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Geological Society, London, Special Publications; 2009; v. 326; p. 85-107;
DOI: 10.1144/SP326.5
© 2009 Geological Society of London

Articles

187Re-187Os geochronology of Precambrian organic-rich sedimentary rocks

Brian Kendall1,2,*, Robert A. Creaser2 & David Selby3

1 School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA
2 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E3
3 Department of Earth Sciences, Science Labs, University of Durham, Durham DH1 3LE, UK

* Corresponding author (e-mail: brian.kendall{at}asu.edu)

Global correlations of Precambrian stratigraphic successions can be hampered by the coarse resolution of biostratigraphic and chemostratigraphic records, and by the scarcity of reliable U–Pb zircon age constraints. The development of the 187Re (rhenium)–187Os (osmium) radioisotope system as an accurate deposition-age geochronometer for organic-rich sedimentary rocks (e.g. black shales) holds great potential for an improved radiometric calibration of the Precambrian rock record. Here, we review Re–Os isotope data obtained for Precambrian black shales and revisit the discrepancy in Re–Os ages for the Neoproterozoic Aralka Formation (central Australia). In addition, we introduce new Re–Os isotope data for the Late Neoproterozoic Doushantuo Formation (South China) that highlights the necessity of a rigorous sampling protocol for depositional age determinations. Improvements in sampling and analytical methodologies have permitted the determination of precise ages (<1%, 2{sigma}) from Late Neoproterozoic to Late Archaean shales. Whole-rock digestion using a CrVI–H2SO4 solution minimizes the release of detrital Re and Os from shale matrices, and selectively attacks organic matter that hosts hydrogenous Re and Os. The Re–Os system in organic-rich sedimentary rocks appears to be robust during hydrocarbon maturation and up to the onset of lowermost greenschist facies metamorphism, but post-depositional hydrothermal fluid flow can result in scattered Re–Os isotope data. The Re–Os black shale geochronometer should find utility for constraining the age of a diverse range of Precambrian geological phenomena. In addition, the initial 187Os/188Os composition determined from Re–Os isochron regressions serves as a tracer for the Os isotope composition of Precambrian sea water.





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