Constraining the H₂O/CO₂ molar ratio, the volume fraction of exsolved volatiles, and the magma compressibility of the 2006 Augustine eruption, Alaska

Abstract

Geodetic modeling of volcano deformation can be used to estimate the volume of magma presumed to be mobilized within a volcanic system. These geodetically modeled subsurface reservoir volume changes are commonly much smaller than simultaneous eruptive volumes, where the eruptive volume is estimated based on geological mapping of units, their thicknesses, and their densities. This discrepancy is thought to be at least partially due to magma compressibility, which describes the phenomena where the volume of a given mass of magma changes as pressure increases/decreases primarily due to the presence of highly compressible exsolved volatiles. In this study, I combine deformation, volcanic gas, and petrologic constraints acquired prior to and during the 2006 eruption of Augustine volcano, Alaska, to estimate the amount of exsolved volatiles present in the magma storage region prior to the eruption and calculate the resulting compressibility of the magma. By doing so, I am able to constrain the H₂O/CO₂ molar ratio of the syn-eruptive gas emissions to between 24 and 59, with my best estimate of 28. My results suggest that for the specific parameters of Augustine's magmatic system, including a pressure of 120-170 MPa, a temperature of 880 ± 13 °C, and 40 ± 2% phenocrysts by volume, an exsolved volatile phase of about 8.2 vol% and a magma compressibility of ~7.1 x 10⁻¹⁰ 1/Pa are required to explain the observed eruptive volume to deformation volume ratio equal to three. The exsolved volatile volume and magma compressibility values determined here agree with results of previous studies of volatile-rich volcanic systems. This study reiterates that magma compressibility is an important factor that must be considered when interpreting deformation data within volatile-saturated volcanic systems.