Abstract
Reactions limiting the stability of garnet + quartz in the CaO-MgO-Al2O3-SiO2 (CMAS) and MgO-Al2O3-SiO2 (MAS) systems have been experimentally reversed in a piston cylinder apparatus at 1000-1300°C and 10-25 kbar. At high temperatures, garnet composition is buffered successively by orthopyroxene + sillimanite, orthopyroxene + anorthite, clinopyroxene + anorthite and clinopyroxene + sillimanite/kyanite as garnet progresses from pure pyrope to more calcic compositions. The buffering assemblages are stable in separate divariant P-T fields, separated by three univariant reactions: At 980°C, the reactions intersect at an invariant point near 14.5 kbar. At 1150°C and 1300°C they take place at 14.8,15.9 and 19.8 kbar, and at 15.1,17.4, and 21.8 kbar, respectively. At 1300°C the mole fractions of pyrope in garnet for the three reversed univariant reactions are 0.95,0.90 and 0.85. The reaction pyrope + quartz = enstatite + sillimanite was reversed at 1000°C (15.4 kbar), 1150°C (17.0 kbar) and 1300°C (18.8 kbar).
The sequence of mineral assemblages determined in this study is consistent with natural parageneses. However, quantitative application of the present results to natural compositions is difficult due to lack of knowledge of exact pyroxene compositions taking place in Reactions (6), (7) and (8), and due to the unavailability of accurate activity-composition models for highly complex natural orthopyroxene, clinopyroxene and garnet.
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