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.

This entry was posted in (U-Th)/He, dealing with data, earth science, geochronology, impending thermochronocracy, pictures, thermochronology. Bookmark the permalink.

21 Responses to FT2008 – Alaska chapter 2

  1. Branden says:

    you’re a smart guy đŸ˜€ i’m just being introduced to geology studies at my local college and i’ve sort of fallen in love with it (96 on my lab exam go me). maybe someday i’ll understand all the technical terms, but for now i’ll stick with sedimentary rocks, one thing at a time right? đŸ˜€

  2. branden – nice work. I got a whopping 30% on my very first geology lab exam, and somehow ended up a professional geologist. If sedimentary rocks are your thing, then some of the better geology blogs are right up your alley, check out my sidebar, and start with clastic detritus!

  3. This comment has been removed by a blog administrator.

  4. BrianR says:

    sounds like quite a nerdfest … good times

  5. Mary Sousa says:

    Hello, I just stumbled onto your blog. I teach English as a foreign language to horticulture students. We just watched a video about the evolution of photosynthesis which included an interview with a GEOCHEMIST. Is your profession related to geochemistry? Apparently geochemists study chemical reactions recorded in stones and search for connections between the past and today (I’m sure that was spoken like a true layperson!)

  6. stars says:

    Geology study is good.Through the study of geology, you can travel in many areas.

  7. jaytung says:

    Wow..Alaska is so great.You is so the lucky man..Im so want to go Alaska.Thank for your information..Nice for your blog.Can you visit to my blog to đŸ˜€http://www.jaytung.blogspot.com

  8. Anonymous says:

    I have recollected geology studies of my graduation during 1968- 72. excellent articles for he benefit of beginners as well as professionals.

  9. Congrats on being “Blog of Note”. Love your humor … your blog is like reading a text book with jokes. And great pictures … I haven’t been to Alaska for years but your pictures brought it all back.Lunch at Zion every day … heaven!Take care!Small Footprintshttp://reducefootprints.blogspot.com

  10. Tiny T says:

    your photos are gorgeous. i’m not really into geology, but i’m a sucker for a good outdoor pic!

  11. This comment has been removed by a blog administrator.

  12. Beth Quist says:

    Beautiful photography. Loved it!

  13. If I can ever make enough money, Alaska is the place I want to send my dad for a week for his dream vacation. Also, I like the pictures showing the wilderness.

  14. Anonymous says:

    This comment has been removed by a blog administrator.

  15. Anonymous says:

    This comment has been removed by a blog administrator.

  16. Neo says:

    you know this place you call home “Alaska” looks alot like my home known as Enceladus

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