Phase relations and mineral chemistry in pyrolitic mantle at 1600-2200 °C under pressures up to the uppermost lower mantle: Phase transitions around the 660-km discontinuity and dynamics of upwelling hot plumes

We determined phase relations of pyrolite at 12-28 GPa and 1600-2200 degrees C using a 6-8 Kawai-type multianvil apparatus. A large number of high pressure and high temperature experiments were conducted to constrain the phase relations especially around 660-km depth, compared with previous studies. The phase relations drastically change between 19 and 23 GPa with temperature as follows. At 19-22 GPa, a phase transition of ringwoodite (Rw) to garnet (Gt) + magnesiowlistite (Mw) occurs at 1800 degrees C, and no Rw is observed at 2200 degrees C. The mineral proportion of Gt increases with increasing temperature due to this decomposition reaction and becomes larger than that of Rw above 1800 degrees C. At 23 GPa, pyrolite crystallizes to a mineral assemblage of bridgmanite (Bm) + Mw + Gt + calcium perovskite by the post-spinel (pSp) transition at 1600-2000 degrees C and the post-garnet (pGt) transition at 2100-2200 degrees C, the latter of which is defined as a phase transition of a part of Gt to Bm. Clapeyron slopes of the pSp transition and the pGt transition are determined to be -1 and 5 MPa/degrees C, respectively, in this study. Gt is stable at any temperatures around the 660-km discontinuity (D660), and Gt-dis-appearance reaction occurs at 25 GPa with no temperature dependence. Densities of pyrolite were calculated in various pressure and temperature conditions by using the mineral proportions and available data of equations of state. The density at 1600 degrees C is higher than that at 1800-2200 degrees C at any pressure-temperature conditions in this study. In particular, the drastic density drop occurs at 19-24 GPa between 1600 and 1800-2200 degrees C because of increase of Gt proportion with increasing temperature. These suggest that upwelling hot plumes composed of pyrolite from the lower mantle do not stagnate and their ascent is promoted across D660. The mineral proportions and the density changes also suggest that the phase transition which causes D660 changes around 1800 degrees C from the pSp to the pGt transitions because of the mineral proportion inversion between Rw and Gt.