Assessing the presence of garnet‐pyroxenite in the mantle sources of basalts through combined hafnium‐neodymium‐thorium isotope systematics

Abstract

The existence of an enriched component in the mantle with a pyroxenitic or eclogitic composition and its importance for basalt genesis has been discussed for over two decades. Inferences about the depth of melting as well as the dynamics of melting based on the presence of garnet and the location of the spinel‐garnet transition are different if garnet‐pyroxenite is present in the peridotitic mantle. Trace element partition coefficients are dependent on composition, and the differences between garnet‐pyroxenite and peridotite are large enough to produce significant differences in trace element fractionation between melts derived from these different lithologies. Melts derived from garnet‐pyroxenite or eclogite‐bearing sources will have small or no 230Th excesses, which are largely independent of melting and upwelling rate. Melts derived from garnet‐peridotite will have significant 230Th excesses, which are dependent on melting and upwelling rate. We show that the combined hafnium‐neodymium‐thorium (Hf‐Nd‐Th) isotope and trace element data can distinguish between melts derived from peridotitic and pyroxenitic or eclogitic sources. We also present new Hf isotope data for Hawaiian basalts and use the combined Hf‐Nd‐Th isotope and trace element systematics to argue against the existence of garnet‐pyroxenite or eclogite in the source of Hawaiian basalts. It is especially the large variation in degree of melting for relatively constant isotopic composition that allows us to rule out garnet‐pyroxenite in the source of the Hawaiian basalts.