Osumilite in the sapphirine–quartz terrane of Enderby Land, Antarctica: implications for osumilite petrogenesis in the granulite facies

Osumilite is an important rock-forming mineral at 10 exposures in the Late Archean Napier complex of coastal Enderby Land (50–52°E), Antarctica. Mineral assemblages of low compositional variance (all with quartz±plagioclase) are osumilite–garnet–sillimanite ± spinel, osumilite–sapphirine–sillimanite, osumilite–garnet–orthopy roxene–sillimanite, osumilite–rthopyroxene–biotite, and in a rock nearly free of iron (atomic Mg/(Mg+Fe) = 0.89) from Reference Peak (67°15′S, 50°29′E), osumilite–enstatite (En₉₁Fs₉)–sillimanite. The Antarctic osumilite departs from the theoretical composition (K, Na) (Mg,Fe,Mn)₂Al₃ (Si₁₀Al₂)O₃₀ in that (Mg+Fe+Mn) ranges from 2.20 to 2.39; Al, 4.45 to 4.67; and Si, 10.05 to 10.30. These variations approximately obey the substitution (Mg+Mn+Fe) + Si = 2Al. Atomic Mg/(Mg+Mn+Fe) ratio (X_Mg) of Antarctic osumilite ranges from 0.79 to 0.97, and is greater than X_Mg of associated cordierite and biotite. Antarctic osumilite-bearing rocks have moderate to high Mg/Fe ratios (atomic Mg/(Mg+Fe) = 0.43 to 0.89) and are peraluminous.

A petrogenetic grid in the system K₂O–FeO–MgO–Al₂O₃–SiO₂ for quartz (present in all assemblages), osumilite, K-feldspar, garnet, sapphirine, cordierite, orthopyroxene, and sillimanite can be constructed given the constraint that the univariant reaction osumilite = cordierite + enstatite + K-feldspar + quartz in the iron-free subsystem intersects the stable portion of the univariant reaction in the MgO–Al₂O₃–SiO₂ system: cordierite = enstatite + sillimanite + quartz. Pressure–temperature slopes of reactions involving osumilite but not cordierite are positive; slopes of reactions involving both osumilite and cordierite are negative, or in one case, parallel to the pressure axis. Osumilite is predicted to be stable in rocks of appropriate bulk composition at temperatures above 750°C and at total (lithostatic) pressures less than 8 or 9 kbar. M. Olesch and F. Seifert’s recent experimental work and Olesch’s calculations indicate that osumilite would be stable under hydrous conditions only when P_H2O = P_Total < 1 kbar; at higher total pressures, H₂O partial pressure must be much less than total pressure. Minimum temperatures for the appearance of osumilite–garnet and osumilite–sillimanite assemblages are in general higher than those needed for osumilite–cordierite–orthopyroxene, and pressures of 5 or 6 kbar may be needed to stabilize osumilite–garnet or osumilite–sillimanite at geologically accessible temperatures. As the physical conditions proposed here for osumilite stability overlap the estimated conditions in many granulite-facies terranes, osumilite can be expected to occur at more localities than have been reported to date.