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Because Ar has a very short half-life, it is guaranteed to be absent in the sample beforehand, so it's a clean indicator of the potassium content.The advantage is that all the information needed for dating the sample comes from the same argon measurement. This method is commonly called "argon-argon dating."The physical procedure for K.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 rock sample to be dated must be chosen very carefully.
Rock samples are recorded, marked, sealed and kept free of contamination and excessive heat on the way to the lab.
These effects must be corrected, and the process is intricate enough to require computers.
Ar-Ar analyses cost around 00 per sample and take several weeks.
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.Finally, the argon atoms are counted in a mass spectrometer, a machine with its own complexities.Three argon isotopes are measured: Ar is determined by comparison to it.That is, a fresh mineral grain has its K-Ar "clock" set at zero.