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 δ18O 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 δ18O between volcanoes. Low values of δ18O are associated with a ‘depleted’ component that is relatively high in 206Pb/204Pb, low in 3He/4He, and anomalously low in 207Pb/204Pb relative to 206Pb/204Pb. This component is preferentially sampled in lavas from the so-called Kea trend volcanoes (Kilauea, Mauna Kea, Kohala and Haleakala).Low δ18O 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 206Pb/204Pb-207Pb/204Pb relationships indicate recent enrichment in U/Pb in this component and further support the hypothesis that this component is young ( < 108 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 δ18O 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 δ18O values within the range typical of the upper mantle.