Geochemistry of Lavas from the Emperor Seamounts, and the Geochemical Evolution of Hawaiian Magmatism from 85 to 42 Ma

Abstract

The Hawaiian–Emperor Seamount Chain (ESC), in the northern Pacific Ocean, was produced during the passage of the Pacific Plate over the Hawaiian hotspot. Major and trace element concentrations and Sr–Nd–Pb isotopic compositions of shield and post-shield lavas from nine of the Emperor Seamounts provide a 43 Myr record of the chemistry of the oldest preserved Hawaiian magmatism during the Late Mesozoic and Early Cenozoic (from 85 to 42 Ma). These data demonstrate that there were large variations in the composition of Hawaiian magmatism over this period. Tholeiitic basalts from Meiji Seamount (85 Ma), at the northernmost end of the ESC, have low concentrations of incompatible trace elements, and unradiogenic Sr isotopic compositions, compared with younger lavas from the volcanoes of the Hawaiian Chain (<43 Ma). Lavas from Detroit Seamount (81 Ma) have highly depleted incompatible trace element and Sr–Nd isotopic compositions, which are similar to those of Pacific mid-ocean ridge basalts. Lavas from the younger Emperor Seamounts (62–42 Ma) have trace element compositions similar to those of lavas from the Hawaiian Islands, but initial 87Sr/86Sr ratios extend to lower values. From 81 to 42 Ma there was a systematic increase in 87Sr/86Sr of both tholeiitic and alkalic lavas. The age of the oceanic lithosphere at the time of seamount formation decreases northwards along the Emperor Seamount Chain, and the oldest Emperor Seamounts were built upon young, thin lithosphere close to a former spreading centre. However, the inferred distance of the Hawaiian plume from a former spreading centre, and the isotopic compositions of the oldest Emperor lavas appear to rule out plume–ridge interaction as an explanation for their depleted compositions. We suggest that the observed temporal chemical and isotopic variations may instead be due to variations in the degree of melting of a heterogeneous mantle, resulting from differences in the thickness of the oceanic lithosphere upon which the Emperor Seamounts were constructed. During the Cretaceous, when the Hawaiian plume was situated beneath young, thin lithosphere, the degree of melting within the plume was greater, and incompatible trace element depleted, refractory mantle components contributed more to melting.