Grain scale processes recorded by oxygen isotopes in olivine-hosted melt inclusions from two MORB samples

Abstract

Although olivine-hosted melt inclusions from mid ocean ridge basalts (MORB) are commonly used as a proxy for mantle composition, these melt inclusions generally show larger elemental and isotopic compositional variation than their host lavas and the origin of these heterogeneities remains disputed. Here we present oxygen isotope data from melt inclusions hosted in olivine from two samples from the Mid-Atlantic ridge. Melt inclusions from different crystals within the same sample show >2.5‰ δ18O variation within each sample, which is nearly eight times the analytical error of 0.3‰ (2 standard deviations) and five times the δ18O range in unaltered MORB. Measured δ18O in melt inclusions do not correlate with common magmatic tracers, and δ18O measured in the host olivines suggest a maximum of 1‰ δ18O source heterogeneity. Less than half of the melt inclusions from each sample are in equilibrium with their host crystals; the remaining melt inclusions have either lower or higher olivine-melt oxygen isotope partition coefficients compared to the theoretical equilibrium values. Here we discuss several potential processes that could contribute to these observations, but none satisfactorily explain the olivine-melt inclusion oxygen disequilibrium that we observe in these samples. Nevertheless, it seems clear that the variability of δ18O in melt inclusion from two MORB samples do not record only common magmatic process(es), but rather a localized grain scale process. Any δ18O variation in melt inclusions should thus be interpreted with caution.