Incipient melt segregation as preserved in subaqueous pyroclasts
Résumé
Melt segregation is the extraction of residual melt from the rigid but permeable network of growing crystals with which it co-evolved. It is recognized as an effective mechanism of igneous differentiation that acts over many geologic time and length scales. Here we present evidence for rapid melt segregation in subaqueous basaltic pyroclasts. Segregation produced intravesicular extrusions, i.e., partly hollow balloons of glass penetrating the walls of earlier formed vesicles set in a microcrystalline groundmass. The segregation process can be described as a form of gas filter pressing, where microlite crystallization produced local gradients in volatile supersaturation and vapor pressure that drove melt extrusion into adjacent vesicles. Unlike in previously described segregation features, the structures presented here are shown to have formed after the extruded melt became enriched in fast-diffusing H2O, but before it became measurably enriched or depleted in slower diffusing major elements. We show that melt segregation of this embryonic type must occur in seconds, rather than in the days to centuries required for similar processes to occur in lava flows or magma chambers, and that it occurs within the short time scales that characterize explosive fragmentation of basaltic melt.