Volcanic environments often represent structurally active settings where strain localisation can promote faulting, frictional deformation, and subsequent melting along fault planes. Such frictional melting is thermodynamically a disequilibrium process initiated by selective melting of individual mineral phases and softening of volcanic glass at its glass transition as a response to rapid frictional heating. The formation of a thin melt layer on a fault plane surface can drastically accelerate or terminate slip during fault motion. A comprehensive understanding of the physical and chemical properties of the frictional melt is required for a full assessment of slip mechanisms, as frictional rheology depends on the contributions from selectively melted mineral and glass phases as well as the physical effects of restite fragments suspended in the frictional melt. Here, we experimentally investigate the impact of host-rock mineralogy on the compositional and textural evolution of a frictional melt during slip. High-velocity rotary shear (HVR)experiments were performed under controlled, volcanically relevant, coseismic conditions (1 m s−1 slip rate and 1 MPa normal stress)using three intermediate dome lavas with contrasting mineral assemblages, sampled from volcanic systems where fault friction is evident: (1)an amphibole-bearing andesite (Soufrière Hills Volcano, Montserrat); (2)an amphibole-poor dacite (Santiaguito dome complex, Guatemala); and (3)an amphibole-free andesite (Volcán de Colima, Mexico). For each sample, five HVR experiments were terminated at different stages of frictional melt evolution, namely: (1)at the onset of melting, (2)upon formation of a steady-state melt layer, and (3)after 5 m, (4)10 m, and (5)15 m of slip at steady-state conditions. Progressive mixing and homogenisation of selective, single-phase melts within the frictional melt layer through double-diffusion convection demonstrates the control of melt composition on slip behaviour. Amphiboles melted preferentially, leading to consistently lower shear stress (∼1 MPa less than amphibole-poor samples)and pronounced shear-weakening during the frictional melting of amphibole-bearing lavas. The results highlight the implications of mineral assemblages on fault slip, including conduit flow processes, which may influence the style of eruptions, and run-out distances of rapid granular flows.
- Chemical homogenisation
- Frictional melt