Ancient recycled mantle lithosphere in the Hawaiian plume: Osmium-Hafnium isotopic evidence from peridotite mantle xenoliths

The Emperor Seamount-Hawaiian island chain is thought to originate from melting of a heterogeneous mantle plume, but the composition of the plume has always been inferred from the compositions of the erupted lavas. It has been suggested that recycled (i.e. previously subducted) basaltic oceanic crust (with or without sediments) is part of this plume and responsible for the enriched isotopic compositions of the Hawaiian lavas. Here we present the first combined Hf-Os isotopic study on peridotite mantle xenoliths from the island of O'ahu, Hawai'i. The clinopyroxene and spinet major element compositions overlap the global range of compositions of abyssal peridotites, suggesting that the O'ahu peridotites are samples from the oceanic mantle and residues of variable degrees of melting. The peridotite xenoliths from Salt Lake Crater have both highly unradiogenic Os (down to Os-187/Os-188=0.1138) and radiogenic Hf (up to epsilon(Hf)= 114) isotopic compositions and minimum rhenium-depletion ages up to similar to 2 Ga. Such extreme Hf and Os isotopic compositions are not observed in Hawaiian lavas and are far removed from the composition of the depleted mantle, as it is sampled by mid oceanic ridge basalts and abyssal peridotites. Importantly, both Hf and Os isotopic compositions correlate with degree of depletion, suggesting that they are related to an ancient melting event. In contrast, peridotites from the Pali and Kaau vents have Os and Hf isotope ratios consistent with an origin from the similar to 100 Ma Pacific lithosphere that ties beneath the Hawaiian islands. We suggest that the Salt Lake Crater peridotites are fragments of an ancient (> 2 Ga) depleted and recycled mantle lithosphere that is part of the upwelling Hawaiian plume. Such depleted peridotites have higher solidus temperature than other more fertile mantle components, so that their contribution to the erupted lavas compositions is minor, if any. The recognition of such isotopically and compositionally depleted materials within the Hawaiian plume suggests that depleted components are more common in mantle plumes than previously realized. (c) 2007 Elsevier B.V. All rights reserved.