低温拉曼光谱分析流体包裹体盐度的条件约束

The Constraints on the Method of Using Cryogenic Raman Spectroscopy to Determine the Salinities of Fluid Inclusions

流体包裹体盐度及类型是分析地质流体作用的重要地球化学参数。NaCl-H2O体系是地质体中最常见的流体体系之一,其盐度通常由显微测温法获得,而盐水合物的低温拉曼光谱不仅可以用来计算流体包裹体盐度,还可以区分盐水类型。理论上讲,低温冷冻条件下的流体包裹体并非均匀体系,单一测点的拉曼光谱具有较强的局限性,由其计算的盐度并不能代表整个流体包裹体的盐度。为了更好地了解低温条件下流体包裹体的相变特征及其拉曼光谱对盐度的响应,本文通过配置五种不同浓度的NaCl溶液,研究了其在低温下的结晶过程及拉曼光谱特征。结果显示,在反复冷冻与升温过程中,冰晶首先形成,而水石盐的形成依赖于盐溶液的浓缩,多形成于冰晶间的缝隙中。水石盐的四个拉曼特征峰中,p1[(3 402±1)cm-1]和p2[(3 419±1)cm-1]相对强度稳定,p3[(3 432±2)cm-1]和p4[(3 535±4)cm-1]相对强度随盐度增加发生大幅度变化,从而导致相同盐度样品不同测点的拉曼特征比值随盐度增加而愈发离散。因此,传统的流体包裹体单一测点低温拉曼测盐误差较大,数据分析显示多点测试统计平均值才能更好的反映流体的真实盐度。相对于强度和半高宽,总峰面积与盐度相关性最好,是计算盐度的首选参数。该研究阐述了低温拉曼测盐的实验操作和数据处理方法,并阐明了其在流体包裹体分析中的应用条件。尽管操作过程较复杂,但其抗干扰强,应用盐度范围广,计算结果可靠,是重要的测盐方法。

Knowledge of the salinity of fluid inclusions,including the type of salts and their amounts,is of great importance for the interpretation of geological fluids and their role in rockforming processes like diagenesis,metamorphism,and hydrothermal processes.One of the most vital binary fluid systems for understanding geological processes is the NaCl-H2O system.The salinity of aqueous solutions in fluid inclusions is commonly determined with microthermometry.Moreover,the cryogenic Raman spectra of hydrates of chloride salts in fluid inclusions can be not only used to calculate salinities,but also used to distinguish different brine types.Theoretically,the salinity of one fluid inclusion can＆#39;t be calculated via the Raman spectrum in a single point because of the heterogeneity of the fluid inclusion at low temperature and the location of focus range of the laser beam.In order to have a better understanding of the characteristics of phase change of fluid inclusions at low temperature and reveal their Raman spectra responses to salinities,this paper studied the crystallization process and Raman spectroscopy of ice and hydrohalite in NaCl solutions with 5 different salinities.The result shows that in the process of freezing and heating,the crystallization of hydrohalite occurs from an interstitial,hypersaline liquid among the prior formed ice crystals.The forming of hydrohalite may depend on the concentration of brine.The relative intensities of p1 [（3 402±1）cm-1] and p2 [（3 419±1）cm-1],which are the two of four characteristic Raman peaks,are very stable,while the variation of the relative intensities of the other two peaks p3 [（3 432±2）cm-1] and p4 [（3 535±4）cm-1] increase with the salinity increasing because of different relative crystal orientations of hydrohalite compared to the polarized laser beam.Therefore,the characteristic ratios of Raman peaks at different points in the same sample are usually variable,and the traditional method of calculating salinities of the fluid inclusion using cryogenic Raman spectrum at one single point makes a big error.Based on large data statistics and analysis,the results indicate that the statistical average value of characteristic Raman peaks maybe reflects the salinity of the fluid inclusion better for the first time.With respect to the relative intensity and the width at half height of the characteristic Raman peaks,the total peaks＇ area shows the best linear dependence on the salinity,which is the first choice for the calculation of the salinity of brine.The results of this study expounds the suitable experimental operation and data analysis methods of the salinity calculation of brine using cryogenic Raman spectroscopy,and illustrates the applied conditions of this method in fluid inclusions analysis.Although it＇s a complex operation process to calculate salinity using cryogenic Raman spectra,this method is very important because of its anti-interference,large applied range and reliability.