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Mechanical controls on fluid flow in healed magmas: Insights from natural and experimental investigation

Honor James, Jackie E. Kendrick, Anthony Lamur, Yan Lavallée

Abstract

The evolution of permeable pathways in magmas is poorly constrained and, as such, our understanding of how pressure may build in the conduit, limiting our ability to accurately forecast what might happen next. Here we focus on understanding how magma healing between two immiscible, yet co-erupted magmas may influence eruptive dynamics. We study the Mono-Inyo domes, in Long Valley caldera, USA that show healed contacts between two physically and mechanically different magmas (fully crystalline dacite vs. aphyric obsidian). These 2 endmembers show a substantial difference in porosity but with high permeabilities that are controlled by the presence of fractures at multiple scales. Healed contacts display a wider response range to increasing confinement than the endmember samples, suggesting a greater variability in the geometry of permeable pathways. Microstructural analysis further reveals undulating contact interfaces crosscut by micro-fractures, suggesting a complex mechanical and cooling history. We then conduct healing experiments in a biaxial press, where we 1) Axially force two glass rods against each other at temperatures above Tg; 2) control simple shear rates and distances to investigate healing kinetics and the geometry of the resulting contact. We show that the healing kinetics remain controlled by a wetting phase followed by a diffusion dominated phase but that the healing efficiency may be influenced by the shear rate. We finally show that the geometry of the healed contact is strongly influenced by both rate and distance. Our results highlight how shear plays a crucial role in closing permeable pathways in volcanic conduits.