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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 performed a series of extrusion experiments on partially molten samples of forsterite plus 10 vol% of an anorthite‐rich melt to investigate melt segregation in a pipe‐extrusion geometry and test the predictions of two‐phase flow theory with viscous anisotropy. The employed flow geometry has not been experimentally investigated for partially molten rocks; however, numerical solutions for a similar, pipe‐Poiseuille geometry are available. Samples were extruded from a 6‐mm diameter reservoir into a 2‐mm diameter channel under a fixed normal stress at 1350°C and 0.1 MPa. The melt distribution in the channel was subsequently mapped with optical and backscattered electron microscopy and analyzed via quantitative image analysis. Melt segregated from the center toward the outer radius of the channel. The melt fraction at the wall increased with increasing extrusion duration and with increasing shear stress. The melt fraction profiles are parabolic with the melt fraction at the wall reaching 0.17–0.66, values 2 to 16 times higher than at the channel center. Segregation of melt toward the wall of the channel is consistent with base‐state melt segregation as predicted by two‐phase flow theory with viscous anisotropy. However, melt‐rich sheets inclined at a low angle to the wall, which are anticipated from two‐phase flow theory, were not observed, indicating that the compaction length is larger than the channel diameter. The results of our experiments are a test of two‐phase flow theory that includes viscous anisotropy, an essential theoretical frame work needed for modeling large‐scale melt migration and segregation in the upper mantle.