ИСТИНА

Dissolved in magma, H2O plays a significant role in generation, evolution, and eruption of arc magmas. Mineral-hosted glassy melt inclusions (MIs) have been used to quantify magma volatile contents for several decades. Recent studies have demonstrated that hydrogen diffusion within a host mineral is rapid, causing MIs to be partially open to H2O loss. Thus, the amount of H2O that MIs contain likely represents a minimum from what was originally dissolved in a particular magma. Nonetheless, when attempting to estimate water dissolved in primitive magmas at arcs, it is still generally interpreted that the water contents of MIs in primitive magmas represent the amount of H2O in primitive arc melts. Here we show there is a physical limit on how much dissolved volatiles a glassy MI can retain. We explored the limits of MIs as hydrous magma recorders in an experimental study on the quenchability of hydrous mafic melts. We show via hydrothermal experiments (supra-liquidus, 1 GPa, piston-cylinder) that the ability to quench a mafic hydrous melt (H2O content ranges from 1 to 21 wt%) to a homogeneous glass at cooling rates relevant to natural samples (20 to 90 K/sec at the glass transition temperature) has a limit of no more than 9 ± 1 wt% of dissolved H2O in the melt. Experiments with water contents greater than 9 wt% have no glass after quench and result in fine-grained mixtures of alteration/devitrification products. Thus, natural MIs with H2O contents above 9 wt% most likely never quench to a glass and form devitrified inclusions or exsolve water that may over-pressurize the inclusion and break the host crystal (decrepitation). Our limit of 9 ± 1 wt% agrees well with the maximum of dissolved H2O contents found in natural mafic glassy MIs (8.5 wt% H2O). The super-hydrous experimental basaltic liquids quench to hydrous phases that include chlorite and smectites. Presence of these minerals in crystallized MIs would be potential indicators of a super-hydrous progenitor magma. This new study raises the question of whether the observed H2O-record of glassy mafic MIs has an upper limit that is partially controlled by the quenching process. Thus, our results elucidate the importance of looking for melt inclusions that contain these low temperature hydrous phases.