Syntectonic Recrystallization of Olivine and Modes of Flow in the Upper Mantle

Abstract

Olivine in dunite, peridotite, and olivine powder has been recrystallized syntectonically during compression tests at temperatures from 950°C to 1350°C, confining pressures from 5 to 30 kb, and strain rates from 10 −3 to 10 −8 /sec. For intragranular recrystallization of the dunite, the orientations of the new grains are controlled by the host orientation. For inter-granular recrystallization and total recrystallization of the dunite and of olivine powder, the orientations of the grains are controlled dominantly by the stress. The fabric produced over the entire range of physical conditions is a [010] maximum parallel to σ 1 and [001] and [100] girdles in the σ 2 = σ 3 plane. With few exceptions, and allowances for the experimental requirement that σ 2 = σ 3 , fabrics in olivine tectonites are similar to those produced experimentally. Such probable upper mantle source materials as ultramafic nodules in diamond-bearing kimberlites and in under-saturated basalts and “Alpine-type” peridotites show these same preferred orientations. We propose, therefore, that Syntectonic recrystallization is an important or dominant mode of flow in the upper mantle at temperatures above about 500°C. The implications of this conclusion, as they bear on seismicity and seismic anisotropy, are considered in light of the two most seriously regarded hypotheses advanced to explain motions of spreading plates of the lithosphere.