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摘要
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 Os-192 (non-radiogenic Os isotope) concentrations, and initial Os-187/Os-188 isotope compositions from Re-Os isochron regressions of ORM can shed insight on the source of Re and Mo to Archean seawater. Seawater Os-187/Os-188 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 Os-187/Os-188 excursions and higher Os-192 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 Os-192 concentrations in Archean ORM that yields unradiogenic initial Os-187/Os-188 may reflect a MHE origin for dissolved Re and Mo in the overlying paleo-seawater. Local oxidative weathering in the presence of photosynthetic O-2 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 Os-187/Os-188. By contrast, an initial Os-187/Os-188 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 Os-192 concentrations in Archean ORM yielding unradiogenic initial Os-187/Os-188 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 O-2 levels. These concepts may help trace the onset of photosynthetic O-2 production and accumulation along Archean ocean margins.
- 出版日期2014-10-14
