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Petrochronological constraints on the origin of the Salt Lake Crater garnet-bearing pyroxenite xenoliths, Oahu, Hawaii Zhang, Zhen


This thesis reports the results of a comprehensive major and trace element study of seven garnet-bearing pyroxenite xenoliths recovered from Oahu, Hawaii. The pyroxenites are dominated by clinopyroxene, but also contain garnet, olivine, orthopyroxene and spinel (Cpx, Gt, Ol, Opx, Sp). Four zircons extracted from two pyroxenites have been dated and the trace element signatures studied in detail. Petrological and geochemical information from optical microscopy, electron microprobe analysis (EMP; 11 major and minor element oxides) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS; 36 trace elements) for major mineral phases indicate that the pyroxenites are cumulates formed through fractional crystallization of magma formed by partial melting of a subcontinental lithospheric mantle fragment entrained in the convecting oceanic lithosphere. Two groups of ages were acquired from zircon core and rim zones, with simultaneous, in situ, determination of trace element concentrations by a cutting-edge Laser Ablation Split Stream-ICP-MS technique. They are the first zircons yielding true ages (non-model ages) for xenoliths recovered from the oceanic lithospheric mantle. The older core (80.8 ± 2 Ma) of Zircon3 yielded clinopyroxene/zircon, garnet/zircon partitioning coefficients for most elements that match literature coefficients for similar magma types. This suggests that clinopyroxene and garnet, the dominant minerals in the xenoliths, formed during a magmatic event ~ 81 Ma, when the zircon cores formed. Younger ages (12.9 ± 0.2 Ma to 14.5 ± 0.2 Ma) given by four zircons (Zircon2 and Zircon4, along with overgrowths on Zircon1 and Zircon3) are inferred to record a event at ~ 13 Ma, possibly due to the presence of an adjacent mantle plume. The result implies that instead of recycled oceanic crust and lithosphere, the removal of mantle lithosphere from below the continents during subduction or asthenosphere upwelling could be an important mechanism that contributes to chemical variability in the mantle.

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