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

Abstract

Magma degassing mechanisms are key determinants of explosive and effusive eruption styles. Paired measurements of H2O content and hydrogen isotopic ratios (e.g. δD) in pyroclastic and effusive products can elucidate endmember degassing mechanisms (e.g. closed and open system) during eruption. Here we present "VolcDeGas", a MatLab program that models δD-H2O degassing trajectories of rhyolitic magma. Operating within an intuitive GUI, VolcDeGas calculates degassing paths based on: Initial δD (in%0), 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 intomulti-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. a closed- to batched-system) provide the best statistical fit to pyroclastic and effusive obsidian geochemical datasets.