How accurate is potassium argon dating Free exhibitions video chat
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.
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.
Thus, the ratio of argon-40 and potassium-40 and radiogenic calcium-40 to potassium-40 in a mineral or rock is a measure of the age of the sample.
The calcium-potassium age method is seldom used, however, because of the great abundance of nonradiogenic calcium in minerals or rocks, which masks the presence of radiogenic calcium.
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.
Any alteration or fracturing means that the potassium or the argon or both have been disturbed.
The target mineral is separated using heavy liquids, then hand-picked under the microscope for the purest possible sample.
This mineral sample is then baked gently overnight in a vacuum furnace.
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.