Weathering has drastically reduced the RbSr and, to a lesser extent, the KAr age of biotite from the Morton Gneiss of southwestern Minnesota. The ages are approximately 75% and 25% lower than the corresponding ages for biotite from the fresh gneiss. The effects of even incipient weathering cannot be neglected in RbSr dating of biotite and, by analogy, of feldspar and whole-rock samples. ?? 1966.
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Effects of weathering on the RbSr and KAr ages of biotite from the Morton Gneiss, Minnesota