"Ultrahigh density" carbonic fluids in ultrahigh-temperature crustal metamorphism

Abstract

Ultrahigh temperature (UHT) granulites in Tonagh Island, Napier Complex, East Antarctica record peak metamorphic pressure-temperature (P-T) conditions of up to 9 kbar and 1100°C. Sapphirine, garnet, orthopyroxene and quartz in these rocks contain very high density fluid inclusions with melting temperatures close to that of pure CO2 (-56.6°C) and homogenization temperatures down to -34.9°C translating into high CO2 densities (up to 1.07 g/cm3). The UHT granulites of Vizianagram in Eastern Ghats Belt, India which were subjected to extreme crustal metamorphism at >1000°C and 8-9 kbar also carry very high density (up to 1.15 g/cm3) pure CO2 inclusions in quartz adjacent to spinel rimmed by various coronas of sillimanite, orthopyroxene and garnet. Garnets in a granulite facies rock from Salem in southern India that equilibrated at peak P-T conditions of 740-800°C and 9-11 kbar carry the highest density (1.17 g/cm3) pure CO2 yet reported from continental crust. This rock also contains very high density CO2 inclusions in plagioclase and quartz. High density (0.998 g/cm3) pure CO2-rich fluid inclusions also occur abundantly within garnets from a mafic granulite in Ampitiya, central Highland Complex, Sri Lanka. The peak P-T conditions of metamorphism of the mafic granulite are around 10.6 kbar and 985°C with subsequent rapid isothermal decompression along a clock-wise path down to 5.5 kbar. In most of the cases above, the representative isochores for the CO2 inclusions either penetrate through or pass very close to the P-T windows defined from mineral phase equilibria indicating that the fluid inclusions were trapped at the time of peak or near post-peak metamorphism. We thus recognize a group of "ultrahigh density" (UHD) CO2-rich fluids that characterize UHT rocks and other granulites formed under extreme crustal metamorphism. These fluids contrast sharply with the lower density (generally <1.0 g/cm3) CO2 inclusions commonly reported from normal granulite facies rocks in various terrains. The presence of "synmetamorphic" UHD CO2 within various minerals is consistent with the low water activities predicted by the mineral assemblages in these rocks. Although the source of the CO2 is equivocal, mantle derived mafic magmas could have provided the heat and volatiles required for crustal metamorphism at extreme conditions displayed by these rocks.