硅酸盐岩中微量碳酸盐的碳氧同位素分析及其地球化学应用  

Carbon and oxygen isotope analyses of minor carbonate in silicate rocks and their geochemical applications.

作　　者：李龙[1] 郑永飞[1] 周根陶[1] 龚冰[1] 傅斌[1] 赵子福[1] 

机构地区：[1]中国科学技术大学地球和空间科学系化学地球动力学研究实验室,合肥230026

出　　处：《岩石学报》2002年第1期109-116,共8页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(40033010);科技部国家重点基础研究发展规划项目(G1999075503);中国科学院知识创新工程项目(KZCX2-107)资助成果.

摘　　要：硅酸盐岩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。通过对比实验证明,微量法通常可将碳含量检出限降低至5μg/g,特殊情况下可至1μg/g。对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,δ13C值为-4.5‰～0‰,指示其原岩为正常海相沉积环境,并与邻近大理岩原岩的灰岩沉积环境不同。正片麻岩的碳含量较低,δ13C值为-23.4‰～-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。不平衡分馏指示其受到过后期退变质流体的影响。不过,变质岩中微量碳酸盐的碳含量和碳同位素比值分析能够对原岩类型提供有效的判别。There exist minor amounts of carbonate in silicate rocks, but no study was carried out for its carbon and oxygen isotope geochemistry because of difficulty in analytical technique. 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 both carbon and oxygen isotope compositions can be quantitatively determined if the carbon concentration of whole-rock samples is greater than 5mug/g in general and even 1 mug/g in particular. A case study is presented for biotite paragneiss and granitic orthogneiss at Shuanghe from Dabieshan. The biotite paragneiss shows high carbon contents with delta(13)C values of -4.5 to 0parts per thousand, pointing to the protolith of sedimentary origin in the normal marine environment. The granitic orthogneiss exhibits much lower carbon contents with very dispersed delta(13)C values of -23.4 to -2.1parts per thousand, indicating comtamination of surface organic carbon to igneous protolith. Oxygen isotope fractionations between micro-carbonate and host-gneiss are at 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 make a distinction in origin between sedimentary and igneous protoliths.

关 键 词：碳同位素 氧同位素 微量碳酸盐 副片麻岩 正片麻岩 退变质作用 大别山 

分 类 号：P588.245[天文地球—岩石学] P597.2[天文地球—地质学]