Carbon concentration and isotope composition of granites from Southeast China

Both concentration and isotope composition of bulk carbon in Phanerozoic gr anites 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 delta C-13 from -37.0 to -7.5 parts per thousand. Apatite has uniformly low VC values of -28.9 to -18.7 parts per thousand with contents of 0.04 to 0. 19wt%. Some of the granites were re-analyzed after HCl leaching, and the re sults show the presence of the different forms of carbon in granites (at le ast carbonate and noncarbonate carbons). There is a correlation in delta C- 13 between apatite and host granite, indicating that the carbon isotope com position of the granitic magmas was not significantly affected by magma deg assing, crustal contamination and hydrothermal alteration during their empl acement and subsolidus crystallization. Therefore, the isotope composition of bulk carbon in the granites reflects the isotopic fingerprint of their s ources. While the low VC values for the apatite are interpreted to be produ ced by the oxidation of organic and/or elemental carbon during magma genera tion, the higher delta C-13 values for the granites suggest the carbon isot ope heterogeneity in their source survived after magma generation. The concentration and isotope composition of bulk carbon exhibit a log-norm al distribution and two-peaks normal distribution, respectively, pointing t o mixing processes between different carbon reservoirs. Along with Nd and S r 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 delta C-13 value of -5 parts per thousand which cor responds to a Sr-87/Sr-86 ratio of 0.704 and a epsilon (Nd)(t) of 0 like th e PREMA, whereas the crustal end-member requires variable delta C-13 values of -35 to -25 parts per thousand which correspond to a large variation in initial Sr-87/Sr-86 ratios of 0.715 to 0.740 but a single epsilon (Nd)(t) v alue of -18. Although the crust-mantle mixing can explain their Nd and Sr i sotopic patterns, the mantle-like VC value cannot be simply ascribed to a p rimary mantle source because such a VC value may be produced by the mass-ba lanced homogenization of sedimentary carbonate and organic carbon in the gr anite precursors. Some of the granite of igneous origin with the initial Sr -87/Sr-86 ratios less than 0.710 also have the low delta C-13 values of -30 to -20 parts per thousand, 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 o f surface organic carbon into the granite sources prior to magma generation . (C) 2001 Elsevier Science Ltd. All rights reserved.