USING THERMODYNAMIC MODELING TO UNDERSTAND DISEQUILIBRIUM TEXTURES PRESERVED IN SILICIC ERUPTIONS

Degassing of a volatile-rich magma can drive plagioclase microlite growth on eruptive timescales, but if magmatic ascent is sufficiently fast, this growth can be inhibited. Previous models have attributed the absence of microlites to indicate rapid ascent of magmas. However, open-system processes, such as magma reheating in response to recharge prior to decompression can also suppress microlite growth, possibly extending the timescales for ascent. Here, we explore the balance between degassing-driven growth and reheating-driven growth on the suppression of microlite formation using a quantitative model implemented using MELTS and based on the chemical affinity between plagioclase and a silicate melt. The chemical affinity represents the degree of 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. We present a range of alternative ascent rates, 0.002-0.006 MPa/s (~0.08-0.25 m/s), from a storage depth of 200 MPa that can suppress microlites growth when compensated by reheating of 50-70 ˚C. We also present a preliminary model that distinguishes between nucleation versus bulk crystallization.