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The phase boundary between spinel ([Gamma] phase) and MgSi[O.sub.3] perovskite + MgO periclase in [[Mg.sub.2] Si[O.sub.4] was determined by in situ x-ray measurements by a combination of the synchrotron radiation source (SPring-8) and a large multianvil high-pressure apparatus. The boundary was determined attemperatures between 1400 ° to 1800 ° C, demonstrating that the postspinel phase boundary has a negative Clapeyron slope as estimated by quench experiments and thermodynamic analyses. The boundary was located at 21.1 (± 0.2) gigapascals, at 1600 ° C, which is ~2 gigapascals lower than earlier estimates based on other high-pressure studies., Olivine [(Mg, Fe).sub.2] Si[O.sub.4], the most abundant mineral in the upper mantle, transforms to modified spinel and spinel structures at high pressure (~13.5 and ~18 GPa, at 1400 ° and [...]

An open question for solid-earth scientists is the amount of water in Earth's interior. The uppermost mantle and lower mantle contain little water because their dominant minerals, olivine and bridgmanite, have limited water storage capacity. In contrast, the mantle transition zone (MTZ) at a depth of 410 to 660 km is considered to be a potential water reservoir because its dominant minerals, wadsleyite and ringwoodite, can contain large amounts of water [up to 3 weight % (wt %)]. However, the actual amount of water in the MTZ is unknown. Given that water incorporated into mantle minerals can lower their viscosity, we evaluate the water content of the MTZ by measuring dislocation mobility, a property that is inversely proportional to viscosity, as a function of temperature and water content in ringwoodite and bridgmanite. We find that dislocation mobility in bridgmanite is faster by two orders of magnitude than in anhydrous ringwoodite but 1.5 orders of magnitude slower than in water-saturated ringwoodite. To fit the observed mantle viscosity profiles, ringwoodite in the MTZ should contain 1 to 2 wt % water. The MTZ should thus be nearly water-saturated globally.