04: It's a plus four charge on all of them, except uranium can have a plus six charge sometimes, which is weird.
• 16:44
  SPEAKER_04: Um, but it so, in other words, the suitability for them to replace it, the size, the charge, it works.
• 16:52
  SPEAKER_04: Basically, it gets there through Ionic substitution.
• 16:55
  SPEAKER_03: That's exactly right.
• 16:56
  SPEAKER_04: So and the other important point, Jesse, is that this it tends to exclude the daughter isotope lead.
• 17:05
  SPEAKER_04: Uh-huh.
• 17:05
  SPEAKER_04: And so, yeah, you know, and that's a really important point.
• 17:08
  SPEAKER_04: Lead's the wrong size, it's the wrong shape, it doesn't fit.
• 17:12
  SPEAKER_03: It's a huge and so it's excluded.
• 17:14
  SPEAKER_03: Lead is usually occurs in a two-plus charge state, and it's a huge uh uh um cation at that at that size, so or at that charge state.
• 17:21
  SPEAKER_03: So it just doesn't fit, right?
• 17:22
  SPEAKER_03: So you're exactly right.
• 17:23
  SPEAKER_03: That was a really nice description of in a in a magma, that's how zircon is going to why uranium and thorium will make their way into zircon is because they can substitute in for the four both zirconium and silica are four plus charges in that zircon structure, ZR SIO4.
• 17:40
  SPEAKER_03: They're both four plus cations in that structure, so uranium and thorium can substitute in there.
• 17:44
  unknown: Yeah.
• 17:45
  SPEAKER_04: Yep.
• 17:45
  SPEAKER_04: So Jesse, let's then use that then.
• 17:48
  SPEAKER_04: Let's talk about what happens to zircon during metamorphism.
• 17:52
  SPEAKER_03: Yeah.
• 17:52
  SPEAKER_03: So you laid out a bunch of different sort of processes like hydrothermal fluids, uh, metamorphic fluids, the grain is damaged and stuff gets in there, uh, new growth.
• 18:03
  SPEAKER_03: You you those are kind of outlined a bunch of different ways that you could alter the uranium-led systematics of a zircon grain, of a part of a zircon grain that we analyze.
• 18:12
  SPEAKER_03: And the way I think about this, Chris, is I break it into kind of two categories.
• 18:16
  SPEAKER_03: One is new growth.
• 18:17
  SPEAKER_03: So we're growing new zircon on the edges.
• 18:20
  SPEAKER_03: The other is modifying existing growth zones.
• 18:25
  SPEAKER_03: Or modifying you know, currently existing growth zones what that are there at the time of the process.
• 18:30
  SPEAKER_03: So modification or new growth, right?
• 18:33
  SPEAKER_03: And the modification one, I think, is well, they're both kind of easy, I think, when you to think about when you break it down that way.
• 18:39
  SPEAKER_03: The modification one, let's think about that.
• 18:41
  SPEAKER_03: You've got a zircon grain, let's say it formed 3.5 billion years ago, and um in that 3.5 billion years, the uranium and thorium have decayed away.
• 18:49
  SPEAKER_03: We talked about this before.
• 18:50
  SPEAKER_03: During that decay process, there's this alpha recoil that blows apart the crystal structure.
• 18:55
  SPEAKER_03: So any zones that had high uranium or thorium are gonna be really radiation damaged.
• 19:00
  SPEAKER_03: Uh, they're gonna not actually be zircon anymore, they're gonna kind of be uh uh not not very crystalline.
• 19:07
  SPEAKER_03: And if they are have crystallinity to them, they're gonna be really damaged in there.
• 19:10
  SPEAKER_03: So it's easy for fluids to get in there.
• 19:12
  SPEAKER_03: It's easy for when you heat it up for it to kind of re-anneal and it'll kick out some of the lead as it regrows, as it heals itself.
• 19:19
  SPEAKER_03: And so there's all sorts of ways to modify existing structure.
• 19:22
  SPEAKER_03: That's like you know, metamorphic, metamorphic processes modifying these.
• 19:26
  SPEAKER_03: The other way is growing new zircons.
• 19:28
  SPEAKER_03: So if you have a zircon that's sitting there and you have a little bit of melt around, and let's say the rock is being metamorphosed, and let's say that initial rock had amphibole in it and biotite in it and quartz and plagiarclase, as that rock gets metamorphosed, the the biotites and the amphiboles will start to recrystallize metamorphically, and those things will often kick out zirconium.
• 19:49
  SPEAKER_03: They'll have little bits of zirconium in them that they'll kick out that zirconium as they break down, and that zirconium will then move and migrate and grow new zircon around existing zircon, sort of using the existing zircon grain as a nucleation site for new growth.
• 20:04
  SPEAKER_03: That's an easier way for it to grow than starting a new zircon grain growing somewhere else in the rock.
• 20:10
  SPEAKER_03: So new growth can happen that way.
• 20:22
  SPEAKER_04: Well, it could be why would it kick out the zirconium?
• 20:25
  SPEAKER_03: It could be you know, biotite breaking down, biotite can often get um uh metamorphosed to chlorite at first and then different minerals.
• 20:31
  SPEAKER_03: So they break down initially and then they get changed in composition.
• 20:35
  SPEAKER_03: And you know, the substitution of zirconium into amphibole is a complicated process, it's temperature dependent in some ways, and it's dependent upon the you know, you and I know amphibole.
• 20:44
  SPEAKER_03: We've seen tons of different amphiboles when we're out rock collecting, and it's a garbage bag mineral, you can fit everything in there, right?
• 20:50
  SPEAKER_03: And so the composition changes, the crystal structure changes, and the amount of zirconium that goes in there changes dramatically.
• 20:56
  SPEAKER_03: So breaking down igneous amphibole into metamorphic amphibole often releases zirconium.
• 21:01
  SPEAKER_03: Okay.
• 21:02
  SPEAKER_04: So, how does this fit into then the construct of interpreting ages, or maybe maybe a better way of saying this is that lower intercept on your Concordia plot.
• 21:14
  SPEAKER_04: How does this fit into that construct?
• 21:16
  SPEAKER_03: Okay, so it's a great question, Chris.
• 21:18
  SPEAKER_03: Like, how are how do we interpret these events, right?
• 21:22
  SPEAKER_03: And I'm gonna paint two quick scenarios for you.
• 21:24
  SPEAKER_03: And I don't know if this is a good way to do this, so please help me help keep me on the rails here.
• 21:29
  SPEAKER_03: Um, I'll do my best.
• 21:30
  SPEAKER_03: The first one, the first one we've kind of already talked about.
• 21:32
  SPEAKER_03: Let's say we have a igneous rock, it crystallizes, we've got a beautiful igneous zircon sitting there.
• 21:36
  SPEAKER_03: A billion years later, that rock undergoes metamorphism.
• 21:41
  SPEAKER_03: Let's say the biotites and the amphiboles in that rock start to break down, release their zirconium.
• 21:46
  SPEAKER_03: That zirconium, those atoms migrate around and nucleate on the existing zircon grain and grow a metamorphic rim of zircon around the original igneous stuff.
• 21:56
  SPEAKER_04: That's so you're gonna have two ages or multiple ages.
• 22:00
  SPEAKER_04: Within that rim decor.
• 22:02
  SPEAKER_03: That's right.
• 22:02
  SPEAKER_03: Young decore, two ages.
• 22:04
  SPEAKER_03: Yep.
• 22:05
  SPEAKER_03: Pretty clean.
• 22:05
  SPEAKER_03: Igneous, and the second one's metamorphic.
• 22:08
  SPEAKER_03: There's another scenario.
• 22:09
  SPEAKER_03: Let's say we take this igneous rock, it crystallizes, and then a billion years later, a magma intrudes, a really hot magma intrudes right nearby.
• 22:21
  SPEAKER_03: That magma will intrude.
• 22:22
  SPEAKER_03: We've talked about this in our Camp Geo Physical Geology Intro to Physical Geology Audio Textbook on our app.
• 22:29
  SPEAKER_03: That process of assimilation where the new magma starts to eat away and dissolve the existing rocks or the rocks around it.
• 22:38
  SPEAKER_03: That can happen.
• 22:39
  SPEAKER_03: And it can happen such that the magma, the new magma will dissolve the entire rock.
• 22:44
  SPEAKER_03: Right.
• 22:46
  SPEAKER_04: Completely reset the clock, then.
• 22:48
  SPEAKER_04: Completely reset everything.
• 22:49
  SPEAKER_03: That is like dissolve everything, assimilate all the ions and the atoms and move them around and just reincorporate them.
• 22:56
  SPEAKER_03: Or zircon's a really resilient mineral, and some magmas don't have the chemical power to dissolve the zircon.
• 23:03
  SPEAKER_03: So they'll dissolve everything but the zircon from that older rock.
• 23:08
  SPEAKER_03: And then you have these little zircon grains that are floating around in a new magma.
• 23:12
  SPEAKER_03: And what happens?
• 23:12
  SPEAKER_03: Well, they serve as nucleation points for new zircon growth, but that new zircon growth will not be metamorphic but will be igneous because they're growing from a magma.
• 23:22
  SPEAKER_03: Yes.
• 23:23
  SPEAKER_03: And in that scenario, you can kind of get the same upper and lower age, you can get the same uranium-led distribution and the same kind of look, but we're dealing with two very different processes there.
• 23:32
  SPEAKER_04: Now, this this process that you're describing has a name, right?
• 23:37
  SPEAKER_04: You you guys, you you people, you call this inheritance.
• 23:42
  SPEAKER_04: No sentence has ever started out, or no good sentence has started out with the term U.
• 23:46
  SPEAKER_04: I mean it in the best possible way, Jess.
• 23:48
  SPEAKER_04: Come on.
• 23:48
  SPEAKER_04: Um But you're talking about inheritance.
• 23:51
  SPEAKER_04: Okay.
• 23:52
  SPEAKER_04: And in the example that you just uh said, there are different kinds of inheritance you can get.
• 23:57
  SPEAKER_04: You just talked about the inheritance of a xenochrist.
• 24:00
  SPEAKER_04: That's spelled with an X, Xenochrist, okay, which is Xenochrist.
• 24:04
  SPEAKER_04: It's a mineral, it's a mineral that that the magma inherited from something that was older.
• 24:11
  SPEAKER_04: And as you said, it the magma didn't have enough heat, it didn't have enough thermal energy to dissolve this zircon grain.
• 24:18
  SPEAKER_04: And so it it kind of inherited it.
• 24:21
  SPEAKER_04: And the minerals that grow out of this magma as being younger than the grain it inherited.
• 24:28
  SPEAKER_04: Okay, that Xenocrist.
• 24:30
  SPEAKER_04: And then there's another.
• 24:31
  SPEAKER_04: Oh, go ahead.
• 24:32
  SPEAKER_03: What would you say the age of that rock is?
• 24:35
  SPEAKER_03: That that first model that we talked about, where it's not it, it's not really a Xenocryst, but the the rock, we have an igneous rock, it's crystallized, and then it gets metamorphosed later on.
• 24:44
  SPEAKER_03: Which age?
• 24:45
  SPEAKER_03: We have two zircon ages.
• 24:46
  SPEAKER_03: We talked about how that would produce that.
• 24:48
  SPEAKER_03: Which of those ages is the age of the rock to you?
• 24:52
  SPEAKER_04: Oh, the one that the protolith.
• 24:55
  SPEAKER_04: The the age of the xenocrist.
• 24:57
  SPEAKER_03: Yes, yes.
• 24:58
  SPEAKER_03: The the age of in that case, it's I don't know the Xenocris term probably would not really apply because it's not actually foreign.
• 25:05
  SPEAKER_03: It's just you know, it's just the the primary, it's a leftover primary fragment.
• 25:09
  SPEAKER_03: But what would be the age in the second scenario, which we're we've also been talking about, with the Xenocryst, where this zircon grain, the whole rock gets dissolved, but the zircons don't, and they kind of float around in the magma and become xenocrists.
• 25:21
  SPEAKER_03: What's the age of the rock then?
• 25:23
  SPEAKER_03: The first age or the second age?
• 25:24
  SPEAKER_03: Oh, yeah.
• 25:26
  SPEAKER_03: In your mind, the second age, okay, in my in my mind.
• 25:28
  SPEAKER_03: Okay, explain that why why you're exactly right.
• 25:31
  SPEAKER_03: Then this is like this is we're spending a lot of time on this because this is the crux of the argument on the oldest rocks in the United States.
• 25:38
  SPEAKER_03: So we're gonna get to that point.
• 25:40
  SPEAKER_03: We're like banging out about Zenokris here, but this is really the crux of the argument.
• 25:44
  SPEAKER_03: So why is the younger age the correct, quote unquote, correct age for that particular rock?
• 25:50
  SPEAKER_04: Because that particular rock would be an igneous rock.
• 25:53
  SPEAKER_04: It's a new rock at that point.
• 25:55
  SPEAKER_04: Whereas a metamorphic rock is not a new rock, it's a rock that was a rock before.
• 26:02
  SPEAKER_04: And that's the distinction that I draw.
• 26:03
  SPEAKER_03: So yeah, you're exactly right.
• 26:05
  SPEAKER_03: And to me, this is a very subtle difference and one that's hard to explain.
• 26:09
  SPEAKER_03: So do you feel the same way?
• 26:10
  SPEAKER_03: Or no, I don't.
• 26:11
  SPEAKER_03: I never thought about it that way, actually.
• 26:13
  SPEAKER_04: I never but I I see what you're saying now, and I think uh I I I I can't help because I I teach young people, so I can't help but think of that that might be a point of confusion for them that is not for me in my own mind, and I need to do a better job of parsing that out.
• 26:30
  SPEAKER_04: Yeah, yeah.
• 26:31
  SPEAKER_03: Okay, so you but you feel like this is we've we've for this podcast, we've done a pretty good job of describing those kind of two scenarios.
• 26:37
  SPEAKER_03: We can move past this the Xenocrist inheritance thing.
• 26:39
  SPEAKER_04: Okay, yeah, and I think, but Jesse, we're gonna circle back to inheritance later on because this is a very it's gonna become an important part of our of Jesse coming to a conclusion or what are America's oldest, what are the United States' oldest rocks?
• 26:54
  SPEAKER_04: Okay, so exactly right.
• 26:56
  SPEAKER_04: We need to move on, Jesse, because we are on a timeline here.
• 26:59
  SPEAKER_03: So we've painted two really uh clear end member scenarios for one that we have uh old grain and a metamorphic part, a metamorphic rim and the rock is old, or we have old zircon that's inherited with new growth that's also new igneous growth, and the old part of the grain is not the old age of the rock.
• 27:19
  SPEAKER_03: So it really comes down to how do we tell what's metamorphic zircon growth and what's igneous zircon growth?
• 27:27
  SPEAKER_03: And that's really a sticky one.
• 27:29
  SPEAKER_03: And I don't I don't know what you feel about going super deep into the weeds on these things.
• 27:34
  SPEAKER_03: We probably can stay pretty shallow, Chris, uh, on this one.
• 27:38
  SPEAKER_03: I think we need to.
• 27:39
  SPEAKER_03: What stood out to you?
• 27:40
  SPEAKER_03: What stands out to you about you know the the the difficulty of determining what's igneous and what's metamorphic?
• 27:46
  SPEAKER_03: Or how would you think about saying, okay, how how I've I'm giving a zircon grain, like this image we're showing, image number two.
• 27:51
  SPEAKER_03: How do I tell what's igneous and zircon?
• 27:53
  SPEAKER_03: Okay, igneous and metamorphic.
• 27:56
  SPEAKER_04: I think that the answer to your question depends.
• 27:58
  SPEAKER_04: Um, are you only given the zircon grain?
• 28:01
  SPEAKER_04: Are you talking about that?
• 28:03
  SPEAKER_04: Like, how would you know if that's all you had?
• 28:06
  SPEAKER_04: Or or did you do the work?
• 28:09
  SPEAKER_03: From the same rock.
• 28:10
  SPEAKER_04: Okay, but I'm saying, or did you do the work?
• 28:12
  SPEAKER_04: Did you have the rock and then you separated the zircon out of the rock?
• 28:18
  SPEAKER_04: You know, because then it would become the ladder.
• 28:21
  SPEAKER_03: The the second, yeah.
• 28:22
  SPEAKER_03: So you have the rock, you have the zircons, but you have a zircon that has like, let's say, three, in this case, three different growth zones or four different growth zones to it.
• 28:31
  SPEAKER_03: How do you tell which ones are igneous and which ones are metamorphic?
• 28:34
  SPEAKER_03: Is the question.
• 28:36
  SPEAKER_04: Well, wouldn't the rock that that's the way I feel too, Chris, is oof.
• 28:41
  SPEAKER_04: It's hard.
• 28:41
  SPEAKER_04: And just in my but doesn't the rock help itself, the the background rock that it came from, doesn't that use that apart.
• 28:48
  SPEAKER_03: Yeah, elaborate.
• 28:49
  SPEAKER_04: Well, I mean, if you're if you're given an igneous rock, you extract the zircons out of that, and then you do your thing in the chronology geochronology lab.
• 28:59
  SPEAKER_04: Um isn't that the the biggest clue you have is to whether that's an igneous zircon or a metamorphic zircon?
• 29:07
  SPEAKER_04: I mean, you know, it's to me it seems fairly straightforward, but yeah, it can it can it it can be.
• 29:13
  SPEAKER_03: Some instances it can be pretty clear.
• 29:16
  SPEAKER_03: Um, we have different tools that we can use.
• 29:18
  SPEAKER_03: We can look at the the the how the grain looks, the morphology of it, these images that we see.
• 29:24
  SPEAKER_03: Like we can tell the zer the metamorphic ones are often a little bit darker, they have a different chemistry, different thorium-uranium ratios, for instance.
• 29:31
  SPEAKER_03: Those can change between metamorphic and igneous.
• 29:34
  SPEAKER_03: So, you know, there are chemical clues that we can use, but it's certainly not easy all the time.
• 29:39
  SPEAKER_03: And there are certain rocks that, like in my PhD, when I was looking at really old old rocks that are really disturbed, there are rocks that I just did not feel I could confidently interpret the ages from.
• 29:49
  SPEAKER_03: So I just I'm like, this is a messy rock, it's got five different zircon populations.
• 29:54
  SPEAKER_03: I don't know what's the ignis age, I don't know what our met what the metamorphic ages are.
• 29:57
  SPEAKER_03: So you're kind of like, ah, I don't know.
• 30:00
  SPEAKER_03: Um, and if the rock's really important, then you go deeper, but if it's not, you kind of like, okay, I'm gonna set that rock aside and focus on the ones that I can understand that are a little bit simpler.
• 30:09
  SPEAKER_04: That's really interesting.
• 30:10
  SPEAKER_04: So do you see down the line with better technology and more information, just more knowledge, a better process.
• 30:21
  SPEAKER_04: Do you see that happening where that rock will actually be able to be interpreted?
• 30:26
  SPEAKER_03: I think so.
• 30:26
  SPEAKER_03: Yeah, there's always new techniques being developed, and we'll we'll come back to this a little bit when we talk about these individual rocks, because I actually have this whole exercise of going through the you know, this this paper by Carol Frost and co-authors talking about this this issue, it really brought up some potential research directions to me.
• 30:43
  SPEAKER_03: Like we have techniques we're developing in our lab that I think we could apply to these zircon grains and help understand, uh help shed some light or a little bit of clarity onto these arguments about what are the oldest rocks here.
• 30:55
  SPEAKER_03: So I do think there's paths forward for sure.
• 30:58
  SPEAKER_03: Um, I'm gonna mention the th I mentioned it briefly, thorium-uranium uh ratio.
• 31:02
  SPEAKER_03: I'm gonna mention that again just really quickly because we'll I there's a plot coming up that shows this when we talk about the oldest rocks, the candidate oldest rocks.
• 31:10
  SPEAKER_03: This is a really rough tool.
• 31:12
  SPEAKER_03: This is like a sledgehammer size tool.
• 31:15
  SPEAKER_03: It's a really blunt force instrument, but thorium-uranium ratio in metamorphic rocks, metamorphic zircons typically have a lower thorium-uranium ratio.
• 31:26
  SPEAKER_03: Igneous zircons typically have a higher thorium-uranium ratio.
• 31:31
  SPEAKER_03: And you know, we kind of use this like 0.1 number as like a dividing line, but there's a huge spectrum in between these things, and it's not a diagnostic tool, but it has been used, and I'll show I show another example of this later on.
• 31:44
  SPEAKER_03: So, should we move into the talking about the three rocks and and settle on which one's the oldest?
• 31:51
  SPEAKER_04: Yeah, I I think we should.
• 31:52
  SPEAKER_04: I absolutely that's a good segue into you know, is this the Morton Nice?
• 31:58
  SPEAKER_04: Is this the the white rocks from the Wyoming province, or is this the Watersmeat from from Michigan?
• 32:03
  SPEAKER_04: Now, I I'm biased already.
• 32:05
  SPEAKER_04: You know, just know what your vote's gonna be.
• 32:08
  SPEAKER_01: Like with zero information, I know what your vote's gonna be.
• 32:11
  SPEAKER_03: Um, this this image number three here, Chris, just a quick map to frame us here where these places are, the Sakawee Gnice out in the Wyoming province.
• 32:19
  SPEAKER_03: We have the Minnesota River Valley um province with the Morton Gnice, and then you said the Watersmeet Gnice in um upper northern uh the western upper peninsula, Michigan.
• 32:28
  SPEAKER_03: Those two latter ones, the Morton Gnice and the Watersmeet Gnice, are both part of the Superior province, which is this huge Canadian um shield province that's all 2.5 billion years and older.
• 32:38
  SPEAKER_03: Let's focus on the most famous ones, probably, which spoil alert are probably not the oldest rocks that we have.
• 32:46
  SPEAKER_03: And this is the Morton Gneisses in the Minnesota River Valley.
• 32:51
  SPEAKER_03: They're really complicated.
• 32:52
  SPEAKER_03: I'm showing here image number four.
• 32:54
  SPEAKER_03: I just took some data from some of these papers that were published on them, and I'm plotting them with igneous and metamorphic thorium uranium ratios.
• 33:02
  SPEAKER_03: So I've color-coded them differently.
• 33:04
  SPEAKER_03: The igneous, quote unquote, thorium uranium ratio ones are all older.
• 33:08
  SPEAKER_03: There's two populations, one up around 3.7 billion years old, 3.6 billion, one around 3.3 billion years old, and then there's a bunch of metamorphic, metamorphic thorium uranium ratio grains that are much lower down 2 billion year events, 2.5 billion year old events.
• 33:25
  SPEAKER_03: So really complicated, not easy to interpret.
• 33:28
  SPEAKER_03: These rocks, quick story, spoke before, and we showed last episode a sign that like said these are Ursulus rocks, 3.8 billion years old.
• 33:36
  SPEAKER_03: That number 3.8 comes from some 1960s and 1970s era whole rock rubidium stratium gyro chronology, which is probably inaccurate.
• 33:46
  SPEAKER_03: Uh, we can say is is probably not the right age.
• 33:48
  SPEAKER_03: It's been a disturbed um isochron age there.
• 33:52
  SPEAKER_03: Zircon work in the 80s also suggested that they could be 3.6 or maybe even 3.7 billion years old, but at least 3.2.
• 34:01
  SPEAKER_03: And then in 2006, further refinement with this big, we showed a photo of this in our Ursulus Rock book in um in our Jack Hill's Zircon sections, the ion microprobe, the secondary ion mass spectrometer, this big room shape, this huge instrument takes up a whole room that you can put tiny pinpricks in, measure the ages very precisely.
• 34:19
  SPEAKER_03: That type of work went up went in and detailed and looked at the grain shapes, the grain character, the grain chemistry, and the grain ages, and teased out these different events.
• 34:30
  SPEAKER_03: So these two populations, the interpretations there.
• 34:33
  SPEAKER_03: This is like Bickford et al.
• 34:35
  SPEAKER_03: 2006 paper, the oldest grains, the oldest two populations, 3.3 and 3.5, have igneous thorum uranium ratios.
• 34:46
  SPEAKER_03: So the conservative interpretation there is that the younger population is the oldest magmatic age.
• 34:53
  SPEAKER_03: And then the oldest population that sees through the metamorphism.
• 34:58
  SPEAKER_03: Yes, that age, the age of the rock through metamorphism, and then the older population of grains, the 3.5, 3.6 population of grains, those would be then inherited.
• 35:08
  SPEAKER_03: So those are grains that were dissolved.
• 35:10
  SPEAKER_03: The the rock, their parent rock was dissolved, and those grains were floating around in a magma and were inherited as xenochris.
• 35:17
  SPEAKER_03: So coming back to that inheritance, why we talked so long about inheritance, it helps inform this.
• 35:23
  SPEAKER_03: So, Chris, what do you think about this argument that the Morton Gneiss in the Minnesota River Valley is not the oldest rock in the world because the old grains are actually inherited?
• 35:32
  SPEAKER_04: I mean, to me, that's pretty cut and dry.
• 35:35
  SPEAKER_04: I I you know I think you're happy that you're happy with that.
• 35:38
  SPEAKER_04: I am happy.
• 35:38
  SPEAKER_04: I'm happy with that.
• 35:39
  SPEAKER_04: Yeah.
• 35:40
  SPEAKER_04: Is uh is the sign still there?
• 35:41
  SPEAKER_04: I think it is.
• 35:43
  SPEAKER_04: The sign, I believe so.
• 35:44
  SPEAKER_04: I I I've not driven up that maybe in pretty long time.
• 35:47
  SPEAKER_04: Like so, you know, I just a real quick thing about that.
• 35:50
  SPEAKER_04: That happens all the time.
• 35:52
  SPEAKER_04: Um, there are things that we don't early on, there were mistakes that were made, and we can't we continue to to you know, we're we're continuing to get better.
• 36:00
  SPEAKER_04: That's that's the process of signs anyway.
• 36:02
  SPEAKER_04: Um so it doesn't that's true.
• 36:04
  SPEAKER_04: But but this that really irks me in a way because it leads to this a still this perception that's wrong that's not correct, right?
• 36:15
  SPEAKER_04: And and we I see this all the national parks have the same thing.
• 36:18
  SPEAKER_03: Um I know if you were the mayor of Hudsonville, Michigan, your town, and Hudsonville, Michigan had an old claim to Earth's oldest rock and a sign out front, would you be the guy who says we're gonna take it down because you know the scientists don't agree with us anymore?
• 36:32
  SPEAKER_03: You would.
• 36:33
  SPEAKER_03: I would.
• 36:34
  SPEAKER_03: I'd be I would be you know, I don't think the town would revolt if you took that sign down.
• 36:38
  SPEAKER_04: That's why I would never be mayor, because I would never get elected.
• 36:43
  SPEAKER_04: Okay, good.
• 36:44
  SPEAKER_04: You you do very unpopular things.
• 36:46
  SPEAKER_03: I do.
• 36:47
  SPEAKER_04: I do try to do what's right, you know.
• 36:50
  SPEAKER_04: I I don't know.
• 36:51
  SPEAKER_04: I don't oh fair enough, fair enough.
• 36:53
  SPEAKER_04: So let's move on to the next one, the the Wyoming province.
• 36:58
  SPEAKER_04: Walk us through the age of of these rocks and and why these are not considered to be the oldest rocks in the US.
• 37:07
  SPEAKER_03: So these these rocks are interesting ones.
• 37:09
  SPEAKER_03: So I'm gonna look at there's two different categories of rocks from the suite of of rocks, these ortho gneisses in the Wyoming province.
• 37:17
  SPEAKER_03: Some are what they call banded gneisses and some are what they call foliated gneisses.
• 37:21
  SPEAKER_03: And the difference is pretty dramatic here and really important.
• 37:24
  SPEAKER_03: I love it.
• 37:26
  SPEAKER_03: I'm showing two examples.
• 37:27
  SPEAKER_03: These are from the Acosta Nice complex, because that's my where I've done a lot of field work, so I have photos of the hold on.
• 37:32
  SPEAKER_04: Can I I'm sorry to interrupt you, Jesse, but can uh just want the if if the listeners are looking at this right now, which one do you think is more complicated?
• 37:41
  SPEAKER_04: Which one do you think is harder to interpret before we get to that?
• 37:45
  SPEAKER_04: Would it be the banded nice or what we call that that's like a migmatite, right, Jesse?
• 37:51
  SPEAKER_04: Um, or just this the foliated nice.
• 37:54
  SPEAKER_04: Which one is more difficult to interpret?
• 37:57
  SPEAKER_04: Okay, it's a great question.
• 37:59
  SPEAKER_04: Jesse.
• 37:59
  SPEAKER_04: The go.
• 38:00
  SPEAKER_04: The teacher in me just came out.
• 38:01
  SPEAKER_03: So it's a great question.
• 38:03
  SPEAKER_03: And the answer is the the easier one is clearly the foliated gneiss.
• 38:07
  SPEAKER_03: I mean, you look at that rock and you're like, okay, it's relatively simple, it's got some fabric, but it's not this complicated, messy migmatite thing.
• 38:14
  SPEAKER_03: And the banded gneiss is more complicated.
• 38:16
  SPEAKER_03: You could imagine three, four, five populations of zircon grains in this banded gneiss.
• 38:21
  SPEAKER_03: Um, in this block, both are present uh and they kind of help inform each other.
• 38:27
  SPEAKER_03: So um the simpler one, the foliated gneiss, they both it's very similar actually, Chris, to the the uh Morton gneisses.
• 38:34
  SPEAKER_03: There are two populations of zircons in these rocks.
• 38:37
  SPEAKER_03: One is a cluster at 3.4 billion years old, one is a cluster, several analyses really more than a cluster, at 3.82 billion years old.
• 38:46
  SPEAKER_03: So really old, 3.82.
• 38:48
  SPEAKER_03: However, the interpretations are that this 3.82 age is actually xenochrystic or inherited as well, because the 3.4 billion year old grains look igneous.
• 39:01
  SPEAKER_03: So again, this is an inheritance.
• 39:04
  SPEAKER_03: There there used to be 3.8 billion year old rocks present in the Wyoming province, but we don't know that there are still.
• 39:11
  SPEAKER_03: They were probably dissolved, and the zero rocks are the only thing that survived, exactly, completely reset.
• 39:18
  SPEAKER_03: So, Chris, you must be happy inside because we're moving to the waters of the ice in Michigan.
• 39:24
  SPEAKER_03: I am, I'm happy.
• 39:25
  SPEAKER_03: Probably has the United States oldest rocks that you can walk across right now that we know of at the moment.
• 39:32
  SPEAKER_03: They were originally known to be kind of quite old as soon as we could, you know, people could do geochronology, but only quote unquote 3.4 billion years old.
• 39:40
  SPEAKER_04: Um, I'm gonna get this image there.
• 39:42
  SPEAKER_04: There we go.
• 39:42
  SPEAKER_04: Yeah, there you go.
• 39:43
  SPEAKER_04: I have a bone to pick with you on this.
• 39:45
  SPEAKER_04: Um, because we were standing right here, you and I together, and we we you were explaining this machine to me.
• 39:53
  SPEAKER_04: Just for it.
• 39:54
  SPEAKER_04: I know.
• 39:54
  SPEAKER_04: I I I I uh thank you for the listeners.
• 39:57
  SPEAKER_04: We're looking at an image of my lab.
• 39:59
  SPEAKER_04: But why you should have said, Chris, go stand in front of that and and let's take a picture of that.
• 40:05
  SPEAKER_04: And that would have been a oh that needs a bull ice in the middle of my lab photo.
• 40:09
  SPEAKER_04: That's ridiculous.
• 40:10
  SPEAKER_04: We need scale, Jesse.
• 40:12
  SPEAKER_04: We need scale.
• 40:12
  SPEAKER_04: Geology photos always need we need awkwardly standing humans.
• 40:16
  SPEAKER_03: That that's that's a that's a good point.
• 40:18
  SPEAKER_03: There are no awkward humans in this.
• 40:20
  SPEAKER_03: But this is a a lab of my the geochemistry lab uh that that we run here at Penn State.
• 40:25
  SPEAKER_03: It's a laser ablation lab.
• 40:26
  SPEAKER_03: So we use a laser, the center instrument here is a laser sampling tool, and it's basically produces a little tiny laser beam that when it hits, we we point that laser beam at mineral grains, and then the laser blows apart the mineral grains and creates a gas.
• 40:43
  SPEAKER_03: So it ablates them, it turns it from a solid to a gas.
• 40:46
  SPEAKER_03: So it takes the zirconium atoms, the silicon atoms, the oxygen atoms, the uranium, thorium, lead atoms, all get turned into a gas, and then the get we sweep, we pass gas, a little bit of argon and nitrogen over that.
• 40:59
  SPEAKER_03: I need kick out of that.
• 41:00
  SPEAKER_03: We pass gas over the sample, Chris, and then we analyze in the the bigger instruments where there's a small one on the table and a big one right in front of us.
• 41:09
  SPEAKER_03: Those are plasma mass spectrometers.
• 41:11
  SPEAKER_03: So we inject those atoms into this plasma, super high, 7,000 Kelvin plasma, everything gets ionized, and then we then as soon as they're ions, we can steer them with electrical fields and pass them through a magnetic field and measure the uranium and the lead and separate out lead and uranium isotopes from each other.
• 41:27
  SPEAKER_03: So this is the type of measurement that has been done on these water meet nice waters meet gneisses, and there's a big age cluster at 3.5 or 3.6, and then another cluster down near 2.6 billion years old.
• 41:41
  SPEAKER_03: For these rocks, I'll turn to our last image, which is the waters meet gneiss.
• 41:45
  SPEAKER_03: And if you regress a line through those, it's a relative, it's kind of like that that image I showed in number one with these relatively simple linear regression.
• 41:54
  SPEAKER_03: If you regress a line through there, you get an upper intercept age of 3.68 billion years old.
• 42:01
  SPEAKER_03: A little bit more conservative estimate, or the most conservative estimate for the age, would be 3.62 billion years old.
• 42:08
  SPEAKER_03: So that's 3.68 or 3.62.
• 42:11
  SPEAKER_03: The answer is probably somewhere in the middle there.
• 42:13
  SPEAKER_03: Uh, but that's almost certainly the original seeing through metamorphism age of this rock from the Watersmeat Knights.
• 42:20
  SPEAKER_03: And because the Morton Gnice, and because the Sakawee block in the Wyoming province, because we think those are inherited, those 3.8 and 3.7 grains are inherited, that makes this the oldest rock in the United States.
• 42:32
  SPEAKER_04: Okay.
• 42:33
  SPEAKER_03: Which only came about because it's Michigan.
• 42:36
  SPEAKER_04: And this only came about this kind of shifting of where are the or which is actually the oldest rock in the United States.
• 42:43
  SPEAKER_04: That came about because of knowledge.
• 42:44
  SPEAKER_04: That came about because of refinement of understanding uh you know what was inherited and what was not.
• 42:50
  SPEAKER_03: You know, and Chris, for me, it shows the value of these technological advancements, like this laser ablation system.
• 42:56
  SPEAKER_03: This is a uh 20-year-old development, and in the last 15 or 10 years, we've seen rapid increases in the development of these laser ablation tools.
• 43:03
  SPEAKER_03: Same with ion microprobes.
• 43:05
  SPEAKER_03: You know, we could only interrogate these things because of these tools, the the technical development of these geochemistry tools.
• 43:12
  SPEAKER_03: And I'm in I'm excited to, I want to kind of pursue a research project on this because we can now do things like depth profiling, which we're doing in our lab a lot, where we can take the grains instead of putting a little spot.
• 43:23
  SPEAKER_03: I'm gonna go back to this last image of the zircon grain.
• 43:26
  SPEAKER_03: Instead of putting a spot in one particular growth zone, we can actually turn the grain on its side and drill through it and measure the data constantly.
• 43:34
  SPEAKER_03: As we just oh my gosh, that's incredible.
• 43:37
  SPEAKER_03: So you can kind of it's like coring through a tree, you can measure the tree ring layers a little bit better there, right?
• 43:43
  SPEAKER_03: And so we could do this with these grains and say, oh, we're in igneous, oh, we're in metamorphic, oh, we're in igneous, and we could see maybe those four growth domains.
• 43:52
  SPEAKER_03: Um, and I think that'd be that might answer some of these questions a little or add a little clarity um to what the oldest are, actually.
• 44:00
  SPEAKER_04: So wow, yeah.
• 44:01
  SPEAKER_03: Very cool.
• 44:02
  SPEAKER_03: Anyway, lots of stuff to do.
• 44:03
  SPEAKER_03: Yep.
• 44:04
  SPEAKER_04: The Jesse took a stand, everyone.
• 44:06
  SPEAKER_04: Jesse took a stand.
• 44:07
  SPEAKER_03: Well, I'm basing this on the great work by Carol Frost and others in that paper that just came out, but um, we'll put a link to that in the show notes.
• 44:14
  SPEAKER_03: But this is uh the oldest rocks in the United States are probably the Waters Meet Nices in Michigan.
• 44:18
  SPEAKER_03: So, you know, there we go.
• 44:20
  SPEAKER_03: Hey, that's a wrap.
• 44:21
  SPEAKER_03: You can find this and all of our other content in our Earth's oldest rocks visual podcast series on our Camp Geo Mobile app.
• 44:26
  SPEAKER_03: First link in your show notes.
• 44:27
  SPEAKER_03: Check it out there.
• 44:28
  SPEAKER_03: We also have tons of content as well in other visual podcast series and our full Camp Geo content that basically introduced geosciences.
• 44:36
  SPEAKER_03: So if you want to learn, get get some uh intro knowledge on the geos geosciences and stay out of the weeds a little bit, you can go there.
• 44:42
  SPEAKER_03: First link in the show notes, send us an email, contact us linked on our website, my geocaps.com, or hit like, subscribe on YouTube, and hit the button button.
• 44:50
  SPEAKER_03: Cheers.