Using Chemical Affinities to Understand Disequilibrium Textures of Plagioclase Preserved in Magmatic Systems

Abstract

AbstractVolcanic textures are strongly related to magma ascent histories. Degassing of a volatile‐rich magma can drive plagioclase microlite growth on eruptive timescales, but such growth can be inhibited when ascent is sufficiently fast. Thus, the absence of microlites is used to determine the minimum ascent rates. However, open‐system processes, such as magma recharge, prior to decompression can also suppress microlite growth, possibly extending the timescales for ascent. Here, we explore the trade‐offs between degassing‐driven growth and reheating‐driven suppression of microlite formation. We develop a quantitative model on the basis of the chemical affinity that accounts for disequilibrium driving crystal growth or dissolution. We test decompression scenarios to understand how net‐crystallization varies with different pressure‐temperature ascent histories for the 2008 Chaitén eruption. Degassing‐driven growth and reheating‐driven dissolution are balanced for ascent rates of 0.002–0.006 MPa/s (∼0.08–0.25 m/s) from 200 MPa storage depth and reheating by 50 °C–70 °C, significantly reducing the minimum ascent rates.