By Andrew Alden The potassium-argon (K-Ar) isotopic dating method is especially useful for determining the age of lavas.
That is, a fresh mineral grain has its K-Ar "clock" set at zero.
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.
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 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.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 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.