|MadSci Network: Earth Sciences|
I'm not familiar with Vishnu Schist, so I can't speak to the specifics of how it has been dated. However, radioactive dating in general is applied as follows: Several common elements (e.g. Carbon, Argon, Potassium, etc.) are somewhat radioactive (one or more of the isotopes of the element are naturally radioactive). As a particular item ages (whether it is a rock, a bone, or a piece of wood), the radioactive material in it decays into it's "daughter products," leaving the nonradioactive portions unchanged. By taking a precise measurement of the ratio of the nonradioactive portion of the item compared to the radioactive portion, the "decay time" of the element, and thus it's age, can be determined. (This is a fairly simplistic model. If you're interested in researching this topic, please go to http://www.radiocarbon.org/ to find more in-depth information). Elements behave differently in the biosphere (Carbon behaves differently than Argon, for instance). Because of these differences, several systems of dating have been developed. Carbon 14, for instance, is a convenient method to date organic materials less than about 50,000 years old. This is because most living organisms acquire Carbon thoughout their lives (steady state conditions, steady state ratio between stable Carbon and Carbon 14), but when they die they stop acquiring Carbon. Thus, the ratio between stable Carbon and Carbon 14 starts to change, as Carbon 14 decays away (it decays to an isotope of Nitrogen). For the Carbon 14 system, the "clock" starts when an organism dies. Because the half-life of Carbon 14 is only about 5,700 years, it cannot be used to date very old materials (essentially all of the Carbon 14 is gone in about 50,000 years, rendering the system useless for materials older than that). In other systems the clock is started in other ways. For instance, a rock which melts will lose any volatile elements that may be present in the rock. Some radioactive materials happen to decay to volatile elements. Therefore, when the rock resolidifies, the clock starts, because the ratio of the radioactive parent to the volatile daughter can be measured and the elapsed time determined. Lots of information and links are available at the website of Journal of Amateur Paleontology: http://www.fossilnews.com/
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