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 end-member 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 ‰), 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.