湖南砂矿金刚石包裹体原位测试:对金刚石成因来源的启示

In-situ Analysis of Mineral Inclusions in Alluvial Diamonds from Hunan: Insights into the Provenance and Origin of the Diamonds

包裹体是探究矿物结晶生长过程及其地球化学环境的重要探针,是反演寄主矿物形成机制及形成环境的有效工具。本文采用显微激光拉曼光谱、电子探针和LA-ICP-MS技术对来自湖南沅水流域的24颗宝石级金刚石的包裹体进行了原位分析。结果显示,湖南砂矿金刚石中橄榄岩型和榴辉岩型包裹体比例接近,橄榄岩型和榴辉岩型包裹体可在同一颗金刚石中＂共存＂;湖南金刚石形成时地幔的温度和压力分别为1109-1237℃和4.05-5.83 GPa,相应的形成深度大致为133-192 km;包裹体成分及组合特征显示,扬子克拉通金刚石的形成环境与华北克拉通金刚石有显著差异,前者的形成与榴辉岩的关系更为密切,暗示有古老的地壳物质参与了金刚石的形成过程。

Mineral inclusions are important probes to explore the crystallizing process, geochemical environment, and forming mechanism for their host minerals. In-situ microscopic laser Raman spectroscopy, electron microprobe and LA-ICP-MS analysis were carried out on mineral inclusions in 24 gem-grade alluvial diamonds from the Yuanjiang Basin in Hunan province. The results showed that the portions of eclogite- and peridotite-type mineral inclusion assemblages are similar and can be coexist in diamond grains. The calculated temperatures, based on nitrogen aggregation states and their portions, range from 1109 to 1237 ℃, and the forming pressures calculated using Raman barometry range from 4.05 to 5.83 GPa for olivine inclusions in the Hunan diamonds, corresponding to a depth of 133-192 km. Furthermore, the mineral inclusions and assemblages reveal that the Hunan diamonds were formed in a lithospheric mantle environment in the Yangtze Craton, which is distinct from the diamonds from the North China Craton. The Hunan diamonds may be formed in a lithospheric mantle containing abundant eclogite, showing the participation of ancient crustal material in the formation of the diamonds.