Distinguishing eclogite from peridotite: EBSD-based calculations of seismic velocities

Seismic velocities were calculated for 11 eclogites from the Western Gneiss Region, Norway, based on electron-backscatter diffraction (EBSD). The P-wave velocities are 8.0-8.5 kms^-1 and the S-wave velocities are 4.5-4.8 kms^-1; V_P/V_S1 (the ratio of P-wave to fast S-wave velocities) is 1.74-1.81. All the eclogites are relatively isotropic, with the higher anisotropies (3- 4 per cent) in micaceous samples. Peridotite is comparatively more anisotropic (4-14 per cent more for P waves and up to 10 per cent more for S waves), and can have anomalously low V_P/V_S1, which may be useful means of distinguishing it from eclogite. Micaceous eclogite may be modelled using hexagonal anisotropy with a slow unique axis, whereas peridotite is most robustly modelled using orthorhombic anisotropy.