Preferred orientation of silicate spinel inferred from experimentally deformed aggregates of trevorite.

Abstract

Syntectonic recrystallization experiments have been done on the synthetic fine-grained aggregate of trevorite (NiFe2O4; a spinel type mineral) analogous to the γ-(Mg, Fe)2SiO4 in the upper mantle by means of a constant strain rate apparatus under various physical conditions: strain rates from 10-3 to 10-6 s-1, homologous temperatures from 0.56 to 0.80, and confining pressure of 0.6 GPa. An X-ray pole figure goniometry is employed to describe the quantitative orientation distribution of spinel crystallites in uniaxially deformed aggregates. The obtained inverse pole figures indicate that two types of preferred orientation have been produced during the axial deformations both in compression and tension. The lower temperature and faster strain rate yield the preferred orientation of type I: [1 1 0]//compression axis and [1 0 0]//tension axis, while the higher temperature and slower strain rate yield type II: [1 0 0]//compression axis and [1 0 0]//tension axis. The generation of the two types of preferred orientation is caused by grain rotation due to slip in constituent grains in an aggregate, and syntectonic recrystallization, respectively. The present results suggest (1)γ-(Mg, Fe)2SiO4 has a considerable degree of preferred orientation in the upper mantle, (2) the preferred orientation is probably of type II rather than of type I under the mantle conditions, and (3) the detectable degree of shear wave anisotropy is expected in the spinel layer of the earth's upper mantle as a result of preferred orientation.