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Abiotic Pyrite Formation Produces a Large Fe Isotope Fractionation

Science  24 Jun 2011:
Vol. 332, Issue 6037, pp. 1548-1551
DOI: 10.1126/science.1202924

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Abstract

The iron isotope composition of sedimentary pyrite has been proposed as a potential proxy to trace microbial metabolism and the redox evolution of the oceans. We demonstrate that Fe isotope fractionation accompanies abiotic pyrite formation in the absence of Fe(II) redox change. Combined fractionation factors between Fe(II)aq, mackinawite, and pyrite permit the generation of pyrite with Fe isotope signatures that nearly encapsulate the full range of sedimentary δ56Fepyrite recorded in Archean to modern sediments. We propose that Archean negative Fe isotope excursions reflect partial Fe(II)aq utilization during abiotic pyrite formation rather than microbial dissimilatory Fe(III) reduction. Late Proterozoic to modern sediments may reflect greater Fe(II)aq utilization and variations in source composition.

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