Experimental study of the high-pressure stability limit of osumilite in the system K2O-MgO-Al2O3-SiO2: implications for high-temperature granulites

Abstract

In experiments on synthetic minerals in piston-cylinder solid-media apparatus Mg-osumilite breaks down to enstatite, sillimanite, K-feldspar and quartz at a pressure between 11 and 12 kbar over a temperature range of 950-1100°C. The reaction has a flat dP/dT slope. The reactions osumilite = cordierite + enstatite + K-feldspar + quartz and osumilite = sapphirine + enstatite + K-feldspar + quartz limit osumilite stability at lower and higher temperatures respectively. Because osumilite in nature is invariably more magnesian than other co-existing ferromagnesian minerals, the magnesian end-member can be predicted to have the greatest stability field. Osumilite, like cordierite, can thus only occur in normal thickness continental crust. The data indicate that in the Napier complex, Enderby Land, Antarctica, where osumilite occurs on a regional scale, an inferred «isograd», marking the disappearance of osumilite, is the trace of a nearly isobaric surface at a paleo-depth of ca. 35 km. On this surface osumilite breaks down to orthopyroxene, sillimanite, K-feldspar and quartz