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Posted by / 21-Nov-2017 04:02

Argon dating

The method relies on satisfying some important assumptions: 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.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.What simplifies things is that potassium is a reactive metal and argon is an inert gas: Potassium is always tightly locked up in minerals whereas argon is not part of any minerals. So assuming that no air gets into a mineral grain when it first forms, it has zero argon content.That is, a fresh mineral grain has its K-Ar "clock" set at zero.Thus all K-Ar and Ar-Ar "dates" of crustal rocks are questionable, as well as fossil "dates" calibrated by them.

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.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 known to cause grave problems in regional geochronology studies.If so, then the K-Ar and Ar-Ar "dating" of crustal rocks would be similarly questionable.When muscovite (a common mineral in crustal rocks) is heated to 740°-860°C under high Ar pressures for periods of 3 to 10.5 hours it absorbs significant quantities of Ar, producing K-Ar "ages" of up to 5 billion years, and the absorbed Ar is indistinguishable from radiogenic argon ( In other experiments muscovite was synthesized from a colloidal gel under similar temperatures and Ar pressures, the resultant muscovite retaining up to 0.5 wt% Ar at 640°C and a vapor pressure of 4,000 atmospheres.

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