Wednesday, February 28, 2007

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.


Lab Lemming said...

Do you do Sm corrections for grains with low actinides?

Thermochronic said...

I do now, in grad school I monitored Sm, but didn't spike for it and therefore never knew the true amount. You really need a boat load of Sm to make a measureable difference, the only apatites I have seen it make a real difference for were some from an oceanic core complex, Pete Reiners presented the data at the 2005 Mineralogical Society of America short course on Thermochronology, but I don't think they have come out in paper form yet.

Now we do measure Sm. It typically changes the ages <<0.1%, but good to keep track of.

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