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Effects of black shale weathering on the mobility of rhenium and platinum group elements

¹ Woods Hole Oceanographic Institution, MS 25, Woods Hole, Massachusetts 02543-1541, USA

Radiogenic Os from the continents dominates the late Cenozoic marine Os budget. A variety of proposed radiogenic sources includes organic-rich sediments, Precambrian shields, and Re-rich sulfides. Identifying the dominant sources of radiogenic Os to the oceans has a strong impact on past climate models because weathering of silicates acts as a net sink of CO₂, whereas weathering of organic matter acts as a source. Here we investigate the effects of black shale weathering on the integrity of the Re-Os isotope system and release of Re and platinum group elements. Time-correlative pairs of fresh (drill core) and weathered (outcrop) black shales of Late Ordovician age (Utica Shale magnafacies, Québec) were analyzed for Re, Os, Ir, Pt, and Pd, as well as total S, N, and C (as C_carb and C_org). The results indicate that 25%–64% of the initial Re budget, 45%–90% of the Os budget, 14%–65% of the Ir budget, 60%–77% of the Pt budget, 69%–86% of the Pd budget, and 44%–96% of the C_org budget are lost during weathering. We use positive correlations of Os, Pt, and Pd with C_org and N to calculate the respective platinum group element concentrations in the unweathered organic matter end member as 3 ng/g Os, 9 ng/g Pt, and 11–24 ng/g Pd. Although black shales make up <1% of the continental crust, mass-balance calculations indicate that such facies contribute significantly to the labile Os inventory of the continental crust.

Key Words: black shale • weathering • osmium • platinum group elements • rhenium

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