Melt migration in crystal mushes by viscous fingering: Insights from high‐temperature, high‐pressure experiments

We conducted experiments to study melt migration in crystal-rich mushes, with application to magma ascent within transcrustal magma reservoirs. Mushes with crystal volume fractions of 0.59-0.83 were prepared by hot-pressing crushed borosilicate glass mixed with different proportions of quartz sand particles. Each experimental sample comprises stacked disks of mush and soda-lime glass, a proxy for crystal-free magma. Samples were subjected to confining pressures of 100-300 MPa and a temperature of 900 degrees C (above the glass transition temperatures of the borosilicate and soda-lime glasses) for up to 6 h. The bottom and circumference of the mush and soda lime disks experience the confining pressure, but the top of the mush disks is at room pressure, resulting in a pore-pressure gradient across the mush layer. Following cooling and decompression, we determined the area fraction and morphology of soda-lime melt that migrated into the mush layer during experiments. Melt fraction is more strongly correlated to crystal fraction than pore-pressure gradient, increasing with crystal fraction before sharply decreasing as crystal fractions exceed 0.8. This change at 0.8 coincides with the transition from crystals in the mush moving during soda-lime migration to crystals forming a continuous rigid network. In our experiments, melt migration occurred by viscous fingering, but near the mobile-to-rigid transition, melt migration is enhanced by additional capillary action. Our results indicate that magma migration may peak when rigid mushes "unlock" to become mobile. This transition may mark an increase in magma migration, a potential precursor to volcanic unrest and eruption.