Potassium 40 dating rocks
They are the lower mantle (below 670km), upper mantle, continental mantle lithosphere, oceanic mantle lithosphere, continental crust and oceanic crust, the latter four constituting the earth's crust. A steady-state upper mantle model has been proposed for mass transfer of rare gases, including Ar.K since their formation, or if some or all of it came from the mantle or from other crustal rocks and minerals.
Thus under certain conditions Ar can be incorporated into minerals which are supposed to exclude Ar when they crystallize. envisage noble gases from the mantle (and the atmosphere) migrating and circulating through the crust, so there should be evidence of excess in crustal rocks and their constituent minerals could well be the norm rather than the exception.
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.