HR: 1330h
AN: V12B-0575    [PDF]
TI: Impact Origin for the Greater Ontong Java Plateau? Geochemical and Petrologic Evidence.
AU: * Ingle, S
EM: single@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8551 Japan
AU: Coffin, M F
EM: mcoffin@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo, 164-8639 Japan
AB: Voluminous basaltic magmatism at $\sim$120 Ma created Earth's biggest large igneous province (LIP), the Greater Ontong Java Plateau (OJP) in the west Pacific Ocean. This volcanic event has been generally attributed to a large mantle plume. However several important lines of geochemical and petrologic evidence are at odds with a plume origin. High concentrations of heavy rare earth elements (HREE) indicate melting at shallow mantle depths, and major and trace element systematics for OJP basalts are consistent with $>$30% batch melting. These conditions and extents of melting are typical at mid-ocean ridges influenced by hot or wet mantle plumes, but pelagic sediments overlie mostly vesicle-free basalts, implying a deep-water environment of emplacement away from a spreading center. H$_{2}$O contents of these basalts are very low indicating that melting was not facilitated by a wet plume. Radiogenic isotopes vary little considering the large size of OJP. Platinum group elements (PGE) are enriched relative to predicted values assuming a primitive mantle source, but OJP basalts are also sulfide-free, leaving PGE levels difficult to explain. An extraterrestrial impact model for OJP seems more consistent with existing data. A bolide $\sim$20 km in diameter would remove $>$60 km of overburden, and instigate extensive decompression melting. High extents of melting in the shallow mantle would explain the geochemical homogeneity of the basalts, high HREE contents and low H$_{2}$O%. Magmas might also entrain residual bolide which would create basalts with relative PGE enrichment. In the impact scenario, OJP was not heated from below by a mantle plume, so neither uplift nor subsidence would have been extensive, thus explaining the submarine emplacement of the lavas. A major bolide impact could have also caused or contributed to some unexplained phenomena coincident with the timing of OJP emplacement including: onset of the Cretaceous normal magnetic polarity superchron, the Selli oceanic anoxic event, marine faunal extinctions and worldwide radiogenic isotopic excursions that suggest a rapid change from terrigenous-dominated values to mantle-dominated values in marine sediments (Sr and Hf).
DE: 1212 Earth's interior--composition and state (8105)
DE: 3640 Igneous petrology
DE: 6022 Impact phenomena
DE: 8450 Planetary volcanism (5480)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting