Stress dependence of the mechanism of the olivine–spinel transformation

Abstract

OLIVINE, α-(Mg,Fe)₂SiO₄, is the most abundant phase in the Earth's upper mantle. It transforms to high-density polymorphs (α, with the spinel structure and β, with a modified spinel structure) under pressure and temperature conditions appropriate to explain the discontinuity in seismic velocities that defines the base of the upper mantle at 400km depth; the transformation is generally believed to be responsible for that discontinuity¹. Knowledge of the mechanism and kinetics of the transformation is important for understanding certain aspects of mantle dynamics, and extensive experimental and theoretical work has been conducted on both natural olivine and several analogous chemical systems. Previously, conflicting results have suggested a reconstructive transformation mechanism^2–10, a martensitic mechanism^{11, 12} and a mechanism that is not entirely consistent with either^13–16. The conflicts cannot be explained as different mechanisms operating in different systems¹⁷. Here we show that the transformation can proceed by two different mechanisms, depending on the level of nonhydrostatic stress; under hydrostatic pressure or low stresses (at sufficiently high temperatures), nucleation occurs by an incoherent, diffusion-dependent, intercrystalline mechanism proposed by Sung and Burns^{2, 3} and first demonstrated by Vaughan et al.⁴ (Fig. 1), whereas under high stresses, nucleation of the high-pressure (γ) polymorph (spinel) occurs by a coherent, shear-induced, intracrystalline mechanism predicted by Poirier¹⁸. Thus we are able to reconcile the previously conflicting results, and consideration of the stress levels likely in natural situations leads us to conclude that both mechanisms are geologically relevant.