Constraints on the Source Components of Lavas Forming the Hawaiian North Arch and Honolulu Volcanics


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Abstract

Hawaiian volcanoes, dominantly shields of tholeiitic basalt, form as the Pacific Plate migrates over a hotspot in the mantle. As these shields migrate away from the hotspot, highly alkalic lavas, forming the rejuvenated stage of volcanism, may erupt after an interval of erosion lasting for 0·25–2·5 Myr. Alkalic lavas with geochemical characteristics similar to rejuvenated- stage lavas erupted on the sea floor north of Oahu along the Hawaiian Arch. The variable Tb/Yb, Sr/Ce, K/Ce, Rb/La, Ba/La, Ti/Eu and Zr/Sm ratios in lavas forming the North Arch and the rejuvenated-stage Honolulu Volcanics were controlled during partial melting by residual garnet, clinopyroxene, Fe–Ti oxides and phlogopite. However, the distinctively high Ba/Th and Sr/Nd ratios of lava forming the North Arch and Honolulu Volcanics reflect source characteristics. These characteristics are also associated with shield tholeiitic basalt; hence they arise from the Hawaiian hotspot, which is interpreted to be a mantle plume. Inversion of the batch melting equation using abundances of highly incompatible elements, such as Th and La, requires enriched sources with 10–55% clinopyroxene and 5–25% garnet for North Arch lavas. The 87Sr/86Sr and 143Nd/144Nd ratios in lavas forming the North Arch and Honolulu Volcanics are consistent with mixing between the Hawaiian plume and a depleted component related to mid-ocean ridge basalts. Specifically, the enrichment of incompatible elements coupled with low 87Sr/86Sr and high 143Nd/144Nd relative to bulk Earth ratios is best explained by derivation from depleted lithosphere recently metasomatized by incipient melt (<2% melting) from the Hawaiian plume. In this metasomatized source, the incompatible element abundances, as well as Sr and Nd isotopic ratios, are controlled by incipient melts. In contrast, the large range of published 187Os/188Os data (0·134–0·176) reflects heterogeneity caused by various proportions of pyroxenite veins residing in a depleted peridotite matrix.

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