“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.