An osmium-based method for assessing the source of dissolved rhenium and molybdenum to Archean seawater

• The Re, Mo, and 192Os concentrations of Archean organic-rich mudrocks are compared.
• Seawater 187Os/188Os reveals the source of dissolved Os in Archean seawater.
• An osmium-based method is used to infer the source of Re and Mo in Archean seawater.
• Negative correlations of Re + Mo versus 192Os suggest oxidative weathering of Re + Mo.

Authigenic enrichments of Re and Mo in 2.7–2.5 Ga organic-rich mudrocks (ORM) indicate that dissolved Re and Mo were present in late Archean seawater. However, it can be difficult to determine if the dissolved Re and Mo were sourced from oxidative crustal weathering or magmatic/hydrothermal/extraterrestrial (MHE) inputs. Comparison of Re, Mo, and 192Os (non-radiogenic Os isotope) concentrations, and initial 187Os/188Os isotope compositions from Re–Os isochron regressions of ORM can shed insight on the source of Re and Mo to Archean seawater. Seawater 187Os/188Os reflects the relative contributions of radiogenic crustal versus unradiogenic MHE Os inputs to seawater. During the Phanerozoic, transient increases in magmatic/hydrothermal activity, marked by unradiogenic 187Os/188Os excursions and higher 192Os concentrations in ORM, are accompanied in some cases by higher Re concentrations. These unradiogenic Os sources could thus have played a greater role in the Archean marine Re (and possibly Mo) budget given that the riverine Re (and Mo) flux at that time was likely smaller than the Phanerozoic flux. Hence, a positive correlation between Re, Mo, and 192Os concentrations in Archean ORM that yields unradiogenic initial 187Os/188Os may reflect a MHE origin for dissolved Re and Mo in the overlying paleo-seawater. Local oxidative weathering in the presence of photosynthetic O2 of mafic-ultramafic rocks and crustal sulfide minerals with low Re/Os ratios is an alternative explanation, but the two scenarios cannot be readily distinguished on the basis of initial 187Os/188Os. By contrast, an initial 187Os/188Os from Archean ORM that is statistically higher than the MHE baseline of 0.10–0.11 points to oxidative mobilization of Re, Os, and Mo from crustal sulfide minerals. Rhenium and Mo concentrations that are negatively correlated with 192Os concentrations in Archean ORM yielding unradiogenic initial 187Os/188Os suggests that some Re and Mo were derived from oxidative weathering. Such negative correlations may reflect an increased marine Os sink (adsorption to oxide minerals) during a mild increase in local environmental O2 levels. These concepts may help trace the onset of photosynthetic O2 production and accumulation along Archean ocean margins.