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

Sebastian Walter,Jonathan Castro

doi:10.30909/vol.03.01.155168

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.

Highlights

1.1 Natural context: rhyolitic volcanismMuch volcanological research over the last decades has been focused on understanding the mechanisms driving silica-rich rhyolite eruptions [e.g. Alfano et al 2011; Castro and Dingwell 2009; Dingwell 1996; Melson and Saenz 1973; Ninkovich et al 1978; Ogburn et al 2015; Wilson 1976]
The program VolcDeGas was written in MatLab and uses the formulae described in the previous sections to calculate entire degassing histories
The program is ideally suited for rhyolitic eruptions, whose glassy eruptive products have been routinely measured to yield δD versus bulk H2O content relations, providing a natural degassing trend to simulate with VolcDeGas

Summary

Introduction

1.1 Natural context: rhyolitic volcanismMuch volcanological research over the last decades has been focused on understanding the mechanisms driving silica-rich rhyolite eruptions [e.g. Alfano et al 2011; Castro and Dingwell 2009; Dingwell 1996; Melson and Saenz 1973; Ninkovich et al 1978; Ogburn et al 2015; Wilson 1976]. Sized rhyolite eruptions (

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