Olivine-hosted melt inclusions in Hawaiian picrites: Equilibration, melting, and plume source characteristics

Olivine-hosted melt inclusions in tholeiitic picrites from five Hawaiian volcanoes (Koolau, Mauna Loa, Kilauea, Loihi, and Hualalai) have major and trace element compositions that illustrate the magmatic characteristics of ocean island volcanoes and the nature of mantle plumes. The geochemistry of these melt inclusions reflects the well known geochemical features that distinguish Hawaiian shield volcanoes, but with considerably greater diversity than whole rock compositions, providing a higher resolution of the magmatic processes contributing to Hawaiian plume magamatism. Naturally quenched inclusions from Kilauea, Mauna Loa, and Hualalai have been modified by crystallization of olivine on the walls of the inclusion and diffusive interaction with the host crystal. In contrast, melt inclusions in two Loihi picrites have not been affected by re-equilibration with their host olivines, reflecting a relatively brief interval between crystallization of the olivines and eruption of these lavas. Corrected major element compositions of experimentally melted inclusions from two Koolau picritic tholeiites are similar to those of erupted lavas from this volcano and document the presence of Koolau melts with at least 14% MgO. Trace element characteristics of melt inclusions from Mauna Loa, Kilauea, and Loihi can be produced by melting of a moderately depleted, garnet lherzolite source. The extent of melting generally increases from Loihi < Kilauea < Mauna Loa, although rare inclusions from Mauna Loa also indicate contributions of relatively small degree (2-4%) melts to these lavas. Extent of melting and isotopically defined source components appear to be linked in the melting regime, with the Mauna Loa component being sampled preferentially at larger degrees of melting. Melt inclusions with trace element characteristics indicating a recycled basaltic eclogite source were not found at Mauna Loa or any of the other volcanoes. Compositions of the Koolau inclusions do require a unique source component, however, possibly reflecting contributions from ancient lithosphere, either within the plume or entrained in the upper mantle. (C) 2002 Elsevier Science B.V. All rights reserved.