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The quantity of the relevant isotopes in the rock will not simply grow without limit with longer and longer exposure to cosmic rays; rather they will tend towards a maximum (a secular equilibrium): the point at which the cosmogenic cosmogenic production of unstable isotopes is equaled by their destruction by decay.In practice, we are not going to be able to tell the difference between a rock which has reached 99.9% of this maximum and one which has reached 99.99%.Until just 20 years ago, when pioneering work in accelerator mass spectrometry (Elmore and Phillips, 1987), cosmogenic isotope systematics (Lal, 1988), and geologic applications (Craig and Poreda, 1986; Kurz, 1986) hit the presses, such conclusions were unreachable because many hypotheses regarding rates and dates of glacial processes were simply unfalsifiable.In two short decades, we have learned so much about when glaciers and ice sheets retreated that it's hard to imagine a world where glacial boulders were not targets for dating.Consequently, the practical limit for the use of cosmogenic surface dating seems to be about 10 million years; after that, one old rock looks much like another.Be production rate is carefully calibrated, for example by correcting for partial attenuation and complete shielding effects.Also, the exposure assumptions must be justified through other means, for example by taking into account clear signs of surface erosion and information consistent with (or suggestive of) 100% shielding prior the exposure event to be dated.
They used protruding mountains as chronometric dipsticks (e.g., Stone et al., 2003), analyzing the Al content of submeter erratics left behind as the East Antarctic Ice Sheet lowered.
The basic principle is that these radionuclides are produced at a known rate, and also decay at a known rate.
Accordingly, by measuring the concentration of these cosmogenic nuclides in a rock sample, and accounting for the flux of the cosmic rays and the half-life of the nuclide, it is possible to estimate how long the sample has been exposed to cosmic rays.
In the article on radiocarbon dating we have already introduced one cosmogenic isotope, Si and which has a half-life of 717,000 years.
Because the isotopes we're using have a short half-life, it follows that if a rock has been buried for a few million years the quantities of these isotopes will be negligible.
Although dating with this method is expensive and the entire process takes a long time, TCN dating has the advantage that the dateable material is produced by the rockslide event itself by exposing fresh material surfaces to the cosmic rays.