On the rheology of partially molten synthetic granite

In an attempt to obviate some of the difficulties inherent in using natural rocks to study experimentally the rheology of partially-molten granitic rocks, we report some preliminary experiments using hot-pressed synthetic aggregates, with quartz as the solid phase and a quartz–albite glass to form the melt phase. Undrained constant displacement rate and stress relaxation experiments were performed at 1100 °C and 300 MPa total confining pressure using nominally 16 and 28 wt% anhydrous melt, and at 980 °C with 12 wt% of water-saturated melt. Strain rate varies non-linearly with stress, corresponding to a stress exponent of 3.6 for the anhydrous melt tests, and 1.9 with the wet melt. Strain rate is enhanced about 160-fold through doubling the melt fraction from 16 wt%. Intracrystalline plasticity appears to contribute insignificantly to the observed deformation, and the preservation of relatively equant grains implies a granular flow process. Microcracking occurs during isostatic hot-pressing, and also contributes to subsequent non-isostatic deformation at high stresses and strain rates. Models of flow of partially molten rock that assume diffusive transfer accommodation of granular flow predict linear viscosity but only at lower strain rates than accessed experimentally. It is suggested that the non-linear granular flow observed in these experiments involved the episodic unsticking of sintered contacts between grains by a rate-dependent cracking and/or dislocation accommodation process.