Evidence for Melting of Garnet Pyroxenite in the Generation of Hawaiian Post-Erosional Lavas: Effects of a Marble-Cake Mantle During Melt Generation

Abstract

Isotopic variations in mantle-derived melts are a primary source of information on the age, origin and magnitude of chemical heterogeneities in the convecting mantle. Understanding the relationship between the melting process and the generation and preservation of chemical heterogeneity in basaltic magmas is therefore of the utmost importance. Preferential melting of mafic veins in predominantly peridotitic mantle is a potential source of chemical and isotopic heterogeneity in mantle-derived melts. Previous studies have proposed a role for melting of garnet pyroxenite in the origin of the garnet signature in MORB [e.g. Hirschmann and Stolper, 1996]. At high degrees of mantle melting, such as that responsible for the generation of MORB, the signature of pyroxenitic melting may be difficult to detect unambiguously. However, because pyroxenites have a lower Tsolidus than peridotites, small degree partial melts should contain a more pronounced signature from pyroxenite melting. We have examined the Os-isotope systematics of Hawaiian post-erosional lavas from the Koloa Volcanic Suite, Kauai and the Honolulu Volcanic Series, Oahu to evaluate the role of pyroxenite melting in the generation of these highly enriched small-degree partial melts.