The join grossularite-pyrope at 30 kb and its petrological significance

Abstract

The pseudobinary join grossularite-pyrope was investigated at 30 kb nominal pressure at temperatures between 1300° and 1725°C, using piston-cylinder apparatus. Grossularite melts incongruently to gehlenite + liquid at 1545°C, and pyrope melts incongruently to spinel+ liquid at 1705°C. Aluminous diopside forms a congruent compound within the join and melts at 1595°C. The join has two binary eutectics, where diopside is in equilibrium with either grossularite-rich garnet or pyrope-rich garnet. All eutectic compositions are on the join, and the join is a thermal barrier at 30 kb within the system CaSiO_3-MgSiO_3-Al_2O_3. The phase relations suggest that nephelinites might have formed at pressures near or above 30 kb, and that nephelinites fractionate toward a thermal minimum on the garnet + diopside liquid surface, with eclogitic mineralogy.

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