Zonation of H₂O and F concentrations around melt inclusions in olivines

Studies of both naturally quenched and experimentally reheated melt inclusions have established that they can lose or gain H₂O after entrapment in their host mineral, before or during eruption. Here we report nanoSIMS analyses of H₂O, Cl and F in olivine around melt inclusions from two natural basaltic samples: one from the Sommata cinder cone on Vulcano Island in the Aeolian arc and the other from the Jorullo cinder cone in the Trans-Mexican Volcanic Belt. Our results constrain olivine/basaltic melt partition coefficients and allow assessment of mechanisms of volatile loss from melt inclusions in natural samples. Cl contents in olivine from both samples are mostly below detection limits (≤0̇03±0̇01ppm), with no detectable variation close to the melt inclusions. Assuming a maximum Cl content of 0̇03 ppm for all olivines, maximum estimates for Cl partition coefficients between olivine and glass are 0̇00002±0̇00002. Olivines from the two localities display contrasting H₂O and F compositions: Sommata olivines contain 27±11ppm H₂O and 0̇28± 0̇07 ppm F, whereasJorullo olivines have lower and proportionately more variable H₂O and F (11±12 ppm and 0̇12±0̇09 ppm, respectively; uncertainties are two standard deviations for the entire population). The variations of H₂Oand Fcontents in the olivines exhibit clear zonation patterns, increasing with proximity to melt inclusions.This pattern was most probably generated during transfer of volatiles out of the inclusions through the host olivine. H₂O concentration gradients surrounding melt inclusions are roughly concentric, but significantly elongated parallel to the crystallographic a-axis of olivine. Because of this preferential crystallographic orientation, this pattern is consistent with H₂O loss that is rate-limited by the 'proton-polaron' mechanism of H diffusion in olivine. Partition coefficients based on olivine compositions immediately adjacent to melt inclusions are 0̇0007±0̇0003 for H₂O and 0̇0005±0̇0003 for F.The H₂O and F diffusion profiles most probably formed in response to a decrease in the respective fugacities in the external melt, owing to either degassing or mixing with volatile-poor melt.Volatile transport out of inclusions might also have been driven in part by increases in the fugacity within the inclusion owing to post-entrapment crystallization. In the case of F, because of the lack of data on F diffusion in olivine, any interpretation of the measured F gradients is speculative. In the case of H₂O, we model the concentration gradients using a numerical model of three-dimensional anisotropic diffusion of H, where initial conditions include both H₂O decrease in the external melt and post-entrapment enrichment of H₂O in the inclusions.The model confirms that external degassing is the dominant driving force, showing that the orientation of the anisotropy in H diffusion is consistent with the proton-polaron diffusion mechanism in olivine. The model also yields an estimate of the initial H₂O content of the Sommata melt inclusions before diffusive loss of 6 wt % H₂O.The findings provide new insights on rapid H₂O loss during magma ascent and improve our ability to assess the fidelity of the H₂O record from melt inclusions.