Formation of metastable assemblages and mechanisms of the grain-size reduction in the postspinel transformation of Mg₂SiO₄

An in situ X-ray observation of the postspinel transformation kinetics was made using intense synchroton radiation. We confirmed that Mg₂SiO₄ spinel transforms into fine lamellae of SiO₂ stishovite and periclase, and/or MgSiO₃ ilmenite and periclase as an intermediate step in the postspinel transformation. These metastable assemblages eventually disappear and form the stable assemblages of MgSiO₃ perovskite and periclase. Initial grain size just after the postspinel transformation drastically changes with overpressure. Viscosity of the subducting slab into the lower mantle, which is thought to be deformed by grain-size-sensitive creep, would depend on overpressure needed for the postspinel transformation at geological time scale.