白云鄂博碳酸岩墙碳氧同位素地球化学  

Carbon and oxygen isotope geochemistry of carbonatite dykes from Bayan Obo, Nei Mongol

作　　者：舒勇[1] 郑永飞[1] 魏春生[1] 周建波[1] 杨学明[1] 杨晓勇[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,安徽合肥230026

出　　处：《地球化学》2001年第2期169-176,共8页Geochimica

基　　金：国家重点基础研究发展规划项目资助! (G1999043204)

摘　　要：对内蒙古白云鄂博 REE- Fe- Nb矿床周围碳酸岩墙中共存的方解石和白云石进行了 C和 O同位素分析。结果表明，方解石和白云石的δ 13C值变化范围一致，均为－ 3.5‰～－ 7.3‰，落在正常地幔δ 13C值范围 (－ 5‰± 2‰ )内；而它们的δ 18O值可分为两组，第Ⅰ 组为 9.5‰～ 18.0‰，第Ⅱ 组为 20.6‰～ 22.6‰，均远大于正常地幔δ 18O值范围 (5.7‰± 1.0‰ )。第Ⅰ 组低δ 18O值样品中共存白云石与方解石之间的 C和 O同位素分馏均为负值，因此处于热力学不平衡状态，指示它们自形成后受到过后期热液蚀变，与先前的岩石学观察一致。相反，第Ⅱ 组高δ 18O值样品中白云石与方解石之间的 C和 O同位素分馏均为正值，处于热力学平衡状态，指示它们自形成后未受到后期热液蚀变，因此可能沉淀于晚期低温高δ 18O值流体。第Ⅰ 组碳酸岩墙中白云石的 C和 O同位素组成不呈线性分布，指示碳酸岩浆并非由幔源碳酸盐与沉积碳酸盐混合形成。应用水-岩交换模型计算得到，第Ⅰ 组碳酸岩在侵位后经历了碳酸岩浆期后热液的不均一蚀变，蚀变温度约在 220～ 800℃之间，蚀变流体的 CO2/H2O比值较小 (1/500)，但水 /岩比值变化较大 (10～ 400)。由于低温下方解石与热液之间的碳氧同位素交换速率大于白云石，导致这部分碳酸?The carbon and oxygen isotope compositions of coexisting calcite and dolomite were measured for carbonatite dykes outside Bayan Obo REE- Fe- Nb deposit in Nei Mongol (Inner Mongolia). The results show that theδ 13C values of the calcite and dolomite vary in the same range of－ 3.5‰ to－ 7.3‰ , being within the normal mantleδ 13C values of－ 5‰± 2‰ ; while theirδ 18O values can be classified into two groups, the first varies from 9.5‰ to 18.0‰ , and the second from 20.6‰ to 22.6‰ , both being significantly higher than the normal mantleδ 18O values of 5.7‰± 1.0‰ . Carbon and oxygen fractionations between dolomite and calcite in the group I are negative and thus in thermodynamic disequilibrium, suggesting that they underwent hydrothermal alteration. In contrast, the fractionations are in thermodynamic equilibrium for the group II carbonatites and thus the coexisting calcite and dolomite probably coprecipitated from the same fluid at lower temperatures. The water- rock exchange modeling suggests that the group I carbonatites underwent heterogeneous alteration by post- carbonatite magmatic fluid. The calculated alteration temperatures are about 220 to 800℃ , CO2/H2O ratio in the fluid is about 1/500 and water/rock ratios are about 10 to 400. Since the rate of oxygen isotope exchange between calcite and fluid is faster than that between dolomite and fluid at low temperatures, the hydrothermal alteration results in the disequilibrium fractionations in the group I carbonatites. The carbon and oxygen isotope compositions of dolomite in the group I vary nonlinearly, precluding the possibility that the carbonatite magma would be produced by a simple binary mixing between mantle- derived carbonate magma and sedimentary limestone.

关 键 词：碳同位素 氧同位素 热液蚀变 方解石 白云石 碳酸盐岩 内蒙古自治区 

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