Rhenium-osmium and molybdenum isotope systematics of black shales from the Lower Cambrian Niutitang Formation, SW China: Evidence of a well oxygenated ocean at ca. 520 Ma

摘要

The Early Cambrian was a key period in Earth's history, and the reconstruction of ancient ocean chemistry on spatial and temporal scales can contribute to a better understanding of events during that period. New Early Cambrian data for Re-Os and Mo isotopic compositions and redox-sensitive elements (RSE) are reported here for the Niutitang Formation, SE Chongqin, SW China. A Re-Os age of 520 +/- 30 Myr (Model 3, 2 sigma, n = 21, MSWD = 62) and a (Os-187/Os-188)(i) ratio of 0.79 +/- 0.11 were obtained, consistent with previously reported data for equivalent strata from other areas. Separate regression of samples with (Os-187/Os-188)(i) of 0.72-0.79 and 0.81-0.89 produce more precise ages of 520.1 +/- 9.5 Myr (Model1, 2 sigma, n = 7, MSWD = 1.0) and 513 +/- 10 Myr (Model 1, 2 sigma, n = 8 MSWD = 0.96) respectively, suggesting heterogeneity in the primary source of Os as the dominant cause of scatter and uncertainty. High initial Os-187/Os-188 of 0.79 +/- 0.11 may indicate a high Os input from oxidative weathering of upper continental crust for Early Cambrian ocean.