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 synchrotron radiation. We confirmed that Mg2SiO4 spinel transforms into fine. lamellae of SiO2 stishovite and periclase, and/or MgS iO3 ilmenite and periclase as an intermediate step in the postspinel transf ormation. These metastable assemblages eventually disappear and form the st able assemblages of MgSiO3 perovskite and periclase. Initial grain size jus t after the postspinel; transformation drastically changes with overpressur e. 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.