Re and Os isotopes in shales are increasingly used for precise and accurate geochronology and for tracking rates of continental weathering in the geologic past. Moreover, trace element compositions of organic-rich shales reveal paleoredox conditions and properties of shale organic matter (OM) useful for lithostratigraphic correlations. Chemical weathering alters the original shale composition and hence may compromise geologic interpretations based on geochemical data. Here we compare Re-Os isotopes, trace elements and OM properties in correlative shales from outcrops and nearby drillcore to quantify the effects of weathering. We define chemical parameters that distinguish weathered from pristine shale in samples that are macroscopically identical.
Black shale from the Ravnefjeld Formation in East Greenland yields disturbed Re-Os isotope systematics for outcrop samples, and highly precise Re-Os isochron ages for drillcore. Geochemical data show that Re, Os and common Os are hosted in the organic matter rather than sulfides. Re and Os both show mobility during weathering. Combined use of Rock-Eval pyrolysis indices and sulfur contents documents oxidation of both OM and pyrite, thereby providing a valuable chemical criterion to assess invisible shale weathering. Further studies on chemically diverse shales will test the widespread application of these criteria as weathering indicators.
Systematic differences in major and trace element content and kerogen quality between weathered and fresh shale samples are used to characterize and quantify weathering effects. The character of the OM in shales is most telling. Ravnefjeld data are compared with data from time correlative shales from the mid-Norwegian shelf [Georgiev et al., 2011. Hot acidic Late Permian seas stifle life in record time. Earth Planet. Sci. Lett., 310, 389-400] to present a strategy for identification of Re-Os isochroneity in advance of Re-Os analytical work.
Our recent compilation and comparison of Upper Permian shales from widely separated sequences on Pangea's margins reveal exceptionally high Re/Os ratios on a global scale. We suggest Upper Permian shales may have significantly influenced the Cenozoic (e.g. Eocene) 187Os/188Os ratio of seawater on their exposure and weathering.
