Carbon concentration and isotope composition of granites from Southeast China

Abstract

Both concentration and isotope composition of bulk carbon in Phanerozoic granites from Southeast China have been determined by EA-MS online techniques. The results show that the carbon content ranges from 0.04 to 0.79wt% and δ13C from −37.0 to −7.5‰. Apatite has uniformly low δ13C values of −28.9 to −18.7‰ with contents of 0.04 to 0.19wt%. Some of the granites were re-analyzed after HCl leaching, and the results show the presence of the different forms of carbon in granites (at least carbonate and noncarbonate carbons). There is a correlation in δ13C between apatite and host granite, indicating that the carbon isotope composition of the granitic magmas was not significantly affected by magma degassing, crustal contamination and hydrothermal alteration during their emplacement and subsolidus crystallization. Therefore, the isotope composition of bulk carbon in the granites reflects the isotopic fingerprint of their sources. While the low δ13C values for the apatite are interpreted to be produced by the oxidation of organic and/or elemental carbon during magma generation, the higher δ13C values for the granites suggest the carbon isotope heterogeneity in their source survived after magma generation.The concentration and isotope composition of bulk carbon exhibit a log-normal distribution and two-peaks normal distribution, respectively, pointing to mixing processes between different carbon reservoirs. Along with Nd and Sr isotope data for the granites, a two end-member mixing model is suggested between the isotope composition and content of bulk carbon in the granites as well as between Nd, Sr and C isotopes. A mantle-like end-member can be represented by a single δ13C value of −5‰ which corresponds to a 87Sr/86Sr ratio of 0.704 and a εNd(t) of 0 like the PREMA, whereas the crustal end-member requires variable δ13C values of −35 to −25‰ which correspond to a large variation in initial 87Sr/86Sr ratios of 0.715 to 0.740 but a single εNd(t) value of −18. Although the crust-mantle mixing can explain their Nd and Sr isotopic patterns, the mantle-like δ13C value cannot be simply ascribed to a primary mantle source because such a δ13C value may be produced by the mass-balanced homogenization of sedimentary carbonate and organic carbon in the granite precursors. Some of the granite of igneous origin with the initial 87Sr/86Sr ratios less than 0.710 also have the low δ13C values of −30 to −20‰, implying that their igneous protoliths would be exposed to surface and suffered the contamination by organic carbon before remelting. Therefore, the carbon isotope study reveals significant input of surface organic carbon into the granite sources prior to magma generation.