APPLICATIONS OF THE RHENIUM-OSMIUM ISOTOPIC SYSTEM, AND PLATINUM AND IRIDIUM ABUNDANCES IN ORGANIC-RICH MUD ROCKS: A GEOCHRONOLOGY, GEOCHEMISTRY, AND REDOX STUDY
Wright, Shawn Curtis 1984-
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This study demonstrates the utility of the Re-Os isotopic system by providing two precise, absolute ages for Permian sedimentary strata and by performing high-resolution chemostratigraphy across the globally-correlated oceanic anoxic event 2 (OAE2). In addition, the majority of Re-Os studies published to date are based on the organophilic behavior of Re and Os, however, there is not presently a thorough understanding of where in the organic matter Re and Os are complexed. In order to begin to provide an understanding of the location of these elements, measurement of Re and Os isotopes in organic (bitumen, kerogen, and oil) and inorganic (pyrite) phases from the Eagle Ford Shale was performed. Absolute ages and calculated initial 187Os/188Os ratios of 264.3 ± 7.5 Ma (187Os/188Osi = 0.46 ± 0.09) for the Permian Brushy Canyon Fm. from the Delaware Basin, west Texas, and 298.0 ± 2.3 Ma (187Os/188Osi = 0.56 ± 0.02) for the Permian Tasmanite Shale provide some insight into the Os isotopic composition of their respective basins at the time of sediment deposition (187Os/188Osi). These data coupled with a previously published compilation of (187Os/188Os)i values provide a first order examination of the Paleozoic seawater 187Os/188Os record. High-resolution chemostratigraphy across OAE2 in the Eagle Ford Shale demonstrates an abrupt negative excursion in (187Os/188Os)i values ~340 kyr prior to OAE2. This represents a drastic shift in the 187Os/188Os of seawater during this anoxic period, likely due to the eruption of basaltic magma during the formation of a large igneous province. This negative excursion, followed ~340 kyr later by a marked enrichment in Pt and Ir within highly oxic bottom-waters demonstrate the variable behavior of these elements in the water column. The variable enrichment of Os and Pt-Ir before and during OAE2 was ultimately caused by changes in insolation controlled by Milankovitch cycles. Re and Os abundances in the various organic and inorganic fractions demonstrate that these elements predominantly reside in the kerogen, with only minor amounts in the bitumen and pyrite. However, these elements could be sequestered into micro-inorganic phases that are tightly held within the kerogen and not thoroughly removed during kerogen isolation.