OSMIUM ISOTOPE SYSTEMATICS OF DRILLED LAVAS FROM MAUNA-LOA, HAWAII

We have investigated the isotopic compositions of Os, Sr, Nd, and Pb i n a suite of primitive Mauna Loa lavas from the upper 280 m of the Haw aii Scientific Drilling Project pilot core drilled near Hilo, Hawaii. These lavas were probably erupted from Mauna Loa's northeast rift. Cor relations between Os (hosted by olivine) and other isotopes indicate t hat olivine crystals in these flows are closely related in space and t ime to the enclosing lava, despite the presence of deformation feature s and Fe/Mg disequilibrium in some olivines. The temporal isotopic evo lution of the lavas matches published data for basalts fi um Mauna Loa 's southwest rift, indicating that the two rifts (as well as the summi t) share a common magma feed zone which is distinct from that of Kilau ea. The composition of the lowermost HSDP Mauna Loa sample shows some isotopic similarities to modern Kilauea compositions and in this respe ct compares well with published data on submarine lavas from Mauna Loa 's southwest rift. The good correlations among the isotopic tracers of compatible (Os) and incompatible (Sr, Nd, Pb) elements indicate that a depleted upper mantle component is very minor or nonexistent in Maun a Loa lavas. The Os isotope results definitively rule out equilibrium porous flow as a means of melt transport through the lithosphere. The isotopic variations in shield-stage lavas are most consistent with par tial melting of two distinct sources within the Hawaiian plume followe d by partial mixing and rapid transport of melts through the oceanic l ithosphere. The passage of the Pacific lithosphere over a heterogeneou s Hawaiian plume can account for the systematic differences in the com positions of volcanoes from the ''Kea'' and ''Loa'' trends as well as the geochemical evolution of individual shields on the island of Hawai i.