Deformation of Dunite at Slow Strain Rates under High Temperature and Pressure

Abstract

The investigation of rock deformation under high pressure and temperature has been made by means of the uniaxial test apparatus at various strain rates ranging from about 10-2sec-1 to 10-6sec-1. The ximum value of confining and axial pressure was 8kb and 20kb respectively. The rocks (Dunite) for the present experiments were collected from Horoman. Hokkaido.The stress-strain curves of Horoman dunite were obtained at temperatures of 900°C, 1000°C and 1100°C under a confining pressure of 8kb and strain rates from 1.2×10-3sec-1 to 6.0×10-7sec-1. Under these conditions the deformation pattern of the dunite showed ductile flow. The microscopic observation showed that the deformation mechanism of dunite was mainly dislocation due to sliding. At the temperatures from 900°C to 1100°C, the kink bands with gliding direction [100] were observed in olivine.The relation between the strain rate and the differential stress was studied by applying Eyring's equation to the present experimental rangesε=Dexp(-E/kT)sinh(ατ/kT) (1)where α is the shear activation volume, ε the strain rate, τ the differential stress, E the activation energy, D the constant, k the Boltzman constant, and T the absolute temperature. The full line in Fig. 10 shows equation (1). From the linear relation as shown in Fig. 10 we determined the values of E and α:α=90cm3/mol, E=77kcal/mol and 99kcal/mol at the temperatures of 900°C and 1100°C, respectively. Using these values and the geologic strain rate 3×10-14sec-1, we estimated the values of viscosity coefficient η and sustaining stress τ for the physical conditions of the upper mantle. η thus obtained is in the order of 1014∼1017 poises and τ becomes less than 1b. This suggests the possibility of the mantle convection which may have been the cause of the continental drift.