Geochemical zoning of volcanic chains associated with Pacific hotspots

Abstract

Recent Hawaiian volcanism is manifest as two geographically and geochemically distinct groups of volcanoes¹, the Loa trend in the south and the Kea trend in the north^2,3. The differences between the Loa and Kea lavas are attributed to spatial variations in the geochemical structure of the underlying Hawaiian mantle plume^4,5,6,7,8,9. In turn, the Hawaiian plume structure is thought to reflect heterogeneities in its mantle source^7,8. Here we compile geochemical data¹⁰ from the Hawaiian and two other volcanic ocean island chains—the Samoan and Marquesas—that formed above mantle plumes upwelling beneath the Pacific plate. We find that the volcanoes at both Samoa¹¹ and the Marquesas¹² show geographic and geochemical trends similar to those observed at Hawaii. Specifically, two subparallel arrays of volcanoes exist at both locations. In each case, the southern trend of volcanoes has higher radiogenic lead isotope ratios, ²⁰⁸Pb^*/²⁰⁶Pb^*, and lower neodymium isotope ratios, ɛ_Nd, than those of the corresponding northern trend. We suggest that geochemical zoning may be a common feature of mantle plumes beneath the Pacific plate. Furthermore, we find that the pattern repeats between island chains, with the highest ²⁰⁸Pb^*/²⁰⁶Pb^* and the lowest ɛ_Ndfound at Samoa in the south and the lowest ²⁰⁸Pb^*/²⁰⁶Pb^* and the highest ɛ_Nd observed at Hawaii in the north. We infer that isotopically enriched material is preferentially distributed in the lower mantle of the Southern Hemisphere, within the Pacific low seismic velocity zone.