Primary carbonates in kimberlites are the main CO 2 carriers in kimberlites and thus can be used to constrain the original carbon and oxygen-isotope composition of kimberlite melts and their deep mantle sources. However, the contribution of syn- and post-emplacement processes to the modification of the C–O-isotope composition of kimberlites is yet to be fully constrained. This study aims to shed new light on this topic through a detailed textural, compositional (major and trace elements), and in situ C–O–Sr isotopic characterisation of carbonates in the Benfontein kimberlite sills (Kimberley, South Africa). Our multi-technique approach not only reveals the petrographic and geochemical complexity of carbonates in kimberlites in unprecedented detail, but also allows identification of the processes that led to their formation, including: (1) magmatic crystallisation of Sr-rich calcite laths and groundmass; (2) crystallisation of late groundmass calcite from hydrothermal fluids; and (3) variable degrees of crustal contamination in carbonate-rich diapirs and secondary veins. These diapirs most likely resulted from a residual C–O–H fluid or carbonate melt with contributions from methane-rich fluids from the Dwyka shale wall rock, leading to higher 87 Sr/ 86 Sr and δ 18 O, but lower δ 13 C values than in pristine magmatic calcite. Before coalescing into the diapiric segregations, these fluids/melts also variably entrained early formed calcite laths and groundmass phases. Comparison between in situ and bulk-carbonate analyses confirms that O isotopic analyses of bulk carbonates from kimberlite rocks are not representative of the original isotopic signature of the kimberlite magma, whereas bulk C-isotope compositions are similar to those of the pristine magmatic carbonates. Calcite laths and most groundmass grains at Benfontein preserve isotopic values ( δ 18 O = 6–8‰ and δ 13 C = − 4 to − 6‰), similar to those of unaltered carbonatites worldwide, which, therefore, probably correspond to those of their parental melts. This narrow range suggests kimberlite derivation from a mantle source with little contribution from recycled crustal material unless the recycled material had isotopic composition indistinguishable from typical mantle values.

中文翻译：

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对来自 Benfontein 窗台（南非）碳酸盐的原位 C-O-Sr 同位素分析的金伯利岩的来源、组成和侵位后改性的新限制

金伯利岩中的原生碳酸盐是金伯利岩中主要的CO 2 载体，因此可用于约束金伯利岩熔体及其深部地幔源的原始碳和氧同位素组成。然而，同步和后入位过程对金伯利岩 C-O 同位素组成改变的贡献尚未完全受到限制。本研究旨在通过对 Benfontein 金伯利岩基岩（南非金伯利）中碳酸盐的详细结构、成分（主要和微量元素）和原位 C-O-Sr 同位素表征来阐明这一主题。我们的多技术方法不仅以前所未有的细节揭示了金伯利岩中碳酸盐岩的岩相学和地球化学复杂性，而且还可以确定导致其形成的过程，包括：(1) 富锶方解石板条和基质的岩浆结晶作用；(2) 热液中晚期地块方解石结晶；(3) 富含碳酸盐的底辟和次生脉中不同程度的地壳污染。这些底辟很可能是由残余的 C-O-H 流体或碳酸盐熔体产生的，其中来自 Dwyka 页岩围岩的富含甲烷的流体的贡献，导致较高的 87 Sr/86 Sr 和 δ 18 O，但较低的 δ 13 C 值比在原始岩浆方解石中。在聚结到底辟偏析中之前，这些流体/熔体也不同程度地夹带了早期形成的方解石板条和地块相。原位和块状碳酸盐分析之间的比较证实，来自金伯利岩岩石的块状碳酸盐的 O 同位素分析不能代表金伯利岩岩浆的原始同位素特征，而大量碳同位素组成与原始岩浆碳酸盐相似。Benfontein 的方解石板条和大多数地块保留了同位素值（δ 18 O = 6–8‰ 和 δ 13 C = − 4 到 − 6‰），类似于世界范围内未改变的碳酸盐岩的同位素值，因此，它们可能对应于他们的父母融化了。这个狭窄的范围表明金伯利岩源自地幔源，而回收地壳材料的贡献很小，除非回收材料的同位素组成与典型的地幔值无法区分。可能对应于他们父母融化的那些。这个狭窄的范围表明金伯利岩源自地幔源，而回收地壳材料的贡献很小，除非回收材料的同位素组成与典型的地幔值无法区分。可能对应于他们父母融化的那些。这个狭窄的范围表明金伯利岩源自地幔源，而回收地壳材料的贡献很小，除非回收材料的同位素组成与典型的地幔值无法区分。