Application of the 187Re-187Os system to black shale geochronometry

Abstract

The decay of 187Re to 187Os provides a tool for determining depositional ages of black shales. Re and Os concentrations and Os isotopic compositions of whole rock samples of the Bakken Shale, a Mississippian/Devonian boundary black shale, yield a whole rock isochron with an age of 354 ± 49 Ma. This age is in agreement with the accepted age of the Bakken Shale (≈360 Ma). The initial 187Os186Os ratio of the isochron is 6.2 ± 3.3. This value is indistinguishable from the 187Os186Os ratios observed in modern Black Sea sediments. The concentration of common Os in Bakken Shale samples is strongly correlated with total nitrogen, indicating that a large fraction of the Os in these samples is associated with a hydrogenous component, which overwhelms any Os supplied to the sediment in association with detrital material or cosmic dust. The dominance of the hydrogenous component imparts a relatively homogeneous initial 187Os186Os ratio to the sediment at the time of deposition. The degree of scatter about the isochron exceeds the degree of scatter expected from either analytical error or from the estimated degree of initial isotopic heterogeneity. The pattern of scatter is consistent with postdepositional mobilization of Re and/ or Os on a small spatial scale. We suggest that this mobilization may be a consequence of petroleum formation and migration in the Bakken Shale.