Rhenium-osmium geochronology of Precambrian organic-rich sedimentary rocks: Systematics and applications

The 187Re-187Os radioisotope system in organic-rich shales represents a precise and accurate deposition-age geochronometer that has widespread utility for Precambrian geological timescale calibration and sedimentary basin analysis. New Re-Os ages of 643.0 +/- 2.4 Ma and 657.2 +/- 5.4 Ma from the Tindelpina Shale Member and Aralka Formation constrain the end of "Sturtian" glaciation in southern and central Australia, respectively. Black shales overlying Sturtian-equivalent diamictites in northwestern Tasmania yield an identical Re-Os age (640.7 +/- 4.7 Ma) as the Tindelpina Shale Member. The "Sturtian" ice age is thus shown to be markedly diachronous and/or there were multiple glaciations at ca. 750-643 Ma. A Re-Os date of 1100 +/- 77 Ma was obtained from Lapa Formation post-glacial shales (Brazil) overlying glacial diamictites previously ascribed to "Sturtian" glaciation.;Joint application of the Re-Os deposition-age geochronometer and the Mo isotope paleoredox proxy has tremendous potential for tracing atmosphere and ocean oxidation over geological time. New Re-Os ages of 1361 +/- 21 Ma and 1417 +/- 29 Ma constrain the depositional age of the Velkerri Formation, Australia. Black shales from the upper Velkerri Formation were deposited from euxinic bottom waters and have an average Mo isotope composition (delta97/95Mo) of 0.72 +/- 0.10‰ that is ∼0.8‰ lighter than present-day seawater. This reflects decreased uptake of Mo onto Mn oxides associated with the expansion of deep-sea euxinia and contraction of oxic marine deposition. A Re-Os age of 2501.1 +/- 8.2 Ma for the Mt. McRae Shale (Western Australia) constrains the timing of mild shallow-ocean oxygenation prior to the 2.45-2.32 Ga Great Oxidation Event. Precambrian seawater Os isotope compositions and Re and Os abundances and 187Re/ 188Os isotope ratios are consistent with hypothesized anoxic Archean atmosphere/oceans, stratified Proterozoic oceans under a weakly to mildly oxygenated atmosphere, and fully oxygenated Late Neoproterozoic and Phanerozoic oceans.;Reconnaissance Re-Os, Mo, and Fe isotope data for the 635-551 Ma Doushantuo Formation (South China) suggest further studies will yield valuable insights regarding inter-relationships between ocean oxygenation, glaciation, and metazoan evolution.;The Re-Os and Mo isotope systematics of black shales are found to be disturbed during post-depositional hydrothermal fluid flow.