Effect of H_2O on Liquidus Relationships in MgO-Al_2O_3-SiO_2 at 30 Kilobars

Abstract

Dehydrated gels with compositions corresponding to mixtures of pyrope-enstatite, pyrope-quartz, and pyrope-quartz-corundum were sealed in platinum capsules and reacted in half-inch piston-cylinder apparatus at 30 kbar. Samples were pyrope composition with variable H_2O, and other compositions with 7.5 wt % H_2O. Highly aluminous metastable orthopyroxenes nucleated from and persisted in mixtures with compositions near pyrope. There is no univariant fusion curve for pyrope-H_2O. Pyrope dissolves incongruently in H_2O subsolidus, and a trace of liquid is produced near 1000°C. A univariant reaction: pyrope + forsterite + vapor = spinel + liquid was located at 1150 ± 20° C. The liquidus for pyrope with excess H_2O is at 1310°C. Orthopyroxenes coexisting with SiO_2-rich liquids contain 0.8-5.7% excess SiO_2 compared with MgSiO_3-Al_2O_3. Liquidus diagrams dry, with 7.5% H_2O, and with excess H_2O illustrate the effect of H_2O on the liquidus relationships. With increasing H_2O: the liquidus volume for Opx expands greatly at the expense of that for Qz, and slightly at the expense of Py; the volume for Fo expands significantly at the expense of that for Opx, and slightly at the expense of Py; liquid coexisting with Fo + Opx + Py is enriched in SiO_2 and Al_2O_3; liquid coexisting with Opx + Py + Qz is enriched greatly in SiO_2 and slightly in Al_2O_3; the range of liquid compositions coexisting with Opx + Py is greatly increased toward enrichment in SiO_2. These results provide boundaries for corresponding phase relationships in CaO-MgO-Al_2O_3-SiO_2-H_2O with applications to the effect of H_2O on partial fusion of subducted oceanic crust (quartz eclogite) and mantle peridotite.

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