Mechanisms of the transformations between the ?, ? and ? polymorphs of Mg2SiO4 at 15 GPa

Abstract

Data on the mechanisms of mantle phase transformations have come primarily from studies of analogue systems reacted experimentally at low pressures. In order to study transformation mechanisms in Mg2SiO4 at mantle pressures, forsterite (α) has been reacted in the stability field of β-phase, at 15 GPa and temperatures up to 900° C, using a multianvil split-sphere apparatus. Transmission electron microscope studies of samples reacted for times ranging from 0.25–5.0 h show that forsterite transforms to β-phase by an incoherent nucleation and growth mechanism involving nucleation on olivine grain boundaries. This mechanism and the resultant microstructures are very similar to those observed at much lower pressures in analogue systems (Mg2GeO4 and Ni2SiO4) as the result of the olivine to spinel (α→γ) transformation. Metastable spinel (γ) also forms from Mg2SiO4 olivine at 15 GPa, in addition to γ-phase, by the incoherent nucleation and growth mechanism. With time, the spinel progressively transforms to the stable β-phase. After 1 h, spinels exhibit a highly striated microstructure along {110}γ and electron diffraction patterns show streaking parallel to [110]γ which indicates a high degree of structural disorder. High resolution imaging shows that the streaking results from thin lamellae of β-phase intergrown with the spinel. The two phases have the orientation relationship [001]β//[001]γ and [010]β//[110]γ so that the quasi cubic-close-packed oxygen sublattices are continuous between both phases. These microstructures are similar to those observed in shocked meteorites and show that spinel transforms to β-phase by a martensitic (shear) mechanism. There is also evidence that the mechanism changes to one involving diffusion-controlled growth at conditions close to equilibrium.