Abstract
An ion-exchange method was developed for extracting microgram quantities of lead from pegmatitic feldspars. The isotopic composition of lead in feldspars ranging from 350 to 2750 million years (m.y.) in age was determined. Model ages were calculated and were generally found to be in good agreement with independent age determinations. The results are compatible with a lead model stipulating relatively short periods of time in a crustal environment for the lead. In one instance where there was a large discrepancy between the model age and the known age, because of excess radiogenic lead in the feldspar, the anomaly was resolved by acid leach experiments which preferentially removed the excess radiogenic lead. The lead content of twenty-five feldspars was measured and found to range from <20 p.p.m. to 500 p.p.m.
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