THE LOW-PRESSURE STABILITY OF PHASE-A, MG7SI2O8(OH)(6)

Phase A, Mg7Si2O8(OH)(6), is a dense hydrous magnesium silicate whose importance as a host of H2O in the Earth's mantle is a subject of deba te. We have investigated the low-pressure stability of phase A in expe riments on the reaction phase A = brucite + forsterite. Experiments we re conducted in piston-cylinder and multi-anvil apparatus, using mixtu res of synthetic phase A, brucite and forsterite. The reaction was bra cketed between 2.60 and 2.75 GPa at 500 degrees C, between 3.25 and 3. 48 GPa at 600 degrees C and between 3.75 and 3.95 GPa at 650 degrees C . These pressures are much Lower than observed in the synthesis experi ments of Yamamoto and Akimoto (1977), At 750 degrees C the stability f ield of brucite + chondrodite was entered. The enthalpy of formation a nd entropy of phase A at 1 bar (10(5) Pa), 298 K, were derived from th e experimental brackets on the reaction phase A = brucite + forsterite using a modified version of tile thermodynamic dataset THERMOCALC of Holland and Powell (1990), which includes a new equation of state of H 2O derived from the molecular dynamics simulations of Brodholt and Woo d (1993). The data for phase A are: Delta H-f degrees = -7126 +/- 8 kJ mol(-1), S degrees = 351 J K-1 mol(-1). Incorporating these data into THERMOCALC allows the positions of other reactions involving phase A to be calculated, for example the reaction phase A + enstatite = forst erite + vapour, which limits the stability of phase A in equilibrium w ith enstatite. The calculated position of this reaction (753 degrees C at 7 GPa to 937 degrees C at 10 GPa) is in excellent agreement with t he experimental brackets of Luth (1995) between 7 and 10 GPa, supporti ng the choice of equation of state of H2O used in THERMOCALC. Comparis on of our results with calculated P-T paths of subducting slabs (Peaco ck et al. 1994) suggests that, in the system MgO-SiO2-H2O phase-A coul d crystallise in compositions with Mg/Si > 2 at pressures as low as 3 GPa. In less Mg rich compositions phase A could crystallise at pressur es above approximately 6 GPa.