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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.

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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 $1000 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.

The potassium-argon (K-Ar) isotopic dating method is especially useful for determining the age of lavas.

Developed in the 1950s, it was important in developing the theory of plate tectonics and in calibrating the geologic time scale.

The Ar-Ar method is considered superior, but some of its problems are avoided in the older K-Ar method.

Also, the cheaper K-Ar method can be used for screening or reconnaissance purposes, saving Ar-Ar for the most demanding or interesting problems.

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.

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