WATER, PHLOGOPITE, AMPHIBOLE, CARBONATITE AND A NEW MODEL FOR THE LITHOSPHERE/ASTHENOSPHERE AND THE LOW VELOCITY ZONE BENEATH OAHU

We present a petrologic model for the lithosphere and lithosphere/asthenosphere transition beneath Oahu based on spinel peridotite and garnet pyroxenite “suite” xenoliths that occur in the Honolulu Volcanics [HV]. Spinel peridotite xenoliths represent variably modified MORB residues that form the bulk of the lithosphere. Harzburgite forms the uppermost 10 km of the lithospheric mantle. Although the garnet-bearing xenoliths have varied modes, most are garnet clinopyroxenites. In some cases, the garnet bearing xenoliths exhibit cumulate texture. Clinopyroxene is the dominant phase; garnet, olivine, spinel, and orthopyroxene occur in variable amounts. Minor phlogopite and amphibole, and melt pockets are also present. Phlogopite occurs as veins and as discrete crystals that are sometimes associated with carbonates, which are interpreted to be carbonatitic melts. The glass associated with quench amphibole crystals have up to ~ 4.8 wt% H2O. Amphibole and glass have Koolau (plume)-like Pb-isotope composition. A single phlogopite crystal also has Koolau like Nd, Sr isotope composition. Garnet pyroxenites, which host the pockets of alkaline glass, amphibole, and carbonate, typically show a HV (i.e., post-erosional volcanism) like isotopic and trace element composition. Consistent with previous proposals, the garnet pyroxenite “suite” xenoliths appear to be dominantly cumulates frm HV parent magmas. Consideration of phase equilibrium and other geophysical constraints leads us to conclude that the seismic low velocity zone below the ~80 km thick Oahu lithosphere is composed of layered cumulates of garnet-bearing assemblages that contain pockets of carbonatitic and hydrous alkaline silicate melts derived from a cooling plume residue at a deeper level. The lithosphere itself is heterogeneous and largely lherzolitic with rare garnet pyroxenite veins in the bottom 70-80 km. Spinel pyroxenite veins in the middle lithosphere are minor.