This is geology B with Oliver's trampled in previous, geology B
episodes. We've talked about using the
radioactive decay of uranium to lead within a Zircon, Crystal to
infer the age of a rock, but the Decay process also produces
another product helium being a gas and much more volatile than
led the helium diffuses out of the Zircon Crystal, even after
it. Eliza's.
So while the lead becomes trapped within the Zircon, as
soon as it forms within a cooling magma at 900 degrees
Centigrade, the magma has to cool all the way down to 200
degrees before. The Zircon can stop the helium
that is produced from leaking out of its crystal.
Lattice by looking at different Decay products of uranium.
We can measure how long ago the rock cooled to the temperature
at which each product became trapped in a mineral crystal and
so obtain, multiple data points of the Rock's cooling history,
Becky flowers is a professor of geological Sciences at the
University of Colorado Boulder. She has a thermal chronology lab
in, which he measures the amounts of the different
radioactive decay products within crystals to figure out
the cooling history of rocks, knowing how rocks cooled gives
us new insights on jalaja call histories and processes of many
parts of the earth. And even of the Moon, Becky
flowers, welcome to geology B. It's for having me.
It's a pleasure to be here. Why is it so valuable to
decipher the thermal histories of rocks?
What can we learn that other observations?
Don't tell us. So thermal histories can yield
really important constraints on the timing duration and rates of
a whole array of Earth processes.
Because so many of these processes are either temperature
dependent or influence temperature.
So for example, these include burial erosion magnetism
metamorphism and deformation Shen.
So I could just give you one example for erosion.
So because temperatures increase with depth beneath the earth's
surface, a rock sitting at 3 kilometers.
Deep in the earth is at warmer temperatures.
It's hotter than a rock sitting at the surface.
So as rocks approach, the Earth's surface due to removal
of overlying rocks by erosion. That rock will cool off.
So if we can reconstruct the cooling history of that rock,
that's a proxy for the erosion history of that rock and that
Turn can give us some insights into Vigor Earth processes such
as when a mountain belt with elevated or win a big canyon.
Like the Grand Canyon was in size and that in turn can give
us some insights into the processes that might have caused
those events. Okay?
Before we talk about the applications of thermal
chronology to various geological problems, I want to ask you
about the methods themselves. In the classic dating of zircon.
Crystals, the daughter nuclide whose abundance is measured is
lead but in your lab you concentrate on a very much
lighter and more volatile daughter nuclide helium.
Why is that? Yes, we're in a deep constrains
mineral cooling rather than the crystallization age of a
mineral. We call this Thermo chronology
and the technique that I use uranium thorium helium is based
on the decay of uranium and thorium to heal him.
So, when in uranium and thorium, they also undergo a long Decay
chain to lead. And in that process, they have a
bunch of alpha decay events, and those Alpha decays generate
alpha particles, which are helium atoms.
And so, in our technique, rather than dating, the lead atoms, we
date the parent, uranium, thorium, as well as the helium
atoms. In the case of the uranium-lead
method in Zircon, the lead when it's produced is stuck in the
crystal ever since its produced. So if you date a Crystal Zircon
with uranium Ledger essentially, dating the crystallization age
of that mineral in contrast in the case of helium helium is an
element that is prone to diffusion out of the crystal.
So although if you're Zircon crystallizes and it's undergoing
Decay and generating helium atoms, if that Zircon is at
elevated temperatures in the crust that helium will
completely diffuse out of the crystal.
So if you went and were able to date a crystal sitting down in
the Earth at 600 degrees C, you would get a zero age for that
Crystal. There's no healing retained in
it. And so it's only after that
mineral has cooled through a certain temperature known as the
closure temperature and begins retaining helium.
That it begins to retain that daughter.
And so if you go and date that mineral in a cooling only
scenario, you date the time that that mineral last cooled through
its closure temperature and that closure temperatures, much lower
than the closer temperature of the lead, because the helium
diffuses out so much more easily than if you like the crystal
lattice has to kind of close up a whole lot more to stop it
leaking out. Yeah, that's exactly the idea
and you can go and work on a single Crystal of zircon and you
can get a uranium-lead date and date.
It's crystallization age. Let's say it's 100 million years
ago and then you could also get uranium thrown helium date for
that same crystal and it might be a younger date.
That might tell us that that rock cooled and was exhuming and
eroding to the surface. Much more recently what minerals
are suitable for the uranium thorium helium from Akron.
G that you do one of the reasons why this technique is so
versatile? Is that pretty much any mineral
that has Trace Amounts of uranium and Thorium in the
crystal structure, has the potential to be used as a
uranium thorium helium thermal chronometer is long as it's
retentive to helium. So, most Studies have used
appetite and Zircon because these are very common minerals
in many rock types. And they constrain thermal
histories at upper crustal and near surface conditions.
And so they're very useful for constraining.
Or Surface erosion histories, but there are a variety of other
minerals that people are developing as helium thermal
chronometers and there's a bunch of reasons for this.
One thing is that if you have lots of different minerals that
you can date from a single Rock and these different minerals,
have different closure temperatures, this allows you to
reconstruct really high resolution thermal histories.
The second thing is that different rocks, have different
minerals in them. So it's useful to have tools
that allow us to reconstruct thermal histories in a whole
range of rock types. And then the third thing is that
commonly, there is a desire to date processes that are
notoriously difficult to date, like weathering, which occurs at
the Earth's surface and is climatically driven, and there's
some minerals that we can date with helium, that allow us to
constrain some of these difficult to date.
Earth processes, and what's awesome.
These other minerals apart from zakone, appetite the iron oxides
hematite and grew tight, because there's are, commonly weathering
products. They develop for example, soil.
Files. So in that case if you have a
mineral that develops at the Earth's surface and you date it,
you might be constraining the time at which that mineral form
and be dating a weathering event.
Sometimes those minerals form on fault surfaces.
And so there's a lot of innovative work that some groups
are doing to constrain the timing of fault Movement by
dating hematite on these fault surfaces.
There are also other minerals like perovskites and badly.
I that occur in rocks that are mafic, rocks, iron and magnesium
retracts that are other Is difficult to date there's
probably like 20 or 30 different minerals that people have tried
to date with this tool with varying success in some of them
show. A lot of promise, I know that a
big part of your research effort is directed to improving.
The thermal chronology methods. Not only to extend them to a
wider range of source rocks, but also to make the results more
reliable and perhaps more precise.
Can you tell us what the main challenges are that you face
when you try and do that? Yes, I think one of the big
challenges that workers in this field are addressing right now,
is trying to better understand how helium, diffusion occurs in
different mineral phases. So as I was just talking about,
you have this healing that's generated and then it diffuses
out of the crystal in order to interpret those data
quantitatively, you need to know the conditions under which the
helium diffuses out of your mineral and some minerals like
appetite and Zircon, the diffusivity of helium out of
your Crystal, how fast it goes out.
Is a function of not only temperature, which is what we
just talked about the things like radiation damage.
So as helium is generated, it's generated by radioactive decay
and that process damages, the Crystal and that affects the
Reit entity of your Crystal. Also Crystal size can affect the
Reit entity or the diffusivity of helium out of your Crystal.
And so, there's a lot of work focused on trying to understand
these different mineral characteristics that control the
loss of helium and to quantify those into A models.
So that we can have kinetic models that we can use to
decipher the significance of our date.
I guess, in the idealized case, you'd have a function, which
should give you the diffusivity as a function of temperature,
radiation damage and Crystal size.
Yes, that's exactly what you want.
That's exactly what you want. But isn't it also further
Complicated by the fact that when you get A rock sample into
your lab. You're actually looking at that
time integration of all the diffusion out of that Crystal
over the geological time. So how do you that's another
issue, isn't it? Yeah.
So if you have proper kinetic models, you can deal with that,
but that's a great point. One simple way that we think
about it is that if you have a fast cooling history at
temperatures above the closure temperature, your helium is
completely lost from the crystal, okay?
And below that temperature, your helium is completely retained.
And that's if you have like a fast cooling history and that's
envisioning. The system is like a light
switch. It's either on or it's off but
in reality it's not like that. It's like a dimmer switch.
So there's actually a temperature range over which
your helium can be partially retained in and partially lost
from the crystal. So, if you have a simple
scenario, let's say you're sitting in the Himalayas and
there's a granite that was in place 25 million years ago, and
then it gets rapidly eroded and brought to the surface.
That's pretty simple, your date is simply.
We going to record when that rock was eroded and got to the
surface. However, because of this
temperature window over which helium is partially retained and
partially lost, you can have much longer histories that
involve erosion to the surface reburial and then erosion back
to the surface. And over that entire history,
your Crystal may be gaining helium and then losing a little
bit and then regaining it. And so this requires them
thinking about some of these so-called dates that we Obtained
for a minerals, actually or something, a reflection of the
total time, integrated history, of helium, accumulation and
diffusion out of your Crystal. These require quantitative
models and increasingly our field is pushing towards looking
at deeper time histories to infer their own erosion
histories over longer time scales.
And that's something that my group does quite a bit of work
on. Let's talk now about some of the
geological problems that you've addressed.
Seeing Thermo chronology. Let's start with a great
unconformity. Yeah, the great unconformity.
So this is an iconic geologic feature.
It marks a major Gap in the geological record of the
continents. So it classically marks.
This nice sharp boundary in the Rock record between unfossilized
first pre-cambrian, Brock. So much older rocks and
fossiliferous layered sedimentary rocks and across
that. Andre at the great unconformity
there's commonly a half billion years up to like three billion
years of missing time. The great unconformity was
originally described that its famous Grand Canyon locality by
John Wesley Powell. But this feature occurs all over
the US you can go out and see it at all kinds of locations around
the United States and also occurs all around the world.
And to Charles Darwin, he talked about the sudden appearance of
complex macroscopic fossils in the Cambrian strata.
The great unconformity is suggesting some kind of singular
Global event that led to the development of this feature and
this missing Gap in time. And so Rosen, across this
feature, it's played a big role in ideas about the Cambrian
explosion of Life. Some people have tried to
develop a causal link between major erosion across the Great
unconformity. And the Cambrian diversification
of life, we actually don't really know how old it is.
So we know the age of sedimentary rocks that sit above
this feature, but Until recently we haven't been able to
constrain the timing of the erosion event that led to the
development of that erosion surface and that's exactly
because this feature marks is huge gap.
In the Rock record so the erosion history that led to its
development, you can't investigate that directly by
study of preserved, Rock units. So this is something that we can
tackle with firmer chronology. A number of models have been
proposed for when and why the Great unconformity The some
models have suggested the great unconformity formed right at the
timing of the Cambrian explosion and others will it occurred 500
million years before that and so if it occurred for 500 million
years before that it probably doesn't have a direct causal
link and so to actually try to understand degrade and conformed
erosion, contribute to the Cambrian explosion, we actually
need to better determine the timing.
And then another big question is did the great unconformity
develop in One Singular, massive?
Erosion event, for example, caused by something like
snowball Earth, or did it form at different times in different
places and have multiple causes? For example, maybe there are
tectonic drivers in different locations, that ultimately led
to development of a feature that looks the same everywhere, but
it actually has a Time integrated history that with
multiple erosion events, that Amalgamated to form, this
geologic Gap in the record. Go about dating that erosion
surface. Then you go and you can collect
samples from right below the Great unconformity.
And the minerals in these rocks, immediately below the great
unconformity can retain the thermal history of that erosion
event that led to the development of that feature.
Even if the rocks that were eroded are now gone.
So, just below the great unconformity, erosion surface,
you pick out a mineral and you see when it's closer
temperature, drop pretty much close to what you'd expect on
the surface. I don't know, 20 degrees 10
degrees, something like that. And that age tells you when that
service was last exposed. Is that the idea?
That's the idea. Yes.
Yeah, it can be more complicated than that because there's all
kinds of stuff that happened since then, that you also have
to see through in order to get back at that erosion event.
But that is the basic idea. Wow, I can see how that could
be. Really powerful.
And so what have you been able to discover about the age of the
great unconformity? Yeah.
So we've been working on the great unconformity in different
locations and it's clear that the timing of the last major
exclamation event that led to that development of the great
unconformity, erosion surface varies regionally.
So for example here in Pikes Peak Colorado we have data that
constrain the development of that surface to before 700
million years ago. Whereas in in the central
Canadian Shield, our data very clearly say that that feature
form there after 600 million years ago.
So it appears that the great unconformity can't be attributed
to a singular sort of global event.
But rather that it formed heterogeneously a different
timing and possibly with different tectonic causes in
different places and so this makes it more difficult to tie.
It directly as a very specific and direct cause of the
Cambrian. First vacation of Life, although
there still may be in direct links to environmental and
biologic change in this interval.
The great unconformity is very conspicuous.
As you mentioned in the Grand, Canyon has your work helped us
figure out the erosion history, there are p a PhD student
working with me required, you throwing helium data for
minerals and rocks, right below the great unconformity at
various locations within the modern Grand Canyon and Is
coupled that data with a variety of geologic.
Constraints and a key Point here is the history that she's
deciphering related to Great unconformity, development
occurred, hundreds of millions of years before the Grand Canyon
itself was in size. So the Grand Canyon is in size
there now, and allows us to see the great unconformity and to go
sample it. But the history that were
inferring doesn't have anything to do with the Grand Canyon
carving itself, but her work reveals that there was likely
paleo topography across that region in the eighth.
Our millionaire to 500 million year time frame, that was
developed due to faulting associated with assembly and
breakup of the rodinia supercontinent.
And so her data suggest for example that there was a high
Paleo High in. What is now the western part of
the Grand Canyon and opilio low in?
What is now eastern part of the Grand Canyon such that the
payload high in the western part might have been shedding
sediments to the area to the East.
And so in these faults in the Grand, Canyon have been known
for a long time to have a history of repeated.
Reactivation. And so our interpretation is
that this faulting and paleo topographic Evolution occurred
during this multi-phase tectonic, activity during the
assembly and breakup of the rodinia supercontinent over
several hundred million year interval.
And so together this complicated history, contributed to the
singular feature that we know as the great unconformity in the
grand Canyon's. That's interesting.
So your thumb or chronology enables us to infer a possible
link between the To pull elevation changes and the
resulting erosion and the very large scale tectonic events
happening. At the time, from the Assembly
of rodinia supercontinent about a billion years ago to its
subsequent breakup about 750 million years ago.
So let's talk about the long-term history of Continental
cores or cretons. The conventional thinking has
been that these ancient blocks of content.
Continental lithosphere have just basically sat there since
they formed some of them over 2 billion years ago, there's a
variety of data that clearly indicate that some of these
areas have undergone a more active history than we
previously realize we have been acquiring you throwing helium
data for samples across the interior of North America, that
show that some of these areas were buried by multiple km of
sedimentary rocks that were eroded and then, Read by
sedimentary rocks. And then those were eroded away
a second time. And so we can use our thermal
chronologic tool to decipher the spatial extent, the thickness
and the erosion history of these.
Now eroded sedimentary units and we're deciphering these
histories of barrel and erosion and places where most people
have assumed nothing has happened in that interval.
So this raises all kinds of interesting questions then like
why were these areas buried in a row did multiple times when
we're in the middle of continents far from Plate
tectonic boundaries that traditionally people have looked
to, to explain, large-scale, burial and erosion events in a
previous podcast, Carolina, Lithgow Bert saloni talked about
Dynamic topography and how the elevated Topography of some
cretons met because at least in part by the force of an
upwelling mantle below. I understand that you've been
able to shed some light on that idea.
Pharmacology. Yeah this is exactly the kind of
problem that we're trying to tackle.
So both with some of our work across North American Interior
as well as in southern Africa. Because if you think of how
elevations vary in different locations, like the East Coast,
like where you are Oliver sitting at sea level.
And me here, Mile High here in Boulder Colorado.
You can either look to differences in the thickness and
the buoyancy structure of the crust and the lithosphere.
So this is like the the buoyant part of the earth or you can
look to deeper mental processes that might be driving
differences in the elevation. And in the same way changes in
the lithosphere buoyancy structure or changes and mantle
flow patterns, which some people call Dynamic topography that can
cause elevation change over time.
And so if you're sitting near the plate margin, like if you're
in the Himalayas or in the Andes, it's easy to say, okay,
we'll plate tectonic processes that are a consequence of plate,
sliding back and forth, and Sighing, the crust are
responsible for the elevation change, but if you're in the
middle of continents, you don't have that as a mechanism.
And so a good place to go, try to decipher the effects of
dynamic, topography are in some of these Continental cores,
where your shielded from the plate boundaries.
So you don't have significant plate tectonic.
Effects, over printing, your signal and southern Africa is
particularly interesting because it's one of the most widely
cited examples of elevated. Dynamic topography on the
continent. So southern Africa is a plateau
and unlike all of the other major plateaus on earth, like
the Colorado Plateau or the Tibetan Plateau, those other
plateaus were in some kind of a contractional plate boundary
setting. So a setting where two plates
are moving together. When they underwent elevation
gain, the southern African Plateau was completely
surrounded by plate boundaries, that were pulling apart when it
became elevated. And so for this reason, people
have pointed towards deeper mantel processes as Possibly
being responsible for elevation gain.
A dynamicists like Carolina have imaged deep structures in the
mantle. That might be responsible for
that. So, how did you go about
disentangling of the causes of uplift and southern Africa.
So our projects was aimed at deciphering the erosion history
across the interior of the Southern African plateau.
And one of the things we specifically did was work on
what are called kimberlite pipes.
So these things are small volume, volatile, rich.
Magma is like volcanoes but they contain most of the world's
diamonds and they come up through these ancient cores of
continents. And these things come blasting
up from deep in the mantle and they sample the lithospheric
mantle and then pieces of the crust and then any of the
sedimentary cover at the surface.
So what we did was acquire thermal chronology data on
samples collected across southern Africa, to decipher the
timing, the history and patterns of erosion across two different
regions there. So, one of these regions had a
very strong record of Lithospheric.
Mantle alteration as recorded by kimberlite pipes that had
erupted through that area. And then the other area had
relatively little modification of the lithospheric mantle
structure. And so what we found is in the
area with substantial lithospheric modification, we
found a phase of pronounced erosion that coincided with win
that lithospheric modification occurred, telling us that
lithospheric modification process likely was responsible
for the Engane and therefore the erosion history that we see in
that area, in contrast in the region that underwent less
lithospheric modification. We see a much more protracted
erosion history. We see a phase of scarp Retreat
over several tens of millions of years across the plateau and
because this area underwent less lithospheric modification.
This points towards deeper mental processes such as Dynamic
topography effects, that probably contributed to the
elevation gain there. So together, these results
indicate that both lithospheric and deeper Dynamic processes,
likely caused the rise of the Southern African Plateau with
the relative influences of those two processes, varying spatially
across the plateau. Wow.
So the bottom line is that, at least under the southern African
Plateau, we really are seeing the effects of dynamic
topography. Yes, Dynamic Topography is real,
but is not the only part of the story related to the rise of the
Southern African Plateau. Most of the work you've talked
about so far has been based on analyzing minerals like Zircon
or appetite. But what about rocks that don't
contain these minerals such as shells and limestone's Shields
and limestone's form, thick sections and sedimentary basins.
But it turns out you know, shales and limestone's.
They don't have any minerals that are easily dated or in the
past have been easily dated with firmer chronology and so we
attempted to Date conodonts in shells and limestone's.
So these are micro fossils. These are Critters that lived
for several hundred million years in the Paleozoic and early
Mesozoic. So for thing, like 500 million
years ago, until maybe 300 million years ago and they're
very common these conodonts, they're actually bio appetite in
many rocks, we date, Crystal, and appetite.
But these are actually appetite. That is biologically formed,
like, your teeth are bio appetite.
We tried to date some of these materials to develop as a
thermal chronometer and it wasn't as easy as other
minerals. One of the challenges is that.
These conodonts, they don't start off with lots of uranium,
and Thorium in them. Like, you don't have lots of
uranium and Thorium in your teeth, these Critters did not
die. They did not have a lot of
uranium, thorium and their bodies.
So what happens is these Critters, die.
And these conodonts are deposited and they acquire,
uranium and thorium as they get buried.
And what this means is that, the uranium and thorium is more
mobile and for a junior chronometer thermal chronometer,
it's very bad you Want your parent isotope to be mobile and
be lost from your Crystal. There are different content
components. And so I have not totally given
up on this idea, because if you could use this to constrain
thermal history, so it would be extremely powerful.
And as I mentioned in the introduction, you're even
working on lunar rocks. What can thermal chronology
reveal about those on the moon? The firm will histories are
induced by meteorite impact processes early in the solar
system, we know. That there were lots of impacts
that this caused melting of the surface and planetary,
resurfacing and at some point in time that impact flux decline to
modern-day background levels. And so a big question is when
and how did this flux Decline? And so in order to figure this
out this requires dating, the history of meteorite impacts on
the moon. So using uranium from Helium, we
can begin to decipher parts of that history, to constrain the
tiny and the thermal effects, the magnitude of these impact
events, right? Have a new NASA project, where
we're doing additional work, on lunar materials, to try to
constrain that history, and it's really cool to be working on
samples, that came back from the Apollo Mission.
But when you have an impact event, you have a momentary,
huge heating. Do you then also have a slow
cooling history that you can track on the moon?
We have these short, as you say, a very intense Heating and
Cooling events. So, whatever kinetic models, we
develop have to be able to handle.
Short-term events, but then on the lunar surface also, it's
pretty hot. So, we also have to consider the
effects of some of these samples sitting at the surface of the
Moon, for a long time, and being relatively warm and deconvolve
that from the effects of these short extreme, heat pulses, and
have you been able to get some results on the bombardment
history of the lunar? Surface data, allow us to detect
a 3.95 billion-year-old impact event. 110 million year impact
event, and they allow us to limit the temperature of any
impact event. That would have occurred between
those two impacts that I just described.
And so that gives us an insight into the impact history in the
thermal history at one spot on the lunar surface.
But then by working on other samples with different
histories, the idea is that by integrating those that will be
able to gain a more comprehensive picture of the
larger lunar history. For my last question, I want to
ask you about the capabilities, you dream about having in your
lab especially if there were no funding constraints.
What I would really want. If I had unlimited funding would
be to invest in human infrastructure.
Just talking with you today. You can see we deal with the
normal spatial scales and temporal scales.
So we worry about nanoscale like how does diffusion occur in our
structure? And there's a variety of light
material characterization tools and all kinds of new tools that
people are Developing in other fields, that might help us
better understand diffusion at that nanoscale.
But at the same time we're trying to extrapolate that to
uplift at the Continental scale or characterizing short
experiments in our lab over S and H and we're trying to
extrapolate that to figure out histories back of the Moon back
to 4.5 billion years ago to Grapple with this, you really
need people who think about that nanoscale.
We need to have improved statistics and uncertainty
characterization and people can do the modeling and the coding
and then people who about those big problems and so thinking
about how to better use the tools we have and to get people
working creatively on all different aspects of the
problem. Like if I had unlimited money, I
would put it into people. Becky, flowers.
Thank you very much. Thank you Oliver.
This is great. I really enjoyed it. for more
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