Application of High T/High P Rock Mechanics to the Problem of Extraction of Granitic Magma from its Protolith

The mechanical deformation behaviour of a partially molten granitic protolith is investigated, implications towards a mechanism for extraction of low melt percentages («30 %) are mathematically explored. For this purpose a series of experiments on Westerly granite (with no added water) are presented in which samples were heated to temperatures between 800 and 1200 °C, at a confining pressure of 250 MPa. Samples were uniaxially deformed to study the effects of constant strain-rate (4 * 10-4 to 2 * 10-7 s-1), creep and stress relaxation tests on the mechanical behaviour. The volumetric percentage of melt (Φ) in samples ranged from 3 % at 800 °C to 77 % a t 1200 °C at heating times between 2.5 and 170 hours. However, none of the samples attained chemical equilibrium, due to the short duration of tests. An equilibrium melt percentage was also not reached. Over this temperature interval the supported strength decreased from 500 MPa to less than 1 MPa (at a constant strain rate of 8 * 10-5 s-1) monotonically. No step-like drop in strength, corresponding to a rheological critical melt percentage (van der Molen & Paterson 1979) was observed. A preliminary flow law for the partially molten rock was obtained to allow extrapolation to lower strain-rates. The viscosity of the melt was estimated at 950 and 1000 °C from distances it could be made to permeate into porous sand under a known pressure gradient. Melt distribution in static tests indicated a very low wetting angle (< 5°) which results in high melt contiguity even at low melt percentages along grain boundaries. Under all conditions, the solid phases deformed by brittle fracture only. Samples with 3 shear enhanced compaction of melt filled voids by analogy with uniaxial compaction of porous sands. In samples with Φ > 45 % grains were carried about passively in the flowing liquid. A simple mathematical model is erected to describe a two stage process of extracting low volume percentage, granitic melt from its partially molten protolith with the aid of non-hydrostatic stress. Shear-enhanced compaction drives melt from the protolith into a series of interconnected melt-filled veins, whereupon porous flow through the high-permeability vein network allows rapid drainage of melt to higher crustal levels. Modeling suggests that melt extraction, with Φ11 Pas, is possible within geologically realistic time periods (10 Ma)