The low-pressure stability of phase A,Mg7Si2O8(OH)6 |
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| Authors: | A R Pawley Bernard J Wood |
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| Institution: | (1) Department of Geology, University of Bristol, Wills Memorial Building, Bristol BS8 1RJ, U.K., GB |
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| Abstract: | 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 debate. We have investigated the low-pressure stability of phase A in experiments
on the reaction phase A=brucite+forsterite. Experiments were conducted in piston-cylinder and multi-anvil apparatus, using
mixtures of synthetic phase A, brucite and forsterite. The reaction was bracketed between 2.60 and 2.75 GPa at 500° C, between
3.25 and 3.48 GPa at 600° C and between 3.75 and 3.95 GPa at 650° C. These pressures are much lower than observed in the synthesis experiments of Yamamoto and Akimoto (1977). At 750° C the stability field of brucite + chondrodite was entered. The enthalpy of formation and entropy of phase A at 1 bar (105 Pa), 298 K, were derived from the experimental brackets on the reaction phase A=brucite+forsterite using a modified version
of the thermodynamic dataset THERMOCALC of Holland and Powell (1990), which includes a new equation of state of H2O derived from the molecular dynamics simulations of Brodholt and Wood (1993). The data for phase A are: ΔH
o
f
=−7126±8 kJ mol-1, S
o=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=forsterite+vapour, which limits the stability of phase A in equilibrium with enstatite.
The calculated position of this reaction (753° C at 7 GPa to 937° C at 10 GPa) is in excellent agreement with the experimental
brackets of Luth (1995) between 7 and 10 GPa, supporting the choice of equation of state of H2O used in THERMOCALC. Comparison of our results with calculated P-T paths of subducting slabs (Peacock et al. 1994) suggests that, in the system MgO–SiO2–H2O, phase A could crystallise in compositions with Mg/Si>2 at pressures as low as 3 GPa. In less Mg rich compositions phase
A could crystallise at pressures above approximately 6 GPa.
Received: 3 July 1995/Accepted: 14 December 1995 |
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