The Times about time: Geochronology themed article in the NY Times

No real analysis here, but I'd like to draw your attention to this recent article by Kenneth Chang in the New York Times about recent work that might shed light on the very early earth. I am highlighting the article both because it deals with geochronology (U-Pb zircon geochronology to be exact), and because it is a rare article dealing explicitly with geology; not modern climate change or some geoscience themed hazard, but straight up geology. The article also has some good quotes from some of the grand high mucky mucks of geology including Mark Harrison, John Valley, and Norm Sleep. I've seen a talk version of some of this data before, and when I get into my office tomorrow I'll check out the Nature paper as well and try to comment, although in all honesty I seriously doubt I'd come up with much more insightful than Professors Harrison, Valley, Sleep, and Mojzsis.

In consideration of self promotion, if you'd like some background on geochronology to help with the NY Times article, check out this earlier post of mine, or any of the background geology posts I have listed on my sidebar.

FT2008 - Alaska chapter 4 and a big thanks

Before I write any more about FT2008, the International Conference on Thermochronometry, I need to take care of one blog related item. Last week I was named a "blog of note" on blogger.com, and have since seen a drastic increase in my readership. Above is a bar graph of my daily page loads from 10/10/2008 until today. Take the statistics challenge, see if you can tell what day I was named a blog of note. I removed the actual numbers, well, mainly because I have been shamed into realizing I was letting my blog suffer tremendously and therefore are unwilling to admit my average readership. But, thanks to the recognition, my page loads really spiked, that first day they were 2 orders of magnitude higher than average, and although they have settled down, are still 20 times what I am used to. Now, based on the comments, many people want to attribute this to my pretty pictures from Alaska, but that is probably only because they are embarrassed to admit how addictive thermochronology can be. I understand gentle readers, but don't be ashamed, it is OK to admit that you are fascinated by thermochronology, that you now want to quit your career and pursue this new passion, that you now try to work in the phrase "thermally activated volume diffusion" into everyday conversations, and you are constantly frustrated when reviewing papers that compare apparently phase-independent "⁴⁰Ar/³⁹ ages" to U-Pb zircon ages like they are the same thing.

Seriously though, thank you to whoever named me a blog of note, and to all of the people who've had so many nice things to say about the blog and my pictures. I appreciate the kind words.

So back to FT2008, the International Conference on Thermochronmetry. In a previous post, I discussed some of the methodoligical advancements I was most interested in. Today I just wanted to highlight a few of the case studies I found most intriguing. Again, if you are interested in these topics, make sure to check out the free and downloadable extended abstracts from the meeting, available from the Union College FT2008 website. Of course, these will be interspersed with random pictures from the field trips, in no particular order.

• There was one talk and a few posters that dealt with apatite fission-track and (U-Th)/He ages from tunnels in the alps. The talk was by Reinecker, and I apologize for not remembering his first name, and the posters were by Glotzbach and Spiegel. All of these papers were in the Alpine Orogen session on the Thursday of the talk. So why tunnels? Well, these tunnels go straight through significant topographic peaks. Isotherms, or surfaces of equal temperature in the earth, tend to mimic topography, especially at relatively shallow levels. In some ways this is a problem in thermochronology. We often would like to know how fast things came to the surface, but that depends on the depth of the closure temperature isotherm, which in turn depends on toppgraphy (and many other things), which we don't necessarily know. Isotherms are deflected up under large topographic peaks, meaning that if you drill sideways through a mountain, you will experience hotter and hotter temperatures towards the core of the mountain. So I mentioned that the deflection of isotherms is a problem for us brave thermochronologists, but used correctly, it could also be a relatively powerful tool. If topography can affect isotherms, then topography should also be recorded in thermochronometers. The tunnel studies should see evidence for the topography being recorded in the low-temperature thermochronometers. Turns out it isn't so obvious, but I'll leave the abstracts for you to read.

A Blue Grouse (I think, correct me if I am wrong)

• In the last few years there have been a number of studies investigating the link between climate and tectonics. Specifically, which drives which? My own personal belief is that it just isn't an either or, but the idea that climate (namely erosion) could drive crustal processes is kind of hard to swallow for many geologists. Some of the evidence for this involves correlations between erosion rates, rainfall, and uplift rates in active mountain belts. This isn't supposed to work everywhere, there are plenty of places that get tons of rain but where nothing is being uplifted (like the Amazon basin), but many people think of it as a major driver in mountainous regions. Frank Lisker presented a paper on some of his results from Sri Lanka, and what struck me is that the southern part of the island has a rather large mountain (2000+ meters) and gets buckets of rain, but has i n c r e d i b a l l y s l o w uplift rates, slow enough they are reported in meters per million years (typically we report uplift rates in kilometers per million years).
  

More massive piles of Late Miocene - Pliocene conglomerates

So I think that is all I'm going to write on this. It gets difficult to decide what talks to highlight and what talks not to highlight. If you have found any of the things I've discussed intriguing, download and enjoy the abstract volume.

More pillow basalts from the Kenai Peninsula. Seriously, they actually look like pillows!

And my last Alaskan fall picture

FT2008 - Alaska chapter 2

As I mentioned in my last post, the scientific program at FT2008 (The International Conference on Thermochronometry in Anchorage) was overall pretty impressive. I thought I'd highlight a few of the presentations that I found most interesting. As a side note, the extended abstracts for this meeting can be downloaded for free from the official meeting website here. The abstracts vary in length, but most are true extended abstracts with color figures. My discussion is by no means exhaustive, and who knows, I might augment it later. Here are some of my highlights, interspersed with random pictures of mine from the field trips. I am primarily sticking to methodological highlights today, I'll save the others for later posts.

• There were many discussions and presentations by one of the meeting sponsors, Autoscan. Autoscan is an Australian company that has been working to develop an automated fission-track counting system. I am still in the early stages of learning to count tracks, but I've observed the process and know most of the basics. Counting fission tracks is exactly what it sounds like, fission-tracks are etched in acid, and then using a microscope you count the number of tracks in your grain (gross oversimplification, I know, but to make a point). So counting tracks can be tedious, you need to count hundreds of them from dozens of grains to beef up your statistics. Anyways, as nice as it would be to have an automated counting system, the mechanics and potential complications of the process make me wary of trusting an algorithm. That being said, the Autoscan demonstrations are pretty convincing. You can download the demonstration and demo images from the Autoscan website here. Andy Gleadow gave the presentations on Autoscan and led the discussions. He went into detail about how the software deals with some of the more specific problems, comparing reflected and transmitted light images, evaluating overlapping tracks, distinguishing tracks from scratches and dust, etc. By the end I was sold. Again, I am not a certified fission track counter [yet], and therefore am undoubtably missing some important caveats, but Autoscan impresses me.
• Speaking of fission-tracks, there were another set of talks and posters by the group from Union College/SUNY Albany (John Garver and his student Matt Montario) about their recent work using a scanning electron microscope to date high track density zircon samples. The problem is this: Fission-track dating works because with time, tracks form in U-bearing minerals due to the spontaneous fission of ²³⁸U. Old and/or U-rich samples can accumulate so many tracks that they become impossible to count; they overlap and obscure each other too much. The Union/Albany group has developed techniques that allow them to count very high density samples. They do this by using a modified etchant (super secret recipe, well, until they get it published that is) and a scanning electron microscope. Typically, fission-tracks are etched with acid so they become large enough to see with an optical microscope. But, if you have a lot of tracks, this is a problem. So by using a less aggressive etchant, and more powerful microscope, they are able to effectively count samples that would otherwise be useless. I am assuming this will all be published soon, so I'll keep you updated.
• Barry Kohn presented some work he has been doing attempting to reduce single-grain apatite (U-Th)/He age spread in quickly cooled samples. Apatite (U-Th)/He thermochronology has been in widespread use for a little over a decade now, and as more and more data sets are collected, we are starting to identify and grapple with recurring problems. Perhaps the most significant issue are irreproducible single-grain ages. These are samples that appear well-suited for analysis, and have easily measurable quantities of U, Th, Sm, and He. Despite this, it is not uncommon for grains from the same hand sample to show significant scatter, well beyond what you'd expect from simple analytical uncertainty. There are many reasons why you'd actually expect significant single-grain scatter in slowly cooled samples. I won't go into it, but instead refer you to Fitzgerald et al., (2006) for a review. For quickly cooled samples, however, there shouldn't be as many complicating factors. Kohn presented results from his experiments where grains are abraded prior to analysis. Air-abrasion removes the outer rind of the crystals, leaving just a rounded core. Air-abrasion has the potential to deal with the "bad neighbor" problem in apatite (U-Th)/He thermochronology. "Bad neighbors" are U, Th, and/or Sm bearing phases that are close to or in contact with the apatite crystal in the rock. Because the He atoms move about 20 microns or so when they are expelled from their parent atom, He produced in neighboring phases can be implanted into the apatite. You end up with "parentless" He, which gives you artificially old ages. So, the idea is that if you abrade off the rind, you remove the region that could have had "parentless" He implanted into it. Kohn isn't trying to say that this is the only answer or that it always works, but in some of the samples he analyzed it certainly had the desired effect. Namely, abraded grains showed less scatter and were more consistent with fission-track ages and/or other constraints. Obviously still a lot of work to be done, but again, very intriguing.
  

So those are some of the presentations I have thought about the most since I got back from Alaska. I'll have more highlights in later posts. I'll also have more pictures, including a special Alaskan wildlife post, and a brief discussion of our stop at the Wasilla town hall. Yes, we stopped in Wasilla. But before I leave, here is a picture I took of the Exit Glacier, I tried to get the glacial striae in the foreground with the big looming wall o' ice in the background. Unfortunately I couldn't Photoshop out the guard rope and warning sign.

Fitzgerald, P. G., S. L. Baldwin, L. E. Webb, and P. B. O'Sullivan (2006), Interpretation of (U-Th)/He single grain ages from slowly cooled crustal terranes: A case study from the Transantarctic Mountains of southern Victoria Land, Chemical Geology, 225, 91-120.

Thermochronologic Sampling

I've posted about thermochronology and mineral separations (twice), but I realized that I haven't yet gone over the field work aspect of thermochronology.

Now, I'll admit up front that the field work I do is not very hard core. There are plenty of thermochronologists who spend months in the classic style, mapping and sampling in some remote part of the planet, I am really not one of them. The most rugged field work I've done was when I was in college, and while the helicopter rides and grizzly bears gave me some geo-street cred, my field work since then has been rather tame. The truth is, as a thermochronologist, you rely on good maps. Your data is meaningless outside of geologic or structural context. Now, it is pretty commong for people doing thermochronology to provide the necessary structural and geologic underpinnings. For much of my work, I've been fortunate enough to work in places that have been mapped very well, and I'm often in field parties with the people who've done the mapping.

Anyways, what I am going to be blogging about assumes you either have or are making the maps necessary for actually understanding your data.

OK, first things first, you can take a thermochronologic sample anywhere there are rocks, but when you plan your sampling, there are some important things to consider.

First, although you can get important information from single samples, the most powerful sampling method involves collecting multiple samples from different structural positions. If cooling and exhumation have been vertical (as is the case with my current project, see this for a more in depth discussion), then this means collecting samples at different elevations. For reasons I'll explain in a later post, it helps to cover as much structural relief as possible without moving too much laterally. The ideal sampling surface in a region with vertical exhumation would be a sheer cliff. In regions like the basin and range, where faults and structures have rotated lithologic units, structural relief might be almost horizontal (like Yerington). Again, this is why you need to already understand your field area.

So lets just say that exhumation has been vertical. Your sampling transect will try to cover the greatest amount of relief possible.

In almost every case, the best lithologies to sample for thermochronology are granites or granodiorites. So to summarize, you are looking for a big granite mountain.

Unless you are attempting some rather specialized analyses, you also want fresh and underformed rocks to sample. Obviously then, avoid altered rocks, weathering varnishes, and rocks that have been faulted or otherwise chewed on. You will also want to avoid the outer few cm's of outcrop because of potential problems caused by forest fires (see Mitchell and Reiners, 2003, Geology v.31, n.12, pp.1025-1028; summary if you don't have access, fires can reset apatites, so don't sample the outer few cm's. They do very interesting things with the data, but I won't go into that right now).

OK, so find your enormous granite mountain made entirely of fresh outcrop (this sounds pipe dreamy, but actually describes some of my current field areas).

So you want to take a whole slew of samples from different elevations. It is pretty standard to sample every 100 meters or so of relief (structural relief or elevation). So, first step, find a place at the right elevation that has fresh outcrop. As below

If the rocks aren't that well exposed or fresh, it helps to spend some quality time with your chisel. I love the ones with the hand guards. If you are sampling granitoids, bring a sledge hammer, not a pick hammer. Sledge hammers (3 or 4 lb head, metal or wood handle) make the job much easier.check out Dr. New Helvetia chiseling away at an oriented sample


If you are interested in mica, hornblende, or K-Feldspar, you usually don't need much sample at all. You can often see the minerals you want to analyze with your hand lens, and can sometimes "field dress" your sample, and take out what you want with your knife (works well especially for books of mica). For apatite, zircon, and monazite, you have a lot more work ahead of you. since the first step in that mineral separation might mean feeding your sample through a jaw crusher, I find it useful to break your sample into bite sized chunks on the outcrop. If you wait to bash up a large sample into smaller pieces back in the lab, you have to spend all kinds of time cleaning up. You don't have to clean up or worry about contamination if you bash things up on the outcrop. Bashing things up also lets you remove the weathered rinds you want to avoid.

Many rocks seem to be either feast or famine when it comes to apatite and zircon abundance. Some rocks I've worked on yield piles and piles of apatite, where I could have gotten away with much less sample. Others yield nothing, and it wouldn't have mattered if I would have brought home a barrel's worth. I usually go for about 3-4 kgs of sample.
So you've now got your mandarin orange sized chunks of amazingly fresh granite, time to bag them. I prefer heavy cloth bags, like those available here at Miners Inc. I don't reuse them, so I label the bajeesus out of the bags with a Sharpie.

I write the sample name, elevation, and some basic info about the geologic context of the sample. I also like to take a picture of the sample bag (with the sample name readable) on the outcrop, kind of a way to help me remember the sample and where I took it. I also take a GPS reading, and plot myself manually on a topographic map. I'll actually check my map location against the GPS whenever possible.

This is also a good way to trick otherwise camera shy members of a field party to have their picture taken. Again, Dr. New Helvetia.
Taking pictures like this also helps me keep my photos organized. For example, I'll have pictures like this below, that I don't write a ton about in my field book (because I am an idiot who is still acquiring skills), but I have it sandwiched in between pictures I have GPS coordinates for, so I'm all good. I was able to recreate the outcrop where I saw these enormous K-feldspars, even though it was towards the end of the day and my field notes weren't as complete as they should have been.

The K-feldspars remind me of salmon swinning upstream.

Now, I usually have a back-and-forth sampling route. Meaning, I'll hike up to the highest point in my field area, take a sample, and then head down, sampling as I go. This saves you from having to carry rocks up hill, which is usually a good thing. I also find it easy to plan time this way. I plan on eating lunch at the highest point, and I know that I can cover ~1300 m of elevation in a day (13 samples) with a good field party (at least 1 other person). You can also note good outcrops on your way up, making the way down more efficient.

FT2008 Early Registration Deadline

The early registration deadline for the 11th International Conference on Thermochronometry is tomorrow! Get the purchase card and sign up now, otherwise your grant will have to pony up another 50$. All the information you'll need can be found here. The meeting is in Anchorage, from September 15-19. I've never been to this conference, but reliable sources tell me they have been excellent in the past.

The FT conferences started out as Fission-Track workshops, but have expanded to include all low-temperature thermochronometers. There will be a wide range of posters and talks at the conference. The conveners are specifically requesting papers that fall into these categories:

1) New analytical developments in helium dating and fission-track analysis
2) Thermochronology of orogenic belts
3) Detrital thermochronology, provenance, and basin analysis
4) Thermochronology of sedimentary basins
5) Kinetics and thermal modeling
6) The thermotectonic framework of Alaska and adjacent areas.

Abstract and final registration deadline is June 13th, still time to crank out some ages!

Incidentally, speaking of abstracts, when writing them I always like to paraphrase our former Secretary of Defense and say "You write abstracts with the data you have, not the data you want."

Great Gift Idea for that Geochronologist in Your Life

I just wanted to mirror a post on the Arizona Geology Blog in case you know any geochronologists with upcoming birthdays, or wanted to get a head start on the Holiday shopping season. This could be the very first gift item aimed directly at us "something"chronologists. And, seeing that this is the world's greatest thermochronology blog, I figured it was appropriate for me to point you here, where you can learn all about the PaleoClock!

Future Geologists

This story appeared in our local paper this morning, and I thought it was worth highlighting. It tells the story of a 5th grader who noticed that a display at the Smithsonian identified the Pre-Cambrian as an "era." He knew this was wrong; the term "era" has a specific use in geologic time, and according to standard time scales the Pre-Cambrian does not qualify. The Cenozoic, Mesozoic, and Paleozoic are eras, the Pre-Cambrian is not. Anyways, many geoscience education posts, especially those dealing with the state of geoscience education in America today, are usually negative or at the very best worrisome. This is good!

And, the fact that there is a 5th grade teacher somewhere that gives proper lessons on geologic time made my day. I doubt I knew anything about geologic time back then. In 5th grade I was an expert on the Chicago Cubs and the 1985 Bears. If the Smithsonian had incorrectly labeled the Cubs 2nd baseman as Ryan Sandberg (instead of) Ryne Sandberg, well hell, I would also have filled out a comment card and made the AP wire. I probably also would have complained about the snack bar.....and so ends the window into 10 year old thermochronic.

Lab Ninjas - ANIMAL edition

I like to think of myself as a lab person. I think on average, I have a decent knowledge of the technical aspects of noble gas analysis. I have built a He extraction line from scatch, I've been involved with upgrades, repairs, routine maintenance, and exploratory surgery on both He and Ar extraction lines. I feel confident in my ability to put together noble gas labs quickly without paying a technician or lab mercenary (that is a special note for all you search committee members out there). I can dumpster dive, resuscitate broken pumps, and leak test with the best of them. Well, that is what I used to think.

Thanks to Dr. Chèvre I recently realized just how much more there is to learn. The good Dr. recently told me about the ANIMAL facility at Auburn University. ANIMAL is an acronym (one that many thermochronologists must be jealous of) for Auburn Noble Isotope Mass Analysis Laboratory. It is the noble gas (mainly Ar/Ar) facility at Auburn, run by Willis Hames and Mehmet Billor (can't find his web page). What makes it amazing though is not just the acronym, it is the fact that the entire apparatus, magnetic sector mass spectrometer included, was built by hand at Auburn, specifically for their lab. Honestly I'd never considered building a mass spectrometer, I figured it was something I'd buy (if I was ever in that position). I've now spent a lot of time on their website, looking at the pictures they have from the development and construction of the machine, looking at their calibration data, it is just amazing. It appears to be a great collaborative effort, involving geologists, material scientists, chemists, physicists, and engineers. Combining experience, resources, and skills, they put this things together from scratch.....and it works! There are even all of these great pictures of students helping to build the thing, polishing the flight tube, wrapping the coil of the magnet, etc.

There are a few things I love about this. First, the design of the machine is actually pretty innovative. Both the extraction line and the mass spectrometer have been optimized in such a way as to significantly reduce the internal volume of the extraction line. The primary modification of the extraction line that impresses me is the lasing system. Most lasing systems have a laser port and sample holder that are attached to the extraction line via stainless steel flex tubes. The laser port is mounted on a motorized stage, so it can be moved under a fixed laser, and samples can be maneuvered into place. this works well, but flex hoses are kind of a pain to deal with. By length, they have much more internal volume than straight tubes, and they are also blessed with much more internal surface area. Both of these facts are negatives from the point of view of signal size and blanks. Some flex hoses are necessary, but it is nice to avoid them when possible. At ANIMAL, the laser port is fixed, using short straight tubing, while the laser beam is mobile.

They call this set up the "flying optic." Basically, the optics of the laser are set up so that both the laser itself and the samples remain stationary, but the mirrors that aim the laser are movable. You can see in the picture above that one of the samples in the sample chamber is glowing hot.

The second great innovation deals with the size of the flight tube. The flight tube of a mass spectrometer can be one of the larger contributions to the overall volume of the extraction line. Large volumes are harder to pump out, but more importantly, the size of your signal is a function of the partial pressure of the gas in your system. Large volumes mean lower pressures, and smaller signals. Many commercial mass spectrometers have flight tube volumes of ~2 liters. ANIMAL is 80% smaller, with an internal volume of ~0.4 liters (that data comes from their website).

So this is all fantastic. One other thing that I think is cool is that they built a viewport into the ion source of the mass spectrometer. That means you can actually see the filament and electrical doohickeys working. I am not sure if this helps in the operation of things, but it is really cool, and doesn't seem to compromise their blanks at all.



So the ANIMAL lab is a good example of Lab Ninjas at work. If you have some time and are interested at all in mass spectrometers it is worth spending some time on their site.

Closure Temperature

I've alluded in some of my previous posts (aquí und hier) to one of the fundamental concepts of thermochronology, the idea of a closure temperature. Today I ended up re-reading some of the first thermochronology papers I ever read, back in the day. Long story short I was thinking about what the term closure temperature means today, and decided that after a year of blogging it was high time the the world's foremost thermochronology blog did something with this fundamental idea.

Most people trace the idea of a closure temperature, or at least the term, to a classic 1973 paper written by Martin Dodson (Dodson, 1973). The issue, as Dodson describes, is that some radioactive daughter products (he was concerned with Sr and Ar, but this also applies to other daughter products) are mobile long after a rock has crystallized. By mobile, I mean that they can readily move out of a crystal. If the daughter products are lost, there is no way to calculate a radiometric age. Well, I take that back, you would calculate an age of zero. Once a mineral cools enough, then the daughter product can accumulate, and the radiometric "clock" has started. Before I go on, I will make another self reference to a post where I point out that OG, Original Geochronologist R.J. Strutt mentioned this problem a century ago. OK, that's done.

It would be nice if this transitional temperature were finite. That is, that there was a set and known temperature above which the daughter product diffused readily, and below which the daughter product is entirely retained. This theoretical temperature is called the closure temperature (usually written as T_c); the temperature below which the mineral is closed to diffusion (in the past few years I've noticed this called a Dodsonian closure temperature, in part recognizing Dodson's contribution but also recognizing the necessary simplifying assumptions his 1973 paper made, more on that in the next paragraph.) In the real world, however, there is actually a broad range of temperatures over which daughter products can, and do, diffuse. This range of temperatures is often referred to as a partial retention zone.

When you start trying to pin down exactly what that means, physically, it gets tricky. The truth is, things like Ar and He never really stop diffusing out of crystals. Even at room temperature, for example, Ar and He diffuse, just so slowly that it really doesn't make much of a difference. Dodson of course realized this, and provides a very zen definition of what he thinks the term closure temperature means:

Closure temperature of a geochronological system may be defined as its temperature at the time corresponding to its apparent age.

So you see, grasshopper, it is what it was. This of course gives no direct way to assign a temperature to an age. Dodson showed the idea graphically this way:

The top graph shows the thermal history of a sample, in the form of temperature decreasing with time. The bottom graph shows the daughter to parent ratio of the sample over the same time interval, with the assumption that at high temperatures the daughter escapes easily, so the ratio is zero. At some temperature, daughters start to be retained, and as cooling continues, the daughter/parent ratio increases with time. If you use the present day ratio to calculate an age, and then see what temperature the rock was at at that time, well, that is the closure temperature of your system. Unfortunately, as I'll talk about in a bit, rocks don't come with the top graph, or even the bottom one for that matter, all we have is the present day daughter to parent ratio.

Mathematically, Dodson worked out a way to calculate the closure temperature, given a few assumptions and with a few other knowns thrown in. Here is the Dodsonian closure temperature equation

The best description I've ever heard of a closure temperature was when I was in grad school, a visiting über-thermochronologist said that a closure temperature is really just thermodynamic shorthand for a whole suite of physical and chemical properties. In the equation above, R is the gas constant, E_a is the activation energy of the daughter product's diffusion in the mineral of interest, A is a geometric factor considering the shape of the host mineral the daughter is diffusing out of, D_o is the frequency factor (another diffusive constant describing the rate the daughter product), a is the size of the diffusion domain in the mineral of interest, and dT/dt is the cooling rate. D_o and E_a are things that are determined through laboratory diffusion experiments, A and a are specified depending on the situation. I'd like to point out a few things about the equation. First, of all the inputs, E_a is the strongest control on the closure temperature; the other variables are locked up in a natural log term. Second, you have to input a cooling rate. But how do you know the cooling rate without thermochronologic ages and closure temperatures? Laboratory measurements of of the diffusive parameters are actually pretty good, but assigning a closure temperature to a system (say He in apatite, or Ar in biotite) still requires knowing a priori the cooling rate. In practice, people often quote a closure temperature assuming an average cooling rate, which is almost as meaningless as saying average geothermal gradient, and which should read geologically reasonable cooling rate (just like it should be a geologically reasonable geothermal gradient,) but I digress. In truth, for a wide range of geologically reasonable cooling rates, the range of closure temperatures isn't that great, certainly within an acceptable range of uncertainties for most applications. Where you run into problems are on the extreme slow cooling end of the spectrum. Minerals that cool very slowly can have drastically reduced closure temperatures (some slowly cooled Chinese samples I worked on have apatite He closure temperatures of about 57°C, about 30% lower that the quoted closure temperature of He in apatite.) The situation gets even more tricky, because this definition of a closure temperature, at least the derivation of the equation, assumes cooling that is linear in 1/T (or that in a time versus inverse temperature plot the thermal history is linear.) Dodson explains why he did this in the paper, and why it is a reasonable assumption for the purpose of the paper, but you can probably guess that rocks that have experienced very complex thermal histories that include perhaps prolonged time at moderate temperatures and/or cycles of heating and cooling will not fit this model.

The upshot of all of this, in my opinion, is that by themselves, thermochronologic ages are meaningless. They are non-unique, and therefore just numbers. Assigning meaning to a thermochronologic age requires external constraints, namely a good understanding of the geology of your study area (good maps, cross sections, and tectonic reconstructions) and multiple thermochronologic controls. There are some excellent software packages that allow you to use many different thermochronometers at once to iteratively constrain each other, usually considering many potential variables are deciding what thermal histories are consistent with all of the available data. My personal favorite is Rich Ketcham's HeFTy. But it all really starts with the geology.

Like many complications in science, the problems inherent in assigning a temperature to a thermochronologic age are increasingly being turned into tools savvy thermochronologists can actually use to their advantage. As I've said before, calculating a geochronologic or thermochronologic age is simple and straightforward. Assigning geologic meaning to that age takes time. It turns out that this problem is especially important to the understanding of the "low temperature" thermochronometers. Since these have seen a huge boom in use and importance in the past decade, it is no wonder that the problem of what closure means is
receiving more and more attention.

I hesitated at including this as the final figure, but decided to include it with a few disclaimers. First, these are all approximate range of closure temperatures for common thermochronologic systems assuming cooling rates of ~10-100 °C/My, and, I do not mean to imply that U-Pb zircon has a classic closure temperature, only that Pb is retained in zircon at exceptionally high temepratures, and typically is more reflective of the time of crystallization than of cooling through a specific temperature. Also, deciding what thermal importance to assign to your particular thermochronologic age requires a great deal of thinking, that is, these temperatures are relevant for the kinds of systems and tectonic settings I typically study, not necessarily those you happen to study. For example, if you work in hydrothermally altered areas, all bets are off. That being said.

References, etc..

Dodson, M.H., 1973, Closure Temperature in Cooling Geochronological and Petrological Systems, Contributions to Mineralogy and Petrology, v. 40, pp. 259-274.

And, although it is focused on the ⁴⁰Ar/³⁹Ar world, Ian McDougall and T. Mark Harrison's excellent book Geochronology and Thermochronology by the ⁴⁰Ar/³⁹Ar Method (Oxford University Press, 1999) gives an excellent discussion of closure temperatures, their meaning, and their derivation.

Mineral Separations Part 2

So I was really interested in all of the comments I got for my first mineral separation post (here). So I decided I'd try to make a list of the various steps people seem to use. This may be a little repetitive considering my last post, but it's been in my head, so if you are tired of mineral separations, go read one of the other fine blogs in my sidebar.

Separating minerals has the end goal of pure piles of individual minerals. You are often searching for relatively small and not all that abundant. Mineral separation techniques are equivalent to burning the haystack to find the needle; taking advantage of various properties that remove minerals you don't want, and leave what you do.

The first step is almost always turning your rock sample into a pile of individual minerals. There are two primary ways people do this.

1. Crushing and grinding. Jaw crushers (like the Bico Chipmunk) reduce fist sized rock chunks into small pellets. Disc mills, or pulverizers (like the Bico UA Pulverizer) break these small pellets into sand. The hope is that the minerals preferentially break along grain boundaries, but themselves stay realtively intact. Realistically it is difficult to tell what percentage of the desired minerals are broken beyond recognition (a real problem in the (U-Th)/He world, as I blogged about here and here), but I would not be surprised if the total yield from a standard pulverizer was 50%. As far as the actual machines go, there really aren't that many options when it comes to the jaw crusher, you can buy large ones and small ones, but they operate in the same way. With the pulverizers, there are two main options in style; you can have them belt-driven or direct-drive. Unfortunately Bico no longer makes the direct drive model, and if anyone knows of another supplier, please let me know. The belt drive models are OK, but much less powerful and much more difficult to set up. Pulverizers work by grinding your sample between two metal plates, and the quality of your grind depends on how far apart the two plates are set. Belt drive plates tend to drift during grinding, whereas the direct drive plates stay set. Think of coffee, you want a standard drip grind, but the belt drives give you some drip, some espresso, and some french press. The other option with pulverizers is the material the grinding plates are made of. You can get various steel alloys, iron, and even ceramic. I have recently started using the super expensive Mo-steel plates and have now become a complete convert. Iron plates leave filings in your sample, which have to then be removed. The Mo-steel plates do not "shed" and consequently do not wear down as fast. Again, I'd be interested in other other experiences.
2. Electric Pulse Disaggregation (EPD). EPD machines are now commercially available, although they are still pretty pricey. They were originally developed for use on lunar samples, where the waste generated during standard crushing and grinding would have just been unacceptable. The company marketing them now is called SelFrag. They have a pretty good website (complete with downloadable video), but don't show enough examples of separated crystals. EPD works by sending an enormous pulse of electricity through your sample, causing it to fall apart along grain boundaries. EPD separates whole crystals, even preserving delicate surface features. Bernhardt Saini-Eidukat at North Dakota State University has a nice page showing images of EPD separated minerals, I think made using a home made device.

Once your rock sample has been turned into a pile of sand, the order of the steps becomes somewhat arbitrary. It depends on preference, what mineral you are aiming to separate, rock type, and sample size. So I'll present the options in the order I tend to do them. The next step, for me, is to concentrate the dense minerals (namely zircon and apatite, but monazite and sphene also count). This is typically done hydrodynamically, using machines that are basically big gold pans. Gemeni tables and Wilfley tables work on the same principle: your sample is slowly introduced onto a sloped and vibrating grooved table that has a constant stream of water running over it. The "heavy minerals" are preferentially caught in the grooves, while the "light minerals" get washed away. You then collect the heavy and light fraction in buckets, and can effectively reduce your pile of sand from a big bucket to a small beaker. At one point in grad school I collected and separated some of the Fish Canyon Tuff. I was unimpressed with the total amount of apatite and zircon I ended up with, and decided to go back to the light fraction from the gemeni table and see if I had missed some substantial amount of apatite. I tried everything and ended up discovering that there was absolutely no apatite or zircon in the gemeni light fraction; they are pretty efficient machines.

I've also heard rumors of skilled geologists using actual gold pans to separate minerals. I am terrible at gold panning, and have never tried it with anything but river sand.

After hydrodynamic separation, some samples may need to be cleaned and/or washed. For granitoids, this means just time in an ultrasonic and rinsings with ultra-pure water. But you may also want to soak in acetic acid (to get rid of carbonate cements), hydrogen peroxide (do dissolve organics), or some other chemical (again, I'd love to hear more examples).

Two of the primary minerals geochronologists are interested in are non-magnetic (apatite and zircon). Minerals have slightly different magnetic properties, so the next step in separations is to take your cleaned heavy fraction from the gemeni table and run it through a Franz magnetic separator. Using a Franz is simple, your sand is slowly let into a vibrating metal channel that runs through a large electromaget. The magnetic field acts on the grains as they move down the channel, pushing the "magnetc" fraction to one side of the channel. You end up with two different streams of mineral grains, the magnetic and the non-magnetic, which can easily be collected once they exit the magnet. You can vary the power of the magnet, and really skilled users can effectively separate out very pure piles of magnetic minerals, including monazite, sphene (yes, I still call it sphene), different micas, amphiboles, etc... When you are done with all of the magnet powers, you are left with the non-magnetic fraction. This is hopefully mainly apatite and zircon, but is usually contaminated with quartz and feldspar that made it through the gemeni table. That means, time for the heavy liquids.

In my last post I mentioned a lot of heavy liquids, but I realized afterwards I should be more systematic in my presentation. So I'll try. First with a list of the heavy liquids I know about, then a brief discussion of the different ways to use them. All heavy liquids separate minerals by floating things less dense than the liquid, and letting the rest sink.

Heavy liquids I know about (most available from GeoLiquids or Sometu):

1. The Tungstates: Sodium polytungstate (SPT), lithium polytungstate (LST of FastFloat) and lithium metatungstate (LMT now discontinued) : ρ=2.5-3.1 g/cm³. These liquids have the distinct advantage over all other products in the fact that they are non-toxic. Many of the other liquids I'll mention are nasty things, but the tungstates don't even require a fume hood. Their only downside is their relatively high viscosity, which means it takes a while for your heavy minerals to sink, and filtering the liquid is kind of a pain. But I don't care, the safety and freedom from the hood is well worth it. SPT, LPT, and LMT will float quartz and feldspar, and sink apatite, zircon, and pyrite (argh, pyrite).
   
2. Tribromomethane or Bromoform : ρ=2.85 g/cm³. Bromoform is not pleasant to work with; it is very toxic and you have to avoid both skin contact and inhalation. It can be especially bad for your liver and kidneys, oh, and even better news, it might be a carcinogen. Bromoform has the same use as the tungstates, which begs the question, why does anyone buy bromoform?
3. Thallium foimate or Clerici Solution : ρ=4.32 g/cm³. I've only heard of this in legend, well, and I've seen a locked cabinet with a "Warning, Clerici Solution" on it. I am guessing you use it to sink zircon and float apatite. Or you use it to destroy your enemies, I am not sure. I think I'd rather hand pick apatites from a pile of sand than use it. From the MSDS "May be fatal if swallowed. May be fatal if absorbed through the skin. Causes respiratory tract irritation. Causes eye and skin irritation. May cause digestive tract irritation. May cause central nervous system effects. May cause liver and kidney damage. May cause cardiac disturbances."
4. Acetylene Tetrabromide or Tetrabromoethane (TBE) : ρ=2.96 g/cm³. TBE, by the way, is also called Muthmann's Liquid, I like that name, had never heard it before tonight, and thought I'd mention it. (could diet coke be called Thermochronic's Solution?) TBE is nasty, but allegedly less nasty than Bromoform, but is dangerous in similar ways, it attacks organs, is an inhalation hazard, and can be easily absorbed through the skin. TBE has the same general uses as Bromoform and the tungstates (separating apatite and zircon from quartz and feldspar). I've used TBE to make "feldspar juice" ρ=2.58 g/cm³, which lets you float k-feldspar and separate them from quartz and plagioclase feldspar. I've been able to get very pure feldspar separates, some of that data I'll be showing later.
   
5. Methylene Iodide or Diiodomethane (MEI) : ρ=3.32 g/cm³. MEI is also nasty, but it has a really low viscosity and you typically don't need to work with large volumes, thereby decreasing the hazard. MEI will float apatite and let zircon sink, which is what it is mainly used for. Using MEI isn't too bad, the real danger is that you wash it with acetone, and the mixture of MEI and acetone is very flammable. If it catches on fire you would rather not be in the vicinity. But that is easy to avoid.

So as far as I can tell there are three primary ways people use the heavy liquids. I have only tried two of them, but here we go:

1. Separatory funnels. These are straightforward to use, the have a valve at the bottom, you fill them with the liquid, dump in your sample, and let things settle. You can then open the valve, let out the dense minerals that have sunk to the bottom, but leaving the light minerals in the funnel. Separatory funnels have the advantage of being simple and easy to buy, but they use a large amount of liquid (50 - 100 mL), and because of their design can often leak. In addition, many are made with plastic valves, which get abraded and can actually have lots of little mineral grains stuck in them, that are almost impossible to clean.
2. Constriction tubes and knitting needles. If you've never tried this, check out the classic paper Dumitru, T.A. and Stockli, D.F., 1998, A Better Way to Separate Apatite From Zircon Using Constriction Tubes, in P. van den Haute and F. De Corte (eds) Advances in Fission-Track Geochronology, p.325-330. These allow you to separate small samples using only a few mL of liquid. Check out the article for a description, and when the web resource describing the technique become available I'll post a link. Any description I try to do will just be confusing.
3. Liquid Nitrogen. I've never tried this, but Ain't From Around Here says she's going to try it, so I am eagerly awaiting the results. The idea is that you put your sample and the liquid in a tube. Some people then centrifuge the tube, but even if you don't, you give things time to settle and separate. You then stick the bottom in liquid nitrogen, freezing the bottom liquid and effectively trapping the heavy fraction. The light fraction is then poured off, and you then just have to wait for the frozen liquid to thaw, and then pour off the heavies. Allegedly the liquids are not damaged by the freezing.

After this you are left with piles of pretty pure individual minerals. Some phases are easier to separate than others, but this is at least a good place to start. Each technique you use, at least for geochronology, has more steps, but they all begin with pure separates. If I missed something please comment or email. Mineral separation is really an amazing thing to watch. Parts of it are a pain, but some of the steps are just incredible. My favorites are the Franz and using MEI. In both of those you immediately see the separation....very gratifying.

The Art of Mineral Separation

UPDATE - My next post is a more complete discussion of mineral separation.

When many people in the geochronology/thermochronology community talk about new gadgets and gizmos on their wishlist they seem to focus almost entirely on the sample analysis side of things; particularly multi-collector noble gas mass spectrometers and various high end lasers. I'll admit, these machines are impressive and could be potentially very exciting, but if I had a pot of money to spend to increase the quality and quantity of the data my labs produce, they would not be the first thing I'd look into.

In a broad sense, doing meaningful thermochronology requires 4 basic techniques.

1. You must be able to identify tectonic and/or geochemical problems that can be be at least partly addressed thermochronologically. This starts with the big picture, but includes consideration of available lithologies and access to the necessary samples.

2. You then need to collect and process the samples. This means turning a 5 kg sack of rocks collected carefully in a very specific location to individual mineral separates ready for your lab.

3. Once you have mineral separates, you need to analyze them in a lab. Although actually collecting data in a lab is fairly trivial, running a lab well enough to insure that your data actually means something is not.

4. Once you have the data, you need to interpret it, again in order to answer the original tectonic and/or geochemical problem you set out to solve in the first place.

Steps 1 and 4 are probably the most complex, and in my opinion are the hardest skills to develop. To design and interpret good projects you need a strong background in basic geology and need to consult all of the experts that relate to the study. In my own work I need enough background to understand what the petrologists, sedimentologists, geophysicists, geomorphologists, structural geologists, and geochemists think. This requirement is not unique to thermochronologists. I'd argue that any geologist who considers tectonic questions is necessarily broad in scope. So steps 1 and 4 I see as general considerations for any earth science study.

Step 3 receives a great deal of attention. I've been a thermchronologist for less than a decade, but even in that time the number of new and expensive machines and techniques has ballooned. I've been involved with building and maintaining labs, and therefore have paid a lot of attention to these advances. As I've gone on in my career, I've started maying more attention to who gets what lab upgrade funded, or what people get with their start-up packages, or what they negotiate for when they have leverage. Right now the flavor of the day seems to be multi-collector noble gas magnetic sector mass spectrometers; these allow for the simultaneous measurement of all of the different isotopes you need to measure for whatever technique you are involved in, thereby cutting down the uncertainty and time lags of changing magnet power, yada yada yada. I won't get started on that.

What I do want to talk about is step 2, sample collection and preparation. In particular I want to talk about turning a rock into an individual mineral, a process called mineral separation. Mineral separation fascinates me, but what really amazes me is how many people either ignore or do not understand the process.

Here is the problem: for almost all analyses you need to analyze pure mineral samples. Techniques which work on small single crystals (fission-track, (U-Th)/He, U-Pb) typically require minerals that are small (100-200 microns in length) and not overly abundant in the average granite (maybe form a 5 kg sample I'll get a few milligrams of apatite). Techniques that work on multiple crystals (biotite, muscovite, and k-spar Ar/Ar), typically require a few milligrams of very pure separates. They both, therefore, require methods of separating a rock into piles of individual minerals. Mineral separation is a blanket term that describes the various ways to turn a rock into a sample. The first step is almost always reducing the rock into individual mineral grains. This is typically done by crushing and grinding the sample, trying to get the minerals to break apart along grain boundaries.

In my experience, the next step is to run your sample over a rogers or gemeni table. These are basically large gold pans that concentrate the denser minerals (apatite and zircon in particular) into a smaller pile. This is then washed and dried, and run through a magnetic separator, basically a large magnet where you can vary the power and separate minerals based on their magnetic susceptibility. This is done is a series of steps, and a skilled mineral separator can obtain almost pure concentrates of the various "magnetic minerals" such as biotite, hornblende, and monazite. When you are done, you are left with a pile of non-magnetic mineral grains, including apatite and zircon.

If you need to get apatite and zircon, you must then enter the world of heavy liquids. Heavy liquids are exactly what they sound like, liquids with very high densities, anywhere from water (1.0 g/cm³) to 4.4 g/cm³. Because minerals have fairly specific densities, they will either sink or float in different heavy liquids. Zircon is very dense (4.6-4.7 g/cm³), and will sink in a liquid like MEI (Methylene Iodide density=3.33 g/cm³), while apatite (density 3.2 g/cm³) will float. Heavy liquids have been used in geology for a long time, but the particular liquids and their methods of use have changed significantly. Many of these liquids are toxic, and therefore kind of a pain to work with. Two of the nastier liquids I have fortunately never worked with, those are Clerici's Solution (Thallium Malonate density=4.36 g/cm³) and Bromoform (Tribromoethane, density=2.89 g/cm³). Clerici's Solution and Bromoform are not all that common anymore, mainly because there are now less toxic alternatives. TBE (tetrabromoethane density=2.95 g/cm³) is also fairly nasty, but is still in use in many labs, primarily because it has a lower viscosity than its non toxic alternatives SPT or LMT (Sodium Polytungstate or Lithium Metatungstate, density 2.85 g/cm³).

I was lucky in my graduate education to learn from one of the masters of mineral separation. While many people have used the same techniques and materials they learned on decades ago, my min sep teacher has continually improved and refined his techniques, trying as best as possible to increase cleanliness, and efficiency, and reduce unnecessary exposure to toxic liquids. He tells me he will soon have a web resource of his methods available, which will be advertised heavily on this blog. I am presently trying to implement some of his methods in my new lab. This is the first time I have had to work with TBE, or with large quantities of MEI, both of which, in my opinion, are completely avoidable.

But what fascinates me is how little attention this necessary step in thermochronology, or geochronology, typically receives. Would anyone dream of asking NSF Facilities for money to upgrade a mineral separation lab? The amount of time and money wasted in mineral separtion is really astonishing. I think though, that one of the reasons these facilities rarely receive the attention they deserve has to do with the hierarchy of the average thermochronology lab. One of the first jobs you delegate with increasing seniority is mineral separation. Right now we have a fleet of undergrads working for us helping crush, grind, and separate minerals. The drive to streamline the procedures is reduced when those of us in charge no longer have to do them. My main reason for trying to improve the set up is primarily because I don't like the idea of an 18 or 19 year old handling liters of TBE on my behalf, especially when there are good alternatives. Although old ways die hard, I think I have convinced a critical mass to support my efforts, and was even able to put in an order with our glassblower this week.

Aside from heavy liquids, I think the most exciting (albeit expensive) recent advancement in the art of mineral separations is the introduction of commercially available electric pulse disaggregators (EPD's). Instead of physically crushing and grinding rocks, EPD's send a pulse of electricity through your rock, which causes minerals to break apart along grain boundaries. The technique has the advantage of retrieving the crystals from the rock intact, that is, you don't run the risk of physically breaking them apart (a huge advantage for separating apatite.) The method was developed originally to work with small and relatively expensive lunar samples, but is amazing in what it can retrieve from a rock. The link above includes a movie showing how quick and easy the process can be. Although it would increase the quality and throughput of samples, they right now are pretty pricey (well, from a geologists point of view, from a college athletics perspective it would cost about 0.33 D1 college football coach yearly salaries, and in my current situation would have only 2 fewer wins.)

I believe wholeheartedly in the garbage in, garbage out philosophy. This is one reason that I think a great deal about mineral separation. I think when it comes to bang for the buck, this could be one of the best ways to improve lab productivity. I'd be interested in other people's experiences with mineral separation, especially if you have used something safer or less toxic that the heavy liquids I described.

The earth, the atmosphere, and 40Ar retention

When I started getting into noble gas thermochronology (i.e. ⁴⁰Ar/³⁹Ar and (U-Th)/He) I realized that there were different types of geoscience literature involving noble gas research. First, there are the studies I was most interested in, involving either the behavior of radiogenic noble gases in common crustal minerals or their application to understanding tectonic problems. The other noble gas studies, which I tended to ignore, used them as geochemical tracers of a whole boatload of earth processes, including whole-earth degassing and formation of the atmosphere. In the past year through collaboration between my research group and some excellent geochemists, and in light of a recent and excellent Nature article announcing some really surprising findings, I have gained a new appreciation for the role of noble gases in geoscience.

The article, which came out in the September 20th issue of Nature, is called ⁴⁰Ar retention in the terrestrial planets. It presents the results of a whole series of experiments examining the behavior of ⁴⁰Ar in forsterite ( Mg₂SiO₄) and enstatite (MgSiO₃), the two minerals that make up most of the mantle. The common perception of noble gases is that they are relatively incompatible in minerals. Not only do they diffuse out quickly, but during partial melting events, the noble gases are strongly partitioned into the melt. The melt then ascends (say at a mid-ocean ridge), the gas exsolves, and escapes into the atmosphere. For ⁴⁰Ar, we should be able to calculate the percent of the planet that has degassed if we know the K content of the planet (⁴⁰K being the radioactive parent of ⁴⁰Ar, and therefore the source of most of the Ar) and the total amount of ⁴⁰Ar in the atmosphere. Some studies have concluded that the earth may be only ~50% degassed. This is confusing if you accept that noble gas diffusion is relatively fast and noble gases are strongly partitioned into melt during partial melting events; why hasn't the whole mantle degassed by now?

The data presented in this paper are discussed in terms both diffusivity (the speed at which ⁴⁰Ar atoms move through the crystal lattice) and solubility (the total amount of ⁴⁰Ar that could be stuffed into a crystal lattice given unlimited time, temperature, and ⁴⁰Ar). The authors took highly polished slabs of minerals (both natural and synthetic crystals) and put them in ⁴⁰Ar rich atmospheres at different pressures and temperatures. After set amounts of time, they removed the samples, and looked at concentration profiles of ⁴⁰Ar in the crystal using Rutherford Backscattering Imaging. From this they are able to construct concentration versus depth plots for all of the various temperature and ⁴⁰Ar pressure scenarios, which are then fit these profiles with equations relating to diffusive uptake, which then allow for the calculation of some of the fundamental parameters of diffusion (diffusivity and solubility). These measurements are different from the bulk loss profiles I am used to, where we infer the concentration profile based on step heating experiments. These instead are direct measurements of the distribution of ⁴⁰Ar in the solid. The paper discusses many of the potential problems of the experiments and measurements, but I won't go into that here. Their punchline is that ⁴⁰Ar solubility is actually fairly high in both forsterite and enstatite, and that ⁴⁰Ar diffusivity is actually fairly low. In fact, during partial melting events, ⁴⁰Ar can almost be thought of as a compatible element, that is, it is not strongly partitioned into the melt at all; both forsterite and enstatite can hold onto significant amounts of their ⁴⁰Ar during partial melting! As I mentioned earlier, this is in contrast to previous thoughts and experiments on the topic, but does at least fit with the suggestion that the earth is not fully degassed.

Like most Nature papers it is not terribly long (they have a maximum of 4 pages to work with), but well worth the read. The implications could be tremendous. Even from a thermochronology perspective, it makes me wonder about the validity of our diffusion experiments that try to infer the concentration profiles of gases in minerals indirectly. Hmmmm.

Because it just got printed I decided not to include any of the figures in this post, but anyone interested should check out the original paper (Nature has the advantage that even most public libraries carry it). Allegedly there is a much more detailed version in the works, I'll keep you posted.

Apparent Return

I have now returned from the field, and am ready to once again rejoin the geoblogosphereo, armed with field pics, a new appreciation for cheese, vertical profiles, and a few hundred pounds of nice fresh granite. First the field pics and a little geology.

For those of you who don't know I am working on a project in the Pyrenees, a very interesting [mainly] Cenozoic convergent orogen. This was my first trip to this area, and I think like many geologists visiting their field site for the first time, my appreciation for the project and of the many papers already written about the geology of the area increased dramatically.

This is also the first time that I've used thermochronology to study the uplift of a convergent orogen. In the past, I have mainly focused on extensional orogens and normal faults. Normal faults are in many ways the most ideal system for thermochronology. This is a highly simplified schematic diagram showing how large rotational normal faults operate. There is no scale on this diagram, but it is meant to depict a fairly large cross section of the brittle upper crust, let's say 8 or 10 km thick.


The three images above depict three different time slices during the extension of some chunk of crust. Normal faults are the kinds of faults that accommodate extension (you can see that the final image, on bottom, has been thinned and stretched, or extended). The faults rotate as they move, and in the end, rocks that were originally very deep in the crust (some of the grey bars), are exhumed, or brought up to the surface. This exhumation can happen relatively rapidly, and results in the cooling of originally hot rocks (hot because they were deep in the crust). This cooling is what we can measure using thermochronometers.

Thrust faults are completely different, not only in terms of what environment they form in, but also with respect to what they do thermally for a rock. The figure below shows time slices of an idealized thrust system

(stolen shamelessly from http://ic.ucsc.edu/~casey/eart150/Lectures/4ThrustFlts/4thrustfaults.htm)

Although thrust systems can make some impressive topography (think the Himalayas, Rockies, and many other major mountain systems), they actually do not exhume rocks. You can see this by tracing how deep any one point is on the diagram above, or on this excellent animation of a thrust fault. Since thrust faults don't exhume rocks, they don't cool rocks (well, that isn't strictly true but we will save that discussion for later), and therefore we can't necessarily use the same thermochronologic approach to study them.

Fortunately for us, the large welts in topography often formed during thrust faulting are often associated with very high rates of erosion. Erosion removes rocks from the top of the thrust sheets, thereby exhuming and cooling the deeply buried rocks we want to study. One of the reasons the Pyrenees are such a great place to work is that the products of this huge erosional event (aka the sediments in the foreland basin) are very well preserved and in many places very well exposed. In fact, many of the older sediments are themselves caught up in younger thrust sheets, which are then eroded and exposed for us to study. Many Spanish and French geologists (along with colleagues from around the world) have done an exceptional job reconstructing the history of deformation and fault activity by mapping these sediments and associated thrust sheets. And, just as a disclaimer I would like to point out that my thermochronology would be completely and entirely meaningless were it not for the labor of the structural geologists and sedimentologists who have studied this area for decades. As another disclaimer I am not a sedimentologist, and can only pass for a structural geologist by association.

On our way to the field site, we passed through places where these sediments are well exposed. One of the more impressive things about the Pyrenees are the piles and piles of conglomerate you see, some like this exposed in thousand meter high cliffs.



these conglomerates were all deposited, then deformed, during the main contractional event in the Pyrenees (early to mid Cenozoic). Some of you who have travelled to Spain may be familiar with similar looking exposures at Montserrat, these are not directly related to the Pyrenean orogen (but instead the younger Catalan Coast Range orogen), but did add to my feeling of this part of Spain as the land of conglomerates.

The thrust sheets are themselves impressive, and although I don't have the information on the unit exposed here, it is a pretty picture


But alas what I came to sample and study were the granites. Most of these granites formed (well, crystallized from a magma) during the Hercynian orogeny (late Paleozoic) where they intruded a Paleozoic sedimentary succession. The Paleozoic sediments are now all deformed and metamorphosed (that will be a later post). Many of the granites were then exhumed to at or near the surface. We know that because we can still find places where the granites are overlain unconformably by Triassic sediments. You can see that in this picture

The reddish layered rocks at the very top lie uncomformably on the granite below (the greyish stuff). The contact is itself folded (don't let anyone every tell you granites don't fold), and has now been eroded nicely for us to sample and study.

In order to do proper thermochronologic studies, you need to sample the greatest amount of structural relief you can. In extensional terrains, this is nice. Because the blocks rotate as they exhume, you can often collect samples from many kilometers of structural relief, sometimes without climbing more than a kilometer of actual altitude. Areas where the exhumation is accomplished mainly by erosion are very different, here, for example, structural relief roughly equals actual relief. So, to sample 15o0 meters of structural relief requires hiking up 1500 meters of elevation.

Not to complain, I was with an excellent field group and was able to do some exceptional (albeit long) hikes. The pyrenees are filled with very nice marked trails that take you up big granitic mountains. I was even able to bag my first 3000 meter Pyrenean Peak (3221 to be exact). But it did give me a whole new appreciation for vertical profiles. They are definitely the way to go for thermochronology (perhaps another post as to why), but they are tough things to collect.

This was also my first trip to Europe, which was incredible. But more on that later.

Anyways, a brief intro to my field season, some of my bajillion pics, and even a little geology.

On-line Thermochronology Community

Lest you think Apparent Dip is the only on-line refuge for Thermochronologists, I'd like to point out the OnTrack Forum, an on-line forum for the international fission-track and thermochronology community. The Forum is edited by Matthias Raab, a research fellow at the University of Melbourne. Although originally a fission-track journal (hence the name), it incorporates all manner of thermochronology, and provides a forum for discussion, networking, meeting announcements, and the like. You must register to take part in any of the discussions, but that is free and worth it if you are a budding thermochronologist.