Other
Scientific paper
Sep 1964
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1964gecoa..28.1419w&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 28, Issue 9, pp.1419-1458
Other
3
Scientific paper
The lead in sedimentary pyrite is typical of dissolved lead in the original depositional environment, provided the isotopic composition is not changed by radioactive decay of uranium or thorium within the samples, or by recrystallization of the samples long after deposition. These conditions appear to be realized, generally, in pyrite from unmetamorphosed sedimentary rocks. In metasedimentary rocks, the origin of lead in pyrite may be quite complex, but in favorable cases the isotopic composition of such lead provides some information about the pre-metamorphic history of the samples. The lead content of thirty samples of pyrite from sedimentary and metasedimentary rocks has been determined by X-ray fluorescence spectrometry. Lead content ranges from a few ppm to several hundred ppm in both the sedimentary and metasedimentary samples. The uranium content of most of the samples was determined by isotope dilution; much of the uranium, and in some cases much of the lead, associated with the samples could be removed by leaching for several days in hot HCl. The U/Pb ratio of most of the leached samples is sufficiently low that the contribution of radiogenic lead by decay of the uranium is negligible. The isotopic abundances of lead, in the samples for which the sedimentary age is known, are related to a two-stage model for the isotopic development of oceanic lead based on the isotopic composition of lead in the modern North Pacific. This model provides reliable minimum ages for the time of deposition of the samples and in many cases this minimum age is within a few hundred million years of the known age. A few samples which show larger deviations apparently originated in restricted basins of deposition. Samples from black schists in Finland contain excess Pb 206 which probably originated in black shale environments prior to metamorphism. Other samples from Finland, and a sample from the Canadian shield, give model ages considerably greater than the latest metamorphism, suggesting intervals of at least several hundred million years between sedimentation and metamorphism.
Kulp Laurence J.
Wampler Marion J.
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