Applications and examination of techniques used to determine magma storage and ascent timescales in arc volcanoes

Abstract

Magmatic volatile elements and compounds in magmas (i.e., H2O, CO2, S,F, and Cl) are linked to the processes that control magma storage and decompression during an eruption. Because of their pressure dependent solubilities, measurements of volatile concentrations in melt can be used to infer storage conditions of the magma. In addition, the fractionation and exsolution of these volatiles during magma decompression have a direct influence on the transportation of magma and explosivity of an eruption. Using crystal-scale analyses and thermodynamic models, I explore how pre-eruptive volatile contents affect the evolution of crystal cargo and how volatile exsolution drives eruption dynamics as recorded in the eruptive history. In the first chapter, I explore thermodynamic drivers of disequilibrium dissolution and growth of plagioclase microlites during ascent by modeling the chemical affinity (the degree of disequilibrium) between plagioclase at set compositions and an evolving silicate melt due to ascent driven exsolution of H2O. The lack of microlites in volcanic samples is often suggested to form due to kinetic limitations of microlite growth during fast magmatic ascent; however, many magmatic systems are disrupted thermally prior to eruption. Such systems may also lack extensive microlite crystallization. Through this work, I show that reheating and ascent significantly reduces any kinetic limitation on microlite formation and that microlite-free eruptions do not require the fast ascent rates imposed by kinetics alone. Next, I leverage olivine hosted melt inclusions (MI) from the 2018 eruption ii at Mount Veniaminof, Alaska, to unravel the storage history preceding the eruption. Volatile measurements correspond to minimum entrapment pressures of 400 MPa and entrapment depths <15 km. Our estimates of magma storage depths are supported by relatively deep precursory long period earthquakes (16- 20 km). In the third chapter, I examine a chronometer for magma ascent based on measurements of volatile gradients in melt embayments (ME), open pockets of melt that remain in contact with the host magma. Past uses of this methodology have been restricted to small (<10) sample sizes because of the rigid requirements for diffusion modeling (i.e., simple, 1-D geometry). Not accounting for more complicated geometry can lead to under or overestimates of magma ascent rates. I perform an in-depth analysis of textures of melt embayments from the 2018 eruption via 3-D volumes derived from micro- computed tomography data to expand the usage of melt embayments to non- conventional samples.