Shear-induced dilation and dike formation during mush deformation

Abstract

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.