The Room Problem (and how Geochronology solves it)

Granite is everywhere — it makes up much of the continental crust beneath our feet; yet, for over a century geologists were stuck on a deceptively simple question: where does all that magma find room? In this episode, Jesse and Dr. Mike Ackerson pick the granite series back up to tackle the "room problem" — the puzzle of how you wedge a magma body tens of kilometers wide into solid crust. Using California's Tuolumne Intrusive Suite (the fruit fly of igneous petrology) and the landmark Coleman et al. 2004 paper, they show that the way out isn't space at all — it's time. These giant plutons weren't emplaced in one molten flash; they were assembled incrementally over millions of years. Along the way: a whirlwind history of geochronology, why the word "lead" makes clean-lab chemists recoil in horror, and the slow death of the "magma chamber."

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Episode Transcript

SPEAKER_00 (00:02):
Welcome to Planet Geo, the podcast where we talk
about our amazing planet, how itworks, and why it matters to
you.
Dr.
Mike Aggerson.
Welcome back.

SPEAKER_01 (00:15):
How are you doing?

SPEAKER_00 (00:16):
I'm great, man.
I'm great.
I get to talk about graniteswith you.
This is so fun.
Uh, what could be wrong in theworld?

SPEAKER_01 (00:22):
We left off your subscriber numbers.
Plummeting.

SPEAKER_00 (00:28):
Just straight plummeting.
People, people, you know, clickon it and like, oh, it's Mike
and Granites.
Now I'm out.
Granite.

SPEAKER_01 (00:36):
Great.
Okay.

SPEAKER_00 (00:37):
That would be interesting to look at the uh
listener.
I don't actually know if we getthat.
Like, I wonder if we can findthere must be a way to find off
when listeners drop off.
And as soon as we talk aboutgranites, more granites.
Second 29 people are jumpingoff.
Well, uh, we have, in allseriousness, we have been doing
uh a series and we've got muchmore of a series to go on

(00:59):
granites.
We're kind of taking a deepdive.
This is, I would say this islike upper level undergrad.
Maybe depending on where you goto undergrad, it might be grad
early grad student level uhigneous petrology, but
definitely upper level undergradin most places.
So that's the kind of the levelwe're pitching here.
If you want to get some of thebasics, like hey, what is a
granite?

(01:19):
What are these guys talkingabout?
Feldspar, Zircon, what are theytalking about?
Are those minerals?
Are those rocks?
What's the deal?
You can go to our Camp GeoMobile app, first link in the
show notes there.
We have a bunch of introductorystuff there.
Um, but we left off last time,Mike.
You gave this beautiful historyof the field of granite
petrology and many of thedebates and arguments, and how

(01:39):
those are really debates aboutthe founding of the field of
geology in many ways.
And that was really fundiscussion.
We left off there talking aboutthe room problem or the space
problem.
And uh maybe it'd be good if youjust give us a recap of of that
problem.

SPEAKER_01 (01:56):
Sure, yeah.
The uh the the cliffhanger fromour last conversation was this
idea of the room problem, right?
So to bring people up to speed,the room problem is the idea
that if you are uh taking amagma, creating a magma
somewhere in the crust or theupper mantle or something, and
bringing it up into the uppercrust, you need to create space

(02:16):
for that magma in the crust.
And really this came from thelast kind of big scientific
shift you and I were talkingabout, which was granitizers
versus magmatists.
So people that thought thatgranitic rocks can be produced
through uh water flowing throughthe system, which are the
granitizers versus magmatists,which are the people like Norman

(02:38):
Bowen who made the observationsthat you could produce uh
granites through magmatism andcrystal fractional
crystallization.
So the magmatic perspective tookhold in the mid-early to
mid-20th century, and that thenbrings with it the room problem
again.
So the granitizers thought theyhad the room problem solved, but

(03:00):
turns out most granites areigneous rocks in some form.
And so then you've got the roomproblem again.
So if granites are magmas, howdo you create room for a magma
in the upper crust, which iswhere we know a lot of the
granites reside?

SPEAKER_00 (03:14):
Yeah.
And so to kind of paint thevisual of this is uh let's let's
imagine, you know, YosemiteNational Park.
This is the rocks under yourfeet in Yosemite National Park.
You can, I mean, most people Ithink can picture Yosemite
National Park.
It's Half Dome, uh, thesebeautiful I think it was one of
the Mac computer, you're not aMac guy, but one of the original
Macs, oh the default backgroundwas Yosemite National Park.

(03:37):
Um that shipped in like you knowthe 20 2010s or something like
that.
Um but you you pictureHalf-Dome, beautiful U-shaped
valley, you know, basicallymountain cut in half by a
glacier, Yosemite National Park.
The rocks underfoot are theTawalumni Intrusive Suite, which
you have worked a lot on, Mike.
Um and is a big, it's a veryfamous, it's like I mean, it's

(04:00):
like the template for one of thetemplates.
It's one of these areas thatpeople have studied for a long,
long, long, long time.
It's like the fruit fly.
We use this analogy when we weretalking about the bishop tough,
as the bishop tuff is the fruitfly of uh volcanism, and the
Tuami Trusa Suite is like thefruit fly of igneous petrology,
right?
Like it's valuable because somany people have studied it

(04:21):
before, it becomes increasinglymore valuable for every study
that's done on it.
But if you're walking across,okay, you're walking across this
entire national park is likegranite, and you walk across it,
it all looks the same.
It's easy to imagine.
If this is all magma all at onetime, it's a huge amount of
space, and that's the roomproblem.
Like, how do you get room in thecrust for liquid, a big liquid

(04:43):
vat of magma that's tens ofkilometers wide, a couple
kilometers thick?
It's a huge batch of magma.

SPEAKER_01 (04:50):
Huge batch of magma.

SPEAKER_00 (04:52):
Yeah, if it intrudes all at one time, and if it's all
magma, we have a room problem.
And what we talked about beforewas like the the knob of if it's
all if it's magma, right?
The granitizers are saying it'snot magma actually, the magma
just saying it is magmaactually, and and that was the
knob that we were discussing.
But but time is a part of this,right?

SPEAKER_01 (05:16):
Sure, sure.
It's absolutely part of it.
And you know, the Tuolie meintrusive suite that that we're
talking about, much like othersimilar rocks in the Sierra
Nevada of California or otherplaces across the globe, you can
actually walk around the edgesof these rocks and you can see
where the granites areinteracting with the

(05:36):
pre-existing wall rock and othergranites.
So you could, yeah, boots on theground, you could walk around
and trace the outline of theTuyumi Intrusive Suite.

SPEAKER_00 (05:45):
Okay, I want to maybe you're going here, but I
want to ask the question why thename Tuamni Intrusive Suite?
What does that mean?
The suite part of it.

SPEAKER_01 (05:52):
Yeah, so you're like a student who asks the question
that's coming up on the nextslide.

SPEAKER_00 (05:57):
Sorry.
Really annoying, extremelyirritating.
Yeah.

SPEAKER_01 (06:00):
No, not at all.
Um so you know, if you go out toTuanyi, you can walk around, you
can see the perimeter, you candefine the perimeter of the
entire intrusive suite.

SPEAKER_00 (06:12):
What's around the intrusive suite?

SPEAKER_01 (06:14):
In some places, it's older igneous rocks, older
granites.
Like uh the L Cap granite, whichis also in Yosemite Valley, is
older than the Tuolumeeintrusive suite.
Uh-huh.
So you can actually kind of mapthe contact between the
Tuolumene and the L-Cap granite.
On the eastern side, thenortheastern side of the
Tuolumni intrusive suite, youcan actually walk uh the

(06:36):
contacts on this amazing placelike called Saddlebag Lake,
where you can hike and you cansee where the igneous rocks are
intruding into uh sedimentary,meta-sedimentary rocks and
things.
It's just that's a beautiful,but you can actually uh see
these contacts and you can walkthe contacts through these
rocks.

SPEAKER_00 (06:53):
Amazing.
Uh-uh.

SPEAKER_01 (06:53):
And it is, yeah, like you're saying, it's tens of
kilometers.
It's it's a massive uh scale ofthis plutonic system.
But why it's called a suite, orsome people call it a complex,
uh, is because if you walk fromthe outside of the Tawang Me to
the inside, the compositions ofthe granites change.
They are uh have more darkmaterial or mafic material on

(07:15):
the outside, and they as you gointerior, the rocks uh get
lighter or more felsic.
So more mafic, darker on theoutside, more felsic, uh lighter
and the interior.
And so you can see this overthis broad period that you go
from darker rocks to lighterrocks.
So this came with it then thisidea from a lot of geologists.

(07:38):
One of the more famous papers isfrom these guys, Bateman and
Chapel, a couple of legends inthe field, where they basically
said, Okay, well, you had theseuh older magmas, like a huge
batch of magma intruding intothe upper crust that was uh
older and more mafic, and thenyounger and more felsic stuff
was intruded to the interior.

(07:59):
But we're talking huge batchesof magma on the scale of
something like a bishop tough orlarger.
So huge volumes of magmaintruded into the upper crust,
kind of one Pluton, one magmachamber gets injected, another
magma chamber gets injected, andthat's how you have like three
or four or five pulses of likemagma coming in to form these

(08:21):
huge bodies.
So, like most people, I don'tthink, ever really thought that
the whole twailum was at onepoint like one huge magma
chamber that was injected atonce, but still huge magma
chambers injected over a coupleof pulses.

SPEAKER_00 (08:37):
So the individual units here, and there's five
dominant units that that we'lltalk about, I mean, broadly that
are that have been broken apart.
There's subdivisions in there aswell, but but broadly a couple
of different units here in thissuite.
And this is a very commonfeature uh amongst granite
complexes.
These are what's called zonedplutonic complexes, they occur

(08:58):
all over the world.
Uh they're very common where youhave sort of more mafic on the
outside, then intermediate inthe in the middle, and very
felsic in the interior.
And this has led to, I think, atthe very intro level, like the
the intro level textbookversion, is that yeah, this
makes sense, right?
Like high temperature magmas aremore mafic, typically.

(09:20):
And so the first batch of magmais mafic, and then maybe there's
some magma source down below, abig magma chamber down at depth
that is the source of this.
And as that cools andfractionally crystallizes,
things go from mafic tointermediate to felsic, and
these zoned plutonic complexesare kind of aggregating, you
know, they're they're recordingpulses of that different
magmatic system down below.

(09:41):
So maybe the magmatic, thefractional crystallization is
not happening in the upper magmachamber, but the upper magma
chamber is recording processesthat are happening down below,
and that would be basicallyfractional crystallization.
A high temperature thing, amafic magma intrudes at depth,
punches a little bit of maficstuff up, then it cools down,
forms more andesiticintermediate composition stuff,

(10:04):
punches that back up to thecrust, that cools down.
Basically, what you're left withwhen you've got, I don't know,
20% of the magma left, you'releft with something felsic, and
you that stuff can also move upinto the upper part of the
crust.
And these are really commoncomplexes globally, and we often
see this is a very commonfeature, these kind of ringed,
you know, mayfic on the outside,felsic on the inside,

(10:24):
intermediate in between.
That's that's pretty common.
So, anyway, the the point herethat we've we talked about
previously that the magmatistsand the granitizers and the you
know, they were debating aboutwhether it was magma or not.
But another like escape hatchfor the room problem is time
because it's easy.
And for me, this was all the waytill I think I referenced this.

(10:46):
Like this was maybe year two ingraduate school when I was like,
wait, magmas or igneous rocksdon't crystallize all at
effectively the same time, youknow?
Like it's not a single pulse ofobviously it takes a bit of
time, but I was always like, Oh,it happens basically
geologically instantaneously.
No, it takes a long time, right?

(11:07):
And it takes actually anincredibly long time, and that's
what we're gonna focus ourdiscussion on here.
The way out of the room problemis really time, it takes a long
time to aggregate these things,and that's how we get around the
room problem.
We're kind of giving up the thepoint of this episode here, but
I think it's an interestingstory in how we got there,
right?
Like we're gonna center ourdiscussion on a key paper.

(11:28):
What's the key paper?
It's near and dear to yourheart, I know, Mike.

SPEAKER_01 (11:32):
Oh, yes.
Uh Coleman uh et al.
2004 from the journal Geology, Ibelieve.

SPEAKER_00 (11:38):
Yeah, it's in it's in geology.
Um, this was kind of one of theframework papers or the initial
papers that kind of documentedtime in this particular igneous
complex Tatuami intrusive suite.
If you'll allow me, uh Mike, Ithink there's an interesting
story here in the history ofgeochronology, in a way, of how
we got the ability to measuretime in a magmatic system.

(12:02):
Like it took a lot of techniquedevelopment, and this was not
possible.
So you're the paper we'rereferencing is a 2004 paper.
This type of science wasprobably not possible in 1970 or
1960.
The techniques did not allow it.
And you made a reference uh lastepisode about like how as our
techniques get better, we getbasically we get to ask better

(12:25):
questions um because thetechniques allow it, and this is
a great example of that.
Like geochronology, so I I Imean, measuring time in magma
chambers is uh a hot field.
I mean, it's something we dovery frequently, lots of
different labs are doing it now,but it used to be really hard,
and I think that was part of theproblem here.

SPEAKER_01 (12:45):
Yeah, it is, it is.
I I will uh if you if you'llallow me to ask you not to call
them magma chambers.

SPEAKER_00 (12:52):
Okay, what should I call them?

SPEAKER_01 (12:53):
Magmatic systems or something.
You know, I I'm just I get alittle, you know, because a
magma chamber, magma chamberevokes the same thing that, you
know, eventually I think we'regonna be talking about is maybe
not what is really happening.
Okay, so oh mag magma chamberevokes churning vat of magma.

SPEAKER_00 (13:09):
All right, all right.

SPEAKER_01 (13:10):
It's a magnetic system.

SPEAKER_00 (13:12):
The word magma chamber evokes the room problem,
and we shouldn't be talkingabout that.
Okay, perfect.
That sounds good.
Let's come back to that point.
I won't call it a magma chamberagain.
I'll call it a magmatic system.

SPEAKER_01 (13:21):
I call it a magma chamber like this.
You know, like the future marksout here.
For for the podcast listeners,quotation marks your own magma
chamber.

SPEAKER_00 (13:29):
Okay, we'll do that.
All right.
Um, I won't do that, and we'llcome back to like what are what
we should visualize when wethink of a magmatic system, um,
and what this data and someother field data uh has told us
Coleman 2004, and that kind ofkicked off a boom in
geochronology uh in this field.
So the the history ofgeochronology, I'm gonna give

(13:49):
some dates, Mike, if you're okaywith that.

SPEAKER_01 (13:51):
Oh, yeah, please.
This is your bailiwick righthere.

SPEAKER_00 (13:55):
I I like this stuff.
It's fun to talk about, and it'sI don't know, the techniques
involved are amazing, I find.
Oh, incredible.
Geochronology is a very oldfield, and it was kind of came
into being.
The first radiometric dates wereactually from uh Ernest
Rutherford, Ernie, in 1904, verysoon after the discovery of
radioactivity.

(14:15):
Um, with these werehelium-uranium dates, and so
helium was determined to be aproduct of the decay of
uranium-238 at that time, and sothe obvious thing was hey, we
can measure helium and uranium,and that's like a chronometer.
Um, they didn't really know theuranium-238 decay rate very
well.
Helium is produced a whole bunchof times throughout the decay,

(14:35):
but they didn't know that, sothey're really rough numbers.
But they're 1904, that's reallyimpressive.
A full century before this magmachamber paper.

SPEAKER_01 (14:42):
Oh, uh, I didn't know this is you know, this is
this is all new information forme because I'm not a
geochronologist, but like thefact that they were immediately
thinking about how you could usedecay to measure time is really
cool.

SPEAKER_00 (14:56):
Like it's really cool.

SPEAKER_01 (14:58):
They're thinking about radioactivity, right?
They're like, wow, thissomething's happening.
You know, we're discoveringalpha particles, and immediately
we're like, okay, how can youuse this to make to measure
time?
Like, that's super cool.

SPEAKER_00 (15:08):
This was in the time period where you know Lord
Kelvin was trying to calculatethe age of the earth from
cooling rates, just simplecooling models, and they didn't
appreciate that Earth wasgenerating its own heat by
radioactivity.
Um, and other people were tryingto calculate the age of the
earth by using the saltiness ofthe ocean and comparing the
salinity of rivers to thesaltiness of the ocean and

(15:29):
saying, like, that's the age ofthe earth, right?
So this was this was a wide openquestion.
So any amount of data wouldwould have been useful in this
scientific discussion.
Um, very soon after, uraniumlead was used to date uranium
ores, so uranium-rich oredeposits.
You could measure uranium andlead from those, just chemistry.
The age of the earth in 1913,Arthur Holmes started to

(15:51):
actually develop some techniquesthat allowed for the measurement
of zircon and feldspar,individual, not individual
mineral grains, but thoseminerals in particular, not
minerals that are loaded withuranium.
And then we really get into massspectrometry improvements.
In 1919, Aston builds a massspectrograph in which you can
measure individual.

SPEAKER_01 (16:11):
Sorry, can I just ask a question there just to get
myself on track?
So you're saying all thesethings up to this point, they're
just bulk measuring theelements.

SPEAKER_00 (16:19):
So the element, it's chemistry.

SPEAKER_01 (16:20):
Uranium and lead concentrations in a big old vat
of zircons that they dissolvedor something.

SPEAKER_00 (16:25):
This is all just chemistry.
So it's like separate uraniumand separate lead from the
minerals of interest and measurethe concentrations of each.
It's not anything to do withisotope geochemistry, which is
using the specific isotopes.
That comes around in 1919 whenAston builds the this mass
spectrograph, which um couldseparate the different isotopes

(16:48):
from one another.
And very soon after that, in the1920s, analyzed lead isotopes,
and this led to the discovery ofthe 235 uranium isotope because
they're looking at lead andsaid, Oh, wait, there's
something called 207 lead here.
There's a 207 lead mass.
Where does that come from?
Looked at uranium and oh wait,there's a tiny little mass of
uranium-235 next to a big 238signal.

(17:11):
And then in the late 30s, Nierimproved this and makes
effectively the template for themodern mass spectrometer and
determines that the Earthcontains lead that is not
produced by the decay ofuranium, basically.
And then we get into this WorldWar II era and and all of the
nuclear physics that goes onassociated with that after the
discovery of fission, fission ofuranium atoms and the focus on

(17:34):
uranium-235 in the ManhattanProject leads to this ability to
separate isotopes from oneanother.
So all of the focus on buildingnuclear weapons was focused on
separating uranium-235 fromuranium-238 and plutonium from
both of those.
But 235, that that technologyallowed for basically an
explosion of geochronology usecases.

(17:55):
Uh, and in large part because wecould separate individual
isotopes from one another, wecould then use those separated,
those pure separated isotopes asa spike.
And we could put that into oursample and say, oh, we're we're
doping our sample with this pureisotope so that we can trace
that throughout our entirechemical and analytical process,

(18:17):
and we can use it to calibratethe ages.
Before that, we were kind ofdealing with uncalibrated
measurements.
Um, and so this is just a way todo what's called isotope
dilution.

SPEAKER_01 (18:27):
Can I pause you again to ask a couple more nerd
questions?

SPEAKER_00 (18:30):
Because this might hold on.
This interlude might become afairly large interlude if this
keeps going on.
But yes, please.

SPEAKER_01 (18:36):
No, no, no.
Well, I mean, you know, I'm theaudience here.
I don't know anything about thisstuff.

SPEAKER_00 (18:40):
Okay, perfect.

SPEAKER_01 (18:41):
So, like, you know, you're talking about was it
near, you said, um, doing thesemeasurements and stuff.

SPEAKER_00 (18:47):
The mass spectrometer, developing mass
spectrometer, yeah, yeah.

SPEAKER_01 (18:50):
So developing mass spectrometers, but were that
they weren't separating for forduring World War II, they were
using they weren't using massspectrometers to separate
isotopes.

SPEAKER_00 (19:00):
Well, it's kind of yes, in a way.
I mean, one of the mechanismsfor separating 235 from 238 was
uh the calutron, which is theBerkeley cyclotron, the cal
cyclotron uh name.
Um, and and this is effectivelya mass spectrometer.
It's an extremely large massspectrometer that can separate
the uranium isotope.

(19:20):
So that was one of the ways.
I forget which plant might havebeen Oak Ridge that did that
used the calutron method.
There are other gaseousdiffusion methods that um you
know relied on uranium-235diffusing slightly faster than
uranium-238.
Uh there's anyway, that that's alittle bit outside, but there
are some of them were using massspectrum, effectively mass
spectrum.

(19:40):
Ionize a thing, spin it througha big magnet, and you get
basically the mass separation,then collect it.
Just put a big bucket at oneside and collect all of it, and
you'd have to do that a couplecycles through to get pure
stuff.
I mean, it's amazing, right?
I mean, but that is so cool.
So I mean, like, this is it'samazing.
So I've heard stories that whenyou turn these isotope separator
machines on, when you are tryingto separate a pure isotope of

(20:03):
one element, you're usuallyafter the smaller isotope, like
not the big isotope.
And when you turn it on, theintensity of the ion beam is so
big that you can like see itglowing sometimes in space.
Like you can see the like theythey are very bright ion beams.
Like there's this huge amount ofmaterial going through these

(20:25):
things, which makes it it'samazing technology.
I would love to see some ofthese things.
Uh, go to a national lab and seethese things, you know,
producing some of these uhisotopes.
It's just awesome.
I mean, it's such cooltechnology.

SPEAKER_01 (20:38):
You know, I guess it just gives me so much respect
for the number of extremelysmart people whose time went
into this stuff because youknow, we're sitting here saying,
Oh, this helps resercons, butlike this is like buoyed entire
economies.

SPEAKER_00 (20:54):
They were doing this before computers, they were
doing this with like extremelycompared to modern stuff,
rudimentary calculationtechnologies.
This is like they they wereusing they were like dumping you
know magnetic sand onto thetemplate and and seeing how it
aligned to map out magneticfields and stuff.
I mean, this is incredibledevelopment.

(21:15):
Like to do this type of work todevelop these things is nuts.
I have so much respect for thesepeople.

SPEAKER_01 (21:20):
Well, let's just ionize a bunch of extremely rare
stuff and then just shoot itthrough a magnet and separate it
by mass.
Like that is where it goes.

SPEAKER_00 (21:28):
I mean, but the fundamentals are so they worked
out these fundamental equationsand then applied them really
strategically.
It's it's a it's an exceptionalthing.
So that the post-war era led toisotope dilution.
We had really enriched isotopesof a single uh a single isotope
of uranium 235.
We could have a pure uranium 235thing, and we could put little

(21:49):
pieces of that into our sample,and therefore we could know much
more precisely how much uraniumwas in our sample with far
smaller uncertainties.
Um, same thing with lead, andlead is.
Kind of the big one for the foruranium blood geochronology.
In the the post-war era, Tiltonand Patterson, who were both at
the University of Chicago as asPhD students, they were working
kind of in tandem on massspectrometer development and in

(22:12):
clean lab technique developmentto do to work on meteorites
basically.
But they they developed techclean lab chemistry techniques
that allowed for much purersampling and means you needed
less sample to go in.
And here's kind of the the grainsizes basically.

(22:38):
So you had to have a sample thathad a ton, you had to have a
huge sample with a lot of leadin it to make a measurement,
just a single measurement.
Wow.
Nier in 1938 needed 10milligrams.
So, you know, several orders ofmagnitude less.
That's about two grains of rice.
That's still a ton of lead bymodern standards.
Like two grains of rice of purelead.

(23:00):
That's not the full sample,that's just lead.
So you couldn't measure you ifyou wanted to measure feldspar,
you had to measure, you had todissolve tons of feldspar to get
that much lead out.
Not tons, literally, but you hadto measure a lot of feldspar.
Lots.
Lots.
Yeah, lots.
Tilton and Patterson, theyneeded 10 micrograms, which is
about the size of one small saltgrain.

(23:21):
So so you know, increasinglythey needed a less and less and
less material to do this with.
In modern techniques arenanograms.
We're talking about the theweight of a human cell, is the
amount of lead we need to make ameasurement.
And so we can dissolve fragmentsof individual mineral grains and
dissolve those and get precisemeasurements now.

(23:43):
I mean, it's crazy stuff.
So in 1955, uranium-235 wasdeclassified from top secret,
and so all this public, allthese publications came out in
in 1955-56 kind of era on thistype of stuff.
Then in the 70s and 80s, wemoved into like the nanogram
range.
So now we're analyzing lead, theuranium and lead.
Lead is really the big one.

(24:05):
And the reason for this isthere's a lot of lead in the
environment.
There's lead in the air, there'slead in the water, there's lead
kind of everywhere.
It's hard to get a lab cleanenough that doesn't have any
lead in it.
So a lot of the techniques weregetting rid of the contaminant
lead so you could analyze a lotsmaller samples and get a useful
precision out of it.
So get rid of the lead in theenvironment, get rid of the lead

(24:26):
in your lab, get rid of the leadin your the whole sample
workflow, which you knowbasically meant clean acids.
The development of Teflon,interesting, by DuPont was a big
step forward here.
The just the invention of Teflonmeant that you could do clean
lab chemistry, you coulddissolve individual zircon
grains in a Teflon beaker verycleanly compared to before.

(24:48):
So there's a bunch of kind oftechnique developments in that
era that led to the ability nowto analyze in the early 2000s
individual single grains ofzircon, or usually like an
aggregate, you'd collect likethree or four zircon grains, or
maybe five or six, and thatwould be one sample.
You would dissolve all that,dissolve it, separate out the

(25:10):
uranium, separate out the lead,and then measure uranium and
lead.
And that would be your sample,would be several zircon grains.
And in some cases, one zircongrain.
So just to calibrate, Mike, likenot when we started our PhDs,
but maybe like a decade beforethat in the early 90s, people
used to collect a fullfive-gallon bucket of rock for

(25:31):
one sample to separate zirconsin order to analyze enough
zircon to make a measurement, orto make like, you know, five or
ten measurements from onesample.
Nowadays, we, you and I, we dothis all the time in my state is
we analyze tiny, tiny, tinyfractions.
We we analyze, take a sand grainand we poke holes in it that are

(25:53):
extremely tiny and get ages outof it.
So the technique development hasjust been exceptional in the
last 40 years.
Uh, in this 2004 paper we'regoing to talk about, you know,
was 20 years ago already, andthere's been technique
development since then.
So um anyway, that that's kindof a short story of how we got
to to 2004.

SPEAKER_01 (26:12):
The amount of material in there, the the the
fact that you were able to makethat so brief is uh pretty
impressive.
Gossed over about 100,000 PhDsright there.

SPEAKER_00 (26:23):
That that's shorthand for that was really
confusing.
You went way too fast, andnobody followed me.

SPEAKER_01 (26:27):
No, no, no, not at all.
My brain is spinning, but in agood way.
Like, I mean, it's it's it'sit's so cool.
I have a a question for you assomebody who's done some of this
and does some of this reallyhigh precision wet chemistry
stuff for with respect to lead.

SPEAKER_00 (26:46):
Yes.

SPEAKER_01 (26:47):
What makes you nervous about lead?
And I'm asking this as somebodywho like I I do experiment, I'm
an experimental petrologist.
We in the laboratory we we weactually use a lot of lead, like
lead foil.

SPEAKER_00 (27:00):
Oh, good, okay, perfect.

SPEAKER_01 (27:02):
So, you know, like lead foil, you know, you wrap
your experiments in lead foil orwhatever.
And when I've been around Tim'speople and talking about using
lead foil, I see them recoilwith smear over my common lead.

SPEAKER_00 (27:18):
Okay.

SPEAKER_01 (27:18):
So yes, does that make you nervous?
It seems like there's a lot oflike um superstitions around
doing this work, like notgetting common lead in your lab.

SPEAKER_00 (27:26):
Yeah, yeah, yeah.
Okay, so this is gonna get alittle bit weedy.
My my real answer will get alittle bit weedy.

SPEAKER_01 (27:33):
Okay.
This is now finally we're gonnaget weedy.
Yeah, yeah, now finally.

SPEAKER_00 (27:37):
We haven't been there yet.
Um, no, I'll give, but I'll giveyou the the I'll give you a
description of what clean labpeople do and don't do in the
lab to keep it clean.
So, first of all, people don'twear wedding rings into the lab
because there's potential formetal contamination.
And actually, people often donot ever wear metal wedding

(27:58):
rings because you know you can'tclean that off your hand
basically once you get in there.
Um, these clean labs are overpressured, so there'll be like
an inner chamber that's atreally high pressure and super
clean, and that blows the allthe air in there comes in
through a bunch of filters, andthat air blows out into an outer
chamber, an outer lab that'slike a slightly less clean room,

(28:20):
and then that air blows into thethe anterchamber that you go in
to change into, and all that airis blowing out into the hallway,
so hallway air never gets intothe clean lab and never, never
gets into the very, very cleanlab in the center.
So that's kind of the we'reworried about airflow for sure
and clean air coming into thelab.
But when people are dissolvingzircons, individual zircons, in

(28:40):
their little tiny, tiny, tinyTeflon beakers, the extent that
they go to, they'll buy newTeflon, brand new Teflon from
the manufacturer.
They'll get it.
They will clean it for a year.
Literally clean it for a year,put very clean acid into it, put
it in for two, two days or aweek, put it in, boil it, take
it out, put a new batch acid in,put it back in for two days or a

(29:03):
week, take it out, do that overand over and over, like a
hundred times over a year beforethey ever let a sample go into
that Teflon.
Then when the Teflon has sampleinto it and the sample it the
cap is open, they will nevermove something over the top of
that thing.
In your little, you know, we'renot only in the clean, clean,
clean chamber, we're in theclean, clean, clean chamber in a

(29:25):
hood that has clean, clean,clean, clean, clean, clean air
blowing over the sample and outof the chamber.
So we're in like this superclean, super, super duper clean
environment.
And they won't like pass thethere's no metal anywhere, no
metal tweezers or anything, butthey won't like pass their hand
over the beaker because thepotential for lead to get in
there is high.
And so the question is, why doyou care so much about lead?

(29:48):
So when you're talking aboutwalking around with lead foils,
people see you as like you arelike the Charlie Brown character
who's just like giving off dustof lead, and like I bet they
won't even shake your handanymore, basically, because
you're just like bleedingcommon, you're just bleeding
lead dust around.

SPEAKER_01 (30:04):
Yeah, you're not gonna see Galena on a on a desk
of one of your colleagues.

SPEAKER_00 (30:08):
Do not bring that anywhere nearby.
Um just this is not a lab I run.
My lab is not like this, uh isnot this style of lab, just to
be clear.
The the reason that it mattersis because if you're making a
sample measurement, we're veryinterested in the 207 lead to
206 lead ratio.
That's where the uranium-ledyour chronology power is um in

(30:30):
some cases, or really the amountof 206 lead.
But in the zircon, in the idealworld, the zircon does not take
any common lead, lead from thebackground.
We're using the term commonlead.
That means lead from theenvironment.
It doesn't take any in when itgrows, but there's always a
little bit of common lead.
And we have to correct for that.

(30:51):
We have to like remove the 206,the amount of 206 that's from
the background.
When we remove that, we don'texactly know what the
composition of that lead is.
We don't know if it's like leadfrom the lab.
We don't know if it's lead fromthe the rock, we don't know if
it's lead from the magma fromwhich the zircon grew.
Like it could be from any one ofthose sources.

(31:13):
So when we do that correction,there's a lot of uncertainty in
doing that correction, unlessyou know exactly where the lead
came from.
And we never really know exactlywhat so you kind of anytime
there's that common lead,there's like there's
environmental lead in youranalysis.
You you can correct for it, butit adds a big batch of

(31:33):
uncertainty to the ultimateresult of your analysis.
So that's why people are talkingabout picagrams of lead in their
you know analysis as being likegood background lead.
Like you want that a tiny, tiny,tiny, tiny amount of background
lead as small as possible.
Um, so that's why people getscared of you and recoil in

(31:54):
horror when you talk about it'snot because of your scientific
ideas, Mike, or your you know,opinions about other things.

SPEAKER_01 (32:00):
Six of one, half of those are the other people.
Yeah, yeah, it could be one.

SPEAKER_00 (32:03):
Anyway, we are very far from the the room problem
right now.
So maybe let's let's like getback to the room problem here.
The room problem.
All right, so we're talkingabout how do we get time out of
a magma chamber, right?
This is the whole rant, my wholerant about geochronology was to
say that it took a lot oftechnique development and a lot
of work to figure out techniquesto get time measurements out of

(32:27):
a magma, magmatic system.
I'm gonna stop saying magmachamber, I promise.
Out of a magmatic system.

SPEAKER_01 (32:33):
I say it too.
Don't don't worry, but it's justit's a default when you're
talking to like non like notyour colleagues.

SPEAKER_00 (32:40):
All right, we're getting time out of a magmatic
system now.
2004, uh, what happens, Mike?
What is the data that that helpsus think about the room problem
differently?

SPEAKER_01 (32:49):
Yeah, so I mean that there's inklings, you know, when
people are doing something, forexample, like a thermal model.
If you know the amount of heatthat's stored in a magma and you
can inject it into the uppercrust, and the upper crust is
going to be relatively coldrelative to the magma, you can
do thermal modeling anddetermine like how likely it is
that a magma can uh can stay inthe upper crust for a long time.

SPEAKER_00 (33:14):
And how long is that?

SPEAKER_01 (33:15):
For like on the order of like less than a
million years, they cool offreally fast.
They cool off really fastbecause it's magma that's hot
being intruded into somethingthat's very cold.
So we're talking like somethingthe size of Tuyumi significantly
less than a million years.
So if if you injected the Tuyumiintrusive suite up into the
crust at once, it would coolvery, very fast.

SPEAKER_00 (33:38):
Gotcha.

SPEAKER_01 (33:39):
So there's this idea then, this kind of background
idea that if the Tuolumeintrusive suite is a magma
chamber, then it should have ifyou you know, theoretically, if
you measure zircons from theoutside to the inside, you would
think that they're older on theoutside, younger on the inside.
Because that's kind of thisconceptual framework, but that

(34:00):
there would be very little timedifference between them.
Basically, basically no timedifference.
So the stuff on the interior uhcrystallized maybe just a
fraction, a couple hundredthousand years or up to a
million years younger than theolder stuff on the outside.
So that was our initial idea,like set set the groundwork.
I don't even know if if inColeman, if the Coleman 2004, if

(34:22):
they talk about like where wherethe baseline was, like maybe
somewhere less than a millionyears for like the entire Tuang
intrusive suite.
Yeah, this was like the idea fora quote unquote magma chamber,
if the Tuang Mi was.

SPEAKER_00 (34:36):
This is useful because the Tuangnee um
intrusive suite, the the age ofthe broad age of emplacement is
roughly around 90 to 80 millionyears that we know that now.
Um, but but that was kind of theage range.
And so just to bring this backto like when we could make these
types of measurements, um tomake a million-year uncertainty
measurement, say that's broadly,you know, just over one percent,

(34:59):
in order to resolve a onemillion year age difference
between 90 and 91 million years,that's about I don't know,
broadly a little over onepercent.
That technology, we only couldreally start to do that in like
around in the 1980s, we could doone percent probably, but but
kind of just, and then thatreally started shrinking in you

(35:21):
know, in the 1980s, into the1990s, when you could start to
answer questions at that onepercent precision range.
That's why I went on this longtangent of like, how do we get
to these ages to resolve thedifference between 90 and 91?
You you had to be at likepost-1980, 1990 to be able to
make those measurements even.
So that's why it kind of took awhile to start to think about

(35:42):
this problem to applygeochronology to this type of
problem.

SPEAKER_01 (35:46):
Exactly.

SPEAKER_00 (35:46):
And and actually, and my lab still, still my lab
could never make thosemeasurements.
Like, you know, laser ablation,not because my lab is bad,
because we use a differenttechnique, these in situ
techniques, laser ablation, um,secondary mass spectrometry,
they're they're nowhere near the1% level now.
Whereas this isotope dilutionthermal ionization mass
spectrometry technique that isused in the the Coleman 2004

(36:08):
paper, and when you dissolve anentire grain or a fraction of a
grain, you can make now sub 0.1%precision measurements.
So um we can get this is easilyaccessible now, and in 2004 was
recently accessible.

SPEAKER_01 (36:23):
Yeah.
And it took, you know, someonelike Drew Coleman to be trained
in a laboratory, do postdocs,get a lab, build a lab, and
answer the questions that theywanted to answer, him and Alan
Glaisner and John Bartley at UNCin um and in Utah to address
this problem.
Right.
So, you know, so the techniquewas around, but it still took,
you know, this is just a sidenote on the necessity for

(36:46):
training people in science toproduce new data.
You know, like you're saying,the technique's been around
since the 90s.
Well, it took someone trained inthat novel field to build their
own lab to answer the questionsthey wanted to answer.

SPEAKER_00 (36:58):
That's right.
That's right.
That's a great point.
Yeah, for sure.
Because there's plenty of otherquestions to answer when you
have precision technique whenyou can make a geochronology
measurement at 1% level.
There's a bunch of other stuffto do with that.
Um, so yeah, anyway, good point.
Okay.
All right, so 1%, the thermalmodeling, which is pretty
reliable, says a million yearsat most uh to cool down this a

(37:19):
batch of magma this big broadly.
Right.
So that's the the defaultexpectation.
If there's a room problem,that's the expectation.

SPEAKER_01 (37:25):
So, you know, I think to a certain extent they
were thinking when they went outto do this study that that ended
up being this paper in 2004 thatthey were going to see
relatively small timescales.
And you know, we have to haveDrew on to answer this question,
but um, but uh you know, I thinkthere was at least a part of
them that was thinking, okay,well, we have the resolution now
for do really good analysis,we'll be able to see this very

(37:48):
narrow time window between theouter units and the inner units.

SPEAKER_00 (37:52):
Yeah.

SPEAKER_01 (37:52):
And then, you know, what ended up happening is they
saw, you know, seven to eight todepending on how you view some
of those outer units, up to 10million years of crystallization
difference between those rocks.

SPEAKER_00 (38:06):
So that's so let's get into the data a little bit
here.
Uh, they take the the fivedifferent units, take a bunch of
different samples from the fivedifferent units, extract zircons
out of them.
This is in an era I went totheir supplements.
This is in an era where theywere doing some in some of the
samples, they're doing singlegrain, one grain gets dissolved
and that becomes onemeasurement.
In some of the samples, uh, theI'm presuming the zircons were

(38:27):
smaller, so they needed multiplegrains in each analysis, in each
dissolution to get enoughuranium and lead in the sample.
And so to make the measurement,to make an accurate measurement,
so they did this for severaldifferent samples, you know,
generated a bunch of data andsaw a big age spectrum in
between the different units, notonly in between the different

(38:47):
units, but within individualunits, there are age ranges,
substantially more than onemillion year age ranges, right?
So that at the face value, doesto you, Mike, does that just
effectively rule out a simplemodel of a big batch of magma
intruding because of thethermal.

(39:08):
I mean, does it does it rule itout?
Does it basically change youropinion immediately?

SPEAKER_01 (39:13):
Well, I mean, you know, I I that's a really hard
question for me to answerbecause I am, as anybody who
might not agree with me whohappens to listen to this, will
say I'm incredibly biased.
Yeah.
So like, you know, this papercame out in 2004.
You know, I started grad schoolin 2008.
So like with those people.

SPEAKER_00 (39:33):
Yeah, yeah, yeah, yeah, yeah.
Measuring Titanites from the 12meters.
Yeah, exactly.

SPEAKER_01 (39:37):
So I'm I'm heavily, heavily biased by by that model.
But I will say when I went toUNC, I was talking to a
colleague, an older colleaguewho had retired from petrologist
from a different institution.
And uh he was like, Oh, you'reworking with those people, huh?
Why I I think that theydefinitely got something wrong

(39:57):
there because their model makesno sense.
And so this is a grizzledpetrologist within the field who
studies uh you know silicicmagmas like granite composition
magmas, who basically was like,Oh, these people are full of
bologna sausage.
Interesting, and I'm sure thereare still plenty of people out
there who can come up with waysto to think that this does not

(40:20):
make sense.

SPEAKER_00 (40:21):
Okay, this is interesting because I I I just
plugged this question intoClaude uh before we were
recording.
Okay, it's which I love it.
Kind of which I don't know, itgives a fun.
I mean, I'm not sure if this isuh this is a bit of a sycophanti
response, but I said, what isthe impact of the Coleman et al.
2004 geology paper on thegeology, geochronology of the
Twilight Munich suite?

SPEAKER_01 (40:39):
All right, certainly it's gonna be a sycophant, yeah.
Claude Claude knows what youwant to know.
Oh, let me tell you how amazingsuper paper.

SPEAKER_00 (40:47):
Spectacular.
Well, it says it's one of themost consequential granite
geochronology papers of the lastfew decades, right?
Okay, good.
It generated some pushback forsure, blah, blah, blah, lots of
different stuff on here.
Uh, the 2004 papers, thecitation that converted, quote
unquote, Plutons as frozen magmachambers into quote unquote
Plutons as incrementallyassembled, rarely more than

(41:07):
mushy systems.
I thought that was quite good.
I thought that was like a quitea good response.
That's pretty good, Claude.
I'll read it again.
The citation that convertedPlutons as frozen magma
chambers, i.e., the roomproblem, into Plutons as
incrementally assembled, rarelymore than mushy systems.
I thought that would that likeframed it on a kind of quite
nicely.

(41:28):
Um, and and then the last thinghere, Mike, which you'll find
interesting.
I I called our last episode thatwe talked about, uh, the the
something along the lines of theGranite Wars, you know, like the
the debates that framed thefield basically.
And it said, um, for thisframing, this is arguably the
paper that started the modernphase of that war.
That was quite that was quitecool, right?

(41:48):
Started the paper that startedthe modern phase of the war.
I like that phrasing.
Anyway, um, it so this is likeuh uh this is a relevant paper,
right?
You you have people disagreeingwith it at the time, you have
people you know thinking hardabout it, and it kind of it kind
of turns the understanding orthe assumption of magmas as
pretty short-lived uh systems orthese intrusive complexes as

(42:12):
relatively short-lived systemsinto something that has to
consider time, and time beingmuch longer than we would have
previously thought.
So it kind of alleviates thisthe room problem.
And why might can you give likepaint a visual for why it
alleviates the room problem forus?

SPEAKER_01 (42:29):
Yeah, sure.
Well, so I hope that theconceptual model is relatively
straightforward in that if youjust visualize the crust as
there's no gaping uh chasms inthe crust, right?
I mean a mammoth cave is notsitting there waiting to be
filled with a magma chamber,right?
There's no there's no there aregigantic vacuums of space in the

(42:50):
crust.
Uh so you have to produce theroom for that.
So if you have a massive tool,amine-sized injection coming in,
they have to solve that withsomething.
So people came up with a wholebunch of mechanisms by which you
could do that inflation of thecrust or uh stoping, which we
one thing that we're really goodat as geologists is utilizing

(43:12):
depth to our advantage becauseif something uh doesn't make
sense with your model, you canjust bury it deeper than you can
observe it, right?

SPEAKER_00 (43:20):
So like oh yeah, it's just down below.
Don't worry about that thing.
It's just down below.
So that's like a common thing.
We all do it.
That's extremely funny.

SPEAKER_01 (43:27):
Well, we all do it.
So, you know, you can stop.
So you stoping means like takinghuge chunks of wall rock and
just dropping them to the bottomand like letting the magma fill
in and circulating these bigmagma chambers.
Um, magma chambers, quotationmarks.
So that is like how that modelworked, but you still uh are
really uh deficient in the roomproblem.

(43:47):
So what incremental emplacementdoes, which is uh a very elegant
idea, is that instead of uhinjecting all that magma or huge
volumes of that magma at onceinto the crust, you can do it uh
in smaller.
Increments over longer periodsof time.
So on periods that align withplate tectonic models.
So when you have subductiongoing on at a place like this

(44:12):
here in Nevada in 90 millionyears ago, you can operate
subduction in a way thatproduces room in the upper crust
that is on par with the longduration, small volumes of
incremental emplacement.

SPEAKER_00 (44:28):
Yeah.

SPEAKER_01 (44:29):
So it kind of makes sense in like a holistic
geochemistry, geochronology,physical model space.
This it all kind of tiestogether in a way that's much
more digestible than trying toreally bend backward to make
sense of a big magma chamber.

SPEAKER_00 (44:46):
Yeah, that's right.
And you kind of put thingstogether.
Magma chamber in quotes.
Yeah, make in quotes.
Uh and the the interesting partabout I think this data, and
here's a a question for you,Mike, about and maybe I'm not
sure if you know the answer tothis, but within this, so in the
uh intrusive suite, you havefive different rock types, let's
say, that are the zoned things,right?

(45:07):
More Mafik on the outside.
In the interior, you have theJohnson Grande Porphyry, which
is um the the most felsic.
This duchronology not only saidthat the the different units
were different times, but alsosaid that within individual
units there were different timesrecorded as well.
So for instance, the um I forgetwhich one it it might have been,
uh, but one of these units tookmaybe there's like a four-year

(45:30):
age difference.
If you go from the outer part ofthe right, the I think it's four
million for the half domeequagranular or half dome granar
diarrhite.

SPEAKER_01 (45:39):
Half dome.

SPEAKER_00 (45:40):
If you go from the outer part of the half dome to
the inner part of the half dome,which is one of these units, you
get four million year agedifference.
And so that struck me as as veryinteresting.
Even not only across a complexthat is zoned and has multiple
different rock types in it, isthere a long time period?
But even within one of them,where there's one it looks like

(46:02):
one kind of really homogeneouscompletely homogeneous, but one
very similar rock unit acrossthe entire thing takes a long
time.
Which it was it more akin tolike a sediment to me, that you
got a sandstone, big sandstonepackage that had some
depositional rate to it.
There's a long time between thetop of the sandstone package and
the bottom of the sandstonepackage.
Like there's time involved inthat.

(46:23):
And in the same way here forthis, do you do you view that as
like the is that 20 years on?
Is that still the state of theart that these individual units
have a big time differencebetween the outer and the inner
parts of them?
Do you know the answer to that?

SPEAKER_01 (46:37):
Yes.
So, well, I mean, you know, themore modern we get, the more
people are probably going todisagree with me, but that's
that's fine.
Hit us up in the comments.

SPEAKER_00 (46:45):
Yeah, yeah, there you go.
Yeah.
Actually, comment, you know,constructive comments can go in
the the comments, you know.
Uh, negative ones go to Mike'semail.
You can find it, find it online.
Good luck.

SPEAKER_01 (46:57):
Yeah, yeah.
So yeah, but no, I uh I I thinkit would be great if people
commented on this.
And so I think you guysdefinitely definitely miss
something here.
Look at these papers orwhatever.

SPEAKER_00 (47:07):
I think you're sure are are missing and
oversimplifying for sure.
That'd be great.
Yeah, totally.

SPEAKER_01 (47:11):
But yeah, so something like you know, you're
talking about the half-domegranite diorite.
I think you know, it's hard tonecessarily say how I don't know
how the history of the mappingof this works specifically, but
when you go, when you go outthere, you know, if if I had,
you know, next time you come tomy office, we can put samples of
each of these units on my desk.
And if you just look at one rockversus the other, they look very

(47:35):
different.
Like the cathedral peakgranadaurite candid places have
potassium false bar crystalsthat are 10, 20 centimeters big.
And the the half-domegranadiorite has has big case
bars and areas, and in otherareas it doesn't.
Some rocks have a lot ofamphiboles, some rocks have

(47:56):
none.
And so you can see how whenyou're trying to map this out,
you can kind of draw boundariesbetween Half-Dome, Cathedral
Peak, and Half Dome and GlenAllen and Kuna Crest and Johnson
Grande Porphyry.
But I'm telling you, when youwalk out there, like when you
come out there and you come outin the field with us, you can
hike between Half Dome andCathedral Peak.

(48:18):
And uh I you would be uh hardpressed to put your finger on a
boundary.
You you really can't.
I mean, there's a lot oftextures that you can see.
You you can see like somegradational repeating
gradational areas in the halfdome, but you I I mean, I would
be you would be reallyhard-pressed to be able to put

(48:39):
your finger on a a unit contactout there.

SPEAKER_00 (48:42):
Oh, cool.
Okay.
So the fact that in thisgeochronology, when you know,
this geochronology plot the sopeople can kind of visualize it,
they're they sort of plotbroadly the the age is on the
y-axis here, going from like 95million to 84 million or 85
million.
And then basically they plotunits on the x-axis.
So from left to right, you'regoing from the outer units to

(49:05):
the inner units.
And just the way this plot isconstructed, it basically looks
linear from 96 million years inthe outer part, top left, to 84
million years in the the innerpart, bottom right.
It's basically a straight line,it's not like a step function
where you're in one unit, it'sall the same age, and then it
drops down in the next unit.
It's not discrete, it's kind ofcontinuous.
And you're saying that makessense in the field.

(49:26):
Like these are relativelycontinuous units across several
of them at least.

SPEAKER_01 (49:30):
Yeah, exactly.
So, you know, the question oflike is like half dome, half
dome to me, that's kind of uhmore or less an artifact of
previous mapping generations.
Like the Half Dome Graniteurite,I mean, I mean, again, that's
something I'm sure a lot ofpeople disagree with.
But if you uh if you think aboutthe the size of the increments

(49:51):
that are being in place, it'snot on the scale, it's the the
increment is not the size of theentirety of Half Dome.
Uh in it geochronologically, I Ithink it's much smaller packets
of magmas that are are probablybeing overprinted by a bunch of
other processes as well.
But you know, if you go out toHalf Dome, you can see some
areas where you can say, oh, Icould convince myself that you

(50:13):
know this 150, 200 meter thickuh package was like an increment
of magma that was injectedcooled.

SPEAKER_00 (50:22):
Okay.

SPEAKER_01 (50:23):
But the difference between maybe 150 to 200 meters
thick uh package versus uh atwo, three, four kilometer thick
package is is a big difference.
And so that's that's I thinkwhere this incremental
emplacement uh really made a bigimpact is just trying thinking
about the volumes of magma assignificantly smaller than like
the map unit.

(50:43):
So Cathedral Peak, you know, inthat model, Cathedral Peak would
not be one gigantic magma pulseinjected into a quote unquote
magma chamber.
It's uh increments of muchsmaller volumes of magma being
intruded into the core of thissystem over time.

SPEAKER_00 (51:00):
And a system that remained broadly the region
stayed hot for a very long time.

SPEAKER_01 (51:04):
Warm, yeah.

SPEAKER_00 (51:05):
And I might maybe we can um we can leave a bit of a
cliffhanger here with anotherterm, which might be magma mush.
Because I think this, you know,and I know there's people who
don't like the magma mush, um,and you're one of them, but the
mushiness kind of system, or Ithink it's a good like visual
for for the uninitiator forpeople who haven't thought about

(51:27):
these things.
Sure, we can argue about theterm exactly, but like imagining
not imagining a magma as a bigvat of m of liquid, but
imagining it as like mostlycrystals with a significant
amount of melt sort ofpercolating through that.
I mean, is that a good visual ora bad visual?
And uh, we can talk about thismore in the next episode, but

(51:50):
but this this visual, whatshould what is the thing when
it's happening?
Like what does it look like whenit's happening in your mind?

SPEAKER_01 (51:57):
Well, well, I think I think maybe to not answer your
question, but to get at kind ofwhat we're talking about,
because you know, we're sittinghere and you and I are both
granite or granitic and icepeople.
What we have been ignoring thiswhole time is that over systems
that are producing granites aresystems that are producing
volcanoes.

(52:17):
So underneath Mount St.
Helens right now is an activemagmatic system.
And so presumably there aregranites being formed underneath
Mount St.
Helens right now.
So we can't just have like thesystem is way more complicated
than just a granite sittingthere being injected as like a
hundred meter thick packet ofmagma and just dying.

(52:40):
It's much more complicated thanthat because you have to have
somewhere in the system, youhave to produce magmas that
erupt with maybe 10% crystals,not 50% crystals.
So, you know, this is this ideaof a magma mush, it's a very
complicated thing that'sprobably not for for this
episode.

SPEAKER_00 (52:59):
But yeah, yeah, yeah, yeah, yeah.

SPEAKER_01 (53:00):
Yeah, well, this would be great to bring a
geophysicist on board for thisconversation.

SPEAKER_00 (53:04):
Yeah, no, for sure, and talk about what actually can
we say about stuff in the crust.
But but it's not, you know, it'seither increments, little small
dikes, things that are intrudingas dikes progressively over
time, or it's some system thatis mostly crystals, um, you
know, in and has melt kind ofpercolating through it over
again, over long periods oftime.
We know it has to be long periodof time now, i.e., the

(53:25):
geochronology we've been talkingabout.
Um, so I think this is uh thisis a really good episode on the
room problem or on the spaceproblem.
Like I I um have we have we tiedit off?
Is there anything else we needto add here?

SPEAKER_01 (53:37):
No, I think I think we probably we beat this one to
death here.

SPEAKER_00 (53:41):
I think we might have overdone it.
Uh it's possible.
If you think we've overdone it,let us know in the comments.
Uh, but if you know, if you ifwe haven't overdone it and
you're interested in more, uh wegot more coming at you.
But please let us know if youare want to hear more on a
specific subject.
We like uh we like gettingcomments, we like getting
emails.
You can follow us on all thesocial medias or at Plant

(54:02):
GeoCast.
Shoot us an email.
There's a contact us link on ourwebsite, plantgeocast.com.
And again, go to our CampGeoMobile app.
First link in your show notes.
We've got a ton of stuff thereuh that gives you kind of the
backstory to what we've beentalking about in the deep dive
we've been doing on the granite.
Mike, appreciate the time.
Always fun talking signs withyou, man.

SPEAKER_01 (54:18):
Yeah, of course, of course, man.
Yeah.
Till till the next one.

SPEAKER_00 (54:21):
Till the next one.
There we go.
Love it.
All right, cheers.