Post-entrapment modification of volatiles and oxygen fugacity in olivine-hosted melt inclusions

Abstract

The solubilities of volatiles (H_2O, CO_2, S, F, and Cl) in basaltic melts are dependent on variables such as temperature, pressure, melt composition, and redox state. Accordingly, volatile concentrations can change dramatically during the various stages of a magma's existence: from generation, to ascent through the mantle and crust, to final eruption at the Earth's surface. Olivine-hosted melt inclusions have the potential to preserve volatile concentrations at the time of entrapment due to the protection afforded by the host olivine against decompression and changes to the oxidation state of the external magma. Recent studies, however, have demonstrated that rapid diffusive re-equilibration of H_2O and oxygen fugacity (f_(O_2)) can occur within olivine-hosted melt inclusions. Here we present volatile, hydrogen isotope, and major element data from dehydration experiments and a quantitative model that assesses proposed mechanisms for diffusive re-equilibration of H_2O and f_(O_2) in olivine-hosted melt inclusions. Our comprehensive set of data for the behavior of common magmatic volatiles (H_2O, CO_2, F, Cl, and S) demonstrates that post-entrapment modification of CO_2, and to a lesser extent S, can also occur. We show that the CO_2 and S concentrations within an included melt decrease with progressive diffusive H_2O loss, and propose that this occurs due to dehydration-induced changes to the internal pressure of the inclusion. Therefore, deriving accurate estimates for pre-eruptive CO_2 and S concentrations from olivine-hosted melt inclusions requires accounting for the amount of CO_2 and S hosted in vapor bubbles. We find, however, that Cl and F concentrations in olivine-hosted melt inclusions are not affected by diffusive re-equilibration through the host olivine nor by dehydration-induced pressure changes within the melt inclusion. Our results indicate that measured H_2O, CO_2 and S concentrations and Fe^(3+)/ΣFe ratios of included melts are not necessarily representative of the melt at the time of entrapment and thus are not reliable proxies for upper mantle conditions.

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