Geochemical Constraints on the Role of Oceanic Lithosphere in Intra-Volcano Heterogeneity at West Maui, Hawaii.

Stratigraphically well-constrained sequences of late shield-building stage lavas from West Maui volcano, Hawaii, show age-dependent compositional variability distinct from that seen in shield-stage lavas from any other Hawaiian volcano. These distinctions are defined by 206Pb/204Pb–208Pb/204Pb variation as well as 87Sr/86Sr correlation with 206Pb/204Pb and trace element compositions. The West Maui lavas from stratigraphically higher in the sequence have major and trace element and Sr–Pb–Hf–Nd isotopic compositions similar to Kea-type lavas sampled at the younger Mauna Kea and Kilauea volcanoes, indicating that the Kea compositional end-member of Hawaiian lavas has remained homogeneous over ∼1·5 Myr. The 87Sr/86Sr–206Pb/204Pb variation in the stratigraphically lowest lavas is orthogonal to the all-Hawaii variation, indicating that it is not the result of mixing between components normally sampled by Hawaiian shield-stage magmas. We compare our West Maui data with observed compositions of Pacific oceanic basaltic and gabbroic crust, and predicted compositions for 2 Ga basaltic and gabbroic oceanic crust. The observed fine-scale compositional variability in the stratigraphically lowest West Maui lavas is consistent with 10–15% mixing of small-degree (2%) partial melts of the Pacific gabbroic oceanic crust with plume-derived, Kea-type magmas. Mass balance considerations indicate that this geochemical signal can be present in no more than 5% of the total volume of lavas erupted at West Maui. [ABSTRACT FROM AUTHOR]