Nominally anhydrous mantle minerals (olivine, pyroxenes, garnets, etc.) in 11 peridotite xenoliths from four different uneconomic and economic Kaapvaal Craton kimberlite pipes (Matsoku, Thaba Putsoa, Pipe 200 and Bultfontein) have been investigated using Fourier transform infrared spectroscopy (FTIR). All xenoliths contain accessories of garnet, diopside, chromite, and phlogopite. High orthopyroxene content (>30 mol vol.%) in most xenoliths from all kimberlites and its interconnected channel-like nature hint towards hydrous siliceous fluid metasomatism. Peridotite xenoliths from uneconomic kimberlites show development of phlogopite and clinopyroxene (± chromite) forming veins and in garnet rims suggesting metasomatism by alkaline silico-carbonatite (possibly kimberlite-related) melt. The xenoliths contain significant H2O in olivine (17–62 ppm), orthopyroxene (21–230 ppm), and clinopyroxene (87–833 ppm), whereas garnets are dry and only show IR absorbance bands at > 3,670 cm−1 for contamination of hydrous minerals. Compared to the economic kimberlites in the Kaapvaal Craton, the uneconomic kimberlite xenoliths from this study have lower orthopyroxene and olivine H2O content. In the xenoliths affected by garnet breakdown metasomatism, the H2O content of orthopyroxene and olivine is higher and lower, respectively. The structural hydroxyl distribution profile across olivine and higher inter-mineral water partition coefficient, suggest diffusion of hydrogen and possible re-equilibration. Statistical analysis of the olivine spectra suggests that hydrogen bands at 3540, 3624, 3638, and 3672 cm−1 are a good discriminant of economic and uneconomic kimberlites and in literature, they are associated with metasomatism, weathering-associated processes, high water activity, and oxygen fugacity. The lower water concentration in xenoliths from uneconomic kimberlite from the margin of the craton than the economic kimberlites from the interior of the Kaapvaal Craton and identified metasomatism hints towards dehydration of xenoliths by water-poor and CO2-rich melts in tectonized cross-lithospheric zones causing diamond resorption and may be responsible for the diamond-poor nature of uneconomic kimberlites in northern Lesotho.

中文翻译：

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来自 Kaapvaal 克拉通金伯利岩的橄榄岩异晶石之间的非均质水分布：对金伯利岩钻石贫瘠性质的限制


使用傅里叶变换红外光谱 （FTIR） 研究了来自四种不同的非经济和经济 Kaapvaal 克拉通金伯利岩管（Matsoku、Thaba Putsoa、Pipe 200 和 Bultfontein）的 11 个橄榄岩异石中名义上的无水地幔矿物（橄榄石、辉石、石榴石等）。所有石榴石都包含石榴石、透辉石、铬铁矿和金云母的配件。所有金伯利岩的大多数异石中的高正辉石含量 （>30 mol vol.%） 及其相互连接的通道状性质暗示了含水硅质流体交代作用。来自不经济的金伯利岩的橄榄岩异石显示金云母和斜辉石（±铬铁矿）形成矿脉和石榴石边缘，表明碱性硅碳酸盐岩（可能与金伯利岩有关）熔融的交代作用。异石在橄榄石 （17-62 ppm）、斜辉石 （21-230 ppm） 和斜辉石 （87-833 ppm） 中含有大量的 H 2 O，而石榴石是干燥的，仅在 > 3,670 cm-1 处显示红外吸光带，用于含水矿物的污染。与 Kaapvaal 克拉通的经济金伯利岩相比，本研究中不经济的金伯利岩异晶石具有较低的斜辉石和橄榄石 H2O 含量。在受石榴石击穿交代作用影响的异石中，斜辉石和橄榄石的 H2O 含量分别较高和较低。橄榄石上的结构羟基分布曲线和较高的矿泉水间分配系数表明氢的扩散和可能的再平衡。 橄榄石光谱的统计分析表明，3540、3624、3638 和 3672 cm-1 处的氢带是经济和非经济金伯利岩的良好区分因素，在文献中，它们与交代作用、风化相关过程、高水活度和氧逸析性有关。来自克拉通边缘的不经济金伯利岩的异晶岩中的水浓度低于来自 Kaapvaal 克拉通内部的经济金伯利岩，并且已确定的交代作用暗示了构造跨岩石圈带中缺水和富含 CO2 的熔体导致异石脱水，导致钻石吸收，这可能是莱索托北部不经济的金伯利岩贫金刚石性质的原因。