Melt Segregation On Explosive Timescales in Subaqueous Eruptions

Melt segregation is the extraction of residual melt from interstices in 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 examine sub-millimetre melt segregation structures in subaqueously erupted basaltic pyroclasts. These "intra-vesicular extrusions" quenched and solidified as hollow balloons of glass penetrating pre-formed vesicles. Physical characterization by x-ray computed microtomography and H2O and major element mapping by fourier transform infrared spectroscopy and electron microprobe demonstrate that microlite crystallization produced local gradients in volatile supersaturation, which drove melt extrusion into adjacent vesicles by gas filter pressing. A crucial observation here is that the entire segregation process occurred after the residual melt had become enriched in fast-diffusing H2O, but before it had become measurably enriched or depleted in slower-diffusing major elements. We use diffusion modeling to 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 timescales that characterize explosive fragmentation of basaltic melt.