The volatile contents of the Galapagos plume; evidence for H 2O and F open system behavior in melt inclusions

Abstract

We investigated the geochemistry of naturally quenched, olivine-hosted melt inclusions from lavas corresponding to two of the four end-member mantle components from the Galápagos Archipelago: the high 3He/ 4He (Fernandina Is.) and the MORB-like (Santiago Is.) mantle components. Melt inclusions from both islands have a large range in chemical compositions and are trapped within olivine phenocrysts that are in FeO–MgO disequilibrium with the matrix glasses. The majority of the inclusions have major and trace element compositions similar to the matrix glasses. The volatile contents (H 2O, CO 2, F, S, Cl) and volatile/refractory trace element ratios (H 2O/Ce, CO 2/Nb, F/Nd, S/Dy, Cl/K) in these inclusions agree with previous values reported for oceanic basalt. Surprisingly, a subset of inclusions with major and trace elements contents different from the matrix glasses have water and fluorine contents that are remarkably constant and comparable to those of the matrix glasses. This produces unusual H 2O/Ce and F/Nd ratios in those inclusions. These results indicate that the inclusions have not remained closed system for H 2O or F. Models of diffusive re-equilibration for H + and FeO–MgO suggest that the inclusions and their host olivines spent days to months within the surrounding magma. These timescales, combined with the depths of melt inclusion entrapment, suggest minimum melt percolation velocities of ~ 10 − 2 to 10 − 4 m/s within the oceanic crust.