重晶石离子探针S-O同位素微区分析标准物质研制、分析方法研发及其在矿床中的应用研究

重晶石成因多样，是各类岩石与热液矿床中最常见且高度稳定的矿物之一。重晶石的S-O同位素组成研究在示踪物源、反演物理化学与生物地质过程、探讨矿床成因等方面具有重要意义。离子探针可实现高精度、高分辨率的微区原位同位素组成分析，但由于仪器质量分馏和基体效应的存在，研发高质量的重晶石标准物质是开展各种地质应用的前提。目前，国际上尚未报道有适用于离子探针S-O同位素分析的重晶石标准物质。本项目已从全球11个地区收集到重晶石矿物晶体，拟通过主微量元素组成、结构和离子探针S-O同位素均一性分析，气体质谱法定值，研发2-3个元素同位素组成均匀的标准物质，建立基于成分变化的基体效应校正方法。研究光轴效应及其校正方法，建立和完善离子探针重晶石微区S-O同位素分析流程。精选典型矿床中的黄铁矿-重晶石矿物对，开展同位素地质温度计精度验证研究，为重晶石在矿床成因研究中的应用奠定基础。

Formed in variety of settings, barite (BaSO4) is a highly stable and widely-distributed mineral found in magmatic, metamorphic, and sedimentary rocks (of all ages), as well as in hydrothermal deposits. Sulfur and oxygen isotope ratios in barites have been shown to be useful for identifying the origin of the solutions, tracing the physicochemical and biochemical processes affecting nucleation and crystal growth, and decoding the genesis of mineral deposits. With modern Secondary Ion Mass Spectrometry (SIMS) instrumentation, rapid in situ determination of 34S/32S and 18O/16O isotope ratios is feasible, with a measurement repeatability in high accuracy and sufficient mass resolution. Yet, SIMS analyses are hampered by an important instrumental mass fractionation (IMF) and the matrix effect of mineral composition. Well-calibrated barite reference materials for normalisation and quality control purposes were used to measure and calibrate the sulfur and oxygen isotope compositions prior to evaluate the extend of such determination for geological applications. Until now, it is still lacking of suitable reference materials for barite are accessible in larger quantities for matrix-matched SIMS S-O isotope analyzing. In this project, we plan to investigate the suitability of big barite crystals from eleven different regions around the world as homogeneous reference materials for matrix-matched S-O isotope geochemistry. First of all, we characterise these materials by means of major and trace element geochemistry, as well as their S-O isotope compositions, in order to show that these barite crystals has not been affected by heterogeneous growth and later-stage fluid-mediated processes. We assess the extent of matrix-induced bias that cased by composition changes. After that, homogeneous crystals will be selected and their sulfur and oxygen isotope ratios are measured with isotope ratio mass spectrometry systems from at least three different labs for per isotope system as high precision and accurate calibration measurements. Furthermore, crystal orientation effects on instrumental bias will be further detected in optimized barite crystals, and corresponding analysis process can be established for SIMS. Measurements of δ34S in grains of barite and pyrite in ores from some special mineral deposits could be conducted to evaluate the suitability of this analytical technique for in situ sulfur isotope thermometry accuracy in appropriate samples, and further to provide a means by which to tackle the genesis of mineral deposit in various geological setting.