Oxygen isotope constraints on the sources of Hawaiian volcanism

Abstract

We have measured oxygen isotope ratios in 99 separates of olivine and 14 separates of plagioclase or glass from Hawaiian lavas. These data confirm that the source(s) of some Hawaiian basalts are lower in δ^(18)O than peridotite xenoliths and the source region for mid-ocean ridge basalts (MORB). Our data document correlations between oxygen and radiogenic isotope ratios and consistent differences in δ^(18)O between volcanoes. Low values of δ^(18)O are associated with a 'depleted' component that is relatively high in ^(206)Pb/^(204)Pb, low in ^3He/^4He, and anomalously low in ^(207)Pb/^(204)Pb relative to ^(206)Pb/^(204)Pb. This component is preferentially sampled in lavas from the so-called Kea trend volcanoes (Kilauea, Mauna Kea, Kohala and Haleakala). Low δ^(18)O values in the 'Kea' component suggest that it is hydrothermally altered oceanic crust. The similarity of the Kea end member to Pacific MORB in terms of Sr, Nd, and Pb isotope ratios further suggests that this component is assimilated from the local Pacific plate in subcrustal magma chambers. Anomalous ^(206)Pb/^(204)Pb-^(207)Pb/^(204)Pb relationships indicate recent enrichment in U/Pb in this component and further support the hypothesis that this component is young ( < 10^8 yr) Pacific crust. The isotopic distinctions between Loa and Kea trend volcanoes implies a systematic difference in the magma supply and plumbing systems of volcanoes on these two trends. Samples from Lanai and Koolau have 'enriched' radiogenic isotope compositions (radiogenic Sr and non-radiogenic Nd and Pb) and higher δ^(18)O than typical upper mantle values, suggesting the incorporation of recycled sediment and/or oceanic crust in their sources. Other isotopic end members to Hawaiian lavas (e.g., high ^3He/^4He and post-erosional lavas) have δ^(18)O values within the range typical of the upper mantle.

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