Cosmogenic dating methods

If a sample is exposed at the surface for a time, no matter what the production rate or how long the exposure, the concentrations of the two nuclides conform to the production ratio.Then if you bury the sample deeply enough to stop new nuclide production, inventories of both nuclides (or at least one of the nuclides, if the other is stable) decrease due to radioactive decay.That of Cl-36 is also fairly accurately known (0.7%). Nearly all these measurements include chemical processing at the University of Washington, and AMS analysis at LLNL-CAMS.

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The target mineral for Cl-36 production is a K-rich feldspar. The sediment source is at 1300 m elevation and is eroding at 3 m/Myr.

This results in a cosmogenic Ne-21 concentration near 10 Matoms/g, which we can measure with approximately 5% precision.

The final ingredient we need is an estimate of the uncertainty in the production ratios of these nuclides.

This is hard to estimate in a general way — it depends on the rock or mineral in question and its composition — so for purposes of the following discussion I’ll assume that we know these ratios accurately (however, this is a major issue for some nuclide pairs).

One thing that is interesting is that it generally pays to pick a stable nuclide (Ne-21) as one of the pair, for three reasons: first, it doesn’t decay, so there’s no loss of measurement precision with burial age; second, it doesn’t decay, so one less half-life uncertainty gets propagated into the burial age; third, its ‘decay constant’ is zero, which maximizes the difference between decay constants (an important part of the uncertainty) relative to anything else you could choose.

So this is a good reason to focus on Ne-21 measurements (or on He-3 measurements, which would work similarly except that He-3 is not retained in quartz).

Regardless, two things are clear, at least in theory for this particular scenario: First, with this scenario, for any time period it is (again, theoretically) possible to improve on the precision of Al-26/Be-10 burial dating by choosing a different nuclide pair.

Mainly this is for two reasons: i) the uncertainty is inversely proportional to the difference between decay constants (this falls out of the math) and the difference between Al-26 and Be-10 decay constants is not as large as for other nuclide pairs; ii) the half-life of Al-26 is the least precisely measured of all the nuclides.

So far, nearly all applications of burial dating have used the Al-26/Be-10 pair in quartz. The half-lives of Al-26 and Be-10 are 0.7 and 1.4 Ma, respectively.

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