The high-pressure stability of Mg-sursassite in a model hydrous peridotite:: a possible mechanism for the deep subduction of significant volumes of H2O

The stability of the high-pressure phase Mg-sursassite. previously MgMgAl-pumpellyite, in ultramafic compositions has been determined in experiments in the system MgO-Al2O3-SiO2H2O (MASH). The breakdown of Mg-sursassite + forsterite + enstatite to pyrope + vapour with increasing temperature was bracketed at 6.0 and 7.0 GPa. Below 6.0 GPa, Mg-sursassite + forsterite + vapour reacts to chlorite + enstatite. This reaction provides a mechanism for transfer of water from chlorite- to Mg-sursassite-bearing assemblages. At pressures of 7.0 GPa and above. the assemblage Mg-sursassite + phase A + enstatite was found. Phase relations involving Mg-sursassite and phase A are considered. For bulk compositions with a low water content, the vapour-absent reaction Mg-sursassite + forsterite = pyrope + phase A + enstatite determines the upper-pressure stability of Mg-sursassite. and provides a mechanism for the complete transfer of water from Mg-sursassite to phase A-bearing assemblages. Mg-sursassite plays an important role in peridotite compositions in the subducting slab because, at temperatures below 700 degreesC, it can transfer water from hydrous phases such as antigorite and chlorite to highpressure stable phases such as phase A.