硅酸盐岩石中微量碳酸盐的碳氧同位素分析及其应用

Carbon and Oxygen Isotope Analyses of Minor Carbonate in Silicate Rocks and Their Applications

硅酸盐岩石中总是或多或少地含有一些微量碳酸盐 ,但是至今尚未对其碳氧同位素地球化学开展研究。本文建立了对硅酸盐岩中微量碳酸盐的碳氧同位素分析方法 ,并以大别山双河地区两种片麻岩为例 ,讨论了其地球化学应用。对比实验证明 ,微量法可以将碳含量检出限降低至 2 2 0 μg。大别山双河两种片麻岩中微量碳酸盐的碳氧同位素测量发现 ,黑云母副片麻岩与花岗质正片麻岩具有明显不同的特征。副片麻岩的碳含量较高 ,δ1 3 C值为 - 4.5‰～ 0‰ ,指示其原岩为正常海相沉积环境 ,并与邻近大理岩的原岩沉积环境不同。正片麻岩的碳含量较低 ,δ1 3 C值为 - 2 3.4‰～ - 2 .1‰ ,反映出地表有机碳对岩浆岩原岩的混染。两种片麻岩中碳酸盐与硅酸盐全岩之间的氧同位素分馏既有处于平衡状态 ,也有处于不平衡状态。

Although there exist minor amounts of carbonate in silicate rocks, no study was carried out for its carbon and oxygen isotope geochemistry because of difficulty in analytical method. A micro-extraction method has been established to analyze the carbon and oxygen isotope compositions of the minor carbonate in silicate rocks. With this approach carbon and oxygen isotopes can be quantitatively measured when the carbon content of whole-rock is greater than 220μg. As a case study, carbon and oxygen isotopes were analyzed for biotite paragneiss and granitic orthogneiss at Shuanghe from Dabieshan. The biotite paragneiss shows high carbon content with δ 13C values of -4.5‰ to 0‰, pointing to the protolith of sedimentary origin in the normal marine environment. The granitic orthogneiss exhibits much lower carbon content with very dispersed δ 13C values of -23.4‰ to -2.1‰, indicating comtamination of surface organic carbon to igneous protolith. Oxygen isotope fractionations between micro-carbonate and host-gneiss show both equlibrium and disequilibrium. It appears that the disequlibrium fractionation reflects secondary alteration by retrograde fluid. Nevertheless, the combined carbon content and isotope analyses of micro-carbonate in metamorphic rocks can be used to provide a powerful distinction between the protolith types of sedimentary and igneous origins.