GEOCHEMICAL AND ISOTOPIC EVOLUTION OF LOIHI VOLCANO, HAWAII

A 680 m thick section from the deeply dissected east flank of Loihi Vo lcano was sampled using the Pisces V submersible to evaluate the volca no's geochemical evolution, Three types of lavas were recovered: thole iitic, weakly alkalic and strongly alkalic. The ratio of alkalic to th oleiitic lavas varies systematically with depth,from predominantly alk alic at the base of the section to tholeiitic at the tap Glasses from these rocks have similar ratios of highly, incompatible elements and P b, Sr and Nd isotopes, but distinct ratios of highly to moderately inc ompatible elements. Partial melting modeling indicates that these thol eiitic and alkalic lavas could be derived by variable degrees of parti al melting of a slightly heterogeneous source. Many distinct parental magmas were generated for each rock type during the 100-150 k.y. that the east flank section was formed. Crystal fractionation and olivine a ccumulation were the dominant processes controlling compositional vari ation among lavas of the same rock type. Magma mixing features were ob served in only a few of the lavas collected. Loihi typifies the preshi eld stage of Hawaiian volcanism when the volcano drifts closer to the focus of the hotspot. The compositional variation in Loihi's east flan k section, which may represent 40% of the volcano's extrusive history, is consistent with the predicted increase in partial melting during t his drift. The transition from dominantly alkalic to tholeiitic volcan ism on Loihi was fitful but relatively rapid and is now nearly complet e. This transition the opposite of that which occurs during the post-s hield stage of Hawaiian volcanism as the volcano migrates away from th e hot-spot focus. Loihi's tholeiitic lavas overlap in ratios of incomp atible trace elements and Pb, Sr and Nd isotopes with lavas from its m ore active neighbor, Kilauea. The small differences in major element c ontents between lavas from these adjacent volcanoes can be explained b y high-pressure orthopyroxene fractionation of Loihi magmas, which may be a consequence of a low magma- supply rate, or by slightly shallowe r depths of melt segregation for Kilauea magmas.