CONFIRMATION OF MINIMAL POST-DEPOSITIONAL DISTURBANCE OF RHENIUM AND OSMIUM IN THE 2.50 GA MT. MCRAE SHALE AND OSMIUM ISOTOPE EVIDENCE FOR LATE ARCHEAN OXIDATIVE WEATHERING

A recent study suggests that geochemical evidence in the 2.50 Ga Mt. McRae Shale (drill core ABDP-9, Hamersley Basin, Western Australia) for the production and accumulation of photosynthetic O₂ can be explained instead by later Paleoproterozoic and Mesoproterozoic metasomatic alteration (Fischer et al., 2014; Goldschmidt Conference abstract). To address these concerns, we measured Re-Os isotope data from four organic-rich mudrock samples at 148.09 m in drill core ABDP-9 where high authigenic enrichments of Re and Mo were observed and suggested to originate from oxidative weathering of crustal sulfide minerals (Anbar et al., 2007; Science 317, 1903-1906). Regression of these four new analyses yields a Re-Os age of 2489 ± 17 Ma (2σ, Mean Square of Weighted Deviates [MSWD] = 0.62; age uncertainty includes the ¹⁸⁷Re decay constant uncertainty). Including the original Re-Os data from single analyses of organic-rich mudrock at 145.22 m, 146.08 m, 147.10 m and 148.32 m (from Anbar et al., 2007) yields an identical, 8-point, Re-Os age of 2488 ± 13 Ma (2σ, MSWD = 0.60). The new Re-Os ages are statistically identical to the Re-Os age of 2495 ± 20 Ma (2σ, MSWD = 0.95) from organic-rich mudrocks at 128.71-129.85 m in ABDP-9, and to U-Pb zircon ages from tuffaceous beds above, within, and below the Mt. McRae Shale. The low MSWD indicates that scatter in the Re-Os regressions can be explained solely by analytical uncertainties. Hence, post-depositional processes have not affected Re, Os, and likely Mo (because of its similar geochemical affinity to Re) at the cm-scale (four samples at 148.09 m) and m-scale (eight samples spanning 145.22-148.32 m). The initial ¹⁸⁷Os/¹⁸⁸Os isotope ratios from the 4-point and 8-point Re-Os regressions are 0.33 ± 0.17 and 0.35 ± 0.14 (2σ), respectively. Both values are statistically higher than the magmatic/hydrothermal (mantle) baseline of 0.11 at 2.50 Ga. Together with the authigenic Re and Mo enrichments in these samples, the initial ¹⁸⁷Os/¹⁸⁸Os suggests oxidative mobilization of Os, Re, and Mo occurred together from crustal sulfide minerals, and that all three metals were transported together to Hamersley Basin seawater. These findings reinforce existing geochemical evidence for mild environmental oxygenation along late Archean ocean margins.