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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.The rock samples are crushed, in clean equipment, to a size that preserves whole grains of the mineral to be dated, then sieved to help concentrate these grains of the target mineral.
That is, a fresh mineral grain has its K-Ar "clock" set at zero.
The method relies on satisfying some important assumptions: Given careful work in the field and in the lab, these assumptions can be met.
The rock sample to be dated must be chosen very carefully.
Any alteration or fracturing means that the potassium or the argon or both have been disturbed.
These steps help remove as much atmospheric Ar from the sample as possible before making the measurement.
Next, the mineral sample is heated to melting in a vacuum furnace, driving out all the gas.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.Young rocks have low levels of Ar, so as much as several kilograms may be needed.Rock samples are recorded, marked, sealed and kept free of contamination and excessive heat on the way to the lab.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.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.Then the gas sample is cleaned of all unwanted gasses such as H, nitrogen and so on until all that remains are the inert gasses, argon among them.Variations in this data may point to errors anywhere in the process, which is why all the steps of preparation are recorded in detail.K-Ar analyses cost several hundred dollars per sample and take a week or two.