K ar dating
Then the gas sample is cleaned of all unwanted gasses such as H A variant of the K-Ar method gives better data by making the overall measurement process simpler.The key is to put the mineral sample in a neutron beam, which converts potassium-39 into argon-39.The site also must be geologically meaningful, clearly related to fossil-bearing rocks or other features that need a good date to join the big story.Lava flows that lie above and below rock beds with ancient human fossils are a good—and true—example.Because Ar has a very short half-life, it is guaranteed to be absent in the sample beforehand, so it's a clear indicator of the potassium content.
A precise amount of argon-38 is added to the gas as a "spike" to help calibrate the measurement, and the gas sample is collected onto activated charcoal cooled by liquid nitrogen.The mineral sanidine, the high-temperature form of potassium feldspar, is the most desirable.But micas, plagioclase, hornblende, clays, and other minerals can yield good data, as can whole-rock analyses.The learning curve has been long and is far from over today.With each increment in quality, more subtle sources of error have been found and taken into account.
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Also, the cheaper K-Ar method can be used for screening or reconnaissance purposes, saving Ar-Ar for the most demanding or interesting problems.