Seismic anisotropy in an ophiolite peridotite: Application to oceanic upper mantle

Abstract

Compressional velocity V p to 8 kbar and 275°C in a homogeneous, 0.5% porosity, little‐altered harzburgite from the Antalya ophiolite complex in Turkey was measured along the three principal kinematic axes as defined from the preferred orientations of olivine and enstatite. The observed V p anisotropy was 0.7 km/s for pressures P from 2 to 8 kbar and temperatures T from 25° to 275°C. The seismic anisotropy at 250°C and 5 kbar (15‐km depth) calculated from the single‐crystal elastic constants and the fabric data were within 3% of the observed velocities in the core directions. Measured values of the P and T derivatives of Vp for the three cores are 14 m/s kbar and −0.6 m/s °C, which are in reasonable agreement with calculated average values from single‐crystal data. These data result in a critical thermal gradient of 7.5°C/km. A model of oceanic crust and uppermost mantle (constructed from the structural data in this ophiolite complex) permits an estimation of the azimuthal seismic anisotropy in the uppermost mantle beneath this paleocrust under assumed conditions of 250°C and 15‐km depth. The result is Δ V p = 0.3 km/s with V p = 8.46 and 8.16 km/s normal and parallel to the ridge, respectively. The azimuthal shear velocity anisotropy is 0.18 km/s. Measurements of V p are also presented for three lherzolite cores from Salt Lake Crater, Hawaii. The T derivatives of V p for these cores agree with those obtained for the harzburgite; however, the P derivatives are consistently higher. The higher values of the P derivatives for the lherzolite sample are considered to result from its higher porosity.