To understand the effects of H2O on the mineral phases forming under the pressure-temperature conditions of the lower mantle, we have conducted laser-heated diamond-anvil cell experiments on hydrous ringwoodite (Mg2SiO4 with 1.1 wt% H2O) at pressures between 29 and 59 GPa and temperatures between 1200 and 2400 K. Our results show that hydrous ringwoodite (hRw) converts to crystalline dense hydrous silica, stishovite (Stv) or CaCl2-type SiO2 (mStv), containing 1 wt% H2O together with Brd and MgO at the pressure-temperature conditions expected for shallow lower-mantle depths between approximately 660 to 1600 km. Considering the lack of sign for melting in our experiments, our preferred interpretation of the observation is that Brd partially breaks down to dense hydrous silica and periclase (Pc), forming the phase assembly Brd + Pc + Stv. The results may provide an explanation for the enigmatic coexistence of Stv and Fp inclusions in lower-mantle diamonds.

The composition of Archean volcanic crust can be characterized by a higher Mg/Si ratio than modern mid-ocean ridge basalt (MORB), because of the higher degree melting from the warmer mantle in the Archean. Although modern MORB may become less dense than the surrounding mantle beneath the mantle transition zone (MTZ), the Mg-rich composition of Archean volcanic crust may result in the different density, and therefore different sinking behavior near the MTZ. In order to understand the compositional effect of Archean volcanic crust on the sinking behaviors and the scale of mantle mixing in the Archean, we investigated the mineralogy and density of Archean volcanic crust near the MTZ (470–910 km-depth). We conducted experiments at 19–34 GPa and 1400–2400 K using the laser-heated diamond anvil cell (LHDAC) combined with in-situ X-ray diffraction (XRD). The in-situ XRD and the chemical analysis revealed that Archean volcanic crust forms garnet and ringwoodite (84 and 16 vol%, respectively), which gradually transforms to Brg and CaPv (82 and 18 vol%, respectively) at 23–25 GPa and 1800 K. Our in-situ XRD experiments allowed us to measure the volumes of stable phases and to estimate their densities at high pressure and temperature. The results suggest that Archean volcanic crust maintains greater density than the pyrolitic mantle in the Archean regardless of temperature at 20–34 GPa (570–850 km-depth), promoting further sinking into the deeper mantle in the Archean. We also considered the density of the subducting slab in the Archean. The density model showed that the subducting slab is still denser or at least equally dense as the surrounding pyrolitic mantle in the Archean.