Carbon isotopic composition of graphite in metamorphic rocks from Lützow-Holm Complex, East Antarctica: Implications for carbon geodynamic cycle in continental crust

Abstract

Organic carbon and carbonate carbon are two important reservoirs that control the carbon geodynamic cycle at convergent margins during plate subduction, arc magmatism and continent building processes. The movement of carbon through different reservoirs in the Earth relating to the global tectonic activities is key in understanding the carbon geodynamic cycle. In this contribution, a comprehensive synthesis on the different types of occurrences of graphite, the purest form of carbon in continental crust, in the Lützow-Holm Complex (LHC), East Antarctica is carried out and carbon isotopic composition is used as a proxy to identify the movement of carbon during orogenesis. Graphite is an important reservoir of carbon in continental crust and occurs in a variety of rock types in the LHC. Based on the mode of occurrence they were classified into several types, disseminated flakes in gneissic rocks, coarse aggregates in leucosomes, graphite concentration in lithological contacts and as monomineralic graphite veins. Disseminated graphite in pelitic gneisses record the lowest carbon isotopic composition (δ13CVPDB values between –25‰ to –15‰), suggesting biogenic signatures, however those in metacarbonate rocks have equilibrated with carbonate carbon during high temperature metamorphism to show heavier values (δ13CVPDB values between –3‰ to –1‰). The carbon isotopic composition of disseminated graphite is modified during prograde metamorphism by devolatilization and also exchange of carbon isotopes with carbonate minerals. Coarse-grained graphite is observed in leucosomes in the migmatized metapelitic rocks. During the high-temperature metamorphism and partial melting of graphite-bearing rocks, graphite decomposes to form COH fluids, part of which, especially the lighter isotope-bearing fluids have escaped the system causing a shift toward heavier values (δ13CVPDB values in the range between –18‰ to –10‰). Based on the field, textural and carbon isotope evidence a model is suggested, where biotite dehydration melting of graphite-bearing rocks caused the dissolution of pre-existing graphite formed from organic materials, and graphite was reprecipitated as coarse aggregates in leucosomes during melt crystallization and cooling. This resulted in the carbon remobilization and isotopic reorganization. Carbon isotopic composition of graphite concentrations in lithological contacts (δ13CVPDB values ranging between –1.8 to –5.7‰) and monomineralic veins (δ13CVPDB values between –3.5 and –6.0‰) suggest that they were precipitated from CO2 fluids locally released through decarbonation reactions. The presence of large volume of skarn mineralization in the contact between carbonate and silicate rocks and similarities of carbon isotopic composition of graphite in contact zones and veins support a local source for CO2 fluids rather than a mantle derived carbon-bearing fluid for vein type graphite. Thus, carbon is recycled and retained as graphite in the continental crust during high-grade metamorphism and anatexis, though its isotopic composition can be considerably modified during orogenesis. In summary, a comprehensive study of carbon isotopic composition of graphite occurrences in the Lützow-Holm Complex, East Antarctica has thus revealed that prograde metamorphism, anatexis and interaction between carbonate lithologies with silicate rocks can modify carbon isotopic composition of graphite in the continental crust. Recycling of carbon within the continental crust during orogenesis where graphite act as "long-term sinks" of carbon has to be considered for envisaging realistic models on Earth’s carbon cycle.