Intra-Volcano Compositional Variability and the Role of Oceanic Lithosphere in West Maui and East Molokai Volcanoes

West Maui and East Molokai, two Hawaiian volcanoes with depleted, Kea-type isotopic compositions, show fine-scale, intra-volcano isotopic correlations in the shield-stage lavas that are distinct from inter-volcano correlations across the Hawaiian chain. Strong isotopic similarities exist in stratigraphic sections through the late shield stage of these two contemporaneous (1.5-2 Ma) Kea-type volcanoes. The depleted Hf, Pb, Sr and Nd isotope compositions of W. Maui and E. Molokai show limited variability and have a strong affinity to the Hawaiian Kea component. ɛ_Hf ranges from +11.5 to +13.5 and +12.6 to +13.3 in W. Maui and E. Molokai, respectively; W. Maui shows an increase in ɛ_Hf with depth. ²⁰⁶Pb/²⁰⁴Pb compositions are 18.34-18.54 and 18.42-18.59 for these same volcanoes. Hf and Pb isotope compositions do not correlate significantly. Even over a limited isotopic range, ɛ_Nd compositions (W. Maui: +6.6 to +7.6; E. Molokai: +7.1 to +7.3) correlate inversely with ⁸⁷Sr/⁸⁶Sr (W. Maui: 0.70337-0.70363; E. Molokai: 0.70341-0.70362). ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb compositions in E. Molokai and deeper samples from W. Maui show a positive, linear correlation. This trend is orthogonal to the inter-volcano Hawaiian trend, and thus does not reflect mixing between components that typically dominate archipelago-scale sampling of magma sources in Hawaii. A positive Sr-Pb isotope correlation may reflect involvement of oceanic lithosphere, the geochemical signature of which is age-dependent. We have modeled the isotopic compositions of upper, hydrothermally altered basaltic segments and lower gabbroic segments of ancient (1.5-2 Ga) and younger (110 Ma) oceanic lithosphere. The fine-scale compositional trends we observe at W. Maui and E. Molokai are consistent with the interaction of plume-generated Kea-type magmas with the local, 110 Ma Pacific lithosphere. These compositional variations are observed only in specific sections of the stratigraphy, and so must result from processes that operate over time scales much shorter than the life of the volcano.