Experimental deformation of partially molten Westerly granite under fluid‐absent conditions, with implications for the extraction of granitic magmas

Abstract

The mechanical behavior of partially molten Westerly granite was investigated in the temperature range 800°–1100°C, 250 MPa confining pressure, by means of constant strain rate, creep, and stress relaxation tests. The only water in the samples came from the breakdown of hydrous phases, biotite, minor chlorite and muscovite and alteration products of feldspars. Thus the amount of melt was controlled by the test temperature and ranged from 3% at 800°C to 50% at 1100°C. Over that temperature range, strength decreased from ≈500 MPa to less than 1 MPa, and a preliminary constitutive flow law for the partially molten rock was obtained to allow extrapolation to low strain rates. The comparative viscosity of the melt alone was estimated at 950° and 1000°C from the distance it could be made to penetrate into a porous sand under a known pressure gradient. Under all conditions, deformation of the matrix of solid grains was by brittle fracture only. Samples containing up to 10 vol % melt failed with the formation of a shear fault zone. At higher melt fractions, melt‐filled “pores” collapsed by shear‐enhanced compaction, squeezing the melt into axial cracks. Above 40 vol % melt, unfractured solid grains were carried about passively in the flowing liquid. There was no evidence of a “rheologically critical melt percentage” in this system. By analogy with the uniaxial compaction of water‐saturated soils, a simple model is erected to describe a two‐stage process for the extraction of granitic melts from their protoliths with the aid of nonhydrostatic stress. Shear‐enhanced compaction is inferred to drive melt into a network of melt‐filled veins, whereupon porous flow through the high‐permeability vein network allows rapid drainage of melt to higher crustal levels.