Constraints on the Composition and Hydrothermal Alteration History of the Pacific Lower Crust beneath the Hawaiian Islands: Geochemical Investigation of Gabbroic Xenoliths from Hualalai Volcano

Understanding the composition and hydrothermal alteration history of the lower oceanic crust (LOC) can help constrain deep hydrothermal circulation at mid-ocean ridges, which may have a substantial impact on the thermal regime and magmatic processes at spreading centers. Previous studies of LOC primarily examined ophiolites or layer-3 gabbros exposed at the seafloor through faulting. These potentially have experienced secondary hydrothermal alteration in response to faulting, uplift and exposure. We examined major and trace element and isotopic compositions of a suite of gabbroic xenoliths derived from the 1800-1801 Kapulehu flow, Hualalai, Hawaii to constrain the composition and 'primary' hydrothermal alteration history of the in situ Pacific crust beneath the Hawaiian Islands (HI). Although most Hualalai gabbros have trace element and isotopic compositions consistent with derivation from Hualalai magmas, a subset has characteristics indicative of an origin from MORB-related melts. These gabbros contain LREE-depleted clinopyroxene, have Sr-Nd-Hf isotopic compositions that overlap the range of EPR basalts, and are geochemically distinct from Hualalai-related xenoliths and lavas. Despite the limited range recorded, plagioclase and clinopyroxene oxygen isotope compositions correlate well for both MORB-related and Hualalai-related gabbroic xenoliths. This suggests clinopyroxene and plagioclase are in equilibrium. The △plag-cpx (~0.6-0.9‰) is consistent with closure temperatures of ~1170-1220 C.δ18Ocpx (+4.9-5.3‰) of the MORB-related gabbros are negatively correlated with cpx 87Sr/86Sr, but not with 143Nd/144Nd or La/Sm. In contrast, δ18Oplag does not correlate with plag 87Sr/86Sr. Cpx Sr-isotopes may be affected by seawater alteration, which is not as apparent in plag due to higher Sr concentrations. However, the MORB-related gabbros have δ18O values that are largely in the range for normal, fresh MORB (δ18Omelt/NMORB = +5.7-6.0‰, △melt-cpx~0.7‰). This suggests that only limited hydrothermal circulation penetrated to the depth of the layer-3 LOC gabbros beneath the HI, which resulted in only minor hydrothermal alteration. This is in contrast with observations from several ophiolite sequences and fault-exposed gabbros, which show significantly greater hydrothermal alteration and larger shifts in δ18O from normal mantle values (e.g., δ18O down to +3.5‰; c.f., [1]). The greater alteration recorded in these samples may result from hydrothermal circulation triggered by faulting/uplift associated with their exposure. The relatively uniform and 'normal' δ18O values of the MORB-related gabbros also suggest that assimilation of Pacific crust by Hawaiian magmas ponding within the lower crust is unlikely to produce significant shifts in the magma oxygen isotope composition, and is therefore unlikely to account for the low δ18O values recorded in some Kea-trend lavas as previously proposed (c.f., [2]). [1] Gregory, R. T. & Taylor Jr, H. P., 1981, J. Geophys. Res., 86, 2737-2755. [2] Eiler, J. M., Farley, K. A., Valley, J. W., Hofmann, A. W. and Stolper, E. M, 1996, Earth Planet. Sci. Lett., 144, 453-468.