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 experiments up to 100 kbar within the system MgO-SiO2-H2O. Their PT-fields differ from results from former synthesis experiments. Bracketing experiments on the reaction phase A + low P-clinoenstatite <-> forsterite + water resulted in a slightly steeper dP/dT-slope compared to earlier experiments for this equilibrium. Clinohumite-OH and chondrodite-OH both have large stability fields which extend over pressure ranges of more than 80 kbar. However, they are hardly relevant as hydrous minerals within the subducted oceanic lithosphere. Both are too Mg-rich for a typical mantle bulk composition. In addition, the dehydration of subducted oceanic lithosphere - due to (forsterite + water)-forming reactions - will occur before the two humite-group phases even become stable. Restricted to the cool region of cold subducting slabs, phase A, however, might be formed via the reactions phase A + low P-high P-clinoenstatite <-> forsterite + water or antigorite + brucite <-> phase A + water, before dehydration of the oceanic lithe,sphere occurs.