Importance of potassium argon dating

K since their formation, or if some or all of it came from the mantle or from other crustal rocks and minerals.Thus all K-Ar and Ar-Ar "dates" of crustal rocks are questionable, as well as fossil "dates" calibrated by them.

Dalrymple, referring to metamorphism and melting of rocks in the crust, has commented: "If the rock is heated or melted at some later time, then some or all the In a recent study 128 Ar isotopic analyses were obtained from ten profiles across biotite grains in high-grade metamorphic rocks, and apparent Ar-Ar "ages" within individual grains ranged from 161Ma-514Ma.is known to cause grave problems in regional geochronology studies.For example, in the Middle Proterozoic Musgrave Block (northern South Australia), a wide scatter of K-Ar mineral "ages" was found, ranging from 343Ma to 4493Ma due to inherited (excess) , permitting inclusion of the gas in the crystallizing minerals.Likewise, when Ar "dating" was attempted on Proterozoic granulite-facies rocks in the Fraser Range (western Australia) and Strangways Range (central Australia), it was found that garnet, sapphirine, and quartz contained excess was probably incorporated at the time of the formation of the minerals, and calculations suggested a partial pressure of ~0.1 atm Ar in the Proterozoic lower crust of Australia, which extends over half the continent.An Ar-Ar "dating" study of high-grade metamorphic rocks in the Broken Hill region (New South Wales) found widely distributed excess will always be retained in those trapping sites in minerals where it is "held" more tightly.

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