中国东南部花岗岩的碳含量和同位素比值

Carbon Content and Isotope Composition of Granites from Southeastern China

本文应用EA-MS连线技术(将元素分析仪(EA)与气体质谱仪(MS)连接起来,进行硅酸盐岩石微量碳的含量和同位素比值测定)首次测定了中国东南部花岗岩及其部分磷灰石的全碳含量和同位素组成,结果发现,这些花岗岩的碳含量和同位素组成分布较宽(碳含量为0.04％～0.79％,δ^(13)C值为-7.5‰～-37.0‰)。磷灰石与全岩的δ^(13)C值呈正相关关系,指示自花岗岩浆上升到结晶以来未受到同化混染或热液蚀变扰动,所以全岩碳同位素组成反映了其源区δ^(13)C值的不均一性。对部分全岩进行HCl溶解后分析,结果表明碳在花岗岩中以不同的形式存在(碳酸盐碳和非碳酸盐碳)。碳含量和同位素组成分别成对数正态分布和双峰正态分布,反映了不同含碳储库的地球化学混合。结合已有的Nd、Sr同位素数据,发现碳同位素组成与钕、锶同位素和碳含量之间的相关性呈非简单壳幔两端元混合特征,其中地幔端元的δ^(13)C值约为-7‰,而地壳端元的δ^(13)C值变化较大(-20‰～-35‰),对应的初始Sr同位素比值变化也较大(0.715～0.735),但初始Nd同位素组成相对均一(ε_(Nd)=-18)。部分初始Sr同位素比值<0.710的Ⅰ型花岗岩同样具有低的δ^(13)C值,可能指示其火成岩原岩曾出露过地表,受到了有机碳的混染。

Both the content and isotope composition of bulk carbon in apatite and the granites of different ages from southeastern China have been determined by using the EA-MS online techniques. The results show that the carbon content ranges from 0. 04% to 0. 79%, and δ13C from -7. 51‰ to - 37. 03‰ for the granites. A positive correlation in δ 13C between apatite and the granite indicates that the granitic magma was not affected by crust contamination or hydrothermal alteration during its uplift and crystallization. so that the isotope composition of bulk carbon reflectes the isotopic feacture of its source. Some of the granites were re-analyzed after the HC1 dissolution, and the results show that there are different forms of carbon in granites (carbonate carbon and noncarbonate carbon). The content and isotope composition of bulk carbon show a log-normal distribution and a two-peak normal distribution, respectively, pointing to mixing processes between different carbon reservoirs. Along with the Nd and Sr isotope data, a non-simple two end-member mixing model is suggested with respect to the carbon isotope composition and carbon content of the granites as well as Nd and Sr isotopes. The two end-members have the isotopic fe-actures of the mantle and the crust, respectively. The mantle end-member has a single δ13C value of - 7‰, whereas the crustal end-member has variable δ13C values from - 35‰ to - 20‰ and also corresponds to large variations in the intial 87Sr/86Sr ratios (0. 715 to 0. 735) but a unified εNd value of -18. Some of the I-type granites with the initial 87Sr/86Sr ratios less than 0. 710 also have the lowerδ 13C values of - 30‰ to -20‰- probably implying that their igneous pro-toliths have been to surface and been suffered contamination of the organic carbon.