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The Washington Post article Scientists discover hundreds of footprints left at the dawn of modern humanity describes the geological dating of stratified layers of mud by analyzing and dating minerals within each layer.

But since floods jumble materials of different origins and ages together, that meant the scientists had to date dozens of different minerals.

A problem is that it takes two separate measurements to get the potassium concentration and the argon isotope ratios, increasing the uncertainty.

Argon-argon dating gets around many of the issues by measuring only multiple isotopes of argon.

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.

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.

As noted in the comments the wikipedia articles (at the time this question was submitted) are contradictory.

There are quite a few steps to the logic of how argon-argon dating works but none are too complicated, although I won't go into all of the possible interferences.

represents primordial Ar carried from source areas in the earth's mantle by the parent magmas, is inherited by the resultant volcanic rocks, and thus has no age significance.

However, are all other rocks in the earth's crust also susceptible to "contamination" by excess emanating from the mantle?

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The trick is to irradiate the sample with neutrons along with samples of known age.

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