| Abstrakt: |
Magma‐water interaction can dramatically influence the explosivity of volcanic eruptions. However, syn‐ and post‐eruptive diffusion of external (non‐magmatic) water into volcanic glass remains poorly constrained and may bias interpretation of water in juvenile products. Hydrogen isotopes in ash from the 2009 eruption of Redoubt Volcano, Alaska, record syn‐eruptive hydration by vaporized glacial meltwater. Both ash aggregation and hydration occurred in the wettest regions of the plume, which resulted in the removal and deposition of the most hydrated ash in proximal areas <50 km from the vent. Diffusion models show that the high temperatures of pyroclast‐water interactions (>400°C) are more important than the cooling rate in facilitating hydration. These observations suggest that syn‐eruptive glass hydration occurred where meltwater was entrained at high temperature, in the plume margins near the vent. Ash in the drier plume interior remained insulated from entrained meltwater until it cooled sufficiently to avoid significant hydration. Plain Language Summary: Explosive volcanic eruptions produce plumes of volcanic ash and gas that commonly mix with water from overlying seawater, glaciers, or hydrothermal systems. Within these plumes, volcanic glass (rapidly cooled magma) can lose its dissolved magmatic water or gain additional water from the surrounding environment. This study uses water concentrations in volcanic glass, and the hydrogen isotopes of that water, to identify if water was lost or gained in ash during the 2009 eruption of Redoubt Volcano, Alaska, USA. Results show that most of the magmatic water was lost, and some external water was gained in samples that fell closest to the volcano. Numerical models show that external water is most easily gained in glass at high temperatures even at the fastest cooling rates. These findings suggest external water was incorporated into the margins of the eruption plumes during the eruption. Ash hydration and aggregation occurred in these wet plume margins near the vent and preferentially deposited it closer to the vent. Ash in the hotter plume core that encounters water at cooler temperatures is erupted to higher altitudes and disperses the drier ash to further distances. Key Points: Water contents and hydrogen isotopes in volcanic ash record syn‐eruptive hydration during the wet 2009 eruption of Redoubt Volcano, AlaskaThe temperature of pyroclast‐water interaction, more than the pyroclast cooling rate, dictates the extent of syn‐eruptive glass hydrationMore extensive hydration of proximal ashfall suggests wetter plume margins and drier, higher transport of the plume interiors [ABSTRACT FROM AUTHOR] |
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