An isotopically depleted lower mantle component is intrinsic to the Hawaiian mantle plume

Abstract

Most ocean island basalts sample an isotopically depleted mantle component, but the origin of this component is unclear. It may come from either the entrained upper mantle or from a reservoir intrinsic to the plume, sourced from the lower mantle. For Hawaii, the isotopically depleted component is primarily sampled during the secondary rejuvenated-stage volcanism, 0.5–2 million years after the initial shield-stage volcanism. However, it is also inferred in shield and post-shield lavas. We analyse the radiogenic isotopic and trace element compositions of a suite of Mauna Kea shield-stage tholeiites, and found that they have the same isotopic compositions as rejuvenated-stage lavas. We use trace element models to show that these shield-stage basalts can be explained as higher degree partial melts of a rejuvenated-stage source. Our data, therefore, show that the depleted rejuvenated-stage component was directly sampled during shield-stage volcanism. The common source for both shield-stage and secondary rejuvenated volcanism implies that the depleted rejuvenated component is intrinsic to the Hawaiian mantle plume. It is further inferred that the mantle region from which the Hawaiian plume originates, probably in the lower mantle, is also isotopically depleted, similar but not identical to the upper mantle. A mantle component that is isotopically depleted is intrinsic to the Hawaiian mantle plume, and is probably sourced from the lower mantle, according to radiogenic isotope analyses on shield-stage tholeitic basalts from Mauna Kea.