低温拉曼光谱对H2O-NaCl-CaCl2体系流体包裹体溶质成分定量分析探索

Fluid inclusions of H2O-NaCl-CaCl2 system are very common in the natural world, and most of them are related to ore-forming processes. Cryogenic Raman spectroscopy has the advantages of being individual, in-situ, and non-destructive to analyze the solutes of fluid inclusions. Previous studies have identified the peaks of various hydrates and established a semi-quantitive equation for XNaCl analysis. However, limited by the accuracy of the Laser Raman and the common phenomena of metastability, most of the previous studies remain quality or semi-quantitive level. The difficulties for quantitive analysis are summarized as followed: 1) The frozen fluid inclusions are composed of aggregates of various compounds, and it is difficult for the traditional microscope to observe their crystals and behaviors; 2) The point-analysis of traditional Raman cannot cover all the compounds of aggregates, which is harmful for a quantitive analysis. With the development of High-Resolution Raman spectroscopy, the proposed project aims to develop a quantitive method for solute analysis. The critical techniques includes: 1) 3D imaging is able to display all parts of the materials in a three-dimensional space, which enables the clear observation of the frozen fluid inclusion aggregates, and discovery of the metastability phenomena; 2) 2D mapping scanning can collect the spectra of the whole aggregates, which is beneficial for quantitive analysis. The proposed quantitive analytical methods with cryogenic Raman is expected to provide more implications for the evolution of ore-forming fluids of H2O-NaCl-CaCl2 system, as well as the genesis of related mineral deposits.

H2O-NaCl-CaCl2体系流体包裹体在自然界中普遍存在，且与多种成矿过程有关。低温激光拉曼技术对流体包裹体溶质成分的研究具有单个、原位、无损分析的优点。前人利用该技术确认了水合物的谱峰、建立了XNaCl的半定量分析公式，但受拉曼光谱采集精度和多元体系流体亚稳现象所限，尚未实现定量分析。具体难点为：1）冻结后的包裹体呈多种化合物集合体形式，传统的显微镜很难清晰地观察其结晶形态；2）传统的激光拉曼点分析不能均匀涵盖集合体中的所有化合物，不利于定量化测试。本项目拟依托高分辨激光拉曼光谱技术探索溶质成分的定量分析，其关键技术为：1）3D成像技术可在三维空间内展示物质的内部组成，有利于冷冻包裹体集合体的观察；2）2D面扫描分析技术可准确获取集合体的拉曼谱峰，对包裹体成分的定量分析具有重要意义。激光拉曼定量分析方法的建立将为该体系体系成矿流体的演化以及矿床成因机制等研究提供科学依据。

H2O-NaCl-CaCl2体系流体在地质环境中普遍存在，研究该体系流体的溶质成分，对探究古地质流体的来源和演化过程有着重要意义。常温下NaCl和CaCl2的水溶液不具有拉曼活性，因此前人多采用低温拉曼研究该体系流体包裹体的成分。但是，冻结的流体包裹体为冰和水合物的集合体且分布不均匀，传统的激光拉曼点分析很难准确获取溶质的成分。共聚焦激光拉曼光谱技术具有高空间分辨率、高光谱分辨率的优点，且允许在短时间内对物质表面进行高质量地面扫描，对包裹体物质成分分析向定量方向发展有着重要意义。. 该研究首先配制了一系列标准溶液并制备成人工流体包裹体，然后在-185oC下利用高分辨共聚焦激光拉曼光谱技术获取水合物和冰的面扫描图像，最后建立了NaCl水合物拉曼谱峰面积比与NaCl所占摩尔浓度的线性方程X(NaCl, m) = 1.1435 Fhydrohalite – 0.0884, （Fhydrohalite 为 NaCl水合物特征谱峰所占面积比)。实验过程中，低温拉曼2D和3D扫描图像均进一步证实，冰和水合物的分布具有不均匀性，并且颗粒叠置现象普遍。. 将该方程应用于天然包裹体中，成功获得了与扫描电镜分析相近的NaCl摩尔浓度结果，证实低温拉曼面扫描方法的可靠性和可行性。多次实验表明将低温拉曼扫描技术方法应用于自然界的包裹体时，可采用多次升降温找到最优回温温度的方式来促进水合物的生成；适当减小扫描步长来减少颗粒间的相互影响；适当增加曝光时间和取峰次数，以获得更高质量的谱峰。该低温拉曼扫描方法可以推广到其他类型溶液的溶质组成定量分析，并有可能成为流体包裹体溶质成分分析的有力工具。