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Oliver Strimpel: This is Geology Bites with Oliver Strimpel. In previous Geology Bites 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 lead, the helium diffuses out of the zircon crystal even after it crystallizes. 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 thermochronology lab in which she 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 into the geological histories and processes of many parts of the Earth, and even of the Moon. Becky Flowers, welcome to Geology Bites.

Becky Flowers: Thanks for having me. It's a pleasure to be here.

Oliver Strimpel: Why is it so valuable to decipher the thermal histories of rocks? What can we learn that other observations don't tell us?

Becky Flowers: 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. For example, these include burial, erosion, magmatism, metamorphism and deformation. I could just give you one example, for erosion. Because temperatures increase with depth beneath the Earth's surface, a rock sitting 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 in turn can give us some insights into big Earth processes, such as when a mountain belt was elevated or when a big canyon like the Grand Canyon was incised. And that in turn can give us some insights into the processes that might have caused those events.

Oliver Strimpel: Okay. Before we talk about the applications of thermochronology 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?

Becky Flowers: Yes. We're interested in constraining mineral cooling, rather than the crystallization age of a mineral. We call this thermochronology, and the technique that I use, uranium–thorium–helium, is based on the decay of uranium and thorium to helium. So uranium and thorium 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 parents, uranium and 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 it's produced. So if you date a zircon crystal with uranium–lead, you're 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 your zircon crystallizes and is 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 were able to go and date a crystal sitting down in the Earth at 600 °C, you would get a zero age for that crystal; there's no helium 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.

Oliver Strimpel: And that closure temperature is much lower than the closure temperature for the lead, because the helium diffuses out so much more easily. So, if you like, the crystal lattice has to kind of close up a whole lot more to stop it leaking out.

Becky Flowers: 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 its crystallization age, let's say it's 100 million years ago, and then you could also get a uranium–thorium–helium date for that same crystal, and it might be a younger date. That might tell us that that rock cooled, and was being exhumed and eroded to the surface, much more recently.

Oliver Strimpel: What minerals are suitable for the uranium–thorium–helium thermochronology that you do?

Becky Flowers: 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 its crystal structure has the potential to be used as a uranium–thorium–helium thermochronometer, as long as it's retentive to helium. Most studies have used apatite and zircon, because these are very common minerals in many rock types, and they constrain thermal histories at upper-crustal and near-surface conditions, so they're very useful for constraining near-surface erosion histories. But there are a variety of other minerals that people are developing as helium thermochronometers, and there's a bunch of reasons for this. One 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 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 are some minerals that we can date with helium that allow us to constrain some of these difficult-to-date Earth processes.

Oliver Strimpel: And what are these other minerals, apart from zircon and apatite?

Becky Flowers: The iron oxides hematite and goethite, because these are common weathering products. They develop, for example, in soil profiles. 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 formed, and 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 perovskite and baddeleyite, that occur in rocks that are mafic, iron- and magnesium-rich rocks, that are otherwise difficult to date. There are probably 20 or 30 different minerals that people have tried to date with this tool, with varying success, and some of them show a lot of promise.

Oliver Strimpel: I know that a big part of your research effort is directed to improving the thermochronology 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?

Becky Flowers: 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. As I was just talking about, you have this helium 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 in some minerals, like apatite and zircon, the diffusivity of helium out of your crystal, how fast it goes out, is a function not only of temperature, which is what we just talked about, but of things like radiation damage. As helium is generated, it's generated by radioactive decay, and that process damages the crystal, and that affects the retentivity of your crystal. Crystal size can also affect the retentivity, 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 in models, so that we can have kinetic models that we can use to decipher the significance of our dates.

Oliver Strimpel: I guess in the idealized case, you'd have a function which would give you the diffusivity as a function of temperature, radiation damage and crystal size.

Becky Flowers: Yes, that's exactly what you want. That's exactly what you want.

Oliver Strimpel: 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 the time integration of all the diffusion out of that crystal over geological time? So that's another issue, isn't it?

Becky Flowers: 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, and below that temperature, your helium is completely retained. That's if you have a fast cooling history, and that's envisioning the system as being 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. 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 emplaced 25 million years ago, and then it gets rapidly eroded and brought to the surface, that's pretty simple. Your date is simply 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 thinking about some of these so-called dates that we obtain for minerals as actually 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 burial and erosion histories over longer timescales, and that's something that my group does quite a bit of work on.

Oliver Strimpel: Let's talk now about some of the geological problems that you've addressed using thermochronology. Let's start with the Great Unconformity.

Becky Flowers: Yeah, the Great Unconformity. So this is an iconic geologic feature. It marks a major gap in the geological record of the continents. It classically marks this nice sharp boundary in the rock record between unfossiliferous Precambrian rock, so much older rocks, and fossiliferous, layered sedimentary rocks, and across that boundary, at the Great Unconformity, there's commonly half a billion years, up to like three billion years, of missing time. The Great Unconformity was originally described at 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 it also occurs all around the world. And going back to Charles Darwin, who talked about the sudden appearance of complex macroscopic fossils in the Cambrian strata, the Great Unconformity has suggested some kind of singular global event that led to the development of this feature and this missing gap in time. And so erosion across this feature has 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. But we actually don't really know how old it is. We know the age of the 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 this 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 thermochronology. A number of models have been proposed for when and why the Great Unconformity formed. Some models have suggested the Great Unconformity formed right at the time of the Cambrian explosion, and others that it occurred 500 million years before that, and if it occurred 500 million years before that, it probably doesn't have a direct causal link. So to actually try to understand whether Great Unconformity erosion contributed to the Cambrian explosion, we 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 the development of a feature that looks the same everywhere, but actually has a time-integrated history, with multiple erosion events that amalgamated to form this geologic gap in the record.

Oliver Strimpel: So how do you go about dating that erosion surface, then?

Becky Flowers: You can go and 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 the erosion event that led to the development of that feature, even if the rocks that were eroded are now gone.

Oliver Strimpel: So just below the Great Unconformity erosion surface, you pick out a mineral, and you see when its temperature dropped pretty much to what you'd expect at the surface, I don't know, 20 degrees, 10 degrees, something like that. And that age tells you when that surface was last exposed. Is that the idea?

Becky Flowers: That's the idea, yes. It can be more complicated than that, because there's all kinds of stuff that has 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.

Oliver Strimpel: 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?

Becky Flowers: Yeah. So we've been working on the Great Unconformity in different locations, and it's clear that the timing of the last major exhumation event that led to the development of the Great Unconformity erosion surface varies regionally. So for example, here at Pikes Peak in Colorado, we have data that constrain the development of that surface to before 700 million years ago, whereas in the central Canadian Shield, our data very clearly say that that feature formed there after 600 million years ago. So it appears that the Great Unconformity can't be attributed to a singular global event, but rather that it formed heterogeneously, at different times 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, to the Cambrian diversification of life, although there still may be indirect links to environmental and biologic change in this interval.

Oliver Strimpel: The Great Unconformity is very conspicuous, as you mentioned, in the Grand Canyon. Has your work helped us figure out the erosion history there?

Becky Flowers: A PhD student working with me acquired uranium–thorium–helium data for minerals in rocks right below the Great Unconformity at various locations within the modern Grand Canyon, and has coupled those data with a variety of geologic constraints. And a key point here is that the history that she's deciphering, related to Great Unconformity development, occurred hundreds of millions of years before the Grand Canyon itself was incised. So the Grand Canyon is incised there now, and allows us to see the Great Unconformity and to go and sample it, but the history that we're inferring doesn't have anything to do with the carving of the Grand Canyon itself. Her work reveals that there was likely paleotopography across that region in the 800-million- to 500-million-year time frame, which was developed due to faulting associated with the assembly and breakup of the Rodinia supercontinent. And so her data suggest, for example, that there was a paleo-high in what is now the western part of the Grand Canyon and a paleo-low in what is now the eastern part of the Grand Canyon, such that the paleo-high in the western part might have been shedding sediments to the area to the east. And the 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 paleotopographic evolution occurred during this multiphase tectonic activity during the assembly and breakup of the Rodinia supercontinent, over a 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.

Oliver Strimpel: That's interesting. So your thermochronology enables us to infer a possible link between the paleoelevation changes and the resulting erosion, and the very large-scale tectonic events happening at the time, from the assembly of the Rodinia supercontinent about a billion years ago to its subsequent breakup about 750 million years ago. Let's talk about the long-term history of continental cores, or cratons. The conventional thinking has been that these ancient blocks of continental lithosphere have just basically sat there since they formed, some of them over 2 billion years ago.

Becky Flowers: There's a variety of data that clearly indicate that some of these areas have undergone a more active history than we previously realized. We have been acquiring uranium–thorium–helium data for samples across the interior of North America that show that some of these areas were buried by multiple kilometers of sedimentary rocks that were eroded, and then reburied by sedimentary rocks, and then those were eroded away a second time. And so we can use our thermochronologic 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 burial and erosion in places where most people have assumed nothing has happened in that interval. So this raises all kinds of interesting questions, like why were these areas buried and eroded multiple times, when they're in the middle of continents, far from the plate tectonic boundaries that people have traditionally looked to to explain large-scale burial and erosion events?

Oliver Strimpel: In a previous podcast, Carolina Lithgow-Bertelloni talked about dynamic topography, and how the elevated topography of some cratons may be caused, 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 using thermochronology.

Becky Flowers: Yeah, this is exactly the kind of problem that we're trying to tackle, both with some of our work across the North American interior, as well as in southern Africa. Because if you think of how elevations vary in different locations, like the East Coast, where you are, Oliver, sitting at sea level, and me here, a mile high, 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 the buoyant part of the Earth, or you can look to deeper mantle processes that might be driving differences in elevation. And in the same way, changes in the lithosphere's buoyancy structure, or changes in mantle flow patterns, which some people call dynamic topography, can cause elevation change over time. And so if you're sitting near a plate margin, like if you're in the Himalayas or in the Andes, it's easy to say, okay, well, plate tectonic processes that are a consequence of plates sliding back and forth and thickening 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 to try to decipher the effects of dynamic topography is some of these continental cores, where you're shielded from the plate boundaries, so you don't have significant plate tectonic effects overprinting your signal. And southern Africa is particularly interesting, because it's one of the most widely cited examples of elevated dynamic topography on the continents. Southern Africa is a plateau, and unlike all of the other major plateaus on Earth, like the Colorado Plateau or the Tibetan Plateau, which were in some kind of contractional plate boundary setting, 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 mantle processes as possibly being responsible for its elevation gain, and geodynamicists like Carolina have imaged deep structures in the mantle that might be responsible for that.

Oliver Strimpel: So how did you go about disentangling the causes of uplift in southern Africa?

Becky Flowers: So our project 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. These are small-volume, volatile-rich magmas, like volcanoes, but they contain most of the world's diamonds, and they come up through these ancient cores of continents. 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 thermochronology data on samples collected across southern Africa, to decipher the timing, the history and the patterns of erosion across two different regions there. 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 the other area had relatively little modification of the lithospheric mantle structure. And what we found is that in the area with substantial lithospheric modification, there was a phase of pronounced erosion that coincided with when that lithospheric modification occurred, telling us that the lithospheric modification process was likely responsible for the elevation gain, 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 mantle 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.

Oliver Strimpel: Wow. So the bottom line is that, at least under the southern African Plateau, we really are seeing the effects of dynamic topography.

Becky Flowers: Yes, dynamic topography is real, but it is not the only part of the story related to the rise of the southern African Plateau.

Oliver Strimpel: Most of the work you've talked about so far has been based on analyzing minerals like zircon or apatite. But what about rocks that don't contain these minerals, such as shales and limestones?

Becky Flowers: Shales and limestones form thick sections in sedimentary basins, but it turns out that shales and limestones don't have any minerals that are easily dated, or that in the past have been easily dated, with thermochronology. And so we attempted to date conodonts in shales and limestones. These are microfossils. These are critters that lived for several hundred million years in the Paleozoic and early Mesozoic, so from something like 500 million years ago until maybe 300 million years ago, and they're very common. These conodonts are actually bioapatite. In many rocks, we date crystalline apatite, but these are actually apatite that is biologically formed, like your teeth are bioapatite. We tried to date some of these materials, to develop them as a thermochronometer, and it wasn't as easy as with other minerals. One of the challenges is that these conodonts 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, when they died, did not have a lot of uranium and thorium in 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 are more mobile, and for a geochronometer or thermochronometer, that's very bad. You don't want your parent isotope to be mobile and be lost from your crystal. There are different conodont components. And so I have not totally given up on this idea, because if you could use this to constrain thermal histories, it would be extremely powerful.

Oliver Strimpel: And as I mentioned in the introduction, you're even working on lunar rocks. What can thermochronology reveal about those, on the Moon?

Becky Flowers: The thermal 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 that at some point in time the impact flux declined to modern-day background levels. And so a big question is, when and how did this flux decline? In order to figure this out, this requires dating the history of meteorite impacts on the Moon. So using uranium–thorium–helium, we can begin to decipher parts of that history, to constrain the timing and the thermal effects, the magnitude, of these impact events. Right now, we 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 missions.

Oliver Strimpel: 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?

Becky Flowers: We have these short, as you say, 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, it's also 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.

Oliver Strimpel: And have you been able to get some results on the bombardment history of the lunar surface?

Becky Flowers: Our data allow us to detect a 3.95-billion-year-old impact event and a 110-million-year-old 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 and 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, we'll be able to gain a more comprehensive picture of the larger lunar history.

Oliver Strimpel: 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.

Becky Flowers: 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 enormous spatial scales and temporal scales. We worry about the nanoscale, like how does diffusion occur in our crystal structure? And there's a variety of 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 we're characterizing short experiments in our lab, over seconds and hours, and we're trying to extrapolate that to figure out histories back on 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 who can do the modeling and the coding, and then people who think about those big problems. And so it's 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.

Oliver Strimpel: Becky Flowers, thank you very much.

Becky Flowers: Thank you, Oliver. This is great. I really enjoyed it.

Oliver Strimpel: For more about Geology Bites, as well as pictures and illustrations that support this podcast, you can go to geologybites.com.