Distribution of dissolved water in magmatic glass records growth and resorption of bubbles

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.