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RESEARCH PRODUCT

VolcDeGas: A program for modelling hydrogen isotope fractionation during degassing of rhyolitic melts

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description

Magma degassing mechanisms are key determinants of explosive and effusive eruption styles. Paired measurements of H2O content and hydrogen isotopic ratios (e.g., dD) in pyroclastic and effusive products can elucidate end-member degassing mechanisms (e.g. closed and open system) during eruption.  Here we present VolcDeGas, a MatLab program that models dD-H2O degassing trajectories of rhyolitic magma.  Operating within an intuitive GUI, VolcDeGas calculates degassing paths based on: initial dD (in ‰), the H2O content of the melt (wt.%), degassing step size, and temperature.  VolcDeGas also calculates hydrous speciation based on either empirical models or analytical data, and incorporates this effect in simulations.  VolcDeGas solves open-, closed-, and batched-system fractionation equations, and also combines these into multi-stage (e.g. closed-to-open) degassing paths. Tests show that degassing is highly sensitive to step size, and multi-stage scenarios involving progressively declining steps (e.g. batched-system) provide the best statistical fit to pyroclastic and effusive obsidian geochemical datasets.