PLUME-LITHOSPHERE INTERACTIONS IN THE OCEAN BASINS - CONSTRAINTS FROMTHE SOURCE MINERALOGY

Trace element relationships of near-primary alkalic lavas from La Gril le volcano, Grande Comore, in the Indian Ocean, as well as those of th e Honolulu volcanic series, Oahu, Hawaii, show that their sources cont ain amphibole and/or phlogopite. Small amounts of each mineral (2% amp hibole in the source of La Grille and 0.5% phlogopite plus; some amphi bole in the source of the Honolulu volcanics) and a range of absolute degrees of partial melting from similar to 1 to similar to 5% for both series are consistent with the observed trace element variation. Amph ibole and phlogopite are not stable at the temperatures of convecting upper mantle or upwelling thermal plumes from the deep mantle; however , they are stable at pressure-temperature conditions of the oceanic li thospheric mantle. Therefore, the presence of amphibole and/or phlogop ite in the magma source region of volcanics is strong evidence for lit hospheric melting, and we conclude that the La Grille and the Honolulu series formed by melting of the oceanic lithospheric mantle. The iden tification of amphibole +/- phlogopite in the source region of both se ries implies that the metasomatism by fluids or volatile-rich melts oc curred prior to melting. The presence of hydrous phases results in a l ower solidus temperature of the lithospheric mantle, which can be reac hed by conductive heating by the thermal plumes. Isotope ratios of the La Grille and the Honolulu series display a restricted range in compo sition and represent compositional end-members for each island. Larger isotopic variations in shield lavas, represented by the contemporaneo us Karthala volcano on Grande Comore and the older Koolau series on Oa hu, reflect interaction of the upwelling thermal plumes with the litho spheric mantle rather than the heterogeneity of deep-seated mantle plu me sources or entrainment of mantle material in the rising plume. Lite rature Os-Sr isotope ratio covariations constrain the process of plume -lithosphere interaction as occurring through mixing of plume melts an d low-degree melts from the metasomatized oceanic lithospheric mantle. The characterization of the lithospheric mantle signature allows the isotopic composition of the deep mantle plume components to be identif ied, and mixing relationships show that the Karthala and Koolau plume end-members have nearly uniform isotopic compositions. Based on indepe ndent arguments, isotopic variations on Heard and Easter islands have been shown to be a result of mixing between deep plume sources having distinct isotopic compositions with lithosphere or shallow asthenosphe ric mantle. To the extent that these case studies are representative o f oceanic island volcanism, they indicate that interaction with oceani c lithospheric mantle plays an important role in the compositions of l avas erupted during the shield-building stage of plume magmatism, and that isotopic compositions of deep mantle plume sources are nearly uni form on the scale that they are sampled by melting. (C) 1997 Elsevier Science B.V.