LEAD ISOTOPES IN CA. 2.32 GA SHALES, SOUTH AFRICA: EVIDENCE FOR OXIDATIVE CONTINENTAL CRUST WEATHERING

The rise of atmospheric oxygen to a level above 10^-6 PAL is constrained between 2.47 and 2.32 Ga based on multiple S isotope analyses of early diagenetic pyrite in shales of the Dales Gorge Member, Western Australia and Rooihoogte-Timeball Hill Formations, South Africa. It is further inferred, that oxidative part of Fe and S cycles started by 2.32 Ga based on S and Fe isotope data for pyrite from the Rooihoogte-Timeball Hill Formations.  However, the nature of the rise of atmospheric oxygen to high levels and whether the oxidative part of the U cycle started at that time is not known.  Here, we present results of Pb isotope study of the same shale samples from the Rooihoogte-Timeball Hill Formation, which provide an independent test whether oxidative weathering of continental crust started prior to their deposition.

Pb isotopic compositions of eleven whole-rock black shales have been determined for the ca. 2.32 Ga Rooihoogte-Timeball Hill Formations of the Transvaal Supergroup, South Africa.  The lead isotopic ratios lie between 16.8 and 20.8 (²⁰⁶Pb/²⁰⁴Pb), 15.8 and 16.2 (²⁰⁷Pb/²⁰⁴Pb), and 35.9 and 39.8 (²⁰⁸Pb/²⁰⁴Pb). The ²⁰⁷Pb/²⁰⁴Pb - ²⁰⁶Pb/²⁰⁴Pb array corresponds to an age of about 2.31 Ga, which is the age of sedimentation. The ²⁰⁸Pb/²⁰⁴Pb - ²⁰⁶Pb/²⁰⁴Pb array has a slope corresponding to an average second-stage Th/U growth of about 2.5.  This is lower than Th/U ratios measured in Archean shale suites or estimated for the average upper crust (Taylor and McLennan, 1985).  This indicates that these samples were enriched to some extent in uranium with respect to thorium and lead at the time of deposition.  In the modern ocean, U-enrichment in reducing environment corresponds to uranyl ion reduction from seawater, which is delivered to the ocean by oxidative weathering of continental crust.  Our data indicate that oxidative part of U cycle also started by ca. 2.32 Ga and suggest sufficient oxygen levels in the atmosphere for oxidative weathering of continental crust.