Geochemical and Isotopic Evolution of Loihi Volcano, Hawaii

Abstract

A 680m thick section from the deeply dissected east flank of Loihi Volcano was sampled using the Pisces V submersible to evaluate the volcano's geochemical evolution. Three types of lavas were recovered: tholeiitic, weakly alkalic and strongly alkalic. The ratio of alkalic to tholeiitic lavas varies systematically with depth, from predominantly alkalic at the base of the section to tholeiitic at the top. Glasses from these rocks have similar ratios of highly incompatible elements and Pb, Sr and Nd isotopes, but distinct ratios of highly to moderately incompatible elements. Partial melting modeling indicates that these tholeiitic and alkalic lavas could be derived by variable degrees of partial 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 accumulation were the dominant processes controlling compositional variation among lavas of the same rock type. Magma mixing features were observed in only a few of the lavas collected. Loihi typifies the preshield stage of Hawaiian volcanism when the volcano drifts closer to the focus of the hotspot. The compositional variation in Loihi's east flank section, which may represent 40% of the volcano's extrusive history, is consistent with the predicted increase in partial melting during this drift. The transition from dominantly alkalic to tholeiitic volcanism on Loihi was fitful but relatively rapid and is now nearly complete. This transition is the opposite of that which occurs during the post-shield stage of Hawaiian volcanism as the volcano migrates away from the hotspot focus. Loihi's tholeiitic lavas overlap in ratios of incompatible trace elements and Pb, Sr and Nd isotopes with lavas from its more active neighbor, Kilauea. The small differences in major element contents between lavas from these adjacent volcanoes can be explained by high-pressure orthopyroxene fractionation of Loihi magmas, which may be a consequence of a low magma-supply rate, or by slightly shallower depths of melt segregation for Kilauea magmas.