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."

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.

O.G. (Original Geochronologist) R.J. Strutt

Blogger's Note - Today we are digging into the archives of Apparent Dip. I am working on my AGU poster and not up for a brand new post, so I thought I'd re-post one of my earliest entries. My audience has grown since I first put this up (1/16/2007), and it is one of my favorites. So enjoy.

As much as I like the idea of being a field geologist, anyone who knows me also knows that the bulk of my graduate (and most likely post-graduate) geology career took place in a lab. Not just any lab, mind you, but a noble gas thermochronology lab. I primarily worked on (U-Th)/He thermochronology. In the past decade, (U-Th)/He thermochronology has exploded in popularity and has become a relatively common and useful thermochronologic tool. Of course, the more we learn the more potential problems and pitfalls we see, which is good, because that means there are plenty of papers left to write. To show you how the techqnique has really taken off, below is a chart showing the number of georef hits for (U-Th)/He by year. (I compiled this data myself rather quickly, so I am sure I am missing some relevant papers.)



In many ways, (U-Th)/He thermochronology is a cutting edge technique. But, it is also the first radiometric geochronometer. As far back as 1905, super-scientists like Ernest Rutherford and R.J. Strutt were estimating the age of rocks and minerals based on their measurements of U, Th, and He. I'd like to focus on one of these papers today, the one that I am most amazed with. It was written by R.J. Strutt in 1910:

Strutt, R.J., 1910, Measurements of the Rate at Which Helium is Produced in Thorianite and Pitchblende, with a Minimum Estimate of their Antiquity: Proceedings of the Royal Society of London, Series A, Containing Papers of a Mathematical and Physical Character, Vol. 84, n. 571, pp. 379-388

I found this paper on JStor, which most academic libraries have access to. Reading this paper and those it references I am first blown away that they could measure U, Th, or He in the first place, especially He. I spent months and months with very fancy equipment trying to accurately measure the amount of He trapped in apatite crystals. Of course, I was trying to measure much smaller quantities with much higher precision, but I am still astounded by the ingenuity with which these labs were built. For example, to meaure the rate at which He was produced, Strutt first dissolved the material in various liquids (usually combinations of acids), and placed the solutions in this contraption


The solution was allowed to sit for some period of time for the helium to accumulate. Then, the helium was gently boiled off and collected in a test tube inverted into a pool of mercury. (I'd love to try to get this experiment approved by the Health and Safety folks at the University nowadays). The collected helium was transferred into this set up



Here, the helium, in the test tube on the left. The gas would be let into the apparatus (evacuated with a mercury pump), and then the tubing would be filled up with more mercury, pushing the helium along until it was confined to area c, which is a cooled charcoal trap used to clean up the gas (an idea still used today in He thermochronology thanks to nifty devices like this



from Janis Cryogenics). After a while the helium is "drawn" into part "d" (not sure how that is done), and part "d" is filled with even more mercury, pushing the helium into the capillary "g" where the volume of helium can be measured using the length of tube the gas occupies and the pressure of mercury that is pushing it up there. As someone who regularly complains about high-tech devices that dare to come without GPIB ports or LabView drivers, this is slightly humbling. So, amazing fact #1 is that they could actually accurately measure helium in the first place.

Amazing fact #2 is that they could measure helium production rates from both U and Th with decent reproducibility.

Amazing fact #3 is that they all didn't die from Mercury poisoning (curiously, however, the lab assistants are never named)

But, the most amazing fact, that would be #4, is that the ages Strutt calculated, and most importantly the conditions he applied to interpreting that age, are really pretty good.

Below are his results from that paper

Strutt refers to these as "minimum ages," according to him "...because helium leaks out from the mineral, to what extent it is impossible to say"

In earlier papers, specifically one called Leakage of Helium from Radio-Active Minerals (Same journal as above, v. 82, n. 553, pp. 166-169), Strutt discusses some of the reasons helium "leaks" out of geologic materials, spending significant time talking about temperature. Thermally activated helium diffusion is of course now the basis for He thermochronology, something he alluded to in 1909.

So, he realizes that these are minimum ages, and his reasons make perfect sense. But his minimum ages are really not that bad. Realistically they are all good minimum ages for the time period they represent (8.4 Ma for a minimum age for the Oligocene, 31.0 Ma for the Eocene, 150 Ma for the Carboniferous, and 710 Ma for the Archean). This both blows me away and makes me wonder why it took me so long to get a lab running! It also makes me thankful that there are now good alternatives to mercury filled McLeod gauges and mercury pumps.

So, tonight I raise a toast to the O.G.'s of this world, the Original Geochronologists. I'll put another plug in for JStor, they have so many of these early papers there for the downloading.

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.

Therapy for staring at ugly apatites all day

I spent time today helping a student pick out apatites for (U-Th)/He analysis. For those of you unfamiliar with this process, it is easily the biggest time sink in the whole method. After you chose a study area, collect samples, and separate out apatites, the really tedious part begins. For a variety of reasons, you have to be very selective when it comes to figuring out which grains to analyze. First and foremost, they must be inclusion-free. Apatites typically have 10's of ppm of U and Th in them, common phases that appear as inclusions (zircon and monazite) can have 1000's of ppm to weight percent U and/or Th, so even tiny inclusions can really screw up your analysis. This is an even bigger problem because the standard method for analyzing apatite involves dissolving them in nitric acid. Apatite dissolves easily in nitric acid, but of course zircon and monazite and absurdly tough to dissolve. So, they will contribute He to the analysis, but the inclusions won't dissolve, and we therefore won't measure the U and Th. Long story, I could go into more detail, but needless to say, your first priority in picking grains is to avoid visible inclusions.

Next, you'd like the crystals to have nice, recognizable crystal faces. Why, you ask? Well, during decay, the He nucleii are shot ~20 microns from the parent nucleus. So, if the parent atom is near the edge of the crystal, there is a good chance it could be ejected out of the crystal and for our purposes, lost. This is called alpha-ejection. We can apply a geometric correction for this, but it assumes a certain crystal form. (For more on alpha ejection see Ken Farley's 1996 paper in Geochimica [v.60 p.4223-4229] or Jeremy Hourigan's 2005 paper in Geochimica [v.69 pp.3349-3365].)

Then, we want them big. The larger the crystal, the smaller the alpha-ejection correction.

This is simplified, but you get the idea. Sometimes you can spend all day on a single sample, trying to find grains that are worth analyzing. Today I spent time looking at very ugly grains, so to boost my morale I am posting some of the pictures I have taken of my favorite apatite crystals. I have picked plenty of ugly separates in my so far short career, but I need a boost, something to re-energize thermochronic.

Also, I signed up for a google alert for the word "apatite" a few weeks ago. The two types of listings I receive from these alerts are:

1. Misspellings of the word appetite (yes, you should still proof-read even if you have spell check), about 6 per day.
   
2. Discount jewelry and/or crystal healing pages (you think if apatite created harmony all of the world's fission track and (U-Th)/He people would be entirely serene), about 3 per day.
   

So, with this post, I should have a listing for beautiful pictures of apatites separated from granitic rocks, some from China, some from Utah. I will also admit up front that I realize I have committed geology sin #1, and not included a scale bar. Take my word for it, they are all 60-150 microns in width.





Those are nice, here is one that has nice form and is huge, but has obvious inclusions


And finally, what do we do with them once we pick them? We pack them! We don't want to heat them directly with a laser, because that can volatilize Th. So, we put them in individual little platinum packets, and heat those. The pictures below show this process, starting with a crystal and Pt tube, and ending with a Pt "microfurnace." Getting pictures taken while holding the tweezers steady enough and in the right place to see everything is one of the greatest accomplishments of my career, hence the need to shamelessly share these all over the web.



Boom, throw it in the laser, dissolve, toss it in the ICP-MS, a little spreadsheet magic and you've got a paper.

How Old is this Rock?

I had originally meant to take on the Just Science week Challenge, where I'd write a new entry every day about science and science only, no Bears, no music of the week, no asinine and unproductive bill brought up by some pandering politician. But, I flaked. Brian of ..Or Something fame kicked this challenge's ass. But, I decided that instead of daily posts, why not one ginormous one, guaranteed not to be read by even my most dedicated friend? So here you go, the science of Geochronology. I intend to come back to different aspect of this topic in the future, just as I have discussed it in the recent past.

Geochronology is an umbrella term for all of the techniques we use to determine the absolute ages of rocks and/or individual minerals. All modern geochronology is based on measuring the stable daughter atoms produced during the radioactive decay of naturally occurring elements, most notably uranium (U), thorium (Th), potassium (K), rubidium (Rb), and lutecium (Lu). It is now common and fairly simple to measure the absolute abundances of these elements in rocks, or in many cases individual crystals, with a high degree of precision. As a geochronologist, the real work comes with figuring out what those ages mean.

The title of this post is a question I've been asked many times, and in truth is impossible to answer. Rocks have very complex lives and change continually on a variety of timescales. Each radioactive parent and stable daughter nuclide has specific characteristics that determine their inclusion and retention in geologic materials. When you date a rock you are really dating some event or process that the rock experienced. Some systems can be interpreted to record the age of formation of a rock. Others record the time when a rock cooled to a certain temperature. I've seen geochronologic data misused by lots of different kind of people. There is of course the young-earth crowd, but I really don't want to waste time on them (in this post). What is more disconcerning from my standpoint is how often I have seen data misused by trained geologists. In the past year I've seen talks where sanidine K-Ar ages from the 1970's were compared with U-Pb ages from the 1950's; I could tell that the speakers grasp on the philosophy and method of geochronology was simply nonexistent. I am more lenient when non-geologists misunderstand radiometric dating techniques, but when the data is something that you depend on, well....What it really comes down to is:

1. What are the chemical and physical properties of the parent and daughter nuclides?
2. When did the mineral that contains the parent and daughter nuclides form? Is it igneous, metamorphic, has it been deformed or recrystallized?

Calculating an age is pretty straightforward, all you need to know if

1. the number of parent atoms
2. the number of daughter atoms
3. the decay rate of the radioactive parent

When I've taught radioactive decay to classes in the past, I've found that one of the biggest mental stumbling blocks that students have to overcome is how a process which is completely random on the scale of an individual atom (whether or not it will disintegrate) can, on a larger scale, be completely predictable. It is of course all a probability thing, each radioactive atom has a specific probability that it will decay (the decay constant). Below is a graph of observed decay rates from a "radioactive parent nuclide," compared to a expected line.

Actually, these particles aren't radioactive, they are chocolate. These are the results of the great M&M experiment from a class I taught in 2004. M&M's, it turns out, are fantastic for illustrating this principle (pennies would work just as well, but for some reason the classes are never as enthusiastic about those.) M&M's have an "M" printed on only one side of the candy. So, you put 100 of these in a paper cup, dump them out onto your desk, and count the number of pieces that landed with the "M" side up. These are considered to have decayed, so you remove them, place the undecayed candies back in the cup, and repeat until all of the M&M's are gone. I even had them calculate an M&M "half life", the result of which you can see in the corner of the graph (± 1 sigma). I'd have groups of students do this with 100 pieces 5 times each. Then we'd compile the data as a class. Most people are comfortable with the idea that you have a 50/50 chance of an M&M landing heads up, but if a particular piece decays is random. This is directly analogous to the decay constant for a radioactive element. Each atom has a specific chance of decaying. The only real difference is that decay constants are in terms of some time value (per year, second, etc..), while the M&M decay constant is in terms of "rounds." These results are always impressive to me, that we can get such good results from ~4000 "atoms," especially when the average zircon used for U-Pb geochronology contains hundreds of trillions of U atoms, boo-yah, counting statistics.

U-Pb geochronology is unique among geochronometers in that it is actually two separate decay systems in one. There are two relatively abundant naturally occurring isotopes of uranium, ²³⁵U (which decays to ²⁰⁷Pb) and ²³⁸U (which decays to ²⁰⁶Pb). ²³⁸U is by far the most common (now), but every crystal or rock or anything that has ²³⁸U also has ²³⁵U. The ²³⁸U/²³⁵U ratio is actually exceedingly well known; it probably ranks up there among the best known numbers in physical science (thanks to the nuclear weapons industry). Why is this number so constant? Well, because different isotopes of U have the exact same chemical behavior, so they are never fractionated. But, fractionation is a topic for another post. What is important though, from a geochronological standpoint, is that every time you measure a U-Pb age, you are actually making two completely independent and separate geochronologic age measurements, one for the ²³⁸U-²⁰⁶Pb system, and another for ²⁰⁷U-²³⁵Pb. ²³⁸U has a longer half-life, and therefore decays more slowly.



This figure shows the evolution of different U and Pb isotopes in a closed system over time. Basically, start out with 1000 atoms of ²³⁸U and ²³⁵U, and let them evolve for 4.5 Ga. In truth, of course, the materials we use have trillions of times more U, and ²³⁸U and ²³⁵U are never equally abundant, so I've normalized the graph. But, you see the point. The solid lines are U, with red being ²³⁵U and blue ²³⁸U. The dashed lines are Pb, with red ²⁰⁷Pb and blue ²⁰⁶Pb. Either one of these can be used to calculate an age. But, the real power of the technique comes when we use both. Because the decay schemes are independant, the ratios evolve separately. But, if ur measurements of the decay rates are correct (which they are), then the ages from the two systems should agree. The collection of agreeable isotopic ratios is referred to as concordia.
The red dots represent specific time values, and the blue line is called concordia. U-Pb age determinations which fall on this line are called "concordant." There are a variety of processes which can cause ages to fall off of concordia (Pb-loss, multiple crystallization events, etc..), which actually lends more power to the technique. So, every time someone calculates U-Pb ages, they are actually also double-checking the decay constants of the isotopes. If these changed over time, as a few non-scientists claim, then concordia could never work. We actually use the above diagram, called a Wetherill plot, and a similar plot


called a Tera-Wasserburg diagram, to evaluate U-Pb data. These two methods have their different strengths, which I won't go into here (right now at least.)

U-Pb geochronology has a lot of other advantages. The primary crystal we analyze, zircon, does not like to incorporate Pb into it's crystal structure, which means that almost all of the Pb we measure is the product of radioactive decay (U likes to substitute for the Zr in the zircon structure.) Zircon is also great, because it is very common in rocks, and, as people like the famous ..Or Something wrote about, is tough enough to survive significant erosion and transportation and therefore pops up in sedimentary rocks all the time, giving a great deal of insight into sources and transport of sediment.

It also turns out that Pb does not diffuse out of zircon very quickly. In many cases, even if the system has experience really high temepratures, high enough to melt many common rock forming minerals (quartz, felspar..), the zircon will survive and retain it's U and Pb. If you have carefully evaluated the textural relationship of the zircon to the rock, you can often say that the zircon U-Pb age marks the time of crystallization of that crystal, and perhaps that rock.

K-Ar dating, or the more powerful Ar/Ar variant, works essentially the same way. Minerals like to take up potassium, and one isotope of potassium is radioactive. Minerals don't generally take up any argon, at least none in the crystal structure. So we measure the amount of K and the amount of Ar and we can calculate an age. The reason Ar/Ar is different is that we can use a proxy for measuring the K content, this makes the measurement process and the information we can recover from a crystal much more powerful. I will post about Ar/Ar geochronology later, I don't want to go into the details. I think one of the most amazing and interesting proofs of the K-Ar system was published in a 1997 paper by Paul Renne et al.,

P. R. Renne, W. D. Sharp, A. L. Deino, G. Orsi, L. Civetta (1997) ⁴⁰Ar/³⁹Ar Dating into the Historical Realm: Calibration Against Pliny the Younger, Science, v. 277, p. 1279-1280.

In this study they dated ash from the 79 A.D. eruption of Mt. Vesuvius. This is one of the hardest kinds of analysis to do, on a geologic time scale barely any time has passed for Ar to accumulate. But, as shown below, they actually nail the date reported by Pliny the Younger (a recording witness) well within error.

Pliny the Younger would have predicted 1918 years, they got 1925 ± 94.

The chemical and physical behavior of Ar makes the K-Ar system very powerful. Unlike Pb, Ar is a light gas, and when a rock is hot, Ar can diffuse out of the system almost instantaneously. We know a lot of Ar exits rocks into the atmosphere, about 1% of what we breathe is argon. Anyways, K-Ar, or Ar/Ar dates don't really tell you when a rock or crystal formed, but more when it cooled to a certain temperature. This is analogous to the (U-Th)/He system I blogged about here. Different minerals retain Ar differently. So, in the same rock, Ar/Ar ages from 2 or 3 different minerals can help chart the cooling history of the sample. Radiometric techniques that are primarily dependent on temperature are called thermochronometers. The elite earth scientists who use thermochronometers are thermochronologists. Think of thermochronologists as the navy seal or olympic athlete of the geology world.

So how old is this rock? Well, it depends. Do you want to know when that crystal formed? When the rock cooled near the surface? When the crystals that make up that sandstone formed? When the sandstone was deposited? All ages are not the same, but interesting things can happen when you hit the geochronic.

Watch as thermochronic speeds through the 5 stages of grief after a horrible performance by the Bears in the superbowl.

1. I can't believe the Bears just lost the superbowl, there must be some mistake, this isn't happening! Someone must have paid the refs! The Colts are on steroids! Were those french figure skating refs? Maybe the real game is next week, yeah, this can't be real, must be a dream, I'll wake up soon.

2. What the bleep! The bleeping defense couldn't stop anything, damn them, damn them to hell! And come on, make a fricking tackle! Catch the fricking ball! What, another fumble! Bleeping overpaid donkeys! My rabbit could block better than that! That's it, I am done with these jackasses, done, D-U-N done, I give up on these losers, overpaid crybaby can't tackle losers!

3. OK, come on, I'll trade you, you can let the Cubs not make the playoffs for the next 100 years, let Cal win the big game, let Duke win the NCAA tourney, I don't care, come on, we can make a deal! Here, take my helium lab, I don't care, I'll make you first author on this paper I am writing, you can have my office, let's just go back and end the game 30 seconds into the first quarter. No one will know! We can do this, just a little favor for a friend!

4. I can't believe this, I'll never be able to recover. I think I'll call in sick tomorrow, drink my sorrows away. It is dark outside, like in my soul. Will the sun ever shine again? Not on me it won't. Darkness, sadness, Mazzy Star and Hank Williams, Stan Rogers' First Christmas, that movie "Leaving Las Vegas", Ben Harpers first album..Sniff sniff, how could this happen, and why does this always happen to me !?!? (now picture me with my fists raised up towards the sky cursing the world, then breaking down into a fetal position and crying, just like Jimmy Bakker, in the rain, alone, with people laughing at me, very very sad).

5. It's OK, it's just a game where millionaires run into each other for 60 minutes, and Prince wasn't that bad, and that "Mapasaurus" commercial, that was kind of funny, and hell, it beats picking apatites all day. I can get through this. Next year, Grossman will be better, Urlacher will be back, our defensive line will be healthy.... I'll be OK. March Madness is almost here, things will be OK. It's just a stupid game, right?

Anybody need $10,000?

I am by no means an expert when it comes to climate change, so I will leave the more technical posts for my fellow geoblogofolks (see the sidebar). What I would like to point out is something Mrs. Apparent-Dip-but-with-a-different-last-name (mad props) told me about and was reported on CNN Money. Turns out that in response to the IPCC report, ExxonMobil has offered some climate scientists $10,000 for critiques of the conclusions (which, for the bajillionth time in the past decade state very clearly that humans are causing climate change). Now of course oil industry funding in the earth sciences is nothing new, after all, geologists do plenty of oil finding. What I think is unique in this case, and disturbing, is the predetermination of the outcome. I know plenty of people doing industry funded studies, I am sure it is common in other branches of science as well, but when is that conditional on the outcome. Sounds very wild west to me, "Look, there's a bounty on that thar climate report! Wanted, dead or alive!" But, on the plus side, if funding is tight, perhaps I can convince them that apatite (U-Th)/He thermochronology proves the earth is cooling, out of control cooling, my latest data says we are heading to runaway global cooling, straight into snowball earth! OK, it doesn't, but 10 grand could set me up at a sweet hotel for GSA. This is disappointing news considering recent good signs from the same company.

(U-Th)/He Thermochronology

UPDATE : If you found this web page while looking for information on labs where you could do (U-Th)/He analyses, please check out the web page for the new He lab at the University of Colorado - Boulder.

In response to a comment I received on an earlier post, I thought it might be a good idea to give a brief overview of (U-Th)/He thermochronology. Chances are I will have pleanty of posts in the future that will touch on the subject, so this could be useful.

Like all radiometric geochronometers, (U-Th)/He chronology is based on the radioactive decay of a naturally occurring parent nuclide (in this case, U and/or Th) to a stable daughter product (in this case He). We typically think of U and Th decaying to Pb, which it does, and this is the basis for another geochronometer. U and Th each decay by what is called "chain decay", meaning that there are many steps and many intermediate nuclides in the decay process, and decay from one of these to the next is accomplished via wither alpha or beta decay. ²³⁸U for instance, has 8 alpha decay steps in its decay chain. During alpha decay, an alpha particle (2 neutrons and 2 protons) is emitted from the nucleus. Alpha particles are He nucleii, as soon as they leave the nucleus they pick up some electrons and shabang, a He atom. ²³⁵U undergoes 7 alpha decays, and ²³²Th produces 6. Below is a schematic diagram of the decay chain for ²³⁸U, the more abundant U isotope. You can see that there are multiple paths the decaying atom can travel, and that it must experience some number of beta decays, along with exactly 8 alpha decays. Beta decays are when a neutron is converted into a proton and an electron, and that electron expelled from the nucleus (simplified, but you get the point), so during a beta decay, the number of neutrons in the nucleus goes down by 1, and the number of protons increase by 1 (the number of protons is what defines something as a single element, changing the number of neutrons just makes something an isotope of the same element).
²³⁵U and ²³²Th have similar decay chains. So, if you measure the amount of the daughter product (He), and the amount of the parents (²³⁸U, ²³⁵U, and ²³²Th), and know the rate at which decay occurs (which we know exceedingly well), then you can calculate the amount of time that the He has been accumulating.

But helium, really, helium in a rock? It seems like a stretch, especially considering our everyday experience with He. It makes up only about 1 ppm (part per million, or .0001%) of the air we breath, it is so light that it can escape our atmosphere. It escapes from balloons, canisters, everything. The idea then was that it must escape from rocks and minerals right away. This was what Strutt and most of the early workers I discussed in an earlier post believed. When they dissolved the minerals in acid of course, the He was released immediately. It turns out that certain minerals actually can retain He relatively well. Apatite and zircon, two common accessory minerals that typically contain 10's to 1000's of ppm of U and Th, can hold onto He for geologically meaningful lengths of time (millions of years), as long as the temperatures don't get too high. Below is an image of some apatite crystals from a granite in western Utah that I worked on, notice the scale bar. It is fairly easy to "date" an individual crystal, typically we measure 4 or 5 individual grains to make sure the ages are reproducible.

He diffuses rapidly out of apatite at temperatures greater than about 70°C (again, talking geologic time scales). Radioactive decay of course doesn't care about temperature (well, not anything close to a temperature found on or in earth), so it is occurring all of the time. At higher temperatures, the He that is being produced is diffusing out of the crystal and leaving the system, into the atmosphere. If you are not retaining any of your daughter atoms, you have a zero age. Once the rock cools, you start to retain the He, so the age you measure with (U-th)/He is not necessarily a formation age, but rather a cooling age, measuring the time since the rock cooled to the temperature at which it could retain He. There area actually many "thermochronometers" like this, systems that geologists use to determine the time-temperature history of a rock or geologic terrain.

Measuring the different nuclides is a little tricky. As far as I know, there is no one machine that can accurately and preceisely measure U, Th, and He. U and Th are often measured together (like using this amazing machine), but He is a different matter.

For a variety of reasons, we tend to measure He from single grains, I work primarily on apatite (Ca-phosphate, the same mineral that makes up our bones and teeth, it is also a common accessory mineral in granites and related rocks). The first step is to get the He that is trapped in the crystal out, so we can measure it. A common way to do this is to heat the crystal with a laser

These are pictures I took of the laser I used in grad school, an Ar-ion laser. Many different kinds of lasers can be used, the Ar-ion has the advantage that the light is in the visible spectrum, so you can take really amazing pictures of it. The He that is generated during the radioactive decay an that we de-gas from the crystal with a laser is ⁴He. To measure the total amount of ⁴He we spike it with a known amount of ³He, and measure the isotopic ratios on a Quadrupole Mass Spectrometer. There are a few intermediate steps, mainly designed to clean up the gas and remove any contaminants, that is where the cryogenic charcoal trap I showed in a previous post comes into play.

The lab that measures He cannot measure U and/or Th. So we then take the crystals, dissolve them in nitric acid (for apatite, nastier stuff for zircon), spike it with known amounts of ²³³U and ²²⁹Th (or some other appropriate isotope),





and measure the U and Th on a magnetic sector ICP-MS.



Once we know the amount of U, Th, and He, we can then calculate an age. All of the steps up until now are pretty straightforward, now of course comes the hard part, interpreting that age. Sometimes that is fairly obvious, sometimes it isn't.

There are many other aspects of this technique that I find interesting, and that are very important for anyone wishing to use it. For a technical discussion I'd direct you all to an excellent review paper written in 2002:

Farley, Kenneth A. (2002) (U-Th)/He dating; techniques, calibrations, and applications. Reviews in Mineralogy and Geochemistry, v. 47, pp. 819-843.

So, I hope this clears up a bit about the technique. I am sure posts in the future will explore the topic a little more, for example the issue of alpha-ejection, He diffusion, etc. If you are interested in geochronology, one additional place I'd recomment you check out is the Earthtime project. Cheers.