红外光谱分析可以有效地厘定金刚石类型和杂质成分, 揭示金刚石形成的物理化学条件及其源区特征, 约束金刚石的形成机制。 对湖南现代河流砂矿金刚石红外光谱原位分析结果显示, 样品以ⅠaAB型为主(93%), 含有少量ⅠaA型(5%)、 ⅠaB型(<1%)和Ⅱa型(1%); 大部分表现为中-低氮含量(35.0~436 μg·g-1)和中-低氮聚集度(3%~57%, 平均转化率为37%), 少数具有高氮特征(517~2 848 μg·g-1, 甚至高达6 829 μg·g-1)。 这些金刚石在地幔中的存储温度集中在1 100~1 230 ℃, 与前人根据金刚石内包裹体矿物得出的形成温度基本一致。 此外, 它们在地幔中的存储时间普遍较短(大部分<0.2 Ga)。 这些特点暗示湖南现代河流砂矿的金刚石可能主要形成于上地幔, 其来源与超基性岩或榴辉岩有关; 少数高氮含量、 低聚集氮特征的金刚石的存在则显示它们与榴辉岩关系更密切。 与扬子克拉通贵州原生金刚石红外光谱数据的对比显示, 贵州金刚石以高比例极低氮含量的Ⅱa型金刚石为主(占约75%), 同时具有破碎度大、 表面熔蚀强烈等特点, 表明其来源较深, 且在岩浆上升过程中经历了高温熔蚀作用。 红外光谱数据显示, 湖南砂矿和贵州原生矿来源的金刚石形成深度及其地球化学环境存在明显的差异, 可能是在扬子克拉通地幔不同深度范围或者不同阶段形成的产物。 进一步分析表明, 湖南砂矿金刚石和贵州原生金刚石可能构成了扬子克拉通内一个完整的金刚石形成序列, 它们分别与不同源区或不同的地球化学环境相对应。 前者搬运距离较短, 推测其源区可能主要来自近源补给区。 现代河流砂矿金刚石的红外光谱分析为进一步探索扬子克拉通金刚石的源区特征和形成环境提供了新的指示和约束。

FTIR spectrometry provides a crucial means to explore the types of diamonds and their impurities, which would to some extent reflect the physical-chemical conditions of the diamonds and also their origin characteristics, revealing the source information and genesis of the diamonds. In this paper, a set of samples of diamonds from the modern river placer in Hunan are analysed by in-situ measurement of FTIR spectrometry. The results show that the diamonds are mainly IaAB type (93%), with minor IaA type (5%), IaB type (<1%) and IIa type (2%). Most diamonds have medium to low nitrogen contents (35.0~436 μg·g-1) and medium to low nitrogen aggregation (3%~57%, ave. 37%), while the others are relatively high (517~2 848 μg·g-1, even up to 6 829 μg·g-1). The probable range of storage temperatures of the diamonds are between 1 100 and 1 230 ℃, similar to the formation temperature of mineral inclusion in diamonds reported by previous literature. In addition, their residence time in the mantle are generally short (most <0.2 Ga), indicating that the diamonds from the modern river placer in Hunan are probably mainly generated in the upper mantle, and their source rocks are related to both ultramafic and eclogitic parageneses. Few diamonds with high nitrogen contents and low nitrogen aggregation features may be more closely related to eclogitic parageneses. By further comparing with another place where we have found primary diamonds, Guizhou, Yangtze Craton, it shows that Guizhou diamonds have characteristics of high proportion of IIa type (75%), high degree of fragmentation, poor integrity, low quality and strong corrosion, indicating that the diamonds from Guizhou are derived from the deeper mantle than Hunan’s. The different types and features of the diamonds from Hunan and Guizhou may show the diversity of their sources. Furthermore, the placer diamonds from Hunan and primary diamonds from Guizhou may constitute a complete mineralization sequence in Yangtze Craton, which may correspond to different environment stages of diamond mineralization, respectively. The haul distance of the diamonds from Hunan placer is relatively short, indicating a near-field source. Therefore, FTIR spectrometry provides an approach to explore the origin features and formation condition of the diamonds in Yangtze Craton.