Carbonic fluid inclusions in ultrahigh-temperature metamorphic rocks from Tonagh Island in the Archean Napier Complex, East Antarctica: A preliminary report

• Main Authors: Toshiaki Tsunogae, M. Santosh, Yasuhito Osanai, Masaaki Owada, Tsuyoshi Toyoshima, Tomokazu Hokada, Warwick A. Crowe, Takashi Miyano
• Format: Report
• Language: English
• Published: Institute of Geoscience, the University of Tsukuba/Department of Natural Environmental Science, Kochi University/Department of Earth Sciences, Okayama University/Department of Earth Sciences, Yamaguchi University/Graduate School of Science and Technology, Niigata University/National Institute of Polar Research/Department of Geology and Geophysics, University of Western Australia/Institute of Geoscience, the University of Tsukuba 2001
• Subjects: carbonic fluid inclusion; ultrahigh-temperature granulite; Tonagh Island; Archean Napier Complex; East Antarctica; Napier; Antarc*; Antarctica; Polar geoscience
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=3083; http://id.nii.ac.jp/1291/00003083/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=3083&item_no=1&attribute_id=18&file_no=1

We report here the occurrence of high density CO_2-rich fluid inclusions in ultrahigh-temperature metamorphic rocks from Tonagh Island of the Archean Napier Complex, East Antarctica. A study on the different lithological units from Tonagh Island (garnet gneiss, sapphirine granulite, garnet-orthopyroxene gneiss, and magnetite-quartz gneiss) shows the common presence of carbonic fluid inclusions entrapped within various minerals. The melting temperatures of fluids in the first three rock types lie in the range of -56.3 to -57.2℃, close to the triple point for pure CO_2 (-56.6℃). Fluid inclusions in magnetite-quartz gneiss show slight depression in their melting temperatures (-56.7～-57.8℃) suggesting traces of additional fluid species such as CH_4 and/or N_2 in the dominantly CO_2-rich fluid. Homogenization of pure CO_2 inclusions in the garnet gneiss, sapphirine granulite, and garnet-orthopyroxene gneiss occurs into the liquid phase at temperatures in the range of -34.9～+4.2℃. This translates into high CO2 densities in the range of 0.9-1.1 g・cm^<-3>. The estimated CO_2 isochore intersects the counterclockwise P-T trajectory of Tonagh Island at around 6-9 kbar at 1100℃, which corresponds to the peak metamorphic conditions of Tonagh Island derived from mineral phase equilibria. We therefore infer that CO_2 was the dominant fluid species present during the ultrahigh-temperature metamorphism in Tonagh Island. The stability of anhydrous minerals such as orthopyroxene in the study area might have been effected by the lowering of H_2O-activity due to the presence of CO_2 at peak metamorphic conditions (>1100℃).