Tracy P. Hamilton
Posts: 1239
Joined: May 2006
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| Quote (JonF @ Nov. 09 2011,12:44) | With a little Googling, it turns out there is such a thing as using radium in dating, with some application to geochronology.
One group of methods are disequilibrium methods. Crudely, that means that they measure how far a system is from the secular equilibrium that is achieved in U decay to Pb after about 5-10 times the longest half-life of a daughter product in the chain. The most commonly used such method is U-Th disequilibrium. Uranium is somewhat soluble in water, but Thorium most definitely is not. So when U decay reaches Th in seawater, the Th precipitates out and the rest of the decay chain happens somewhere else. When U gets bound in a solid, such as being incorporated into coral or migrating into a buried bone from groundwater or being incorporated into a fish or many other things, the system starts to approach secular equilibrium with the entire decay chain taking place in the same place. As long as secular equilibrium is not reached, the "freezing" of uranium can be dated by how far the system is from secular equilibrium.
U-Th dating is good to about 350,000 years. Pb-Ra disequilibrium dating is good to a few thousand years.
There's also Ra-Th isochrons with Ba (chemically similar to Ra) as the normalizing isotope, good for several hundred years, but there are issues with the requirement of cogenetic samples having the same Ra/Ba ratios at solidification.
Plenty of technical explanation at Radium Isotope Systematics in Nature Applications in Geochronology and Hydrogeochemistry
None of these methods are useful for samples approaching the age of the Earth or a few orders of magnitude less. |
Speaking of equilibrium, finding a decay series in secular equilibrium is an argument against a young earth. I don't recall that point being explicitly made before.
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