CHONDRITIC 187OS/188OS IN LATE ARCHEAN – EARLY PALEOPROTEROZOIC SEAWATER INDICATES LOW RIVERINE FLUX OF OS UNDER ANOXIC WEATHERING CONDITIONS

Re-Os analyses of synsedimentary or early diagenetic pyrite from sulfidic black shales permit tracking of changes in Os cycling in surface environments with the rise of atmospheric oxygen. Modern seawater, with ¹⁸⁷Os/¹⁸⁸Os approaching 1, derives dissolved Os largely from oxidative weathering of black shale. In contrast, our results suggest that Late Archean – Early Paleoproterozoic seawater was dominated by submarine hydrothermal sources, with chondritic ¹⁸⁷Os/¹⁸⁸Os. Because Re and Os are soluble only under oxidizing conditions, low levels of atmospheric oxygen will greatly inhibit transfer of dissolved Re and Os from the continents to the oceans. Moreover, anoxic seawater will have low concentrations of both elements. Pyrite samples from a 3-meter section of shale straddling the contact of the Timeball Hill and Rooihoogte formations, Transvaal Supergroup, South Africa, yield a 7-point isochron with an age of 2316 ± 7 Ma and initial ¹⁸⁷Os/¹⁸⁸Os of 0.1121 ± 0.0012 (MSWD=0.43; Hannah et al., in press*). Re and Os concentrations range from 17 to 51 and 0.7 to 31 ppb, respectively. The chondritic initial ratio may reflect minimal riverine flux of radiogenic Os during the Archean. Alternatively, despite lack of supporting geologic evidence, the chondritic ratio could result from a restricted basin or a locally active hydrothermal source.

New data from the Royal Shale Member of the ca. 2.6 Ga Jeerinah Formation, Fortescue Group, Western Australia, support the first interpretation of the South African data. Four samples of pyritic layers and nodules from an 8 m section of drill core yield much lower Re and Os concentrations (0.5 to 7 ppb and 16 to 227 ppt, respectively), confirming proposed low concentrations in Archean seawater. Assuming an initial ratio of 0.1095 (chondritic at 2650 Ma), three samples yield model ages within the currently established range of 2684 to 2629 Ma. The fourth sample has the lowest Os concentration and a large blank correction, with correspondingly large uncertainties. Low concentrations and the probable chondritic initial ¹⁸⁷Os/¹⁸⁸Os ratio indicate low flux of radiogenic Os from continental sources and low Re and Os concentrations in anoxic Archean seawater.

*Hannah,JL, Bekker,A, Stein,HJ Markey,RJ and Holland,HD, Earth & Planetary Science Letters, in press.