the two‐faced mica

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Abstract
We explain two common 40Ar/39Ar age spectra shapes in biotite. One features a dip in ages at mid‐temperature regions of the spectrum. The other has exactly the opposite characteristic, i.e. a bump upwards in age at the same position. The explanation assumes that the vast majority of biotites consist of two almost completely independent K‐Ar phases. During laboratory step‐heating of irradiated biotite, the argon from these two phases is separated via release over two different temperature ranges. Biotite is therefore a clock with two faces.
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