Osmium isotope systematics of drilled lavas from Mauna Loa, Hawaii

Abstract

We have investigated the isotopic compositions of Os, Sr, Nd, and Pb in a suite of primitive Mauna Loa lavas from the upper 280 m of the Hawaii Scientific Drilling Project pilot core drilled near Hilo, Hawaii. These lavas were probably erupted from Mauna Loa's northeast rift. Correlations between Os (hosted by olivine) and other isotopes indicate that olivine crystals in these flows are closely related in space and time to the enclosing lava, despite the presence of deformation features and Fe/Mg disequilibrium in some olivines. The temporal isotopic evolution of the lavas matches published data for basalts from Mauna Loa's southwest rift, indicating that the two rifts (as well as the summit) share a common magma feed zone which is distinct from that of Kilauea. The composition of the lowermost HSDP Mauna Loa sample shows some isotopic similarities to modern Kilauea compositions and in this respect 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 Mauna 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 partial melting of two distinct sources within the Hawaiian plume followed by partial mixing and rapid transport of melts through the oceanic lithosphere. The passage of the Pacific lithosphere over a heterogeneous Hawaiian plume can account for the systematic differences in the compositions of volcanoes from the “Kea” and “Loa” trends as well as the geochemical evolution of individual shields on the island of Hawaii.