激光显微取样技术在川东北飞仙关组鲕粒白云岩碳氧同位素特征研究中的应用

Application of laser micro-sampling technique to analysis of C and O isotopes of oolitic dolomites in Feixianguan Formation, Northeast Sichuan

显微取样碳氧同位素分析技术利用高聚焦激光束与碳酸盐岩样品作用,是一种在显微薄片下对岩石特定组构产生CO2的有效方法,其空间分辨率20～50μm,δ13C和δ18O的最好分析精度可达±0.22‰(σ),可以为碳酸盐岩碳氧同位素提供“高分辨率、原位”数据,微区分析是同位素测试发展的重要方向。用该技术对川东北飞仙关组鲕粒白云岩地球化学特征进行的研究表明,它可避免样品不纯带来的解释上的误差,为解释鲕粒白云岩成因提供了较为准确的地球化学信息,可进一步认识川东北飞仙关组鲕粒白云岩的成因问题。各种产状白云石的δ13C‰(PDB)值均大于当时海水值,说明白云石化流体没有淡水注入;而δ18O‰(PDB)分布较为离散,伴随白云石晶体大小增加,δ18O‰(PDB)值明显偏负,说明白云石化作用过程经历了埋藏加深、温度升高的过程,并且白云石化流体远离卤水源,其咸化程度减弱,为鲕粒白云岩的卤水渗透回流成因提供了有力的证据。

Laser micro-sampling technique for analysis of C and O isotope in carbonate minerals is a useful tool to focus a high-power laser beam onto a carbonate sample to liberate CO2 gas. It is an effectice way to use a special carbonate structure in the slice to product CO2 gas, with a spatial resolution 20- 50μm, and optimal precision approximately ±0.22‰（σ） for both δ^13C and δ^18O in carbonate standard materials. The technique can provide a lot of high resolution and in situ data, and the micro-sampling technique is becoming an important developing tendency in δ^13C and δ^18O analysis. Geochemical characteristics of the oolitic dolomites in Feixianguan Formation, Northeast Sichuan are studied by using this technique. The results show that the interpreting errors resulted from impure samples are avoided due to the precise micro-sampling technique and the precise geochemical data may lay a foundation of the genesis of the oolitic dolomites. The δ^13C‰（PDB） values of the oolitic dolomites are more than that of the marine water at the time when the oolitic dolomites formed, illustrating that the dolomitization fluids were not mixed by the meteoric water. The δ^13O‰（PDB） values of the oolitic dolomites vary with the micro-sampling sites. They are lighter with the increasing of the dolomite crystalline size, illustrating the process of dolomitization underwent processes of deep burial and gradually rising temperature of the strata. The δ^18O‰（PDB） values are also lighter with decreasing salinity in dolomitization fluid. The laser micro-sampling technique can provide the geochemical evidence for explaining brine reflux origin in oolitic dolomites.