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Are These Earth's Oldest Rocks? · PlanetGeo: The Geology Podcast

• 0:02
  SPEAKER_01: Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you.
• 0:14
  SPEAKER_01: I got the I got Chris full screen view.
• 0:16
  SPEAKER_01: Chris Boy's taking up most of my most of my real estate here on my monitor.
• 0:20
  SPEAKER_01: That's good.
• 0:21
  SPEAKER_01: Apparently that's recording.
• 0:23
  SPEAKER_02: As soon as I jumped on, you started ripping on me about my outfit.
• 0:26
  SPEAKER_01: Like I've no.
• 0:28
  SPEAKER_01: I was wrong though, because this is your Boston Marathon outfit.
• 0:30
  SPEAKER_01: So you know I was wrong.
• 0:31
  SPEAKER_02: Well, it's not an outfit.
• 0:33
  SPEAKER_02: It's a Boston marathon quarter zip, you know, and you started ripping on me.
• 0:37
  SPEAKER_01: Well, here's why I was ripping on you, Chris, because it goes along well with your coach glasses.
• 0:42
  SPEAKER_01: You know, your sunglasses, the wraparound sunglasses.
• 0:44
  SPEAKER_01: You're in coach mode when you got the the quarter zip and the glasses on.
• 0:48
  SPEAKER_01: Coach Bull.
• 0:50
  SPEAKER_02: Actually, you referred to them as my gym teacher um oh right.
• 0:54
  SPEAKER_02: Yeah.
• 0:54
  SPEAKER_02: Sunglasses, which should be offensive.
• 0:57
  SPEAKER_02: You should not actually ever say that because that was what your mom did for quite a long time, actually.
• 1:02
  SPEAKER_02: So that's yeah, that I think that gives me permission.
• 1:04
  SPEAKER_02: No, I don't think permission to rip them.
• 1:07
  SPEAKER_02: Oh I think she's gonna be offended by that.
• 1:10
  SPEAKER_01: Well, Chris, I remember you uh you coached track when I was in high school, and uh for a long time, for a long time you did afterwards too.
• 1:16
  SPEAKER_01: And uh you would always go this is where we thought we had intense Mr.
• 1:21
  SPEAKER_01: Bull Heist during geology class, but that's when really intense Mr.
• 1:24
  SPEAKER_01: Boyce came out was during track season.
• 1:26
  SPEAKER_01: And I remember it's a spring sport, and uh we'd everybody would, you know, spring break would happen in between track season, like I don't know, a third of the way into the season, and everybody'd go and wouldn't work out and you would always check everybody's heart rate.
• 1:38
  SPEAKER_01: I remember we'd do like, I don't know, a 400-meter run or something, and then you'd go around and and check everybody's heart rate and make sure that it wasn't too high.
• 1:45
  SPEAKER_01: And after track season, or excuse me, after spring break, uh, if people had a whole bunch of high heart rates, you got really angry at us one year.
• 1:52
  SPEAKER_00: I think it was my senior year, a whole bunch of people didn't do any running, and you were like, You guys didn't do any running, get back in shape.
• 1:59
  SPEAKER_00: We're gonna have a terrible season.
• 2:02
  SPEAKER_02: I did, because uh what I was checking was how how quickly your pulse would return back down to an acceptable level.
• 2:09
  SPEAKER_02: Yeah, and we were just getting into shape, and then after spring break, I I have you do something, and your heart rates were staying up there for 15 minutes.
• 2:17
  SPEAKER_02: You guys didn't do a thing.
• 2:19
  SPEAKER_01: You didn't do any work at all.
• 2:20
  SPEAKER_01: And I my I I have a uh very low heart rate, a very slow heart rate, uh, even on a good day, even when I'm working out.
• 2:26
  SPEAKER_01: And so I was a pretty terrible monitor of weather of everybody else.
• 2:32
  SPEAKER_02: That's right.
• 2:32
  SPEAKER_02: I remember that actually.
• 2:33
  SPEAKER_02: Yeah, well, plus you were just incredibly slow, so it was terribly slow.
• 2:39
  SPEAKER_01: You were so slow.
• 2:41
  SPEAKER_01: I what did I run?
• 2:42
  SPEAKER_01: I ran like this 300 meter intermediate hurdles.
• 2:45
  SPEAKER_01: Well, I was on the four by four hundred relay, I think, too, but I was the worst leg.
• 2:49
  SPEAKER_01: Like, what is it?
• 2:49
  SPEAKER_01: Did you put the the slowest person in second on the relay or second or third?
• 2:53
  SPEAKER_02: Yeah, yeah.
• 2:54
  SPEAKER_02: Yeah, that's where I was.
• 2:55
  SPEAKER_01: I was I was by far the slowest of the four.
• 2:58
  SPEAKER_01: I wasn't anchoring anything.
• 3:00
  SPEAKER_02: Yeah.
• 3:01
  SPEAKER_02: Oh man.
• 3:02
  SPEAKER_02: Yeah, the the the the Chris that you saw there is because you have 120 guys on your team, you know it's a large team, and you have just so many things going on at the same time.
• 3:11
  SPEAKER_02: So you had intense Chris at that time.
• 3:13
  SPEAKER_01: Intense, intense mr Mr.
• 3:15
  SPEAKER_01: Boyce at the time.
• 3:15
  SPEAKER_01: Yes, Mr.
• 3:16
  SPEAKER_01: Boy.
• 3:17
  SPEAKER_01: So glad I'm done.
• 3:18
  SPEAKER_01: Well, you have intense Dr.
• 3:20
  SPEAKER_01: Rhyming today, I think, about this topic.
• 3:21
  SPEAKER_01: When we're worried about today, a little deep.
• 3:24
  SPEAKER_01: I worry about today.
• 3:25
  SPEAKER_01: Um, scale of one to ten.
• 3:26
  SPEAKER_01: How worried are you?
• 3:27
  SPEAKER_02: I'm a little concerned.
• 3:28
  SPEAKER_02: I mean, I think it's hold on, sorry.
• 3:30
  SPEAKER_02: I tipped my I've got to keep my feet warm and I have my little heater down there and I just tipped it over.
• 3:35
  SPEAKER_02: Um so anyway, yeah, I'm a little worried about it because it like I think this is really interesting stuff.
• 3:41
  SPEAKER_02: And and okay, so just to be forthright, we are going weeds today.
• 3:46
  SPEAKER_02: We're getting in and we're getting in deep.
• 3:48
  SPEAKER_02: So just let's just say that from the from the onset of this episode.
• 3:53
  SPEAKER_02: Um we're diving in.
• 3:55
  SPEAKER_02: You know, this is this is the most recent in and and maybe Jesse, is this the last?
• 4:00
  SPEAKER_02: We might have a little wrap-up uh episode.
• 4:04
  SPEAKER_02: Earth's oldest materials, earth's oldest rocks.
• 4:07
  SPEAKER_02: This is the last of the series, at least that we have planned so far.
• 4:11
  SPEAKER_02: And I think easily the the deepest that we're gonna go.
• 4:16
  SPEAKER_01: You agree?
• 4:16
  SPEAKER_01: I I think so.
• 4:17
  SPEAKER_01: And here can I give you the pitch as to why, Chris?
• 4:20
  SPEAKER_01: Like why I went in the weeds.
• 4:21
  SPEAKER_01: This is this is weedy.
• 4:22
  SPEAKER_01: This is I think very interesting and very important.
• 4:25
  SPEAKER_01: Let me give you a pitch for why I think it's important, and you tell me what you think about it.
• 4:29
  SPEAKER_01: Um, what we're gonna get into, and the reason we're gonna get really into the weeds here in isotope systems, is there is currently a debate since about since 2008, when the first paper came out, there's a debate about what the oldest rocks on our planet are.
• 4:43
  SPEAKER_01: And the debate is not about Acosta.
• 4:45
  SPEAKER_01: We talked, we had two episodes about Acosta.
• 4:47
  SPEAKER_01: We know how old those rocks are, really widely accepted how old those rocks are, that they're 4.02 billion years old.
• 4:53
  SPEAKER_01: So those are one of, if not the oldest rocks on the planet.
• 4:57
  SPEAKER_01: The rocks we're going to talk about today, which are up in northern Quebec, this image here, this first image just shows a map.
• 5:02
  SPEAKER_01: It's on the the east side of the Hudson Bay in what's called the Nuva Wagatuck Greenstone Belt.
• 5:09
  SPEAKER_01: The debate here, why this is so weedy, is because the debate centers on these rocks.
• 5:14
  SPEAKER_01: Are these rocks old, like 4.25 billion years old, or are they younger, more like 3.8 billion years old?
• 5:22
  SPEAKER_01: So the debate is really about these rocks.
• 5:25
  SPEAKER_01: And the question is huge.
• 5:26
  SPEAKER_01: What are the oldest rocks on the planet that we know of?
• 5:28
  SPEAKER_01: Like that's a that's a really important topic, I think, and it's worth getting into the weeds, I think, because of that.
• 5:35
  SPEAKER_01: So I don't know, what's your response, Chris?
• 5:38
  SPEAKER_02: No, I totally agree.
• 5:39
  SPEAKER_02: And just to set the record straight, too, we are gonna settle the debate today.
• 5:43
  SPEAKER_02: We're gonna answer the question for everybody.
• 5:45
  SPEAKER_02: Right, right?
• 5:47
  SPEAKER_02: It's our podcast, it's our podcast, yes, we can.
• 5:50
  SPEAKER_01: These two guys, these two random guys here are gonna answer the debate.
• 5:53
  SPEAKER_01: We're gonna answer this.
• 5:54
  SPEAKER_02: It goes back to 2008 and it's gonna end in 2025 because we're gonna settle it right here today.
• 6:00
  SPEAKER_01: Yeah, exactly.
• 6:01
  SPEAKER_01: Exactly.
• 6:02
  SPEAKER_01: I mean, so how do you think it's worth going?
• 6:06
  SPEAKER_01: I mean, what's your take on that?
• 6:08
  SPEAKER_01: I mean, like, if we're gonna dive into the weeds on a topic, is this a good one?
• 6:11
  SPEAKER_02: Yeah, absolutely.
• 6:12
  SPEAKER_02: There are some really interesting things that we're gonna hit upon later on in the episode when we when we get to the age of the rocks.
• 6:17
  SPEAKER_02: So, what we're gonna do here first is we're gonna talk about the rocks themselves.
• 6:20
  SPEAKER_02: What you know, what what are the rocks involved in this like suite of rocks?
• 6:25
  SPEAKER_02: And we're gonna get a little weedy on that too, but then we're gonna get into the the dating of these rocks, why it's so complicated, why there's a debate, and then we're gonna come up with our answer.
• 6:36
  SPEAKER_01: Yeah, there we go.
• 6:37
  SPEAKER_01: We're gonna give you the answer.
• 6:39
  SPEAKER_01: So um just Chris, this is this is something that when I was in graduate school, this was kind of I started graduate school in 2010.
• 6:46
  SPEAKER_01: So this was this debate was had just started ramping up a little bit when I was in graduate school.
• 6:51
  SPEAKER_01: And I took a class, and then I eventually taught the same class called Precambrian Geology, which was a grad level class, dive into the weeds on pre-Cambrian geology, early earth geology.
• 7:01
  SPEAKER_01: And we had one, it was a three-hour lecture, Tuesday nights, like 6 to 9 p.m.
• 7:07
  SPEAKER_01: I mean, classic fall sleep time for students, right?
• 7:10
  SPEAKER_01: But there was every year they would have we he would have like debates, you know.
• 7:14
  SPEAKER_01: The professor Larry Heeman, who was on my advisory committee, would kind of have these scientific debates where he would pick a student or he'd ask for volunteers, or he'd bring in a faculty member, and they'd kind of take two sides of the debate and they'd kind of make an argument either way.
• 7:27
  SPEAKER_01: And I, when I was a student in the class, I took an Acosta side, so we had this what are Urs Oldest Rock's debate.
• 7:33
  SPEAKER_01: And then all through graduate school, I would like join back in the class and like participate in the debate.
• 7:37
  SPEAKER_01: And sometimes I would take the Acosta side of the argument, and sometimes he would he would like say, No, we're gonna switch it up, you're gonna take the new vlog side of the argument, and we'd have like a debate in front of the students, and you know, students would weigh in and vote and things like that.
• 7:50
  SPEAKER_01: And so it's really this is this type of topic.
• 7:53
  SPEAKER_01: People have this discussion, and I'm perhaps a little biased because I've worked with people who argue that these rocks are quite old.
• 8:02
  SPEAKER_01: My postdoc supervisor was one of the authors on this first paper and subsequent papers arguing that these these what we're gonna call the faux amphibolite are actually 4.25 billion years old.
• 8:13
  SPEAKER_01: So acknowledge that out front.
• 8:15
  SPEAKER_01: Um, but I'm gonna try and give both sides of this argument because I have done that in classes and Jesse.
• 8:20
  SPEAKER_02: So you know what?
• 8:21
  SPEAKER_02: It is okay to have a bias because you know what, it is our podcast, and we're allowed to have that, and it is our you know, it's okay.
• 8:27
  SPEAKER_02: Yeah, we're gonna have it.
• 8:29
  SPEAKER_01: It's not settled science for sure, but what is actually, and so a lot of it comes down to interpretation, and uh we'll show some data and get dive into the weeds on all this.
• 8:37
  SPEAKER_02: So that's like so.
• 8:39
  SPEAKER_02: I didn't know this.
• 8:40
  SPEAKER_02: You actually you've been familiar with the Nuvogatic rocks for the better part of 15 years, actually.
• 8:46
  SPEAKER_02: Maybe maybe that's uh that's right.
• 8:48
  SPEAKER_02: I didn't know that.
• 8:49
  SPEAKER_02: Okay, this is really Jesse.
• 8:52
  SPEAKER_01: I have a piece, I have a I have a piece of it on my desk too.
• 8:55
  SPEAKER_01: It's a beautiful rock, a little, little small piece that Rick Carlson gave me.
• 8:58
  SPEAKER_02: But good deal.
• 8:59
  SPEAKER_02: Um, yeah, so this is gonna be uh obviously, this is your show, this is your realm, it's the world you live in.
• 9:06
  SPEAKER_02: So I'm gonna try to keep up and uh jump in whenever I can, you know.
• 9:10
  SPEAKER_02: But this is this is gonna be your thing.
• 9:12
  SPEAKER_01: So I think this will be fine.
• 9:14
  SPEAKER_01: So image number one, Chris.
• 9:15
  SPEAKER_01: I just wanted to point to this image.
• 9:17
  SPEAKER_01: We're just zooming in on the the rocks in the Hudson Bay area.
• 9:20
  SPEAKER_01: They're kind of folded.
• 9:21
  SPEAKER_01: I mean, they well, they're not kind of folded, they are folded, they're like all of these old rocks, they've been metamorphosed multiple times.
• 9:27
  SPEAKER_01: This whole package has now been tilted and bent, and if you're looking at it on the map view, it looks like a big A, big A frame.
• 9:35
  SPEAKER_01: It's been folded and twisted and gnarled.
• 9:37
  SPEAKER_01: And we're gonna look at a little bit of a geological map gif later on as well.
• 9:41
  SPEAKER_01: But that's what those rocks look like.
• 9:42
  SPEAKER_01: I'm gonna look at image number two here.
• 9:43
  SPEAKER_01: This is a picture from the web, just shows what these again look like.
• 9:48
  SPEAKER_01: This is looking out towards the bay, nice, beautiful day there.
• 9:52
  SPEAKER_01: Layered package of gneisses that are complicated, they're folded, they're twisted.
• 9:57
  SPEAKER_01: We've got light colored ones, we've got dark colored ones, we've got a variety of different colors of gray in here, and this is what the rocks look like.
• 10:04
  SPEAKER_01: They are twisted, gnarled, tortured.
• 10:06
  SPEAKER_02: That's the so the absence of trees, by assume that we're too far north, we're we're well above the tree line here.
• 10:11
  SPEAKER_01: So well above tree line here, yep, yep, exactly right.
• 10:14
  SPEAKER_02: So you get maybe some mosses and grasses and lichens and things like that, and that's the about ex that's the extent of the biology there.
• 10:22
  SPEAKER_01: That's that's exactly right.
• 10:24
  SPEAKER_01: Probably some seabirds and some fish and things like that, um, and some nice ice-scoured outcrops right near Hudson Bay, and then as you go inland, it's like totally covered with lichen and things like that.
• 10:33
  SPEAKER_01: So you know, very typical kind of tundra rocks.
• 10:37
  SPEAKER_02: Well, let's let's just jump into talking about the rocks then.
• 10:40
  SPEAKER_02: Um, you know, when I look at this, then these look very similar to the Issuwa rocks that we talked about in the previous episode.
• 10:46
  SPEAKER_02: In fact, a lot of the rocks themselves are, I mean, we're back to greenstones again, right?
• 10:51
  SPEAKER_02: For for one thing.
• 10:53
  SPEAKER_02: We're back to Biffs.
• 10:54
  SPEAKER_02: We have a couple of other things that are going on with them, but they look really similar.
• 10:58
  SPEAKER_02: I mean, we have pillow lavas, and you know, it's like uh this is uh a repetitive theme here with these super, super old rocks.
• 11:07
  SPEAKER_01: That's exactly right, Chris.
• 11:08
  SPEAKER_01: And and it's a weird thing because the deformation changes, and especially if you I'm gonna skip ahead here to the image that has a geological map, and again, this is a folded belt.
• 11:18
  SPEAKER_01: And one of the weird things, or not one of the weird things, one of the things we know pretty well is that if you want to find, if you're in a folded area, rocks are big fold structure.
• 11:28
  SPEAKER_01: If if you want to find the least deformed rocks, the ones where cross-cutting relationships might be preserved, you don't go to the limbs of the fold, the edges of the fold.
• 11:39
  SPEAKER_01: Um, you go to the hinge.
• 11:41
  SPEAKER_01: The hinge point is where things can be preserved a little bit better because they the the limbs of the fold often get stretched and like kind of pulled like putty.
• 11:49
  SPEAKER_01: I don't know what what a good analogy for this is, Chris.
• 11:53
  SPEAKER_01: But you preserve stuff in the hinge a little bit better.
• 11:56
  SPEAKER_01: Meaning if you take your sheets of stacked sheets of paper and you fold the edges down, the the part in the bend is going to be a little bit better preserved.
• 12:03
  SPEAKER_01: And so you can just you just kind of have to walk around these rocks to find areas that there's a little bit better preservation than that.
• 12:11
  SPEAKER_02: Which is pretty much true.
• 12:13
  SPEAKER_02: That's pretty much true of all geology, right?
• 12:15
  SPEAKER_02: If you're doing geology in the field, you just have to walk the contact and you have to find the contact and you're just walking back and forth.
• 12:22
  SPEAKER_02: It's a very time-intensive and it's a it's a step-intensive process for sure.
• 12:27
  SPEAKER_01: So, Chris, do you find this?
• 12:29
  SPEAKER_01: Do you have to?
• 12:30
  SPEAKER_01: I mean, I I noticed this with the undergrads in field camp, and I'm wondering if you've noticed the same thing with the high schoolers.
• 12:35
  SPEAKER_01: Do you notice that the instinct, the human instinct is to when you walk up to a big outcrop, let's say, the instinct is to just stay in one spot and zoom in really detailed on one spot and not walk the outcrop.
• 12:46
  SPEAKER_01: Do you always have to remind students to walk the area first?
• 12:51
  SPEAKER_02: A hundred percent.
• 12:52
  SPEAKER_02: Yeah, they wanna they want to stay in that one spot.
• 12:55
  SPEAKER_02: I I think I understand why.
• 12:57
  SPEAKER_02: I don't think it's just laziness, I don't want to walk anymore, you know.
• 13:00
  SPEAKER_02: I'm done.
• 13:01
  SPEAKER_02: I don't think that's it.
• 13:02
  SPEAKER_02: I think that they don't understand the scale of geology.
• 13:06
  SPEAKER_02: These are young students in in my in my classes.
• 13:09
  SPEAKER_02: These are these are not advanced well into their geology degrees or anything.
• 13:12
  SPEAKER_02: These are like this is their first true geoscience course.
• 13:16
  SPEAKER_02: So they don't understand how big it is and the necessity then to get out and see how big this outcrop is or this host of rocks.
• 13:23
  SPEAKER_01: Do you think it comes a little bit from an assumption that every outcrop is kind of gonna look the same in the little area and they don't recognize how different different outcrops can look, even though if they're the same rock?
• 13:32
  SPEAKER_02: Perhaps I think that would be shocking because when you're approaching the outcrop, you know, you're driving in a van, or you, you know, in my case, I'm I'm in a bus or a van, and you you're pointing out things out as you go to the outcrop where you're gonna actually stop and have them get out, and so on.
• 13:48
  SPEAKER_02: And and how could they, with the diversity of outcrops that you see, you know, going 55 miles an hour, how could you assume that?
• 13:56
  SPEAKER_02: But perhaps, perhaps that's the case.
• 13:58
  SPEAKER_01: I don't know.
• 13:59
  SPEAKER_01: Is that what you think?
• 14:00
  SPEAKER_01: Uh that's kind of what I think is just like you say, oh, we're we're looking at a sandstone.
• 14:04
  SPEAKER_01: They just kind of think, oh, the sandstone's gonna look the same all the way across the sandstone, at least that you know, new instinct.
• 14:09
  SPEAKER_01: And I think this is a skill that it requires being in the field to learn how to do this, to to really like first thing I do is okay, I know this is the outcrop that I'm gonna come back to and focus on, but I'm gonna look at every little outcrop I can that's around here and get a f a full sense of of these rocks, and then come back.
• 14:25
  SPEAKER_01: And maybe I'll find a better outcrop somewhere else or a little piece of information somewhere else that helps inform what I'm looking at at the outcrop that I want to look at.
• 14:31
  SPEAKER_02: Anyway, it's a good point.
• 14:32
  SPEAKER_02: It's a good point.
• 14:33
  SPEAKER_02: I mean, field geology when you're first doing this is truly like drinking water out of a fire hose.
• 14:38
  SPEAKER_02: I mean, it it is it's overwhelming to them, and and I get it.
• 14:42
  SPEAKER_02: I remember when it overwhelmed me.
• 14:44
  SPEAKER_02: I I felt like I wanted to just go home, curl up in bed, and and like forget about I it was just yeah, it was very overwhelming.
• 14:56
  SPEAKER_01: And then you know, you have an appreciation for how complicated stuff is and what you need to do to kind of work around that.
• 15:02
  SPEAKER_01: So back to this map.
• 15:03
  SPEAKER_01: Again, this just like last time when I we showed a map of the issues super crustal belt, this is super simplified.
• 15:10
  SPEAKER_01: I have simplified this thing down into a few basic units.
• 15:13
  SPEAKER_01: We have the basic geological map, all the units outlined here.
• 15:16
  SPEAKER_01: We have 3.8 billion year old gneisses.
• 15:18
  SPEAKER_01: These are ones that are pretty easy to date.
• 15:20
  SPEAKER_01: They've got zircon in them, so we know that they're 3.8 billion years old.
• 15:24
  SPEAKER_01: Then we also have here in this unit a gabbro sill, which we're gonna come back to an end on.
• 15:29
  SPEAKER_01: There's a gabbro sill that cross-cuts other stuff.
• 15:33
  SPEAKER_01: Then we have bandanaro formations.
• 15:34
  SPEAKER_01: We'll briefly touch on that.
• 15:35
  SPEAKER_01: That's just mostly to highlight this as a supra crustal package.
• 15:39
  SPEAKER_01: This is a green stone belt, like most other Archean greenstone belts.
• 15:42
  SPEAKER_01: And then we have this last thing called the Ujaraluk formation, which is what we're really gonna focus on.
• 15:47
  SPEAKER_01: That's what might be the really, really old rocks, this Ujaraluck unit.
• 15:51
  SPEAKER_02: Okay.
• 15:52
  SPEAKER_02: I was gonna ask you a question about the SIL, but I'll just I'm gonna save that.
• 15:55
  SPEAKER_02: I'm gonna put it in my pocket and we'll come back to it later on.
• 15:58
  SPEAKER_02: Okay, tuck it away in there.
• 16:00
  SPEAKER_02: Well, I didn't want to say put a pin in it because apparently that's my phrase, and you know, oh, you're working on new phrases.
• 16:05
  SPEAKER_02: I like put it in my pocket.
• 16:08
  SPEAKER_02: Put it in your pocket.
• 16:09
  SPEAKER_02: Hold on, hold on.
• 16:10
  SPEAKER_02: I'm gonna I'm gonna put it in my bill fold.
• 16:14
  SPEAKER_01: What an old man.
• 16:15
  SPEAKER_01: Oh, the bill fold.
• 16:17
  SPEAKER_01: That was so funny.
• 16:18
  SPEAKER_02: I did tell you I got called out on that, right, by one of my students.
• 16:20
  SPEAKER_02: You did, yeah, by your students.
• 16:21
  SPEAKER_02: That's great.
• 16:22
  SPEAKER_02: As you should.
• 16:23
  SPEAKER_02: Did you really say that?
• 16:24
  SPEAKER_02: Are you oh my god?
• 16:25
  SPEAKER_02: Rightfully so, man.
• 16:27
  SPEAKER_02: Oh my goodness.
• 16:28
  SPEAKER_02: So, Jesse, let's talk about some of the rocks then that go along with this host of rocks.
• 16:34
  SPEAKER_02: We've got the MetaBasalts, right?
• 16:36
  SPEAKER_02: We talked about those with the issua.
• 16:38
  SPEAKER_02: So it's a green stone again.
• 16:40
  SPEAKER_02: We have these biff layers, but then we also, Jesse, we have these kind of intermingled tonalites, and this is not tonalite is kind of a niche rock.
• 16:49
  SPEAKER_02: I mean, you don't have this rock.
• 16:51
  SPEAKER_02: Uh uh, do you don't have your beginning students identify tonalites, do you?
• 16:55
  SPEAKER_01: No, no, it's like no, definitely not.
• 16:58
  SPEAKER_01: I mean, it's um it's common, but you don't need to know it that specifically, right?
• 17:02
  SPEAKER_01: Like you're I'm I'm guessing you don't have your intro students to I don't.
• 17:06
  SPEAKER_02: I mean, I mean this is it's a felsic rock.
• 17:08
  SPEAKER_02: It's generally phaneritic in texture, which means coarse grained.
• 17:12
  SPEAKER_02: The minerals are big enough to see in hand sample without any magnification for that for that matter.
• 17:17
  SPEAKER_02: It's got quartz, it's got feldspar or plagioclase feldspars, it has alkali feldspars, and then it has, you know, maybe I don't know, what's the breakdown of mafic minerals in it?
• 17:28
  SPEAKER_02: Maybe up to it.
• 17:29
  SPEAKER_01: Well, yeah, it has more mafic minerals.
• 17:32
  SPEAKER_01: It is a it's more of an intermediate to felsic rock.
• 17:35
  SPEAKER_01: You know, it's an intermediate rock.
• 17:36
  SPEAKER_01: The main thing is that the feldspars in it are mostly plagioclase.
• 17:40
  SPEAKER_01: There's very little potassium-rich felds bar.
• 17:42
  SPEAKER_01: It's a sodium-rich rock, has less potassium in it than a granite.
• 17:46
  SPEAKER_01: A granite will have a lot more potassium in it, and so therefore we'll have more potassium-rich feldspars in it.
• 17:50
  SPEAKER_01: So a tonalite, but it's on the spectrum of granitic rocks.
• 17:54
  SPEAKER_01: Like you put it into this granitic rock or granitoid rock category, you'll find it the outer part of the towalium the intrusive suite in the Sierra Nevadas in Yosemite National Park is tonalite.
• 18:05
  SPEAKER_02: It's a rock similar to diorite, it just has these slight chemical variations that make it not a diorite, instead, it's a tonalite because of the lactose.
• 18:14
  SPEAKER_02: That's right.
• 18:14
  SPEAKER_01: It's got more quartz in it, it's got less biotite or amphibolite.
• 18:16
  SPEAKER_01: You know, it's slight variations that us geochemists worry about, but mostly you don't need to worry about.
• 18:22
  SPEAKER_02: So anyway, it has those tonalites in it.
• 18:24
  SPEAKER_02: But then, but then Jesse, you we're gonna focus on two main rock types when we talk about this host of rocks, and the first of one is this um this faux amphibolite.
• 18:36
  SPEAKER_02: And we'll talk about that.
• 18:39
  SPEAKER_02: Yeah, and then the other one uh is the gabbro sill that you mentioned earlier, and which is super important in terms of dating this thing out.
• 18:46
  SPEAKER_01: And that's because this is where the geochronology debate focuses on.
• 18:50
  SPEAKER_01: The question is, is this what we call a faux amphibolite?
• 18:54
  SPEAKER_01: And we'll get into what that means really quickly.
• 18:55
  SPEAKER_01: This is an image of one, and uh, if you're on the podcast and don't have access to images, this is it's a it's a kind of a weird rock.
• 19:01
  SPEAKER_01: Chris, when you look at this, or maybe Millie asks, what do you think of when you think of amphibolite?
• 19:07
  SPEAKER_01: What does the rock in your head look like when you think of amphibolite?
• 19:10
  SPEAKER_01: When I say, Chris, remember the Gore Mountain amphibolites, or what what comes to mind?
• 19:16
  SPEAKER_02: Well, I think it's to me it's gonna be a really dark-looking rock, mafic in composition, and you're gonna have this kind of like um I don't know, the minerals are gonna show this.
• 19:27
  SPEAKER_02: Oh this it's tortured.
• 19:29
  SPEAKER_02: It looks tortured to me.
• 19:30
  SPEAKER_02: Yeah, there's a there's an alignment to these like long, skinny mafic minerals.
• 19:36
  SPEAKER_01: That's right.
• 19:36
  SPEAKER_01: When I think of amphibolite, I think black rock.
• 19:39
  SPEAKER_01: Black rock with light specks in it, or maybe red garnets as well.
• 19:43
  SPEAKER_01: But it's black.
• 19:44
  SPEAKER_01: In the black, this rock here, what we're looking at, this faux amphibolite, is not black.
• 19:50
  SPEAKER_01: It's a lot lighter colored, it's kind of reddish, especially in these weathered faces, it's kind of reddish.
• 19:55
  SPEAKER_01: It has this kind of greasier color to it when you have a cut surface or a broken surface.
• 20:01
  SPEAKER_01: It's not as black as a a typical amphibolite would be.
• 20:05
  SPEAKER_01: And the black in a normal amphibolite comes from the mineral horn blend.
• 20:08
  SPEAKER_01: And Chris, did you?
• 20:10
  SPEAKER_01: I'm guessing you had to memorize the mineral horn blend, the composition of the mineral horn blend.
• 20:14
  SPEAKER_01: Do you still have I think I had to, or maybe you didn't require it?
• 20:17
  SPEAKER_02: I don't require, I put it on so what I do is I have the name of the mineral, I give it to them in a kind of a spreadsheet form.
• 20:26
  SPEAKER_02: The where they have the names of the minerals and then the properties follow on the spreadsheet, but I also give the chemical composition of, I don't make them memorize anything regarding that.
• 20:34
  SPEAKER_02: I just want them to see the complexity of some of these super common minerals.
• 20:39
  SPEAKER_02: I mean, because horn blend is it's really, really complicated, you know?
• 20:43
  SPEAKER_01: It's insane.
• 20:44
  SPEAKER_01: Yeah, it's like second only to Termaline of like how complicated, how many different mineral uh how many different elements can fit in this mineral?
• 20:51
  SPEAKER_01: It's insanely complicated.
• 20:53
  SPEAKER_01: It's the garbage bag amphibole mineral.
• 20:55
  SPEAKER_01: So amphibable is a class of minerals that all have kind of the same structure, different compositions, different end members.
• 21:01
  SPEAKER_01: It's a lot like Garnet in that regard.
• 21:03
  SPEAKER_01: Hornblende is kind of the one in the center, it's the garbage bag.
• 21:06
  SPEAKER_01: It can hold a little bit of calcium, a little bit of magnesium, a little bit of sodium, a little bit of iron, a lot of silica, aluminum.
• 21:12
  SPEAKER_01: It can just hold all that stuff, it can put them on different sites.
• 21:14
  SPEAKER_01: It's really complicated.
• 21:15
  SPEAKER_01: Yeah.
• 21:16
  SPEAKER_02: All right, so Jesse, I want to uh ask you this then.
• 21:18
  SPEAKER_02: How maybe you can't?
• 21:20
  SPEAKER_02: How do you identify faux amphibolite in the field?
• 21:24
  SPEAKER_02: Or do you have to wait until you get it back for chemical analysis from the lab?
• 21:27
  SPEAKER_02: I mean, I I don't honestly, Jesse, I don't think that I would walk up to this rock and say, Oh, there's the first faux amphibolite I've ever seen in real life.
• 21:36
  SPEAKER_02: You know, I no, I agree completely.
• 21:37
  SPEAKER_01: I agree completely.
• 21:38
  SPEAKER_01: I think you would, I think, Chris, when you walked up to this rock, you would be like, Oh, this kind of looks like an amphibolite.
• 21:46
  SPEAKER_01: I can see plagiuclase in it.
• 21:48
  SPEAKER_01: I can see something that is looking like hornblende, but it's the wrong color.
• 21:52
  SPEAKER_01: It's you know, looks like an amphibole.
• 21:54
  SPEAKER_01: I can see garnet in it, but I don't I think you'd think, oh, this is a weird looking rock.
• 21:59
  SPEAKER_01: And then you'd Take it back to the lab.
• 22:00
  SPEAKER_01: And if you've walked across a bunch of different high-grade rocks and you've seen a bunch of different amphibole group minerals, you might be able to identify that this is a different amphibole in this rock than your normal horn blend.
• 22:12
  SPEAKER_01: But I I would certainly need to bring this back to the lab and analyze it and look at the minerals in thin section to identify what these things are.
• 22:19
  SPEAKER_02: I have a follow-up question then on this.
• 22:21
  SPEAKER_02: Um, let's say that you have just a regular amphibolite and you see this in the field.
• 22:26
  SPEAKER_02: Would it be acceptable to you if your students picked that rock up and they knew that this was a metamorphic mineral because you can see the tortured grains in it?
• 22:35
  SPEAKER_02: Would you be okay if they called it a horn blend schist?
• 22:39
  SPEAKER_01: Yeah, I think given the it depends on which kind of naming scheme you teach them in metamorphic petrology, there's like a bunch of different schemes that you can use.
• 22:47
  SPEAKER_01: But yeah, a horn blend schist would be completely fine in many ways, because that's kind of what it is.
• 22:53
  SPEAKER_01: And fibolite is a slightly different classification scheme for rock name, for metamorphic rock naming.
• 22:59
  SPEAKER_02: But yeah, those would be kind of, I would treat those kind of as a field, as a field analysis, that would suffice, right?
• 23:05
  SPEAKER_01: Because you're getting the mineral ID right and you're looking at the thing.
• 23:08
  SPEAKER_01: So you if there's garnet in it, you'd say it's a garnet hornblend schist or something like that.
• 23:12
  SPEAKER_01: And this would be, you would look at this, you'd say, This is a garnet, and here's a mineral name coming at you, Chris.
• 23:17
  SPEAKER_01: Coming tonight is a mineral name.
• 23:20
  SPEAKER_01: Uh, and this is an actual mineral name named after Cummington, New Jersey, or Cummington, Connecticut, excuse me.
• 23:27
  SPEAKER_01: So it's an actual mineral name, and we won't dive into it any more than that.
• 23:30
  SPEAKER_01: But let's say that in in metamorphic petrology class, students they take a while to get past.
• 23:36
  SPEAKER_02: I I understand.
• 23:37
  SPEAKER_01: I'm having a hard time with it right now.
• 23:39
  SPEAKER_02: Um like this is professional that you have with you right now.
• 23:43
  SPEAKER_01: So we could just move past it probably here.
• 23:45
  SPEAKER_02: I think because if we linger here too long, then you're gonna get non-professional.
• 23:50
  SPEAKER_02: Okay.
• 23:50
  SPEAKER_01: All right, fair enough.
• 23:51
  SPEAKER_01: So this mineral, this is an amphibole.
• 23:54
  SPEAKER_01: This is an amphibole that has a different composition from horn blend.
• 23:58
  SPEAKER_01: It basically has a lot of magnesium and iron in it and a little bit of aluminum, but not much calcium.
• 24:05
  SPEAKER_01: So it's a horn blend.
• 24:07
  SPEAKER_02: Because hold on, real quick, because horn blend typically has calcium and sodium, and they substitute for each other as the as the mineral grows.
• 24:16
  SPEAKER_02: But this doesn't have that.
• 24:18
  SPEAKER_02: Coming tonight doesn't have to be.
• 24:24
  SPEAKER_01: It's a lot of iron and magnesium.
• 24:26
  SPEAKER_01: And this faux amphibolite has this mineral coming tonight, and also a similar mineral called anthophylite.
• 24:32
  SPEAKER_01: So it's a coming tonight anthophylite schist, is what this rock is.
• 24:35
  SPEAKER_01: It's got a lot of amphibole in it, just a different composition of amphibole.
• 24:39
  SPEAKER_01: And the key here is lots of magnesium, very little calcium, very little calcium, very little aluminum.
• 24:45
  SPEAKER_01: And the question is, how do you get something that forms that composition?
• 24:48
  SPEAKER_02: Right.
• 24:49
  SPEAKER_02: So is this I here's what I want to know, and I because I don't know the answer to this question, Jesse.
• 24:54
  SPEAKER_02: Is this some sort of substitution that takes place after you had regular amphibolite?
• 25:02
  SPEAKER_01: Oh, is this like a metamorphic?
• 25:04
  SPEAKER_01: No.
• 25:05
  SPEAKER_01: Is this like a replacement thing where you're where it changes later on?
• 25:11
  SPEAKER_01: It's a good question.
• 25:12
  SPEAKER_01: That's a very good question.
• 25:13
  SPEAKER_01: And the answer is no.
• 25:14
  SPEAKER_01: The mineral difference we're seeing between this and a typical amphibolite, horn blend plagioclase amphibolite, is that the the bulk rock chemistry is different.
• 25:24
  SPEAKER_01: So the the starting composition is different.
• 25:28
  SPEAKER_01: Now, the question is when did that chemistry change and how did that chemistry change?
• 25:32
  SPEAKER_01: But it's not like you had horn blend and then you metamorphosed it, removed all the calcium, sodium, aluminum, and created coming tonight anthophylite.
• 25:39
  SPEAKER_01: That's not what's happening.
• 25:41
  SPEAKER_01: We're starting with something and then we're metamorphosing it, and the minerals are different because the starting composition was different.
• 25:47
  SPEAKER_01: The recipe was different.
• 25:49
  SPEAKER_02: Oh, okay.
• 25:49
  SPEAKER_02: All right, okay.
• 25:50
  SPEAKER_02: That makes sense.
• 25:51
  SPEAKER_02: So there wasn't a lot of calcium and sodium and aluminum in the soup that that formed these minerals originally.
• 25:58
  SPEAKER_02: So that's a where does this happen, or is that not an important question?
• 26:03
  SPEAKER_01: I think it's a pretty important question.
• 26:05
  SPEAKER_01: I think it and it's I think it's an interesting hypothesis for how you get that because it looks like a basalt, except it doesn't have much calcium, sodium, aluminum in it.
• 26:13
  SPEAKER_01: Apart from that, looks very similar to kind of a basalt chemistry.
• 26:17
  SPEAKER_01: And so the current theory, I'm gonna skip ahead to this image, which I think this is you see, you said this is a famous image, right?
• 26:24
  SPEAKER_02: I think so.
• 26:24
  SPEAKER_02: I use it all the time, actually.
• 26:26
  SPEAKER_02: I have the same image.
• 26:27
  SPEAKER_02: It's just a this is what the ocean floor looks like, you know.
• 26:30
  SPEAKER_02: Is this from Alvin, this image from the um I I think so, but I'm not sure.
• 26:36
  SPEAKER_02: Okay.
• 26:36
  SPEAKER_02: Uh yeah, I don't know how far offshore this is.
• 26:39
  SPEAKER_02: This could have been a Hawaii, I think this probably is like off the coast of Hawaii.
• 26:43
  SPEAKER_01: The way on the modern earth that we get compositions like this is by taking basalt and hydrothermally altering it, where that hot water circulating through it scavenges out, dissolves out all the calcium, sodium, aluminum, those minerals get dissolved out.
• 26:57
  SPEAKER_01: What's left, what stays behind, is magnesium iron rich.
• 27:00
  SPEAKER_01: So hydrothermal hot water alteration, which often happens on the sea floor in the modern environment, can kind of create a composition, a soup, like what we see in this faux amphibolite.
• 27:14
  SPEAKER_02: Okay, hold on though.
• 27:15
  SPEAKER_02: Hold on.
• 27:16
  SPEAKER_02: So does that hydrothermal alteration take place after the basalt formed?
• 27:21
  SPEAKER_01: Yeah, probably very close to when the basalt formed.
• 27:25
  SPEAKER_01: Okay, like it's after it's crystallized, you're fluxing water through it because what I mean, and this happens at mid-ocean ridges all the time.
• 27:32
  SPEAKER_01: Why does it happen at mid-ocean ridge?
• 27:35
  SPEAKER_02: Super hot salty water circulating through busted up rock.
• 27:37
  SPEAKER_01: You know, it's and and through cracks in the rock, and you know, that water is kind of percolating in down below, getting heated up, and then circulating through.
• 27:45
  SPEAKER_01: So you do all sorts of really intense alteration in the seafloor.
• 27:50
  SPEAKER_02: All right, that makes sense.
• 27:52
  SPEAKER_02: So I I I don't think I asked my question or stated my hypothesis about alteration later on or substitution later on.
• 28:00
  SPEAKER_02: It is getting altered after it forms, just not by substitution.
• 28:05
  SPEAKER_01: Okay, that's more like it's getting leached out.
• 28:08
  SPEAKER_02: These these ions are getting along.
• 28:09
  SPEAKER_01: Yeah, that's perfect.
• 28:10
  SPEAKER_01: That's perfect, Chris.
• 28:12
  SPEAKER_01: It's getting leached out by hydrothermal alteration.
• 28:14
  SPEAKER_01: Exactly.
• 28:14
  SPEAKER_01: I mean, it it's it's more like chemical weathering in a way, in a kind of a weird way.
• 28:18
  SPEAKER_01: It's it's this chemical alteration by hot water fluxing through the rock.
• 28:22
  SPEAKER_01: So yeah.
• 28:23
  SPEAKER_02: Okay, exactly.
• 28:23
  SPEAKER_02: And then the other thing with this image, image number five here, this just shows pillow basalts, which is this is about as textbook as you're ever going to see in terms of how these things form.
• 28:33
  SPEAKER_02: And and they just they'll they'll stack up on top of each other as more flows come over top of this.
• 28:39
  SPEAKER_02: And so you can get these like layered pillow basalts.
• 28:41
  SPEAKER_02: We have this in the upper peninsula in Michigan, actually, where you can have tens of feet thick, these layered pillow basalts as one flow on top of another flow and so on and so on and so on.
• 28:52
  SPEAKER_02: Right.
• 28:52
  SPEAKER_02: Um my question is, Doctor, hold on, sorry.
• 28:56
  SPEAKER_02: Is that a typical process then at mid-ocean ridges or on the ocean floor to have this kind of leaching of calcium, sodium, and aluminum getting removed out of the basalt?
• 29:08
  SPEAKER_02: Is that common?
• 29:09
  SPEAKER_01: Yes, it is in certain locations.
• 29:12
  SPEAKER_01: Um, you know, near these black smokers and things like this, you get all sorts of you get you get both the leaching out of certain minerals and you also get deposition of other minerals.
• 29:21
  SPEAKER_01: And these are because these become really in the ancient earth, these greenstone belts, these archaean greenstone belts, are economic resources because of these sort of ancient vent systems that are dumping sulfides in.
• 29:32
  SPEAKER_01: There's gold involved in there, and there's all sorts of platinum group elements that get sequestered into these sulfides.
• 29:38
  SPEAKER_01: It's a huge like alteration, lots of chem, lots of high temperature chemistry or you know, medium temperature chemistry going on there.
• 29:44
  SPEAKER_01: So, yes, the answer is yes.
• 29:46
  SPEAKER_01: And then I was also wondering if the entire ocean floor, though, would would necessarily look like this.
• 29:50
  SPEAKER_02: I was wondering maybe if this was a specific kind of thing to this early earth.
• 29:56
  SPEAKER_01: Yeah, it's a good question.
• 29:58
  SPEAKER_01: You know, I I've gone a little bit out on a limb here, but but we just don't have enough numbers to know how prevalent if this is more prevalent, you know, back in time than it was, than it is in the modern earth.
• 30:10
  SPEAKER_01: There's probably some indications that it was slightly different because seawater chemistry was different, but I I haven't seen an argument that it's been there, might be one out there, but I just haven't seen it that it's super different in the early earth than the modern.
• 30:21
  SPEAKER_01: Okay.
• 30:22
  SPEAKER_02: All right.
• 30:22
  SPEAKER_02: And that gives me the segue award for the day because it's time to move.
• 30:26
  SPEAKER_02: Speaking of early earth and uh maybe different times, it's time to talk about Jesse, the dating of these rocks.
• 30:34
  SPEAKER_02: So let's let's get into that a little bit in terms of how are we doing this, and then we'll end this with the controversy or the debate.
• 30:42
  SPEAKER_01: The controversy.
• 30:43
  SPEAKER_01: Yeah.
• 30:43
  SPEAKER_01: So, Chris, you thought we were in the weeds earlier.
• 30:46
  SPEAKER_01: If you thought that, you're wrong.
• 30:48
  SPEAKER_01: Because now we're going into the weeds.
• 30:50
  SPEAKER_01: And this image number six here, this is an image I want to just describe this quickly.
• 30:54
  SPEAKER_01: What this is showing is this is Earth history going from left to right, 4.568 billion years, left to right.
• 30:59
  SPEAKER_01: What I'm showing here, what at this plot I've made, is if you take a particular isotope system and you you say, okay, the Earth started with X number of atoms of uranium 238, that equals 100.
• 31:11
  SPEAKER_01: I just want to know what percentage of that decays over Earth history.
• 31:15
  SPEAKER_01: How much do we have on the far right of this plot at zero, at zero age today?
• 31:19
  SPEAKER_01: And if we look at this, Chris, I've plotted the uranium isotopes, the two uranium isotopes that we've talked about a lot.
• 31:25
  SPEAKER_01: I've also plotted two samarium isotopes, which is a samarium is a rare earth element that has two different decay rates, much like uranium.
• 31:34
  SPEAKER_02: This is a really good graph, by the way.
• 31:35
  SPEAKER_02: You did this, I think.
• 31:36
  SPEAKER_02: You made this, right?
• 31:37
  SPEAKER_02: Yeah, you like this?
• 31:38
  SPEAKER_02: I I was I I I was hoping it'd land.
• 31:41
  SPEAKER_02: It lands with me.
• 31:42
  SPEAKER_02: Um, I love it.
• 31:43
  SPEAKER_02: If the data is really clear on this, yeah, for sure.
• 31:46
  SPEAKER_01: Okay, cool.
• 31:47
  SPEAKER_01: So what we see here is uranium-238, the long-lived uranium isotope, has about 50% of the original atoms we had have decayed over Earth history from the start of Earth 4.5 to now.
• 32:02
  SPEAKER_01: And the reason for that is because we've gone through about a billion years old, yes, yes.
• 32:06
  SPEAKER_01: 4.5 billion years, about half of the 238 uranium atoms that we started with have decayed away.
• 32:11
  SPEAKER_01: And the reason is because we've gone through around about one half-life.
• 32:14
  SPEAKER_01: The half-life of 238 uranium is about the same as Earth history.
• 32:18
  SPEAKER_01: 235 uranium is almost extinct.
• 32:22
  SPEAKER_01: We've lost almost all of it because the half-life of 235 uranium, we've gone through almost with less than 0.7%, I think, right?
• 32:32
  SPEAKER_02: Or is it less than that?
• 32:33
  SPEAKER_02: No.
• 32:33
  SPEAKER_01: 0.7% of uranium is 235 uranium.
• 32:38
  SPEAKER_01: Uh well, I'd have to do the calculation.
• 32:40
  SPEAKER_01: Uh no, never mind.
• 32:41
  SPEAKER_02: That's what I that's what I meant.
• 32:42
  SPEAKER_02: I didn't state it precisely as I could have should have, yeah.
• 32:45
  SPEAKER_02: Absolutely.
• 32:45
  SPEAKER_02: Okay.
• 32:46
  SPEAKER_02: 0.7% of the all the uranium is U235.
• 32:51
  SPEAKER_01: Is 235, yeah, that's right.
• 32:53
  SPEAKER_01: So well, we've lost basically we've lost almost all of it.
• 32:56
  SPEAKER_01: We're down to fraction, you know, we're down to a few percent of it left because we've gone through a whole bunch of half-lives.
• 33:02
  SPEAKER_01: 147 samarium is the typical what people have traditionally used for long-lived radiogenic isotope systems.
• 33:10
  SPEAKER_01: 147 samerium decays by alpha decay to 143 neodymium.
• 33:14
  SPEAKER_01: That's a system that's been used for decades, was kind of known about and started being used by geochronologists in the 80s.
• 33:21
  SPEAKER_01: That has a half-life of billions and billions, like 14 billion year half-life.
• 33:25
  SPEAKER_01: So on our scale, we've lost very little of the initial 147 samarium that we started with.
• 33:30
  SPEAKER_01: Like we've barely lost any, maybe a couple percent, right?
• 33:33
  SPEAKER_01: It looks like a line.
• 33:35
  SPEAKER_01: That looks like a line to us because we've zoomed super far in to what we know is an exponential decay.
• 33:42
  SPEAKER_02: It's a very gentle sloping line.
• 33:43
  SPEAKER_01: Yep.
• 33:44
  SPEAKER_01: Very gentle sloping line.
• 33:45
  SPEAKER_01: You can't see you can hardly tell that there's any curve to it.
• 33:48
  SPEAKER_01: It's kind of like looking at the horizon, you know, standing at your house.
• 33:52
  SPEAKER_01: You can't tell there's a curve to it, but there really is a curve there because it's it's decay.
• 33:57
  SPEAKER_01: There's another isotope of samarium that used to exist that no longer does, called 146 Sumerium.
• 34:04
  SPEAKER_01: And that was around during the formation of the solar system when Earth was young.
• 34:09
  SPEAKER_01: This was around, but it's what we call an extinct nuclide system.
• 34:14
  SPEAKER_01: It has decayed away because the half- Hold on.
• 34:17
  SPEAKER_02: That's gonna be confusing.
• 34:18
  SPEAKER_02: That's gonna be confusing.
• 34:19
  SPEAKER_02: So it's can we just you're getting all geochemistry on us right now and geochronologists?
• 34:25
  SPEAKER_02: Simplify it.
• 34:26
  SPEAKER_02: Extinct isotope system.
• 34:28
  SPEAKER_01: Okay, perfect.
• 34:29
  SPEAKER_01: Yeah, okay, extinct isotope system.
• 34:30
  SPEAKER_02: There's no more Sumerium 146 around.
• 34:34
  SPEAKER_02: It's gone.
• 34:34
  SPEAKER_01: It's decayed into its daughter.
• 34:36
  SPEAKER_01: What's that?
• 34:37
  SPEAKER_01: Yeah, and why why is it extinct?
• 34:39
  SPEAKER_02: I think that's kind of the because it has such a short half-life.
• 34:43
  SPEAKER_01: Beautiful.
• 34:43
  SPEAKER_02: Okay, beautiful, Chris.
• 34:44
  SPEAKER_01: Nice.
• 34:45
  SPEAKER_01: Yes, okay.
• 34:46
  SPEAKER_01: Okay, perfect.
• 34:47
  SPEAKER_01: That's that's the only important point.
• 34:49
  SPEAKER_01: It's extinct because it decayed, it was so unstable that it decayed really quickly.
• 34:55
  SPEAKER_01: And it decayed to an isotope of neodymium called 142, which we'll talk about in a minute.
• 34:59
  SPEAKER_01: But Chris, let me ask you a question.
• 35:00
  SPEAKER_01: Just by looking at this plot, which of these systems, which of these isotopes and their their daughter or product isotope, which of these would be more useful, do you think, for looking at old rocks, for understanding old rocks and dating old rocks?
• 35:16
  SPEAKER_02: Uh, the Sumerium-146, and maybe even uranium-235, because even that one with a half-life of around 700 million years is gonna, you're gonna get a lot of decay in the Hadean part of the of the early Earth.
• 35:31
  SPEAKER_01: Perfect.
• 35:31
  SPEAKER_01: Okay, yeah, that's exactly right.
• 35:33
  SPEAKER_01: All the decay, all the action, all the decay is happening early on in the process.
• 35:38
  SPEAKER_01: So it's really valuable for early earth processes.
• 35:41
  SPEAKER_01: And the other, the flip side of that is that there's very little decay happening for like uranium-235 in the last three billion years, only 25% of the decay has happened.
• 35:50
  SPEAKER_01: For 146 samerium, none of the decay has happened in the last three billion years.
• 35:55
  SPEAKER_01: So all these metamorphic events that alter rocks and and deform them and alter them, they don't have any impact on these isotope systems.
• 36:03
  SPEAKER_01: So we can use these extinct systems to see through metamorphism that happened in the last three billion years, which is incredible.
• 36:13
  SPEAKER_01: This is why this is powerful.
• 36:15
  SPEAKER_01: And Chris, we've talked about my mass spectrometer development project.
• 36:18
  SPEAKER_01: That whole project, lots of money thrown out and time thrown at that, was all to make better use of the 146 samerium decay system because it's so valuable for early Earth rocks.
• 36:27
  SPEAKER_01: For early Earth rounds, that's right.
• 36:28
  SPEAKER_02: I just want to I want to drive home this point because we're talking about another tool in the toolbox of a geochronologist here using isotopes that have shorter half-lives and therefore became extinct in the early part of Earth's history, which is seems maybe counterintuitive to some people because you would think, well, wait a minute, we need to use uranium-238 to see this because you're still gonna have some left.
• 36:57
  SPEAKER_01: Right?
• 36:57
  SPEAKER_02: But we're only focusing on this bracket of early time and we're trying to then see what happened during that time.
• 37:06
  SPEAKER_02: We need to use appropriate isotopes, these radiogenic isotopes to do that.
• 37:12
  SPEAKER_02: It's just like um we're looking for something different here.
• 37:15
  SPEAKER_02: We're not does that make sense?
• 37:17
  SPEAKER_01: I I'm not saying that's exactly right, I'm not no, let me let me try and I think I know what kind of point you're trying to make here, and let me make a different.
• 37:23
  SPEAKER_01: It's kind of the inverse of radiocarbon.
• 37:26
  SPEAKER_01: Radiocarbon's great for young stuff, it's terrible for old stuff.
• 37:30
  SPEAKER_01: It's kind of the opposite of that.
• 37:32
  SPEAKER_01: These systems, because they decayed away early on in Earth history, they're great for the old stuff, and we don't have them anymore.
• 37:38
  SPEAKER_01: And so that means that the rocks have kind of locked in that isotope signature early on in their history and it doesn't get overprinted.
• 37:46
  SPEAKER_01: So it's kind of like radiocarbon, except for the old box.
• 37:50
  SPEAKER_02: So maybe here, let's let's let's go with this, okay?
• 37:53
  SPEAKER_02: Because I think this is really cool.
• 37:55
  SPEAKER_02: So if you take uh let's say a rock that's 3.5 billion years old, it won't have any of the daughter isotope for samarium 146 in it because samerium wasn't there.
• 38:09
  SPEAKER_02: Does that make sense?
• 38:10
  SPEAKER_02: This is the presence of the daughter isotope from samarium 146 in a rock indicates that this rock has to be really, really, really old.
• 38:21
  SPEAKER_01: That's exactly right.
• 38:22
  SPEAKER_01: Yes, that's it, that's exactly it, Chris.
• 38:24
  SPEAKER_01: And so we can use now this becomes a tool.
• 38:28
  SPEAKER_01: That framing right there is perfect.
• 38:30
  SPEAKER_01: Now, if we had a rock that had a ton of 146 samerium in it initially, we would now see a lot of the daughter, the product isotope, which is 142 neodymium.
• 38:38
  SPEAKER_01: And that's a beautiful segue into the data from let's go back to this.
• 38:42
  SPEAKER_01: Can we go back to the rocks in the data?
• 38:44
  SPEAKER_01: Or do we have to do that?
• 38:44
  SPEAKER_01: I have one other question though.
• 38:46
  SPEAKER_02: Does all right, so neodymium-142, does that get there any other way?
• 38:52
  SPEAKER_02: Is it a natural isotope or is it only there by decay of Sumerium 146?
• 38:57
  SPEAKER_01: Gosh, this is such a great point, Chris.
• 38:59
  SPEAKER_01: You're getting me all fired up because I love this stuff.
• 39:01
  SPEAKER_01: Yes, 142 neodymium gets there, is a naturally occurring isotope, too.
• 39:05
  SPEAKER_01: So there was a bunch of 142 in the solar system, 142 neodymium at the start of the solar system.
• 39:10
  SPEAKER_01: We have a bunch of it, which makes this difficult.
• 39:14
  SPEAKER_01: Really, really difficult, because there's a lot of 142 already there in the rock.
• 39:18
  SPEAKER_01: There's a lot of one.
• 39:19
  SPEAKER_01: If you pick up a rock, there's gonna be some neodymium in it.
• 39:22
  SPEAKER_01: Uh forget the percentages, something like 25% of the neodymium is gonna be 142 neodymium, like a lot of it.
• 39:28
  SPEAKER_01: I should probably know that offhand.
• 39:30
  SPEAKER_01: I've worked so much on neodymium.
• 39:32
  SPEAKER_01: I should probably know the percentage, but I don't.
• 39:34
  SPEAKER_01: Um, so what we're looking for is either an excess in 142, more than there should be, or a depletion, a lack of 142 neodymium, meaning that there was there's more than there should be, or there's less than there should be.
• 39:47
  SPEAKER_01: And that's a really, really small amount, which means we need really, really precise mass spectrometers to detect this tiny variation in 142 neodymium, which tells us was there 146 samerium in this rock, or was there less 146 samerium, or was there like average?
• 40:04
  SPEAKER_01: That's kind of what we're looking at.
• 40:05
  SPEAKER_01: We're looking, is it is it similar to the Earth average?
• 40:08
  SPEAKER_01: Is it a little bit above it or a little bit below it?
• 40:11
  SPEAKER_01: And if it's above or below, it means that the rock used to have 146 samerium in it, either a lot of it or a little bit of it.
• 40:20
  SPEAKER_01: But either way, the rock was old because there was one, it was it was formed when 146 samerium was alive, which is as this chart shows, the first 600 million years of earth history or something.
• 40:30
  SPEAKER_02: Okay, so we uh Jesse, I'm gonna move this along.
• 40:33
  SPEAKER_02: We we got into the weeds there on samarium and neodymium.
• 40:38
  SPEAKER_02: And I'm not it was this super fun for you and I to have this discussion, but we need to bring this back to relevancy in terms of what we're talking about.
• 40:46
  SPEAKER_02: So let's go to the next image, and we need to talk a little bit about why this discussion is important, differentiating between neodymium 142 that was there just because it's there, and the 142 that gets there by decay from Sumerium 146.
• 41:02
  SPEAKER_01: Yeah, um, what as an intro, okay.
• 41:05
  SPEAKER_01: So, this image, this image number seven that we're that we're looking at here, that you can look at shows data, shows real data.
• 41:12
  SPEAKER_01: What I want to refer to first, though, is we've had several discussions, Chris, about isochrons.
• 41:16
  SPEAKER_01: We had it most recently, probably in our Geology of Garnet episode.
• 41:20
  SPEAKER_02: We did it in Issua, I think.
• 41:21
  SPEAKER_01: In Issua, we did too.
• 41:22
  SPEAKER_01: Geology of Garnet, we showed an isochron, an image of an isochron.
• 41:26
  SPEAKER_01: So you can go to the app and look at the images there.
• 41:28
  SPEAKER_01: Our YouTube channel has has those images too.
• 41:31
  SPEAKER_01: So you can look at those.
• 41:32
  SPEAKER_01: We had this discussion with Rachel Phillips about what an isochron is and how it works.
• 41:36
  SPEAKER_01: This plot right here that we're showing is kind of a pseudo-isochron.
• 41:39
  SPEAKER_01: It's not exactly an isochron because we can't make one because we don't have the parent isotope around anymore.
• 41:46
  SPEAKER_01: We don't have 146 samarium around anymore.
• 41:49
  SPEAKER_01: All we're looking for is variability in the daughter or product isotope, variability in the 142 neodymium, the amount of 142 neodymium in a rock, or more precisely, the isotope ratio of 142.
• 42:01
  SPEAKER_02: Can I I want to interject, Jesse, that it's okay that we don't have samarium 146 because we do have samarium 147.
• 42:09
  SPEAKER_02: And when the crystals are growing, 147 and 146 samerium are so similar that they're gonna, it's not gonna differentiate between them.
• 42:20
  SPEAKER_02: So we have we can normalize the data.
• 42:22
  SPEAKER_02: Is that okay?
• 42:23
  SPEAKER_02: Can I use that term?
• 42:24
  SPEAKER_02: Yeah, absolutely.
• 42:25
  SPEAKER_02: That's perfect.
• 42:25
  SPEAKER_01: Absolutely.
• 42:26
  SPEAKER_01: That's the term.
• 42:26
  SPEAKER_01: Yep.
• 42:27
  SPEAKER_02: So we can normalize the data using samarium 147.
• 42:31
  SPEAKER_02: All right.
• 42:31
  SPEAKER_01: So that's exactly right.
• 42:33
  SPEAKER_01: Go ahead.
• 42:33
  SPEAKER_01: Beautiful, beautifully said, beautifully said.
• 42:35
  SPEAKER_01: So I'm just go ahead.
• 42:38
  SPEAKER_01: Uh no, that's beautifully said.
• 42:39
  SPEAKER_01: I think the important point on this chart here is showing that most rocks on Earth have a particular ratio of 142 neodymium divided by 144 neodymium.
• 42:50
  SPEAKER_01: And we can we call that zero.
• 42:51
  SPEAKER_01: We just divide it by the average and call it zero.
• 42:54
  SPEAKER_01: So they've got zero, which means that they all have the appropriate amount of one for neodymium in them.
• 43:00
  SPEAKER_01: These ujaraluck unit, these faux amphibolite unit, I'm plotting some real data here.
• 43:06
  SPEAKER_01: There's a bunch of different kinds of subdivisions in the ujaraluk unit, different titanium, whatever, it doesn't matter, but there's different subdivisions within that unit, and they all have most of them have too little 142.
• 43:18
  SPEAKER_01: They don't have enough 142 neodymium in them compared to most other rocks on Earth.
• 43:23
  SPEAKER_01: And the question is, what does that tell us?
• 43:25
  SPEAKER_01: Nobody debates the data.
• 43:27
  SPEAKER_01: The data's real.
• 43:28
  SPEAKER_01: We agree with this.
• 43:29
  SPEAKER_01: There's less 142 neodymium than we would expect in this rock.
• 43:32
  SPEAKER_02: And you're clearly getting a slope of a line with this.
• 43:35
  SPEAKER_02: Oh, okay.
• 43:36
  SPEAKER_01: So, Chris, this is this is the important point.
• 43:38
  SPEAKER_01: This is where the debate comes in.
• 43:40
  SPEAKER_01: Is there's a linear relationship here in how little 142 neodymium is compared to, so if there's there's less 142 neodymium in these rocks, the depletion, the lack of 142, is correlated to the samarium to neodymium ratio.
• 43:55
  SPEAKER_01: So if there's lower 147, less samarium in the rock, then there's Less 142 neodymium in the rock as well.
• 44:02
  SPEAKER_01: And those are kind of those are correlated.
• 44:05
  SPEAKER_01: That makes sense.
• 44:06
  SPEAKER_01: Okay, why does that make sense?
• 44:07
  SPEAKER_01: Why does that make sense, Chris?
• 44:08
  SPEAKER_02: Well, because all right.
• 44:10
  SPEAKER_02: Because if there's less 147 samerium, then there's gonna there you also would have had the extinct isotope, the extinct parent isotope, samarium 146.
• 44:22
  SPEAKER_02: So if there was less 147 there, there would have been less or no 146 there to decay into the daughter isotope that we care about.
• 44:30
  SPEAKER_01: Right, exactly.
• 44:32
  SPEAKER_01: As beautifully put, Chris, exactly.
• 44:34
  SPEAKER_01: So that's the argument for these rocks being old.
• 44:37
  SPEAKER_01: Is all we do is it's not a perfect isochron, so the slope doesn't exactly correlate to the age.
• 44:43
  SPEAKER_01: We have to do an additional level of calculation that's way beyond what we need to go into.
• 44:46
  SPEAKER_01: But it's a pseudo-isotron, we can plot a line through that, we get a slope that eventually gives us an age.
• 44:51
  SPEAKER_01: That age is 4.25 billion years old, if we think of it like that.
• 44:55
  SPEAKER_01: That this rock has remained closed systems.
• 44:57
  SPEAKER_02: Let that digest a second.
• 44:59
  SPEAKER_02: 4.1 again?
• 45:01
  SPEAKER_02: Say that again.
• 45:02
  SPEAKER_02: 4.25 billion years old.
• 45:04
  SPEAKER_02: Is it just 4.25?
• 45:06
  SPEAKER_02: Are we gonna go to that the other million years matters?
• 45:09
  SPEAKER_01: No, because it's because this is an extinct system, and look at the error bars on the y-axis here.
• 45:15
  SPEAKER_01: There are pretty big air bars.
• 45:16
  SPEAKER_01: There are large uncertainties in this, but it's 4.25 plus or minus, I don't forget what they are, in the tens of millions of years.
• 45:23
  SPEAKER_01: We don't need to go more than 4.25.
• 45:24
  SPEAKER_01: But Chris, this is amazing.
• 45:26
  SPEAKER_01: We are using an isotope system that is no longer active on Earth, that was happening early on in the system.
• 45:33
  SPEAKER_01: We're using mass spectrometers to measure a ratio to the fifth and sixth decimal place.
• 45:38
  SPEAKER_01: Like very, very this is really precise, really, really precise data.
• 45:43
  SPEAKER_01: And we're using that to argue that these rocks are really old, 4.25 billion years old.
• 45:49
  SPEAKER_02: This is Hadian.
• 45:51
  SPEAKER_02: You know, this is 4.5 to 4 billion years old.
• 45:55
  SPEAKER_02: You the rocks like that are so exceedingly rare.
• 45:59
  SPEAKER_02: I mean, and this this is so this is different from the Jack Hill Zircons from a prior episode because the Jack Hill zircons, those were detrital grains, those were zircon grains that formed and were transported and deposited in a sedimentary rock.
• 46:16
  SPEAKER_02: These are rocks that are in place that we're talking about here.
• 46:21
  SPEAKER_01: So we have the rock itself.
• 46:22
  SPEAKER_01: Now, okay, that's the argument for these rocks being old.
• 46:26
  SPEAKER_01: Okay, that hey, these rocks are 4.25 billion years old.
• 46:29
  SPEAKER_01: This 142 neodymium system is recording that Hadean process, that the they're recording the Hadean formation age of the rocks.
• 46:36
  SPEAKER_01: There's another people.
• 46:39
  SPEAKER_02: So now we're gonna get into the debate.
• 46:41
  SPEAKER_01: What's the debate?
• 46:41
  SPEAKER_01: A bit of yep, the debate.
• 46:43
  SPEAKER_01: The debate is what how to explain this data in this linear relationship.
• 46:47
  SPEAKER_01: The argument is really, is that linear relationship does that represent the age of the rock, or does it represent a mixing line?
• 46:55
  SPEAKER_01: And isochrons are risky because you can make if you if I take an old granite and a young basalt and I mix them together and create different mixtures of those rocks, I can create an isochron that makes it look like the rock is old, that this suite of rocks is old.
• 47:13
  SPEAKER_01: So with mixing, you can create false isochrons.
• 47:17
  SPEAKER_01: If you have two different rocks of two different ages and two different compositions, you mix them together, you can get a mixing line that looks like an isochron and gives you a wrong age.
• 47:25
  SPEAKER_02: Okay, we know that to be true.
• 47:26
  SPEAKER_02: Okay, and we might have mixing going on because we did talk about this these felsic rocks that are part of this of the suite, the tonalites, and then you have the the the metabasalts or the green stones, the pillow basalts.
• 47:39
  SPEAKER_02: Um, so there you have the ingredients for mixing, right?
• 47:43
  SPEAKER_02: That's exactly right.
• 47:44
  SPEAKER_01: We that okay there are potentially some we don't have the N members, we don't have preserved today the N members here that would explain this as a mixing line.
• 47:54
  SPEAKER_01: Like we what we would need is kind of a weird composition.
• 47:56
  SPEAKER_01: We would have to have modern rocks or or say, let's say we'd have to have 3.8 billion year old mantle, normal mantle, on one side of the mixing, and what we'd have to mix it with is Hadean enriched mantle.
• 48:10
  SPEAKER_01: An enriched mantle is mantle that's had fluids added to it, or had kind of volatiles added to it, or had a little crust added back into it, and that would have had to be Hadean.
• 48:20
  SPEAKER_01: We have to have something Hadean in this system, so we have to have a Hadean component somewhere to explain the variability in 142 neodymium.
• 48:29
  SPEAKER_01: The debate is really are these rocks Hadean, or did these rocks form from a Hadean source?
• 48:36
  SPEAKER_01: Was the mantle Hadean and the rocks are 3.8 billion years old?
• 48:39
  SPEAKER_01: Okay, or is the rock?
• 48:40
  SPEAKER_02: Can we talk about that a second?
• 48:42
  SPEAKER_02: Would it make sense?
• 48:44
  SPEAKER_02: It does, it does, but I get that.
• 48:46
  SPEAKER_02: I I do because in order even if you're mixing something, the only way you get a 4.25 billion year old, you have to have that somewhere then.
• 48:54
  SPEAKER_02: That age has to be there.
• 48:56
  SPEAKER_02: So that makes sense to me.
• 48:57
  SPEAKER_02: But maybe what doesn't make sense is this is so early on.
• 49:03
  SPEAKER_02: Earth is still maybe differentiating.
• 49:06
  SPEAKER_02: You would think that the oldest rocks would only be crustal rocks and not deeper in the mantle.
• 49:12
  SPEAKER_02: Am I like is Yeah.
• 49:15
  SPEAKER_01: Well, so it's a good question.
• 49:18
  SPEAKER_01: Given the high amounts of radioactive heating of the early earth, the earth was probably giving off a lot more heat back in time.
• 49:27
  SPEAKER_01: And the one way to give off heat is to form crust and recycle cold crust back into the mantle.
• 49:32
  SPEAKER_01: So there is probably a high kind of resurfacing rate in the early earth.
• 49:36
  SPEAKER_01: So you're you're making crust, you're destroying it.
• 49:38
  SPEAKER_01: That probably was happening.
• 49:40
  SPEAKER_01: But you're exactly right.
• 49:41
  SPEAKER_01: There is evidence for a Hadian something in this system, in these rocks.
• 49:46
  SPEAKER_01: The question is, are the rocks themselves the thing, or are they just passively recording a really old source?
• 49:53
  SPEAKER_01: There are there's more nuances, like there are more debates about these rocks, but I'm trying to distill it down into the simplest version of the argument.
• 50:01
  SPEAKER_01: Right, I get the simplest version.
• 50:02
  SPEAKER_02: I have a question though.
• 50:04
  SPEAKER_02: Like, what what makes the the people that want to debate the 4.25 billion, where does that come from?
• 50:14
  SPEAKER_02: That's what I want to ask.
• 50:15
  SPEAKER_02: Where does that skepticism come from?
• 50:19
  SPEAKER_02: Does it step on the toes of other research that says that those rocks are not that old?
• 50:24
  SPEAKER_02: Or like you know what I mean?
• 50:26
  SPEAKER_01: No, okay.
• 50:26
  SPEAKER_01: What's the source of it?
• 50:27
  SPEAKER_01: I I I think it's just simply a I think it's kind of the way science should work.
• 50:32
  SPEAKER_01: Some people propose something and they kind of push the limits.
• 50:35
  SPEAKER_01: This is this is not a surefire thing.
• 50:37
  SPEAKER_01: Even if you ask the the authors who would say that this is an old rock, I think what they would say is this the simplest argument is that this is an old rock.
• 50:47
  SPEAKER_01: That's the simplest way to explain this data is that this is actually an old rock.
• 50:51
  SPEAKER_01: It's a more complicated story.
• 50:53
  SPEAKER_01: If the source is old and the rocks are young, and you know it's a mixing relationship, that's like a more complicated explanation of this data.
• 51:01
  SPEAKER_01: So it's it's really like two sides of the same coin.
• 51:06
  SPEAKER_01: That's my that's my reading of the interpretation.
• 51:08
  SPEAKER_02: So my follow-up on that is then remember, remember earlier on, Jesse?
• 51:13
  SPEAKER_02: I said I'm gonna put something in my pocket, I'm gonna pull it out.
• 51:15
  SPEAKER_02: Okay, get it out of there, bring it out.
• 51:18
  SPEAKER_02: So this gabbro sill, okay.
• 51:21
  SPEAKER_02: First of all, is it truly a sill?
• 51:24
  SPEAKER_02: Is this parallel to the sedimentary rocks that that existed there?
• 51:28
  SPEAKER_02: Or like is it is it a sill?
• 51:30
  SPEAKER_01: So I have not walked across these rocks and seen this contact relationship, but the the field geologists who worked in this area argue that the the sill is actually a sill and it cross-cuts the eujarelak unit.
• 51:43
  SPEAKER_01: So this sill, this gabbro sill, which is looks much more like a typical gabbro we would see.
• 51:49
  SPEAKER_01: It's got horn blend and plagiaris in it, and it cuts across the eujareluk unit, which means, as we talked about when we were talking about isua, it means that this is younger.
• 51:58
  SPEAKER_01: If it cross-cuts, it has to be younger, right?
• 52:02
  SPEAKER_01: So another piece of evidence.
• 52:05
  SPEAKER_02: Well, hold on, hold on, Jesse.
• 52:06
  SPEAKER_02: Let's finish that thought off.
• 52:08
  SPEAKER_02: So the dating of the gabbro sill seems should be pretty straightforward, yes?
• 52:15
  SPEAKER_01: Yeah, you just take a bunch of samples of the gabbro sill.
• 52:18
  SPEAKER_01: Hopefully, you get some spread in the geochronology.
• 52:20
  SPEAKER_01: And what's that come out?
• 52:21
  SPEAKER_01: Spread in the composition.
• 52:23
  SPEAKER_01: And so here's a chart, Chris.
• 52:24
  SPEAKER_01: This is this this image number eight here, the last complicated isotope geochemistry plot that we'll show.
• 52:30
  SPEAKER_01: But this is a traditional isochron.
• 52:32
  SPEAKER_01: So this is the long-lived system, 147 samerium against 143 neodymium, the long-lived system, the traditional one.
• 52:40
  SPEAKER_01: And you can see why this is interesting.
• 52:45
  SPEAKER_01: Yeah.
• 52:45
  SPEAKER_02: Can I can I interrupt you a second?
• 52:47
  SPEAKER_01: So please do.
• 52:48
  SPEAKER_02: Uh the gabbro cell comes in at 4.115 billion years old, plus or minus 100.
• 52:55
  SPEAKER_02: So it's had that's a big margin of of uncertainty there.
• 52:58
  SPEAKER_01: About 4.1 billion, plus or minus 100 billion, or plus or minus 100 million, yeah.
• 53:04
  SPEAKER_02: That sets a minimum age for the rock, right?
• 53:07
  SPEAKER_02: That's what we're saying.
• 53:08
  SPEAKER_02: Because it cuts across the eujarloc, which then had to exist before the sill could intrude it.
• 53:15
  SPEAKER_02: Um, and so that like if we're pretty good on the age of the SIL, then that really helps us bracket this down a little bit more.
• 53:27
  SPEAKER_01: That's right.
• 53:28
  SPEAKER_02: Is there how much like how how comfortable are we with the age of the SIL?
• 53:32
  SPEAKER_01: Well, it's a good question, Chris.
• 53:33
  SPEAKER_01: So I I mean, the age how comfortable are we with the age of the SIL?
• 53:38
  SPEAKER_01: If we use the same system, the long-live system, on the Ujarluck unit itself, we get across all the Ujarluck samples, you get an age of 3.6 billion years old.
• 53:48
  SPEAKER_01: Um, plus or minus 200, bunch of scatter.
• 53:51
  SPEAKER_01: Not really a valid.
• 53:52
  SPEAKER_01: Yeah, it's not really a valid line.
• 53:54
  SPEAKER_01: It's it's a big scatter.
• 53:56
  SPEAKER_01: If we break those down by different units, we even get shallower lines and and lower numbers.
• 54:01
  SPEAKER_01: We get 2.5 and 2.8 billion years old with big uncertainty.
• 54:05
  SPEAKER_01: So in the Ujarluck unit itself, this long-lived system is not agreeing with the short-lived system.
• 54:13
  SPEAKER_01: It's the short-lived system says this might be 4.25 billion years old.
• 54:17
  SPEAKER_01: The long-lived system says, well, it's actually a lot younger than that, maybe even super young.
• 54:21
  SPEAKER_01: This is a metamorphic rock, though, and the garnets are growing and moving samarium and endymium around.
• 54:27
  SPEAKER_01: And so this is later metamorphism altering the samarium neodymium isotope systematics of these rocks.
• 54:34
  SPEAKER_01: Of the eujarloc.
• 54:35
  SPEAKER_01: Yes.
• 54:36
  SPEAKER_01: Of the eujarloc.
• 54:37
  SPEAKER_01: And if we look at the SIL, the SIL gives an age of 4.1 billion years old in the long-lived system, where that might have been overprinted by metamorphism, but for some reason it's not overprinted by the metamorphic event that probably experienced that that probably hit this area and disturbed the Ujariluk unit 2.7 billion years ago.
• 55:00
  SPEAKER_01: The gabbro sill was somehow insulated from that alteration.
• 55:05
  SPEAKER_01: So on one side of the debate, I'm gonna try, I'm gonna try and and and remain you know agnostic here.
• 55:11
  SPEAKER_01: On one side of the debate, people would say this is the nail in the coffin.
• 55:16
  SPEAKER_01: The gabbro seal is 4.1 billion years old, it cross-cuts the eudarrelec unit, therefore the eudarelic unit has to be old, and the age we get out of the short-lived system, 142 neudium, is is right.
• 55:26
  SPEAKER_01: The other side says, look at your rocks, they're totally deformed and altered.
• 55:30
  SPEAKER_01: The the long-lived system is disturbed in these rocks and and it's not giving you the right age.
• 55:34
  SPEAKER_01: So we don't really care what the Gabbro Sill tells you because it's not right.
• 55:38
  SPEAKER_01: It's deformed and altered and metamorphosed.
• 55:40
  SPEAKER_01: And and then you're depending upon an interpretation of cross-cutting relationships, which these are really good field geologists, I believe.
• 55:46
  SPEAKER_01: When they see cross-cutting relationships, I I think they are probably seeing cross-cutting relationships.
• 55:51
  SPEAKER_01: But this is where the debate is kind of centered, right?
• 55:54
  SPEAKER_01: It's how do we interpret this extinct isotope system, and then what does the long-lived system tell us?
• 56:00
  SPEAKER_01: It's complicated, it's really in the weeds, man.
• 56:03
  SPEAKER_01: It's really in the weeds.
• 56:04
  SPEAKER_02: Okay.
• 56:04
  SPEAKER_02: All right, Jesse.
• 56:05
  SPEAKER_02: I'd like to do that.
• 56:06
  SPEAKER_02: I mean, do you want my opinion?
• 56:07
  SPEAKER_01: Uh like I do.
• 56:08
  SPEAKER_02: That's that's I think we need to wrap this up.
• 56:10
  SPEAKER_02: So let's give the elevator pitch.
• 56:14
  SPEAKER_02: I mean, clearly we got a problem.
• 56:16
  SPEAKER_02: I mean, this doesn't sit well.
• 56:18
  SPEAKER_02: And my yeah, like my stomach right now is not happy.
• 56:22
  SPEAKER_02: Okay.
• 56:22
  SPEAKER_02: Well, I want to know what you've we've gone really far in the weeds.
• 56:26
  SPEAKER_02: Like, I know we have, but what one of these or maybe all of them are wrong.
• 56:31
  SPEAKER_02: I like you, you know, you have a big discrepancy with the eudarlic because that should be older than the gabbro cell, because the gabbro cell cuts across it, and but the numbers don't bear out.
• 56:43
  SPEAKER_02: So it comes down to which of those two data sets are do you trust more?
• 56:49
  SPEAKER_01: Yeah.
• 56:49
  SPEAKER_01: And the eudarlic unit doesn't agree with itself in age, right?
• 56:52
  SPEAKER_01: If you look at the the individual units within it, they get different.
• 56:56
  SPEAKER_01: And that might be expected, actually, right?
• 56:58
  SPEAKER_01: Because metamorphism, if you think about how metamorphism works in a rock, the minerals will be in chemical communication, they'll be exchanging neodymium and samarium.
• 57:07
  SPEAKER_01: But this rock on one side of the outcrop might not be exchanging samarium in neodymium for the other side of the outcrop.
• 57:14
  SPEAKER_01: So you could have it's a it's like depends on how completely the rocks get overprinted.
• 57:20
  SPEAKER_01: So you can look, individual minerals can give ages that are 1.9 billion years old, while the rock itself gives an age that's 3.5.
• 57:27
  SPEAKER_01: We saw that in Acosta, and that's fine.
• 57:29
  SPEAKER_01: It's a different tool, even though we're using the same isotope system, we're it's still a different tool because it's like a slightly different size screwdriver.
• 57:38
  SPEAKER_01: Because you know, we're we're using a Phillipset screwdriver over here for a different size or scale than the little tiny one we're using to fix our computer.
• 57:46
  SPEAKER_01: It's a lot of people.
• 57:46
  SPEAKER_02: But why can't we just use uranium 238 then to see through all of it and go back to the original age of the of the original rock?
• 57:54
  SPEAKER_01: The problem with uranium lead system is that lead is really mobile in fluids.
• 57:59
  SPEAKER_01: So the uranium-lead system in these rocks is really perturbed.
• 58:02
  SPEAKER_01: So nobody uses it for whole rocks.
• 58:04
  SPEAKER_01: This is like take a rock, crush it up, dissolve the rock.
• 58:07
  SPEAKER_01: That's this data.
• 58:08
  SPEAKER_01: It's not individual minerals and things like that.
• 58:11
  SPEAKER_01: So I my opinion on this debate is I think the simplest explanation is that these rocks are old.
• 58:21
  SPEAKER_01: That these are in fact the oldest rocks we have on earth.
• 58:24
  SPEAKER_01: Am I totally convinced by that?
• 58:26
  SPEAKER_01: No.
• 58:26
  SPEAKER_01: I would be maybe two-thirds.
• 58:29
  SPEAKER_01: If I had to put a number on it, I'd be like, yeah, I'm 60 or 70% confident that these are the oldest rocks on earth, these are 4.25 billion years old, because that's the simplest explanation for the existing data that we have.
• 58:40
  SPEAKER_01: Would I bet my anything real on it?
• 58:43
  SPEAKER_01: No, I wouldn't.
• 58:44
  SPEAKER_02: That's it's interesting then.
• 58:45
  SPEAKER_02: Uh, here's my question.
• 58:47
  SPEAKER_02: How does this definitively get solved?
• 58:51
  SPEAKER_02: Is there a tool that we're missing?
• 58:53
  SPEAKER_02: Could more research solve the qu you know, answer the question?
• 58:58
  SPEAKER_02: Or is there a different tool that maybe we haven't used before, haven't thought of yet that we could use?
• 59:04
  SPEAKER_02: What do you think about that?
• 59:05
  SPEAKER_01: There is one that we could people have looked in these rocks for zircon, and what they get is zircon that's metamorphic.
• 59:12
  SPEAKER_01: All the zircon in here is metamorphic and young.
• 59:14
  SPEAKER_01: So it doesn't really tell you much about the age of the rock because it's metamorphic zircon, it's like 2.8 or 3.4 billion years old.
• 59:20
  SPEAKER_01: Our like gold standard one is kind of thrown out.
• 59:23
  SPEAKER_01: Doesn't work for these rocks because they're mafic.
• 59:25
  SPEAKER_01: What you could do is you could measure the 142 neodymium isotope composition of individual minerals in the Ujariluk unit, and that's a really hard task.
• 59:37
  SPEAKER_01: If they're the same, if it's all the same 142 neodymium isotope composition, if the the anthophylite and the coming type and the plagioclase are all the same composition, it doesn't really tell you anything because it it could have been mixed up, they could have started different and mixed up, but if they're different, if the plagioclase and the coming type anthophylite have different 142 neodymium isotope compositions, the rock has to be old because it tells you that the minerals are old, they have different systems.
• 1:00:03
  SPEAKER_01: They they grew, those minerals grew with different decay systems.
• 1:00:07
  SPEAKER_01: They had to be old.
• 1:00:08
  SPEAKER_02: And that has not been done.
• 1:00:10
  SPEAKER_01: That's not been done.
• 1:00:11
  SPEAKER_01: It's a really hard task.
• 1:00:12
  SPEAKER_01: I I don't know if it could be done or not.
• 1:00:14
  SPEAKER_01: It would be a a huge you just have to dissolve a lot, you have to do some really crazy chemistry to separate it out, and then the mass spectrometry is really hard.
• 1:00:22
  SPEAKER_01: So you need to separate out enough neodymium to make the measurement, and that's a difficult task at times.
• 1:00:26
  SPEAKER_02: Alright, so answer the question then.
• 1:00:28
  SPEAKER_02: Do you think this is ever gonna get solved?
• 1:00:30
  SPEAKER_01: Man, does it is it ever gonna get solved?
• 1:00:33
  SPEAKER_01: I'm not sure.
• 1:00:34
  SPEAKER_01: That's a really good question.
• 1:00:35
  SPEAKER_01: I've never actually thought about that.
• 1:00:37
  SPEAKER_01: I think it will.
• 1:00:38
  SPEAKER_01: I think it will.
• 1:00:39
  SPEAKER_01: I don't know how though.
• 1:00:41
  SPEAKER_02: Alright, okay.
• 1:00:42
  SPEAKER_02: That's fair, that's fair.
• 1:00:43
  SPEAKER_02: That's that was a part of my question.
• 1:00:45
  SPEAKER_02: You know, there's something that we haven't maybe thought of yet.
• 1:00:48
  SPEAKER_01: That and that's yeah, or some other some other system that people could use or more precise data.
• 1:00:54
  SPEAKER_01: Okay.
• 1:00:54
  SPEAKER_02: Final question then.
• 1:00:56
  SPEAKER_02: Why is why would this be important?
• 1:00:58
  SPEAKER_02: Just okay, you said or from the onset, because they're the oldest rocks on the earth, and therefore they're important.
• 1:01:03
  SPEAKER_02: But is there anything else?
• 1:01:05
  SPEAKER_01: Well, back to our discussion of Issua, these are this is a super crustal belt.
• 1:01:10
  SPEAKER_01: These are rocks that interacted with seawater, probably, ancient seawater.
• 1:01:14
  SPEAKER_01: And the question is, are we looking at ocean chemistry 3.8 billion years ago, or are we looking at it 4.25 billion years ago?
• 1:01:21
  SPEAKER_01: That's five or almost 500 million years ago.
• 1:01:23
  SPEAKER_01: Yeah, that's a long time.
• 1:01:24
  SPEAKER_01: Difference.
• 1:01:24
  SPEAKER_01: That's a that's a big difference as far as the importance.
• 1:01:27
  SPEAKER_01: So because these rocks are supercrustal, there's banded iron formations in there, we're getting a a view of the surface environment.
• 1:01:34
  SPEAKER_01: We're getting a little tiny window into the surface environment of Earth.
• 1:01:38
  SPEAKER_01: Either we have data like this from Ishua, or this is a totally unique data set that pushes back our understanding of the Earth system 500 million years more.
• 1:01:47
  SPEAKER_02: Like ocean chemistry and atmospheric chemistry, too, you think, or or not?
• 1:01:51
  SPEAKER_01: Yeah, like by inference, you'd you might get ocean chemistry, signs of life, perhaps.
• 1:01:56
  SPEAKER_01: You know, is there evidence of life here?
• 1:01:58
  SPEAKER_01: It's basically do we have another data point like Isua, or do we have a data point that's 500 million years older than Issua?
• 1:02:04
  SPEAKER_01: That's scientifically important.
• 1:02:06
  SPEAKER_02: Anything else that you can think of that would be like uh is this gonna advance any other research that that may lead to other important questions?
• 1:02:16
  SPEAKER_01: Uh there's been I'm trying to think through the the list of papers that have been published on this.
• 1:02:22
  SPEAKER_01: That's a hard question.
• 1:02:22
  SPEAKER_01: Yeah, it's a hard question.
• 1:02:24
  SPEAKER_01: Yes, almost certainly.
• 1:02:26
  SPEAKER_01: I just don't again know exactly which way.
• 1:02:28
  SPEAKER_01: Okay.
• 1:02:28
  SPEAKER_01: Um because when you have old rocks, a lot of different fields get interested in it.
• 1:02:33
  SPEAKER_01: The astrobiology community, you know, they're the biologists get interested in the old samples, um, understanding all the different kinds of reaction pathways that could occur.
• 1:02:43
  SPEAKER_01: The the atmospheric chemists, the ocean chemists get interested, and I don't know those fields.
• 1:02:48
  SPEAKER_01: I'm not an ocean chemist, so like what they're interested in and why is kind of like I'm like, ah, I've got rocks for you.
• 1:02:56
  SPEAKER_01: You know, you there's like a collaboration there, and I'm just not familiar enough with those fields to know like what they're thinking of or what they could do with new toys like these old rocks.
• 1:03:05
  SPEAKER_01: So all right.
• 1:03:06
  SPEAKER_01: So potentially first oldest rocks question mark.
• 1:03:10
  SPEAKER_01: Um, you know, it's a compelling case they made that they are.
• 1:03:15
  SPEAKER_02: I said we were gonna answer the question, and Jesse says yes.
• 1:03:18
  SPEAKER_02: These are the old it's the simplest explanation.
• 1:03:20
  SPEAKER_01: It's a they make it they make a compelling case that they are old.
• 1:03:23
  SPEAKER_01: I would say that.
• 1:03:24
  SPEAKER_01: Um so hey, I don't know.
• 1:03:27
  SPEAKER_01: What do you think, Chris?
• 1:03:28
  SPEAKER_01: That's we've should we pick our heads out of the weeds?
• 1:03:30
  SPEAKER_01: We've been down there for like an hour.
• 1:03:31
  SPEAKER_02: Well, with Joyce is she's got a really solid nap in by now.
• 1:03:35
  SPEAKER_02: Oh this is she she's listening.
• 1:03:38
  SPEAKER_02: I went over there.
• 1:03:39
  SPEAKER_02: Uh I just went there yesterday, and as I walk in, she's like, Oh, I'm just listening to but to you guys talk about garnets right now.
• 1:03:46
  SPEAKER_02: And oh, perfect.
• 1:03:47
  SPEAKER_02: Okay, excellent.
• 1:03:48
  SPEAKER_01: I'm glad to hear.
• 1:03:48
  SPEAKER_01: Well, Joyce, I hope you missed it.
• 1:03:50
  SPEAKER_02: I guess I'm a little bit behind.
• 1:03:52
  SPEAKER_01: Yeah, I hope the nap went well.
• 1:03:54
  SPEAKER_01: And uh maybe she's down for the night after this one.
• 1:03:56
  SPEAKER_01: Maybe just put her out completely after this.
• 1:03:58
  SPEAKER_01: It could, it could be, yeah.
• 1:04:00
  SPEAKER_01: I I Chris, you know, I must say, I I'm uh prepare for a compliment, you know, Midwestern Chris.
• 1:04:05
  SPEAKER_01: Prepare for a compliment.
• 1:04:06
  SPEAKER_01: It always impresses me how we don't talk about this stuff that like the this level of detail or this much in the weeds.
• 1:04:14
  SPEAKER_01: This is really complicated, isotope geochemistry.
• 1:04:16
  SPEAKER_01: Like, I can't belabor this enough.
• 1:04:18
  SPEAKER_01: This is this is advanced graduate student isotope geochemistry.
• 1:04:21
  SPEAKER_01: You pick up on this stuff really quick, you like put the pieces together really quickly from these these plots and just like putting this the story together.
• 1:04:29
  SPEAKER_01: So well, thanks.
• 1:04:30
  SPEAKER_01: That means a lot.
• 1:04:31
  SPEAKER_02: That means a lot, especially coming from you because you're the doctor in the house and this is your field.
• 1:04:35
  SPEAKER_02: I do try to keep up, you know.
• 1:04:37
  SPEAKER_02: Um, but this is something that I I'm enthralled with.
• 1:04:41
  SPEAKER_02: I love this stuff.
• 1:04:42
  SPEAKER_02: Age of the earth is super important to me.
• 1:04:44
  SPEAKER_02: You know that.
• 1:04:45
  SPEAKER_02: Um, it's interesting, Jesse.
• 1:04:47
  SPEAKER_02: I was at a Grand Valley brought in a speaker who works, he's a Grand Rapidian, so he's from Southwest Michigan, but he works for NASA and he's a planetary geologist.
• 1:04:59
  SPEAKER_02: And so he gave a lecture on um like the moon and and places where we could possibly put humans for for an extensive amount of time, right?
• 1:05:10
  SPEAKER_02: And they were actually looking at lava tubes because if they can find a way to take a lava tube and pressurize it, then think about the structures, the infrastructure you don't have to build.
• 1:05:20
  SPEAKER_02: Oh, yeah.
• 1:05:21
  SPEAKER_02: You know, and so it was a really interesting talk.
• 1:05:23
  SPEAKER_02: Anyway, that's not the point.
• 1:05:24
  SPEAKER_02: The point is that I was talking to one of the professors at Grand Valley before the lecture started, and and he had a book in his hand, and I immediately recognized it.
• 1:05:34
  SPEAKER_02: It was The Age of the Earth by Del Ripple.
• 1:05:36
  SPEAKER_02: And I love that book.
• 1:05:38
  SPEAKER_02: That book has set me on a path.
• 1:05:40
  SPEAKER_02: Um, I I read that book actually, I don't know, maybe 15 years ago, as I was getting ready to give a uh a talk with Dr.
• 1:05:48
  SPEAKER_02: Maddox and Lansing at um one of the science conventions on the age of the earth.
• 1:05:54
  SPEAKER_02: And I loved that book, and I'm like, why are you you know, I first of all, I love that book, why are you reading it?
• 1:06:00
  SPEAKER_02: And he was reading it to prepare to go into one of the nearby prisons to give a lecture, and what it was was really it was a trial run because they want to offer courses for the prisoners, and the options are a I think it was a chemistry course, a physics course, or a geology course, and it was kind of like this competition to see which one they thought would garner the most interest.
• 1:06:30
  SPEAKER_02: And he was he was reading that book to prepare for that talk that he was gonna give.
• 1:06:36
  SPEAKER_01: Yeah, that's super interesting.
• 1:06:37
  SPEAKER_01: Wow, that's very cool.
• 1:06:39
  SPEAKER_02: So I am I'm super interested in this stuff.
• 1:06:41
  SPEAKER_02: You know, I wrote this lab, that isochron lab where the students have to account for initial daughter isotopes.
• 1:06:47
  SPEAKER_02: So I I just I appreciate the compliment, but it's kind of like I love it.
• 1:06:51
  SPEAKER_02: I it's this is super interesting stuff to me.
• 1:06:54
  SPEAKER_02: I rip on you all the time, but that's because I can.
• 1:06:56
  SPEAKER_02: I have a license to do that.
• 1:06:58
  SPEAKER_01: So I mean, I just I I say that because it's this is deep stuff.
• 1:07:02
  SPEAKER_01: This is deep patrol, this is deep isotope geochemistry, this is heavy stuff.
• 1:07:06
  SPEAKER_01: So if you didn't follow everything in this episode, that's quite all right.
• 1:07:09
  SPEAKER_01: Um it's you know, it takes a while to to sort of uh flex these muscles like this.
• 1:07:15
  SPEAKER_02: I did have a student that just uh either two days ago, I think, she said she's one of my top-notch students, like of all time that I've ever had.
• 1:07:24
  SPEAKER_02: She's like, Mr.
• 1:07:24
  SPEAKER_02: Bullheis, you guys talk about the weeds a lot, and she said, Can you just go into the weeds?
• 1:07:30
  SPEAKER_02: Like, you know, she she said dancing around it and just do it.
• 1:07:35
  SPEAKER_02: Go into the weeds.
• 1:07:36
  SPEAKER_02: Yeah.
• 1:07:36
  SPEAKER_02: Um I feel like we did that today.
• 1:07:38
  SPEAKER_02: You know, we did that today.
• 1:07:39
  SPEAKER_02: We'll keep doing it.
• 1:07:40
  SPEAKER_01: We'll keep doing it.
• 1:07:41
  SPEAKER_01: We love the weeds, absolutely.
• 1:07:42
  SPEAKER_02: Maybe we just need to go niche, Jesse.
• 1:07:44
  SPEAKER_02: Just yeah, that's right.
• 1:07:45
  SPEAKER_02: That's right.
• 1:07:46
  SPEAKER_02: Lean into it.
• 1:07:47
  SPEAKER_01: So yeah, totally.
• 1:07:48
  SPEAKER_01: All right.
• 1:07:49
  SPEAKER_01: Well, hey, that's a wrap on this.
• 1:07:50
  SPEAKER_01: You can find the images, all the images we discussed.
• 1:07:53
  SPEAKER_01: You can find all and and really, I think these help you understand the weeds, understand the geochronology, understand the debates that are that are ongoing about Earth's oldest rocks and our Earth's oldest rocks video podcast series on the Camp Geo app.
• 1:08:04
  SPEAKER_01: First link in your show notes.
• 1:08:05
  SPEAKER_01: Send us an email, please send us an email.
• 1:08:07
  SPEAKER_01: If stuff like this isn't making sense, if you got questions, you want follow up stuff, send us neoplangeocast at gmail.com.
• 1:08:12
  SPEAKER_01: You can follow us on all the social media at Geocast.
• 1:08:16
  SPEAKER_01: Cheers.