Origin and orientation of microporosity in eclogites of different microstructure studied by ultrasound and microfabric analysis

Abstract

This work contributes to the experimental investigations of the origin and 3-D orientation of micropores in low porosity crystalline rocks. The origin and spatial orientation of microporosity in two eclogites with different microstructures were studied by 1) quantitative and qualitative microstructural analysis of grains and grain boundaries, 2) measurement of lattice preferred orientation using the SEM-EBSD method and 3) experimental measurement of velocity of elastic P-waves in spherical samples in 132 directions under confining pressures up to 400 MPa. Results show good correlation between the elastic properties and the orientation of grain boundaries and cleavage planes in clinopyroxene. The magnitude and anisotropy of velocity change with pressure shows that microporosity in the fine-grained sample is relatively large and strongly preferentially oriented, whereas it is significantly lower and less preferentially oriented in the coarse-grained sample. Seeing that the lattice preferred orientation of clinopyroxene is similar in both samples we can deduce from velocity changes that the grain size of the rock forming minerals controls the amount of microporosity. Also, the orientation of microporosity depends mostly on preferred orientation of grain boundaries and somewhat less on the orientation of cleavage planes. Grain boundaries are therefore the most important contributors to the bulk microporosity in the studied rocks.