Experimental deformation of partially-melted granite
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An account is given of the experimental deformation of partially-melted granite with melt fractions up to 25% at 800 °C and 300 MPa confining pressure in constant strainrate tests between 10−3 and 10−6 S−1, creep tests and cycling tests. Microscopic study reveals that under these conditions most of the uniform deformation prior to macroscopic shear failure is accomplished by melt redistribution into films perpendicular to the least compressive stress, and by axial fracturing of grains, the latter occurring even at low macroscopic differential stress. The strenght of the partially-melted rock at 10−5 S−1 is found to decrease gradually from about 250 MPa at 5 vol.% melt to about 60 MPa at 15 % melt, and then to drop rapidly to less than 1 MPa at 24% melt. The critical melt fraction separating granular-framework-controlled flow behaviour from suspension-like behaviour is deduced to be approximately 30 to 35 vol.%. At low melt fractions the deforming rock tends to take up externally available melt by a mechanism of dilatancy pumping. The relevance of these results to natural conditions involving partially-melted rocks is discussed.
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