This is Java g. B, with all of us trampled to
address many important geological questions.
We often need to determine the history of a crystalline Rock.
When it initially, formed out of a melt and what temperature and
pressures, it may have been subjected to between its
formation and the present day. One important way of doing this
is to look within the rock to see if it contains minerals that
are especially useful for this purpose.
In previous episodes. We've talked about Zircon
crystals and how we can use the decay of uranium to lead to
obtain their crystallization ages.
But in addition to their value as Gio chronometers zircons and
other crystals such as those of monazite Titanite and rutile
contain a number of elements in Trace Amounts.
This makes them valuable as targets for geochemical
analysis, as well. By measuring these Trace
abundances, we can learn quite a bit about the environment in
which the crystal formed this contributes to our understanding
of the Rocks history. Both as to the origin of the
Melt art, of which the crystals formed, as well as the
temperatures and pressures that prevailed when the crystal was
forming, Two challenges have long, been devil, geochronology
and geochemistry, the first is the sheer amount of labor
involved in obtaining a corpus of trustworthy, radiometric,
dates or accurate Trace, element abundances.
The second is the difficulty of ensuring that the geochronology
and the geochemistry analyses actually, apply to the same part
of a crystal and are not biased by variations within a crystal.
This can be especially difficult when a crystal contains pains
several tiny, domains each reflecting a different phase for
the crystals history. John quartal and his team have
developed a technique that has largely overcome.
These hurdles, this has led to a revolution in our ability to
unravel metamorphic histories and has given rise to a new
field called petrol. Chronology.
John Carter is a professor in the department of earth science
at the University of California. Santa Barbara Giancarlo.
Welcome to geology B. Thank you Oliver.
It's a real pleasure to be here and to talk with you about this
really exciting topic. I said in my introduction, That
one of the challenges faced by geochronology and geochemistry
is the sheer amount of work involved in getting to a result.
I have first-hand experience of dating, Luca granite's from the
Himalaya, this way the method I use called thermal ionization
Mass spectrometry was certainly laborious.
I started by Smashing up the rock samples, separating out the
Reza Khan Sometimes, using highly toxic heavy liquids and
reeling them in a furnace, dissolving them in hydrofluoric
acid. And then mounting, the dissolved
material is a gel onto a filament to be inserted as the
cathode inside, a mass spectrometer.
All the, while I had to pay obsessive attention to
cleanliness as at the end of the day, when is just measuring mere
picograms of that. Led Decay products of uranium, I
presume obtaining accurate abundances of Trace elements.
Within the crystal can also be very labor-intensive, that's
right. It's a very similar process to
obtain Trace elements in. Is minerals.
You need to take the mineral and dissolve it, and then separate
out the individual elements using complex column chemistry,
or you need to use some Institute technique that you
might be able to avoid some of those complicated processes and
really get directly at individual parts of the crystal.
So can you tell us how the fields is Advanced since then?
Yeah. So initially we would take parts
or whole crystals and dissolve them up and separate out the
elements of Interest. And really we've moved on then
to try and make in-situ measurements of parts of
crystals down to the Micron scale.
And it really started with electron probe micro analyzers
which are relatively old technology, but useful for
measuring Trace element concentrations down to about 100
or so parts per million subsequent to that a couple of
other techniques were developed primarily laser ablation
inductively, coupled, plasma mass spectrometry.
And also secondary. Ionization Mass spectrometry
were both used. The advantages of those are that
they can measure Trace elements down to the parts per million
and in some cases even down to parts per billion concentrations
of Trace elements. Okay, let's concentrate on the
laser ablation techniques since that's the one that you've
refined what is the role of the laser ablation system?
A laser ablation system, really is just a way of sampling, the
material, it just takes the mineral itself and disaggregate
it into relatively small or relatively fine particles and
then we take them. Fine particles and we transport
them to the Argon plasma. So you can think of the laser
ablation system as a way of sampling or drilling out
material and disaggregating enter very small Crystal sport
parts of crystals to then separate those using the plasma
Source itself which is an inductively coupled plasma, and
what is its role here. So inductively, coupled plasma
is, if you take an Argon gas, and you heat that Argon gas to
around 11,000 K using an electrical source.
You can ionize that argon and turn it into a plasma.
If you take then solid pieces or liquid material, you can pass
that into that plasma and you can essentially take that
material and ionize it in that argon plasma.
And then you can produce ions of individual elements and then you
can use a mass spectrometer to separate those individual ions
and measure their relative abundances.
Okay. I see, so the inductive coupling
of the Argon is the way you heat up the Argon gas to the
extremely high temperatures. Eleven thousand Kelvin, when all
the electrons are stripped off and it becomes a plasma.
That's right. So, you take your solid pieces
of zircon or monocyte, and you have those as very small
particle size and you take those particles and you pass them into
a plasma. And essentially you strip
electrons from those particles and eventually you strip enough
electrons out of them. And so you produce individual
ions and so, you're going from a solid piece of mineral all the
way through to individual ions and essentially the The laser
does the initial part of that process by taking into small
particles, but the islands themselves are made actually
inside that argon plasma. Okay, so you'll get a spot of a
laser onto a sample, get a fatty good spatial resolution because
of the size of the laser beam, which is pretty small about 10
to 30 microns and then you can get it into a mass spectrometer.
So why did these instruments that already existed?
Not adequately. The challenges that I mentioned
earlier, namely the amount of labor involved and the problem
of making geochronology and geochemistry results consistent
with each other. So typically these laser
instruments have been in use since around 1995 or so, but
those early instruments and actually up until relatively
recently would either measure the age of a mineral, or they
would measure the trace element concentrations.
Only relatively recently. Have we figured out how to
measure both the ages? Has as well as the trace element
concentrations together on the same volume of material and
really the advances come from new generations of instruments
that are more sensitive and having a better ability to
separate out those different ions.
And so the better sensitivity allows us to use smaller spot
size but also measure those. Trace elements that are in sub
PPM concentrations, things that we simply couldn't detect
because of the sensitivity of those early instruments.
But from what, I gather those instruments have to be used.
Separate phases. You did the two separate
analyses in two separate phases and so you still went quite sure
if you were actually dealing with the same actual sample.
That's right. And so typically, when I did my
PhD and I did a lot of laser ablation analyses.
The first thing I would do is I would take my sample to an
electron microprobe and I would measure the major and Trace
elements in those minerals. And I would then take my sample
to a laser ablation system and measure the age.
And so, we could never be entirely sure.
That we were measuring exactly the same.
Mmmmm composition or the same domain within a crystal.
And so there's always this mismatch between the different
data that you generate from the different instruments.
Yeah. And as I mentioned in the
introduction, that can be quite a big problem.
If there are variations across a crystal on a fairly small
spatial scale, which is often the case, especially in the kind
of complex situations that you're trying to unravel.
I imagine. So how did you overcome this
problem? So instead of doing sequential
measurements, what we did is we took to mass spectrometers and
we took the Stream of particles that comes from a laser and we
took their stereo particles. And we split that stream of
particles. We sent them simultaneously to
to mass spectrometers. So one Mass spectrometer would
measure the age and the other Mass spectrometer would measure
the trace element concentrations or other isotopic traces.
For example, like hafnium and zirconium or neodymium and
monocyte things like that. But an isotopic or Elemental
Tracer along with the age of the mineral.
The real advantage of that then is that you You can be certain
that the age you're getting has coupled directly to some form of
Trace element or isotopic Tracer.
And so when you get an age you get a direct correlation between
those other crucial indicators of what the age actually means
and a geologic context. That's brilliant.
You have one laser. And then what comes off you
split into two streams. So it's coming from the same
spot so there's absolutely no difference between what's going
into the age determination. And what's going into the
geochemical? Trace element, determination.
When I put it like that it seems kind of obvious.
Why did nobody do it before, was it very difficult?
There's a couple of things that prevented people doing.
This earlier one is simply the cost involved of having to mass
spectrometers in the same laboratory space.
There's a bunch of infrastructure issues.
The amount of gas that gets used the power supplies that are
necessary. The other part of that is that
only relatively recently in the last decade or so that we've
actually been able to get the instruments with.
Sensitivity such that when you reduce the amount of material by
a half when you split it to to mass spectrometers that you
still get sufficient amounts of signal to actually measure those
concentrations and those isotopic, ratios add a
Precision, that's good enough to solve the geologic.
Problem is this dream coming from a laser split up as soon as
it comes off the sample. So the way the laser system
works is the sample is at atmospheric pressure, and so the
laser samples, the material atmospheric pressure, but it's
in a Liam atmosphere. So we have an enclosed cell or
chamber, and we pump that chamber with helium gas, and
that helium gas essentially, takes that aerosol material,
that's being ablated and extract it from the cell, and then we
mixed that helium and that sample with some Argon gas and
that drives it into the mass spectrometer.
So, the useful thing about that process is that it's relatively
quick in the sense that every time, the laser hits the sample
at excavates, a small amount of sample and we immediately remove
that sample. Or that material from the sample
site and so you can actually get relatively good spatial
resolution by rapidly. Removing that material, as you
go down a whole or as you go along a crystal.
And so, when I say rapidly, I'm talking about much less than a
second between the material, being a ejected, from the sample
site and it reaching the mass spectrometer.
So you get relatively good to depth, resolution as well as
good spatial resolution in the x y direction.
I'm impressed that you can go from an atmospheric type
pressure of will It filled with helium and introduce Argan.
But inside the mass spectrometer itself, you have a very hard
vacuum, don't you? That's right.
So inductively couple plasma has are unique in that sense that
the sample starts at atmospheric pressure.
And then there's a rapid decrease in pressure across that
Supple interface and that plasma interface to get down to 10 to
the minus 9 or 10 to the minus 10, millibars to actually be
able to get those ions to travel to the detector.
So, part of the thing, we really focused on was trying to
understand how those vacuum systems can be improved.
And we spent a lot of time working with mass spectrometry
companies to actually make those vacuum systems a lot better and
the better you make the vacuum, the better, the sensitivity of
the instrument, alternatively, if you want to think about it,
another way, if you have enough signal, you can actually use a
smaller spot size. In other words, use less
material for the same amount of precision.
So depending on what you're trying to do, we really isolated
the biggest Improvement we could make really was to that vacuum
system. And so we spent a lot of time
trying to develop that what Minerals are amenable to this
laser ablation split stream technique and do use different
minerals because sometimes a rock will only have one of
those. And so you just have to use what
you've got or because each mineral can provide information
on something different about the Rocks history.
We really try to tailor the mineral, we measure to the
problem. We're trying to answer.
For example, in igneous, systems will often use Zircon because
that's the primary carrier of uranium.
But in other geologic, Systems, for example, metamorphic rocks
will often measure the monocyte and the appetite the Xena time
and maybe the titanate as well. And the reasons for that are
primarily that they may tell us about different parts of the
metamorphic history of that rock.
But also they tend to taken Trace elements at different
concentrations because of the compositions of those minerals
and so different. Trace elements might tell us
different things about how those rocks evolved.
If the mineral has about half to about 1, PPM uranium.
And it has some ready Jake lead in it and that's not swamped by
Common lead, then we can usually measure an age.
And if the mineral has greater than about half a PPM of any
Trace element, we can use you measure that to relatively good
precision as well. So this new term that we're
using in connection with the simultaneous measurement of the
age and the trace element abundances is petrol chronology.
So I understand the chronology bit because we've talked a lot
on the podcast earlier about uranium-lead dating of zircons
and You get a date from the ratio of the daughter nuclides
to the parent, nuclides, roughly speaking.
And, and with uranium LED, you have two clocks, so that makes
it even better. But on the petrol side if you
like or only Trace element abundance measurement side.
What exactly does that tell you about the Crystal and about the
rock within, which it Formed. Yeah, so one of the challenges
we've faced with minimal Fat Rocks but also with igneous
rocks, as well as if you have a date or a series of dates from a
mineral. Or different minerals in a rock.
The fundamental question is, what does that date mean in the
context of that rock and in the context of the broader?
Geology and so in a minute, morphic Rock, if you were to
date a monocyte, for example, you might typically see ranges
of Ages, that might be five to maybe 10 or 20 million years
spread within an individual Crystal, or with an individual
Rock. For example, monocyte is a great
mineral because it takes in Rare Earth elements and those Rare
Earth elements can be used to tell you about what else is
happening. Burning in that rock.
For example, the mineral Garnet has a big control on the rear
Earth element budget of a rock and so if a guarded is growing
it will compete with monocyte for Rare Earth elements and so
if we think about the way Rare Earth elements are partitioned
between Say Ghana and monocyte. Then if Garnet is growing the
monocyte that grows at the same time might well be depleted in
certain Trace elements. You can think about the trace
elements in your mineral as being a reflection of what's
going on in the geochemistry of The Rock.
The petrology of that rock. And so, being able to make a
quantitative link between your date and your Trace element,
concentrations to what's happening in the Rock allows you
to tie it to, for example, things like defamation, but also
the pressure, temperature conditions have been a morphism
and you can do a similar kind of thing and igneous rocks as well.
If you understand something about the phase relationships,
amongst different minerals. So really the key is to take it
from a qualitative interpretation, which is what
people were doing previously to making some quantitative
estimates of what the The conditions of that age, actually
are relative to the history of that rock.
Oh, I see. So just to expand on the
example, you gave with Garnet. If you can identify that at a
particular age, there was a certain depletion in certain
Rare Earth elements that you're measuring.
You would infer that Garnet was growing at that time in that
rock or from that melt. And therefore the fact that
Garnet is growing tells you something about where you are on
the pressure temperature graph, It's right.
So one really good example, is the element Atrium.
If you take a metamorphic rock and as you minimal, foes that
rock. You increase the pressure, you
increase the temperature and as The Rock gets to maybe about 500
degrees or so, monocyte might, well start to crystallize and it
might start to crystallize either at the same time or
potentially, even earlier than garnets dust to nucleate any
monocyte that grows prior to any gun at forming will be
relatively enriched in the element yttrium and so earlier
formed monocyte would Be relatively high in a tree.
Mm, but then if you continue to metamorphose that rock and you
nucleate Garnet that Garnet will tend to take up most of the
Eritrean, The Rock. And so any monocyte that grows
will be relatively depleted in Atrium.
And so you can track the growth of Garnet in that rock by
actually looking at the yttrium concentration of the monocyte
and often what you'll see is at the very end of the metamorphic
cycle. Some of that Garnet might
actually start to break down or I will It'll be the rock, might
start to melt, but in either case, you might see breakdown of
garnet. And so that liberates a bunch of
yttrium and any monocyte that grows during that period will be
relatively enriched in a tree. And there's a bunch of interim
available in the system. And so, the monocyte will take
that up and so you can make a link between the atrium
concentration or the other Rare Earth elements between your
Manas 8th Annual Garnet. Now, there are complications
because there are other minerals in the Rock, which you also have
yttrium and them for examples in a team or appetite.
But if you know, do something about the modal abundances of
those other minerals, you can then start to calculate what the
effect of those minerals might be but the principle really is
to try and get at whereabouts. Are you on that minimal for
path? In both your monocyte and as
well as your garnet and then you can calculate the pressure
temperature conditions you can calculate the age and you can
then get a rate and that's fundamentally what most people
really care about when they think about metamorphic.
Rocks is the absolute conditions are minimal for them but then
also the rates of that process as well.
It's the right that we dominantly care about in terms
of s scale of it geologic process.
How long does it take you to obtain a date and it Trace
element abundance measurement for a particular location within
a crystal. An individual sport can take
anywhere from five seconds to about 20 seconds to measure so
that would give you one analysis for a typical monocyte that
maybe has 3 or 4 different age domains.
We might make 2240 measurements on an individual.
One has a crystal and Or an individual Rock.
We might measure 10 to 12 miles out crystals.
So we might end up with on the order of 100 to 200 analysis per
sample, and that might take one to three hours.
By the time, you measure some, secondary standards, and some
primary calibration materials as well to make sure you're getting
a precise and accurate Point. That's incredible compared to
weeks and weeks of work for one single family and ization.
Mass spectrometry measurement, Toki different style of working.
That's right. So, when you want to unravel,
say, the history of An entire outcrop.
How many mineral grains would you typically use?
And how many spot measurements would be used for a typical
outcrop? You might date maybe four to
five rocks. We would date the metamorphic
rocks prison. And if they're at cross-cutting
igneous rocks, you might date those as well.
And so typically within a day or so, a 12-hour day on the
instrument, you might be able to date 526 rocks.
Really the limitation of this technique is how long are you
prepared to run the instrument for?
We have people who come to our lab and they'll be essentially
awake for Four hours a day for seven days a week.
And in that time, you can get literally thousands to several
thousand. Analyses offer a range of
different rocks. Now it's great because the more
data you have the bigger picture, you see, but that comes
at a cost and you mentioned thermal ionization Mass
spectrometry. So I think it's really important
to note that both of those techniques have really important
parts to play. If you want the ultimate
Precision date, then you will never beat a thermal ionization,
Mass spectrometry date. They're the most precise.
They're the most accurate. And instead what As we tried
some of that accuracy, we trade some of that Precision for
number of analyses. And for that special resolution,
we can measure individual parts of a crystal and so I hear a lot
of people in the community. They have one or the other, but
to me, they're both tools. And depending on the question,
you want to answer, you should choose one or the other or
ideally you might use both. Can you give us some examples of
how the use of this bitstream method has enabled us to
deconvolve? Some metamorphic histories that
were Out Of Reach with the Methods.
Yes, most of my research over the last 20 years, or so has
been focused on the Himalaya and really trying to understand the
metamorphic history of the Himalaya, but also the magmatic
history as well and the spatial scales of metamorphism and
melting as well. And so, when I first started out
of my PhD, we really considered the mid crustal rocks in the
Himalaya, these rocks the, the greater Himalayan series, these
rocks, make up the highest peaks of the Himalaya.
And we always imagined that there was this 20 to 40 km,
thick sequence of this. High grade metamorphic rocks
that formed a pretty coherent package but subsequent to that
we've dated literally hundreds to thousands of these
metamorphic, rocks with collaborators across the
orogenic front and we realize that actually, the internal
structure of that series of mood crustal.
Rocks is a lot more complicated. And I think we only really were
able to figure that out by literally dating, tens of rocks
on an individual transect and then doing multiple transects
across different parts of the Himalaya and you start to see
that the timing of metamorphism is subtly different, but also
the conditions of metamorphism a subtly different.
And so not only can you see differences in when these rocks
are metamorphosed, but we're in the crust, they would be in a
more focused and how they were juxtaposed against one another.
And if I think about my PhD, I dated I think something like 15
rocks. It took me 34 years of really
hard work to do that and then we would go back 5 or 10 years
later with this new technique, we could date that number of
rocks in an afternoon on the instrument.
And so with a really concerted effort, Get large amounts of
data and only then, do you actually start to see that
really fine scale resolution of how that mid crust was
constructed? It really changed the way I
thought about how you actually make the mid crushed in the
Himalaya. It's not really this.
Homogeneous package of metamorphic rocks, actually has
a significant amount of variation and that you only
really are able to see when you actually get these really large
data sets that link the geochemistry and therefore the
conditions of metamorphism to those ages.
Do we have an understanding yet of what caused that?
That heterogeneity within the greater Himalayan series, one
way of considering, it is when you make the mid crust or when
you alter them at crossed, there might actually be a series of
stacks of thrust slices that get accumulated relative to one
another. And when you take those stress
slices and you put them together, you then take all of
that material and you start to extrude it Southward towards the
Indian front. We imagine that that process of
taking that made crust. And moving it Southward over,
the subcommittee are happened is Parent block was actually, I
think we see now is that it's happening at a finer scale.
There are individuals thrust sheets, that are actually being
moved relative to one another to then accumulate together to
produce that McChrystal package. And so to me it's a question of
scale. We thought it was one giant
package of rocks and now I think we realize that actually it's
smaller packages with their own individual histories, which we
can start to tease out there any other examples of places where
you've applied these methods, there are working on subduction
related. Nice rocks in Antarctica.
Antarctica is incredibly difficult place to get to, but
there is a really large number of rocks available from people
who have previously been to Antarctica and many of those
rocks were dated by isotope dilution techniques, is that the
same? As the thermal ionization
method? We were talking about earlier,
that's right. Yes.
And the problem with those minerals is particularly the
Antarctic. Granite's are relatively high in
uranium. And so prior to the mid 2000s,
one of the main limiting factors of thermal ionization technique
was that Feels Iran has led loss.
If it has high uranium and the crystal lattice is damaged, then
that accumulated rated eunuch LED can leak out of the Crystal.
And so, when you measure the uranium-lead ratio, that date is
younger than you might expect, because it's lost that daughter
product. And so a lot of those dates were
really imprecise or they were subject to lead loss.
Now, the useful thing about the laser technique is that when you
take a look at your Crystal, you can actually isolate domains.
That have undergone led loss and domains.
That have not undergone their lies.
You can see it, When you do multiple spots and individual
Crystal. And so you can essentially
eliminate any of the domains of underground lead Lawson only
select the parts of your Crystal that are concordant or where
there's been no disturbance to the uranium lead system.
And so in Antarctica, we were able to date several hundred
rocks across the orogenic front and essentially eliminate all of
that led loss and really get at the truth timing of igneous
intrusion. But also, the duration of that
metamorphism, we're kind of we're able to redefine what the
history of magnetism in Antarctica.
Looks like the The time scales of magnetism and Link that to
things like hafnium Isotopes in the same analyze volume that
would allow us to understand the sources of those Magma's but
also the processes that might modify those Magma's.
So from an individual spot, we would get the age but also the
isotopic composition and we could then say something about
these rocks. Are they dominated by mental
components, or they're dominated by recycling of old crust?
People argue a lot about subduction zones, whether they
represent sites of crustal, recycling or crosstalk?
Growth. But unless you have those really
precise age, constraints, coupled to some isotope Tracer,
it's really difficult to get at that.
So, that's another area that we're really done this campaign
style, geochronology of literally hundreds of dates,
across, thousands of kilometers of section to really get at what
that magnetic history looks like what period are we talking about
in Antarctica, the magnetic rocks Antarctica range from
about 560 million years all the way through to about 490 million
years. Most of the magnetism ranges
between about 15 in about 490. But one of the interesting
things were actually able to show is that magnetism is much
older and much younger than we previously thought because much
of that variation is hidden by that led last that was in those
older analyses. And so with the laser technique
you can see older magnetism that's concordant and you can
see younger dates that are concordant as well.
But yeah, roughly that kind of Canberra ordovician period.
Are you working on any other parts of the world with these
techniques? We're trying to think about
other places on the planet where we see metamorphic rocks that
either have more than one phase of metamorphism and have
previously been either interpreted in the light of
dates or ages, that may be mixtures of those multiple
metamorphic events and places where we need a large amount of
data to actually see Trends and minimal forecast area.
So in particular had PhD students working in New Zealand
to try and understand the Cretaceous metamorphic history
of zealandia as it's breaking up from parts of gondwana and
particularly in place. Like New Zealand.
It's really challenging because there are older and younger
faces. Have been a morphism that you
might also have to contend with. And so that high spatial
resolution technique is really useful for dick involving the
different scales have been a morphism, the different imprints
of metamorphism on those rocks. It's fantastic.
Several people I've spoken to have said this is really
creating a revolution in the field.
I can imagine that all kinds of people would be falling over
themselves, to be able to use your methods, are your
instruments. I think we're really actually,
only at the beginning of our Our thinking about how do you best
take that information and how do you really get the most out of
that, in terms of a geologic interpretation?
That requires people who are interested in more the data
science side of things as well as the isotope geochemistry is
also metamorphic petrology. So for me that's the most
exciting thing is. I think we've got more people
involved in the chronology Community to bring their
expertise to these problems. John cockerill.
Thank you very much. Thank you very much.
It's been a real pleasure. For more about geology b, as
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