Age dating of basalt

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.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.

But micas, plagioclase, hornblende, clays and other minerals can yield good data, as can whole-rock analyses.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.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.

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.