Role of the deep mantle in generating the compositional asymmetry of the Hawaiian mantle plume

Abstract

Volcanoes formed above the Hawaiian mantle plume exhibit a striking contrast in the geochemical characteristics of the lavas erupted at the northern Kea compared with the southern Loa volcanoes. Isotopic data show that these trends have persisted for more than 5 million years and may mirror compositional heterogeneities in the deep mantle. Linear chains of volcanic ocean islands are one of the most distinctive features on our planet. The longest, the Hawaiian–Emperor Chain, has been active for more than 80 million years, and is thought to have formed as the Pacific Plate moved across the Hawaiian mantle plume, the hottest and most productive of Earth's plumes. Volcanoes fed by the plume today form two adjacent trends, including Mauna Kea and Mauna Loa, that exhibit strikingly different geochemical characteristics. An extensive data set of isotopic analyses shows that lavas with these distinct characteristics have erupted in parallel along the Kea and Loa trends for at least 5 million years. Seismological data suggest that the Hawaiian mantle plume, when projected into the deep mantle, overlies the boundary between typical Pacific lower mantle and a sharply defined layer of apparently different material. This layer exhibits low seismic shear velocities and occurs on the Loa side of the plume. We conclude that the geochemical differences between the Kea and Loa trends reflect preferential sampling of these two distinct sources of deep mantle material. Similar indications of preferential sampling at the limit of a large anomalous low-velocity zone are found in Kerguelen and Tristan da Cunha basalts in the Indian and Atlantic oceans, respectively. We infer that the anomalous low-velocity zones at the core–mantle boundary are storing geochemical anomalies that are enriched in recycled material and sampled by strong mantle plumes.