Saturday, July 16, 2005

New radiometrics for surface rocks

The US-European joint effort CRONUS is trying to standardize decay times for new radionuclides useful for dating surface rocks. (great acronym, by the way.) The series they're hoping to establish would be useful for an enormous time range- between 50 and 10 millon years, or nearly 6 orders of magnitude.

The nuclides of interest are generated by cosmic rays hitting the top 3 meters of the Earth's surface. (Spaceref.com, my ultimate source for this, had a supernova tie-in. ) The high-energy rays generate a whole panoply of exotics- 3He, 10Be, 21Ne, 26Al and 36Cl-- that are not generated in deeper rocks. Ratios of these nuclides with respect to decay products, i.e. 10Be/9Be, could in effect say when the rocks left the surface.

I guess this approach would be limited to surface rocks that get shielded, by 3 m or more of overlying rock, in a short time relative to the age of the rocks. I was thinking about the 40,000 year old mexican footprints, which were left in volcanic ash and possibly promptly buried.

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