This is Jala, g b with all of us trampled.
In a recent episode na jibun spoke about newly discovered
Zircon crystals that form during the late hadean and early,
archean, when the Earth was between 500 million and a
billion years old, the zircons revealed information about
processes occurring in the earth's Nason crust, casting,
light on, when, and how modern day plate, tectonics may have
started. In this episode, we talk about a
very different source of information about the LEF
namely, the abundances of noble gases occurring within
present-day basalts. It turns out that these can
probe the Earth's mantle and atmosphere even further back in
time to the first hundred million years of Earth history.
Sujoy. Mukhopadhyay leads a team of
researchers who have developed new techniques for measuring.
The abundances of noble gas Isotopes in a variety of earth
materials by combining the results of these measurements
with geochemical models. He has shed light on questions
about the very early Earth and Planet formation that have
challenged researchers for decades here.
We focus on one of these. Do any structures originating
from the very early Earth survive in today's.
Tall. Sujoy mukhopadhyay is Professor
of geochemistry at the University of California.
Davis, sujoy mukhopadhyay. Welcome to geology B.
Thank you, Oliver. I'm delighted to join you.
And I am really excited to chat with you.
How is it that we are able to use noble gases to probe the
very early Earth is it to do with the fact that they are
chemically inert? So yes, you're absolutely right.
It is got to do with their chemical inertness.
So in that respect the keeper memory of these very early
events and likewise we can combine the variety of Isotopes
from the different noble gases to probe the Earth on different
time scales. The noble gases form a group in
the periodic table from Helium being the lightest through neon
argon Krypton Xenon and radon, how were these elements
produced. So in the Noble gases, we have
isotopes that we called primordial, which means these
are the Isotopes that were produced in stars and then the
solar system acquired these elements from neighboring stars.
And then there are set of isotopes that are produced by
radioactive decay such as from elements, like, uranium and
thorium, and potassium. And the Really fascinating
aspect about this is that the half-lives of Of these isotopes
that ultimately end up producing these noble.
Gases range from something like 14, billion years, for thorium
all the way down to something like 16 million years for an
isotope of iodine that produces a particular isotope of xenon.
So, in many respects, this very wide range of these.
Half-lives, give us the sense to Pro or it on these very
different time. Scales you focused on Particular
isotope of xenon to probe the early Earth, and that was 129
Xenon. Can you explain why?
Yes. So 129 Xenon comes from the
radioactive decay of 129 iodine. And this particular isotope was
present when the solar system formed, but it's Half-Life was
16 million years and typically the The useful range or useful
time range for any particular radioisotope is roughly six
times its Half-Life. So that means that after about
100 million years, I dine 129 is no longer around.
All of, it has been converted to 129 Xenon.
So, therefore Earth is able to keep a memory of this
radioactive decay happening within the first 100 million
years, because After 100 million years, there was not going to be
any new production of 129 Xenon. So any kind of variability that
people see on the Earth would have to relate back to this
first hundred million years. I said, in my introduction that
you can use such noble, gas, abundance, measurements to shed
light on whether today's mantle retains any of the original
structure from which it first differentiated, how do you
connect measurements of a Xenon isotopes?
Isotope with the mantle. So what we do is we start
probing the products of melting of the modern-day mantle.
So for example, if I go to places like Hawaii or Iceland
which are really nice places to visit on its own, but also
happened to be location of mantle plumes or hotspots.
And in these mantle plumes, we now have evidence from
seismology that there. Derived from Material very near
the core-mantle boundary. So, in that respect these plumes
are acting as chemical probes of the composition of the deepest
parts of the Earth's mantle and likewise, when we go to a
mid-ocean ridge spreading Center where there is new oceanic crust
being created the partial melting, that we see is being
derived from Material, that's present in the Shell or part of
the mantle. So then, bye.
Comparing the composition of basalts in the mantle, plumes
versus the composition of the salts from the mid-ocean ridge.
We're effectively probing different depth within the
Earth's mantle. And so if we see differences in
the 129 Xenon, composition between these two different
regions of the mantle, we can immediately say aha.
The system could have only changed within the first 100
million years. The fact that See, differences
are pointing to the fact that it had to have come from that first
100 million years. So that's how we connect our
measurements of mantle. Drive basalts to what's actually
within the Earth's interior. That's fascinating, I'm struck
by the fact that you can actually measure the abundances
of noble gases in various kinds of battles.
So how do noble gases get into the Rock in the first place?
I mean being inert. They can't really form minerals
or react with the minerals. We in any way, can they you're
right? They can't really chemical you
react with the minerals and because of that, they don't
actually like to be in a mineral because they are not forming any
kind of chemical bond with their neighboring constituents.
So often that tend to exist in what might be potentially
distorted sites within a mineral lattice, or they might be
present in very large. Ring structure in certain Lines
of minerals like amphiboles but those sites are rare and because
of that, they would rather be in a liquid phase where there are
more opportunities for these noble gases to be present and
compared to liquid phase of vapor phase is their preferred
host because then they're really happy.
So what happens is when the mantle starts partially melting
The noble gases are one of the first elements to leave the
mineral lattice and go into the Melt but there are other
elements other compounds such as maybe CO2 or water.
Which also typically do not like to be in minerals and want to
effectively go from the mineral to these males that are forming
as the parcel of mantle starts to rise to the surface, and the
pressure release causes this partial melting.
And then the male starts, Early rising to the very surface of
the Earth and as the mouth starts Rising ultimately, a CO2
Vapor bubble might form. And then the noble gases will
right away, go from the Melt, into the CO2 Vapor bubble.
And when this melt erupts onto the sea floor, it immediately
quenches because it is in contact with really cold ocean
water and it forms a class and then glass effectively
encapsulate, these Vapor bubbles So, we then take these glass
samples into our laboratory, and we crush them under vacuum and
release the vapor bubbles, which releases the noble gases.
And that's what we end up measuring in our laboratory.
That's fascinating that they come out into the gas phase, but
that makes me wonder. Why did these noble gases go
into the solid phase at all, to begin with.
When the earth was formed, why didn't they just all stay in an
atmosphere? That's a really fascinating
question, and we can kind of speculate as to why Why?
So in the earlier that at some point there was a magma ocean
where maybe most of the mantle was molten.
And then this magma ocean will eventually D gas to form the
very earliest atmosphere and the noble gases are going to
partition between this magma Ocean and the atmosphere but
there are solubility in this magma is very low but it's not
zero. So there's still going to be a
finite amount. That is going to be in the
magma. The ocean.
And then, as the planet cools by losing heat, to space the magma,
ocean is going to slowly start to crystallize.
And basically, the noble gases are prisoners at this point,
because as the minerals are forming, they are being shoved
into these minerals because they can't escape.
So, they are present in very low abundances in these melts.
And then, when the Melt starts to solidify the Enter or are
forced to enter the minerals. But again, in very low
abundances and so the first opportunity to get the try and
escape, and that first opportunity is re melting of the
mantle. So, now we have these gas
bubbles, which are say in the case of the mid-ocean ridge.
Basalt Vex, spewing out onto the sea floor, and in the case of
the ocean Islands, they're coming up in volcanoes like in
Hawaii. But in that point they must
start being affected by in one case contamination by seawater
and in the other case, weathering with the atmosphere
on the islands. So the atmosphere itself also
has some noble gases. We hear quite a bit about the
helium in the atmosphere and then we know that radon is
generated in our basement sometimes, if our houses are
built on granite. And we got to watch out for
that. Mmm.
So, do these gases, contaminate these gas bubbles, and how do
you get at the actual mantle of Vince's, so yes, this is one of
the frustrations that we have to deal with in these measurements
that we often don't get a sample.
That is completely devoid of contamination of the atmospheric
gases. And, as you point out, yes, when
you rupt the basalt out, to the sea floor, they might get
contaminated by the ocean water, which is some dissolved noble
gases in them for. Or the mantle plumes.
We also actually try and collect the soil samples that erupt
underwater as opposed to the lavas that are up straight into
the atmosphere, sub-area lie. And the reason is that, if they
erupt underwater pressure, there's still a chance that the
gas bubbles would be trapped in the liquid itself.
Whereas if they erupt straight into the atmosphere, the chances
are that the bubbles would be lost very quickly to Sphere and
be solidified, Basalt sample won't have a lot of gas bubbles,
but independent of this, both the whom basalts and the
mid-ocean ridge basalts would have a little bit of atmospheric
contamination. So, the way we try and tease
this out is by crushing the sample in multiple steps,
because the expectation is that they're going to be point so
weaknesses in these Salt samples, like fractures through,
which the condemnation is going to enter the sample and so gas
bubbles, that might be close to microfracture, might actually
have a lot more of this atmospheric contamination, or as
little tiny gas. Bubbles that are much further
away from the fractures may have very little or none.
So when we start cracking the sample open the first set of
bubbles that would be released would The ones that are very
close to the fracture patterns because the samples would break
along these pre-existing Plains of weaknesses.
So typically what we see is, when we first start, crushing
the sample, most of the measured noble, gas composition look
largely similar to what you would expect in seawater, which
is the atmospheric signature dissolved in seawater.
And then as we progressively crack the sample into smaller
and smaller bits, we start to see the composition Isshin move
away from the atmospheric, composition towards the mantle
composition. And very interestingly, what we
find is that for both the mid-ocean ridge basalts and the
mantle plume resources start up at the same point in their
initial Crushers, which is the atmospheric composition.
But then, as we progressively crush the sample, the trend
starts diverging in two different directions, which are
indicated. That whatever composition is
being contaminated, it's not the same.
Because if it was the same, both sets of rocks, would have
defined a single or Trend because it would be a single
mantle composition contaminated by a single atmospheric
composition. But we see two distinct Trends
and that's telling us there are two distinct sources within the
Earth's interior. How do you actually we measure
the noble gas abundances, you crush the basalt in a vacuum
chamber. And then how do you get the
abundance of a noble gas? I'm curious because it's so
inert. So, again, that's a fascinating
question. So, when we crush the basalt and
we release the gas bubbles, most of the gas is actually CO2
dissolved in that CO2 at the level of parts per trillion.
Parts for quadrillion are these noble gases.
So one of First things he have to do is to get rid of the CO2,
get rid of any nitrogen, any little bits of methane or water,
that might be present. So the way we do this is we
expose all of the gases to a highly reactive surface and that
highly reactive surface can be absorbs all of these
constituents like CO2 water nitrogen and so on but the noble
gases are left behind because they're chemically inert and so
That's how we purify the noble gases and then we can separate
them out from each other. Using a very cold, cryogenic
trap where the noble gases, get trapped on to a metallic
surface. And then we progressively
increase the temperature of that metal surface and that will
slowly release first helium and the neon and then are gone.
And so on. And so as one of the gases gets
released from this very cold, cryogenic trap, we take that gas
and we put it To a mass spectrometer to measure how many
atoms of helium we have. And what its isotopic
composition is and then we repeat it for all of the other
noble gases, like neon and argon and Krypton and Xenon.
So, let's come back to the progressive crushing.
Then you start out with the first bubbles being essentially
reflective of are of the atmosphere, and then, as you
crush, more and more than abundance is diverge, until I
guess the very very last crushes.
Has are more reflective of the actual mantle abundances on the
one hand of mid-ocean ridge Basalt and on the other of ocean
Island Basalt. So what do those results show?
So that's where this half-life of the parent radioisotope
iodine 129 comes in because what we are detecting are these
different signatures of 129. Xenon in the ocean Island
basalts and in the middle should enrich basalts and the
difference could be created in only two ways.
One is through differences that we are inherently present in the
mantle early on because of differences in let's say the
degree of degassing of the Earth's interior or they might
be created by subduction of atmosphere.
And so what these He's two different Trends.
Tell us is that subduction of atmosphere.
Can't possibly explain this difference between the mid-ocean
ridge basalts and the ocean Island basalts because they
don't lie on the same mixing Trend.
And so our only option in terms of an interpretation is that the
differences in Xenon, 129 comes from the decay of iodine 129 in
the And as I pointed out initially in the podcast that
the iodine 129 is only around for the first hundred million
years of Earth's history. And so that means that, then
this difference in the Xenon 129 could only have been produced
within that first hundred million years.
And that's the maximum range, which of the two kinds of Basalt
has the lower amount of xenon. John 1:29.
So it turns out that the lower amount of xenon 129 actually is
present in the Gloom mantle or the ocean Island Basalt mantle.
That's the one, then that retains an identity, that goes
all the way back to the first. 100 million years of Earth
history. Is that right?
So that's right. We think that its identity is
related to its history within that first hundred million years
and that it's telling us Something about the differences
in the degree of gas laws, from these two different regions of
the Earth's mantle. Because again, one critical
point to remember, is that Xenon or any of the other noble gases,
don't just Escape out of The Mantel on their own.
They usually are taking a ride on some other gas species like
CO2 or maybe water. So their history is Only tied to
the history of these other gases like carbon dioxide or water,
okay? So the ocean island, or the
basalt that we think was sampled from much much deeper in the
earth than the mid-ocean ridge Basalt, which we think comes
from middle or perhaps upper levels of the mantle has less of
the Xenon 129. And therefore, it is what
retains the memory of the very, very earliest formation of the
Earth. So does that tell us that in the
mantle it has not in fact, experienced any mixing with the
material that forms the mid-ocean ridge Basalt.
I don't think it indicates that it has no mixing.
I think what it indicates is that it has a limited amount of
mixing or another way of saying it.
Is that after 100 million years, the earth has Never been
homogenized. Because if it had been
homogenized such as by mixing, then we wouldn't see this
difference in Xenon 129 because once we homogenize it and mix
away this difference after 100 million years, I don't have a
mechanism. Now to regenerate the
difference. So the way I think about it is
that yes there has been mixing between the mid-ocean ridge.
Called source and the ocean Island Basalt Source but it's
limited. So if I was able to dial Back
Time all the way back to 4.4 billion years, I would predict
that the differences that I see compared to today would have
been much, much larger. And so over time that difference
is being whittled away by mixing, but it has not been
erased. So, how might such a separate
Source material at the bottom? Bottom of the mantle of formed
so early on and maintained. Its separate identity at least
in part throughout four and a half billion years of Earth's
history. So that is one of the million
dollar questions and one potential mechanism.
For creating these differences might be related to Earth's
violent birth where the planet was being bombarded by other
end. Heroes are protoplanets
generating very deep magma, oceans and more recent Studies
have shown that one of the products of crystallization of
these mag motions could be iron and magnesium Rich phases which
settled to the bottom of the mantle and those phases.
Now are going to be inherently denser than the rest of the
mantle so one could imagine in a scenario where there was a giant
impact during Earth's growth that melted the planet may be
all the way down to the core-mantle boundary and you
crystallized this iron and magnesium Rich layer which was
inherently denser and then subsequent magma, oceans created
by other impacts never reached down to that depth and so that
inherently denser material has effectively survived All through
the planets history. So that's one potential
explanation. I don't think we have a definite
answer at this point to how exactly this Reservoir was
produced. And I think that is one of the
challenges in terms of figuring out, how did we produce it?
And why is it still around for such a long time?
Why didn't later giant impacts such as Maybe?
Be the moon forming, giant impact, wipe this out.
It's reminds me of a couple of earlier podcasts in which two
speakers. In fact, I'm and McNamara.
And Matt Jackson and particular spoke about these very large
structures. We see, when we make seismic
images of the earth that called large low Shear, velocity
provinces, where it appears that the shear waves are traveling, a
little bit slower, which one can interpret because they're little
warmer than the surrounding. Ring or alternatively or maybe
better both of their made of a different material.
Is it possible that these reservoirs that you're talking
about that have the separate identity correspond to these
large low, Shear wave velocity provinces that we can actually
see. So, most likely the answer to
that is, yes, because again, as we look at recent, seismic,
evidences. We see that these ocean Island,
besides these mantle, plumes are being derived from these low.
Whoa, large Shear wave velocity provinces, and I'm of the group
that actually believes that these large, no Shear wave
velocity. Provinces are not just warmer
that they're actually also denser than the surrounding
mantle. And so they're not made out of
entirely material that have existed in its pristine form,
for over four and a half billion years, but they have at least
chunks of material that go all the way back.
Back to this 4.5 billion years, but there might be other
material that has been recirculated from the surface
into these, large low, Shear wave, velocity provinces.
And we actually do see some chemical evidence that that has
happened and have people built Judah, Dynamic models.
And I think Alan McNamara showed some of that with the
simulations that he did. That can generate stable
patterns that are consistent with what we know about how
slabs subduct and sink to very great.
Depths and then possibly recycled from somewhere in quite
far down near the core-mantle boundary.
So, yes, I think Alan's work actually do show that these
features could be stable for the entire time of Earth's history
of about four and a half billion years and there are other groups
that are likewise shown that. But I'm not sure this point
there is a general consensus that these provinces which are
maybe a little bit bigger than the size of Let's continents on
the surface actually, go all the way back to four and a half
billion years. There's another group of thought
that says these are created entirely through subduction of
earth's tectonic plates. But now we know that can't
really be the case, right? Because of the work you just
described. That's correct.
So I would argue that when we put together the seismic
evidence that these mantle plumes are coming from these
large low, Shear wave velocity provinces.
With the Xenon composition that is different between mid-ocean
ridge basalts and these ocean Island basalts.
I would say there must be a component in this material that
has to go all the way back to four and a half billion years.
It doesn't rule out that maybe the vast majority is recycled
but there is chunks of material. That is really really old.
I'm struck by the coincidence that there happens to be.
Noble gas, radiogenic isotopes. That was produced on a time
scale, that's very short compared to the age of the Earth
but still the order of time over which geodynamic or processes
can occur. IE a few tens of millions of
years. That is what enables us to
distinguish a reservoir that was isolated during a Time less than
a few half-lives of the corresponding Decay.
I so early on in Earth, history, are there any other Hopes, whose
abundances we can measure that are produced by radiogenic.
Decay was still shorter half-lives specifically half
live shorter than about 10 million years which might enable
us to probe still closer to the formation of the earth.
So yes, there are and one of these Isotopes is hafnium 182
which decays to tungsten 182. And this system is particularly
interesting because hafnium likes being in the mantle
tongue. In the Decay product likes being
in the corps. So hafnium, tungsten has been
used extensively to look at the differentiation between a
planet's metallic core and its silicate mantle.
And interestingly, there are some recent studies now that
show that there are differences in tungsten isotopic,
composition between ocean Island basalts and mid-ocean ridge
basalts. And again this is an indication
that that signature might have been created within the time
scale that hafnium was alive and because the half-life of hafnium
is shorter than iodine, 129 the half life is about 8.9 million
years. The total amount of time
available is 60 million years and now there is some growing
debate as to what might be the implication of this tungsten
isotope difference between ocean Alan Basalt in mid-ocean ridge
basalts and could it potentially also be an indicator of a
chemical interaction between the Earth's core and its silicate
mantle because the core-mantle boundary is a fascinating
region. It's probably one of the most
reactive boundaries we have on the earth.
It is super hot and on one side we have pure iron metal or iron
metal with a little bit of other.
Tough. And on the other side we have
silicate material so one should expect that.
There are some chemical reactions that are ongoing.
And there are other Isotopes such as Palladium which the case
to Silver. The half-life associated with
that is even shorter six and a half million years.
There is an isotope of Manganese manganese 53 that decays to
chromium that has a half-life of three million years and recently
this manganese chromium system was utilized to look at the
time. Scales of magma ocean.
Crystallization for Mars. So there are other isotopes that
might allow us to probe the Earth's formation much further
back in time and one of the wave forward in my view is to
actually make all of these measurements on the same set of
samples. So we can build a much more
complete picture of what might be going on.
So, so far, we are still measuring these different
isotopes on different samples. So we are not necessarily always
comparing an apple to an Apple. It might be an apples to oranges
comparison. I'd like to ask you final
question. If you could command a limitless
budget for either space-based or terrestrial Research into the
early Earth, how would you use it?
So, I think I would design a mission to a comet to bring some
Comet samples back to Earth, so we can study comments in much
more detail. And the reason I say that is
because I'm fascinated by the question of what was the
composition of the early atmosphere and where did the
early atmosphere come from? So in the noble gases we now see
evidence that shows that the Earth's early atmosphere cannot
be generated simply by outgassing of its interior, but
we also don't have a meteorite, or a carbonaceous chondrite
necessarily that we can put our finger on say That particular
meteorite has all of the necessary characteristics that
can explain the atmospheric fingerprint.
And so the noble gases are extremely useful in terms of
fingerprinting, what kind of materials might have contributed
to the early atmosphere? And if we know that we have a
much better sense of what the overall composition of the
atmosphere might have been in terms of.
Was it CO2 dominated or was it a CO dominated?
Here, what was the proportion of hydrogen in the atmosphere
compared to H 2 O. And so we know, so little of
commentary noble gases and comments in general, I would
argue that that's where I believe.
I would focus. My attention is to bring a
little bit of a comment back to my laboratory.
Do you have your eye on any specific comments that are
likely to come into our solar neighborhood anytime soon?
I cannot give you a direct answer in terms of a specific
comment other than that we would want.
To sample. Both the icy part of the Comet
as well as the rocky part of the Comet.
So a lot of our observations, so far of comets are actually from
the icy part which supplements as the Comets come into the
inner solar system, we have very little information about the
rocky part. We do have some amount of
material that was returned by the Stardust Mission but not
enough for somebody like me to make very precise measurements
of all of the noble gases. Sujoy mukhopadhyay.
Thank you very much. Thank you.
All of our for having me. This was wonderful.
It was a delight chatting with you for more about geology b, as
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