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Lamont Geological Observatory Contribution No. 572.

Abstract

The lead content of Precambrian carbonate rocks is about 1 part per million, and the U/Pb ratio is often much lower than that of average crustal rocks. Hence isotope studies on these rocks can provide information about lead in the environment in which these rocks formed.

The measured lead-isotope ratios must be corrected for radiogenic lead produced from the trace quantities of uranium within the carbonates; correction for other sources of more radiogenic lead (e.g., ground water) is apparently unnecessary.

Model ages based on the corrected 206/207 ratios of lead from ancient marbles of Southern Rhodesia and Finland are equal to or greater than the metamorphic ages of the samples. This model should provide reliable minimum ages for the time of sedimentation of carbonates.

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