A Petrologic, Trace Element, and Isotopic Investigation intothe Upper Mantle Xenoliths From Oahu, Hawaii

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Current models of Hawaiian magma genesis postulate the involvement of at least two or more isotopically and chemically distinct end member mantle sources, one of which is a primitive hot plume and another one is the lithosphere. The relative roles of the lithosphere and the plume have been a matter of debate in these models, which are generally based on the composition of the erupted lavas. Fragments of the lithosphere occur in abundance as xenoliths in the post-erosional Honolulu lavas of Oahu, which provide a window into the Hawaiian lithosphere, and a critical examination of these rocks may help resolve some of these disputes. The present project is concerned with the second phase of an ongoing study of upper mantle xenoliths from Oahu, which includes a detailed isotopic and trace element investigation into the mineral phases of several xenoliths selected on the basis of their mineralogy and petrography (and, in some cases, trace elements). In the first phase of this research, petrographic and mineral-chemical characterization was carried out on a large number of xenoliths from number of vents that cut across the koolau shield volcano. The proposed isotopic and trace element investigation, in conjunction with the earlier petrographic- mineralogic study, is expected to offer an insight into the nature of the lithosphere and the fluids which have crystallized the hydrous phases. Comparative studies of the compositional the hydrous phases. Comparative studies of the compositional variations within and between xenoliths and the erupted Koolau (tholeiitic) and Honolulu (alkalic) lavas will help evaluate models of Hawaiian magma generation and help evaluate models of Hawaiian magmas generation and lithosphere-magma interaction processes during hot spot magmatism. //