Phase relationships in the join grossularite-pyrope-7.5% H_2O at 30 KB

Abstract

Experiments were conducted in piston-cylinder apparatus using gels with added H_2O between 900° and 1525°C. The synthesis results were checked by two-stage reversal runs. Compositions of garnet (Ga) and clinopyroxene (Cpx) solid solutions were measured by microprobe. With 7.5 percent H_2O, pyrope (Py) melts incongruently producing spinel (Sp), and grossularite (Gr) melts congruently, with brackets 1500° to 1525°C and 1300° to 1325°C, respectively. A field for Cpx + L has temperature maximum on the liquidus at 1380°C, very near the composition Gr_2Py_1 where the join diopside-Ca-Tscherrnak's molecule (Di-CaTs) crosses the garnet join (33.3 mole percent Py). This field is bounded by two liquidus piercing points for the coexistence of Ga + Cpx + L with liquid compositions and temperatures of 25 mole percent Py-1275°C and 48 mole percent Py-1295°C. The Cpx field is bounded with decreasing temperature by a reaction interval for Cpx + Ga+ L and terminated at 1110°C by the reaction: Cpx_(ss) + L = Gr_(ss). At lower temperatures, solid solution in garnets is continuous from grossularite to pyrope. The compositions of clinopyroxenes coexisting with liquid are near Gr_2Py_1 : with decreasing temperature the Al_2O_3 content increases, and for bulk compositions richer in MgO the Cpx is enriched in enstatite (En) component. As Cpx reacts to produce Ga with decreasing temperature, the clinopyroxene component En decreases, and CaTs increases. The results obtained with 7.5 percent H_2O present are compared in detail with the few previous studies in the dry system at 30 kb.

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