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Pressure–temperature stability of pyrope in the system MgO–Al₂O₃–SiO₂–H₂O

Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany

^* Corresponding author, e-mail: thomas.fockenberg{at}rub.de

The pressure–temperature stability field of pyrope was experimentally determined in reversed equilibrium experiments up to 10 GPa within the system MgO–Al₂O₃–SiO₂–H₂O. The lower pressure limit of pyrope is defined by reactions to aluminous enstatite + sapphirine + kyanite (1.5 GPa, 950–1050 °C) and to aluminous enstatite + corundum (1.5–1.7 GPa, 800–950 °C). Between 1.7 and 1.8 GPa pyrope is formed from the assemblage enstatite + chlorite + Mg-staurolite. The curve talc + Mg-staurolite + kyanite = pyrope + H₂O marks the pyrope in field between 1.8 and 1.9 GPa, and at higher pressures up to 4 GPa pyrope forms from chlorite + talc + kyanite. Beyond 4 GPa, the assemblage talc + chlorite + Mg-chloritoid defines the lower temperature limit around 600 °C. At pressures higher than 5 GPa, pyrope forms via the reactions low-clinoenstatite + chlorite + Mg-sursassite = pyrope + H₂O and high-clinoenstatite + forsterite + Mg-sursassite = pyrope + H₂O (with increasing pressure). The bracketing results indicate that all these reactions have a high positive dP/dT-slope. Pyrope is a high-pressure phase stable only at mantle depths.

Key-words: pyrope, garnet, phase-equilibrium experiments, MASH system, high pressure.