The PGE Signature of OAE1a: No Evidence of a Bolide Trigger for OJP Volcanism

Details The PGE Signature of OAE1a: No Evidence of a Bolide Trigger for OJP Volcanism The PGE Signature of OAE1a: No Evidence of a Bolide Trigger for OJP Volcanism

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.v23h..02t&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #V23H-02

Mathematics

Logic

1051 Sedimentary Geochemistry, 4802 Anoxic Environments (0404, 1803, 4834, 4902), 4924 Geochemical Tracers, 8137 Hotspots, Large Igneous Provinces, And Flood Basalt Volcanism, 8409 Atmospheric Effects (0370)

Scientific paper

The large-volume eruption and the short-period of emplacement shown by the geochronological, geological, and geophysical studies of the Ontong Java Plateau (OJP) require some explanation beyond what is already known from plate tectonic processes. Plume impact, bolide impact, and their variants have been proposed to account for the origin of these features. The two end member models (bolide vs. plume) cannot be distinguished from one another based solely on the Os isotopic compositions of the OJP lavas because both meteorites and mantle have unradiogenic Os isotope signature. Additional evidence to test the models can be derived from the platinum group element (PGE) abundances in sedimentary rocks deposited before and during the LIP emplacement. We analyzed the platinum group element (PGE) contents of the Selli Level horizon at Gorgo a Cerbara, central Italy, which was previously analyzed for Os isotope composition, to further test the mantle plume vs. bolide impact models for the origin of the OJP and the Early Cretaceous oceanic anoxic event, OAE1a. We found that Ir concentrations are low compared to other known large impact horizons. Only one black shale bed yielded high Ir concentration of 544 ppt but it is not at the base of the Selli Level interval. In addition chondrite-normalized Os/Ir and Pt/Ir values of 1.4-21 and 7.7-31, respectively, within the Selli Level horizon are nonchondritic, including the bed that yielded the highest concentration of Re and other PGEs. Previous analysis of the Os isotopic compositions of the organic fractions of these sedimentary units showed a major interval of unradiogenic Os isotopic compositions coinciding with the Selli Level (Tejada et al., 2008). The absence of positive correlation between enrichment of PGE and the unradiogenic Os isotopic compositions suggests that the OAE1a event represented by the Selli Level horizon was triggered by a significant amount of mantle input into the marine environment and not by a bolide impact. Highly variable Os/Ir ratios that are uncorrelated with initial 187Os/188Os are very difficult to reconcile with the impact hypothesis. Our PGE data further corroborate our previous results that favor a mantle plume origin for the OJP and that the main phase of plateau volcanism triggered the OAE1a event. Reference: Tejada, M. L. G., Suzuki, K., Kuroda, J., Coccioni, R., Ohkouchi, N., Sakamoto, T., Tatsumi, Y., and Mahoney, J., 2008. Ontong Java Plateau eruption as a trigger for the Early Aptian oceanic anoxic event. Geology, submitted.

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