Tuesday, December 04, 2007

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.


Chris R said...

And look at those wonderful apparatus figures - my lack of artistic ability might have been a serious problem 100 years ago!

BrianR said...

Another value of refining the technique and making the ages better is that I can just re-analyze samples i've already collected (he he). Sometimes new ages can really fly in the face of long-standing paradigms in tectonics ... gets everyone debating and discussing, which is exciting.

I heard that zircon U/Pb SHRIMP ages are systematically too young...is this just a rumor?

Thermochronic said...

Juicy rumor, I am not sure. I have heard that using a slope Franz to separate zircons can bias the population. I am not sure which way it goes, I think radiation damaged zircons can be pulled off in one of the stronger magnetic fractions, but don't quote me on that. I recently looked at a Frzna magnetic fraction (2.2 Amps, I am not sure the slope) that was entirely apatite, kind of threw me for a loop.

Jim said...

Ok, I'm a little bit confused, so hopefully you can teach me something here. How exactly is the helium measured and how does that result help us determine how old the rocks are, especially considering that the helium leaks from the rocks at an unknown rate?

Thermochronic said...

Excellent question. It turns out that helium does leak out of geologic materials rather easily, but in some minerals, especially single crystals, helium loss or retention is strictly governed by thermally activated volume diffusion and is in fact very predictable. The temperatures required to diffuse helium out of apatite (common accessory mineral in granitic rocks) you really only need to get to ~70°C or so (on geologic timescales). But, geologists didn't really understand thermally activated volume diffusion in geologic materials until the 1960's, when the potassium-argon geochronometer was being developed. Now it is well understood (an excellent review can be found in a book called "Geochronology and thermochronology by the 40Ar/39Ar method" by McDougall and Harrison.)
He is measured nowadays from single crystals, most often of apatite or zircon. They are heated with a laser, and the evolved gas measured on a quadrupole mass spectrometer.
I think this weekend I'll work up a post describing the process, with some figures and everything. Hope this makes sense..

BrianR said...

I look forward to the Helium post...good times

Jim said...

That definitely helps - thanks. Hopefully, your expansion on the process will help clarify further.

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