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Porous-permeable evolution of sintering polydisperse, fragmental, hydrous magma

^^ Julia Schunke1^^ , Janine Birnbaum1, Jackie E. Kendrick1, Anthony Lamur1, Fabian B. Wadsworth1, Jonathan Castro2, Yan Lavallée1

Abstract

Understanding the evolution of porosity and permeability is essential to constrain the development of magma buoyancy and so, its eruptibility. At silicic volcanoes, explosive processes may produce deposits of hydrous magma fragments whose porous-permeable networks continue to evolve after deposition; that is, fragments may vesiculate (causing volumetric expansion and dehydration), diffusively outgas (causing contraction and further dehydration) and sinter, (causing sealing of the system). We see that these fragmental systems are often polydisperse, comprising of fragments with a wide range of grain sizes. During our laboratory experiments, we induce temporal changes in porosity and permeability due to sintering of polydisperse fragments which simultaneously vesiculate and diffusively outgas. When a higher fraction of coarse grains are present, greater intra-clast vesicularity is achieved causing a stronger reduction in connected porosity and permeability. Smaller fragments densify faster due to diffusive outgassing, leaving no proof of vesiculation. More polydisperse systems have better packing and the larger contact area accelerates sintering. In our ongoing work, we compare the results to polydisperse sintering at different stages in the field (Mono-Inyo Craters, California). Several outcrops with sintered, highly vesiculated clasts on fracture surfaces are analysed in terms of their grain sizes, water content, inter-clast interactions and the rind thickness. We exploit the variation in clast size and the variable degrees of completion of sintering and diffusive outgassing to unravel the thermal history of the deposits.