Penetration of Warm Slabs Across the 660 km Boundary: The Potential Role of Hydrous Stishovite

We investigate the phase boundary of hydrous/anhydrous post‐spinel transition at conditions relevant to the 660 km boundary. For the hydrous ringwoodite decomposition, the slope is found to range from negative to positive values depending on the distribution of water between the final products, namely, bridgmanite, periclase (mostly anhydrous) and stishovite. The water partition coefficient () between the products, namely, stishovite (Sti) and bridgmanite (Brg), is found to increase with pressure but decrease with temperature. Consequently, under warm slab conditions, the magnitude of is such that the slope of the transition has a small positive value (+0.42 MPa/K at 1500K). Formation of hydrous stishovite upon ringwoodite breakdown hence: (a) may facilitate the descent of warm slabs into the lower mantle past the 660 km boundary by favorably tailoring the Clapeyron slope and (b) acts as a means to carry water deep into the lower mantle.