EQUILIBRIUM EXPERIMENTS IN THE SYSTEM MGO-SIO2-H2O (MSH) - STABILITY FIELDS OF CLINOHUMITE-OH [MG9SI4O16(OH)(2)], CHONDRODITE-OH [MG5SI2O8(OH)(2)] AND PHASE-A (MG7SI2O8(OH)(6))

The water-pressure and temperature stability fields of clinohumite-OH, chondrodite-OH and phase A were determined in reversed equilibrium ex periments up to 100 kbar within the system MgO-SiO2-H2O. Their PT-fiel ds differ from results from former synthesis experiments. Bracketing e xperiments on the reaction phase A + low P-clinoenstatite <-> forsteri te + water resulted in a slightly steeper dP/dT-slope compared to earl ier experiments for this equilibrium. Clinohumite-OH and chondrodite-O H both have large stability fields which extend over pressure ranges o f more than 80 kbar. However, they are hardly relevant as hydrous mine rals within the subducted oceanic lithosphere. Both are too Mg-rich fo r a typical mantle bulk composition. In addition, the dehydration of s ubducted oceanic lithosphere - due to (forsterite + water)-forming rea ctions - will occur before the two humite-group phases even become sta ble. Restricted to the cool region of cold subducting slabs, phase A, however, might be formed via the reactions phase A + low P-high P-clin oenstatite <-> forsterite + water or antigorite + brucite <-> phase A + water, before dehydration of the oceanic lithe,sphere occurs.