Re-Os systematics in syn-sedimentary/diagenetic pyrite: Precise ages and Os cycling

摘要

Two studies of Re-Os systematics in syn-sedimentary and diagenetic pyrites demonstrate both the promise and limitations of the Re-Os system for addressing questions of depositional age and metal sources in sedimentary rocks. Deltaic/lacustrine sandstones of the ca. 140 Ma Morrison Formation (Colorado and New Mexico, USA) that experienced diagenesis in a continental setting, show open system exchange of Re and Os with oxygenated groundwater. Nevertheless, useful age information can be extracted by analyzing multiple samples from small volumes of rock, minimizing any potential spatial variability in ~(187)Os/~(188)Os ratios. Application of this approach to syn-sedimentary pyrite from highly carbonaceous shales of the Paleopro-terozoic Rooihoogte and Timeball Hill Formations (Transvaal Sequence, South Africa) yields a precise isochron age (2322 +- 15 Ma, initial ~(187)Os/~(188)Os = 0.1087 +- 0.0063) for the deposition of the shales. The chon-dritic initial ratio requires that Os in Paleoproterozoic seawater at the site of Rooihoogte-Timeball Hill deposition was dominated by input from hydrothermal sources rather than continental runoff, a possible consequence of anoxic atmospheric conditions throughout the Archean.