Abstract
Volcanic eruptions are driven by the growth of gas bubbles in magma. Bubbles grow when dissolved volatile species, principally water, diffuse through the silicate melt and exsolve at the bubble wall. On rapid cooling, the melt quenches to glass, preserving the spatial distribution of water concentration around the bubbles (now vesicles), offering a window into pre-eruptive conditions. We measure the water distribution around vesicles in experimentally-vesiculated samples, with high spatial resolution. We find that, contrary to expectation, water concentration increases towards vesicles, indicating that water is resorbed from bubbles during cooling; textural evidence suggests that resorption occurs largely before the melt solidifies. Speciation data indicate that the molecular water distribution records resorption, whilst the hydroxyl distribution records earlier decompressive growth. Our results challenge the emerging paradigm that resorption indicates fluctuating pressure conditions, and lay the foundations for a new tool for reconstructing the eruptive history of natural volcanic products.
Highlights
Bubbles nucleate when magmatic volatiles exsolve from a supersaturated melt
We find that all vesicles, in all samples, have higher far is minimum H2Ot in far-field (H2Ot) concentrations adjacent to the vesicle walls than in the far-field (Table 2)
The secondary ion mass spectrometry (SIMS)-calibrated H2Ot concentration gradients are steepest at the vesicle wall and decay to a far-field value over a few tens of microns from the vesicle wall (Fig. 5), corresponding to the edge of the observed halos in BSEM images
Summary
Bubbles nucleate when magmatic volatiles (species such as water, CO2 and SO2, that are only weakly soluble in the silicate melt) exsolve from a supersaturated melt. Water is the most important volatile because it is usually the most abundant and because it strongly affects melt viscosity (Hess and Dingwell, 1996). It is dissolved in the melt as two principal species: molecular water, H2Om, and hydroxyl groups, OH. Of volatiles from the melt (Sparks, 1978) Together these processes control the bubble growth rate which, in turn, controls or influences almost every aspect of magma ascent and eruption, including: magma vesicularity, buoyancy, rheology and permeability; the pressure gradient that drives the eruption; and the onset of magma fragmentation. Understanding and quantifying bubble growth is, one of the most fundamental challenges in physical volcanology
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