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We present the results of high-temperature (900°C), high-pressure (200 MPa) deformation experiments that identify the processes and deformation conditions leading to melt migration in crystal-rich mushes. This study is relevant to transport of magmas in transcrustal magma reservoirs. Experimental samples comprise juxtaposed pieces of soda-lime glass and densified mixtures of borosilicate glass and quartz sand, which, at elevated temperatures and pressures, have melt and shear viscosities similar to natural silicate melts and crystal-rich mushes. The synthetic mushes have crystal fractions of 0.60 to 0.83. Samples were deformed in torsion at shear strain rates of 10-5 to 10-4 s-1 to shear strains up to 2.7. Image analysis of experimental samples shows melt migrates into the mush during shear. In mushes with crystal fractions ≥ 0.75 shearing causes melt-filled mm-scale dikes to form and propagate into the mush. To our knowledge, these features are the first dikes formed in high-temperature, high-pressure deformation experiments. Dike formation results from shear-induced dilation, which causes the mush to become underpressurized relative to the melt, at an estimated pressure differential of 10 MPa. Experimental conditions indicate shear-induced dilation and diking occur while the mush is still viscous (i.e., Weissenberg number < 10-2). We apply our results to Soufrière Hills Volcano (Montserrat, West Indies) and use our analysis to predict the deformation conditions that would lead to diking and rapid, voluminous melt migration in that active volcanic system. 

Data accompanying manuscript of the same name. Here we present results of high-temperature, high-pressure experiments that test the conditions leading to melt migration in mushes. Our samples were made up of juxtaposed pierces of soda-lime glass and a densified mixture of borosilicate glass and quartz sand (X = 0.65 to 0.83). When these materials are subjected to high temperatures and confining pressures (900°C, 200 MPa) they are proxies natural silicate melts and mushes, respectively (Ryan et al., 2022). We deformed these samples in torsion and observed migration of melt into the mush as a result of shear. In samples with intermediate (X = 0.75) and high (X = 0.83) mush crystal fractions melt-filled dikes formed and propagated within the mush. To our knowledge these are the first instances of dike formation and propagation in high-temperature, high-pressure deformation experiments. The dikes formed as a result of shear-induced dilation, a process that was recognized in other granular media ∼150 years ago (Reynolds, 1885) but is rarely invoked as a potential deformation behavior for mushes (Petford et al., 2020). We use our experimental results to identify the conditions for shear-induced dilation and diking in mushes, apply this analysis to an active volcanic system (Soufrière Hills Volcano, Montserrat, W.I.) and, finally, consider the role of dike formation and propagation in mushes in the rapid generation and transport of crystal-poor magmas." Imaging: BSE mosaics of transverse sections of each experimental product were captured using a JEOL JXA-8530FPlus Electron Probe Microanalyzer (15 kV, 10 nA). Compositional differences between quartz, olivine, soda lime and borosilicate mean each phase is distinguishable based on its greyscale. Each sample was ground, polished and imaged four to ten times to produce serial sections. The area fraction of soda-lime glass that migrated into the mush (A) was quantified by thresholding and filtering BSE mosaics using ImageJ (Abramoff et al., 2004). Euclidean distance maps were thresholded to identify regions of soda-lime glass that have dimensions less than and greater than the estimated interparticle distance (40 μm; Supplement 2). Aintru is the area fraction of soda-lime glass in the mush with dimensions greater than the interparticle distance. The spatial distribution of soda-lime glass in the mush was quantified by overlaying rectangular grids on the BSE mosaics and measuring the area fractions greater and less than the interparticle distance (Supplement 2). 

Abstract 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°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.