Abstract Eclogite mantle xenoliths from various kimberlite occurrences on the Kaapvaal craton show evidence for depth- and redox-dependent metasomatic events that led to variable base metal sulphide and incompatible element enrichments. Eclogite xenoliths from the Roberts Victor, Jagersfontein, Kimberley (Kamfersdam) and Premier kimberlites were investigated for their silicate and base metal sulphide geochemistry, stable oxygen isotope compositions and oxybarometry. The variably metasomatised eclogites had basaltic, picritic and gabbroic protolith compositions and have garnet δ18O values that range from +3.3 to +7.9‰, which, when coupled with the trace element characteristics, indicate oceanic lithosphere protoliths that had undergone variable degrees of seawater alteration. The deepest equilibrated eclogites (175–220 km depth) from near the base of the Kaapvaal craton lithosphere are the most refractory and feature significant light rare earth element (LREE) depletions. They show the most oxidised redox compositions with ∆logƒO2 values of FMQ-3.9 to FMQ-1.5. Subtle metasomatic overprinting of these eclogites resulted in base metal sulphide formation with relatively depleted and highly fractionated HSE compositions. These deepest eclogites and their included base metal sulphides suggest interaction with relatively oxidised melts or fluids, which, based on their HSE characteristics, could be related to precursor kimberlite metasomatism that was widespread within the Kaapvaal craton mantle lithosphere. In contrast, eclogites that reside at shallower, “mid-lithospheric” depths (140–180 km) have been enriched in LREE and secondary diopside/phlogopite. Importantly, they host abundant metasomatic base metal sulphides, which have higher HSE contents than those in the deeper eclogites at the lithosphere base. The mid-lithospheric eclogites have more reducing redox compositions (∆logfO2 = FMQ-5.3 − FMQ-3.3) than the eclogites from the lowermost Kaapvaal lithosphere. The compositional overprint of the shallower mantle eclogites resembles basaltic rather than kimberlitic/carbonatitic metasomatism, which is also supported by their relatively reducing redox state. Base metal sulphides from the mid-lithospheric eclogites have HSE abundances and distributions that are similar to Karoo flood basalts from southern Africa, suggesting a link between the identified shallow mantle metasomatism of the Kaapvaal cratonic lithosphere and the Karoo large igneous event during the Mesozoic. The sulphide-hosted platinum group element abundances of the mid-lithospheric eclogites are higher compared with their analogues from the deeper lithospheric eclogites, which in combination with their contrasting oxidation states, may imply redox-controlled HSE mobility during sulphur-rich metasomatism of continental mantle lithosphere.

中文翻译：

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Kaapvaal克拉通榴辉岩的富硫地幔交代作用及其在氧化还原控制铂族元素迁移中的作用

摘要 Kaapvaal 克拉通上各种金伯利岩矿点的榴辉岩地幔包体显示了依赖于深度和氧化还原的交代事件的证据，这些交代事件导致了可变的贱金属硫化物和不相容的元素富集。研究了来自 Roberts Victor、Jagersfontein、Kimberley (Kamfersdam) 和 Premier 金伯利岩的榴辉岩捕虏体的硅酸盐和贱金属硫化物地球化学、稳定的氧同位素组成和氧气压计。变质交代榴辉岩具有玄武质、苦味质和辉长质原岩成分，石榴石δ18O值范围为+3.3～+7.9‰，结合微量元素特征，表明大洋岩石圈原岩经历了不同程度的海水蚀变。来自 Kaapvaal 克拉通岩石圈底部附近的最深的平衡榴辉岩（175-220 公里深度）是最难熔的，并具有显着的轻稀土元素 (LREE) 枯竭。它们显示了最氧化的氧化还原组合物，ΔlogƒO2 值从 FMQ-3.9 到 FMQ-1.5。这些榴辉岩的微妙交代叠印导致贱金属硫化物形成，其中 HSE 成分相对贫化和高度分馏。这些最深的榴辉岩及其包含的贱金属硫化物表明与相对氧化的熔体或流体相互作用，根据它们的 HSE 特征，这可能与 Kaapvaal 克拉通地幔岩石圈内广泛存在的前体金伯利岩交代作用有关。相比之下，位于较浅层的榴辉岩，“中岩石圈”深度（140-180 公里）富含轻稀土元素和次生透辉石/金云母。重要的是，它们拥有丰富的交代贱金属硫化物，其 HSE 含量高于岩石圈底部较深的榴辉岩中的硫化物。中间岩石圈榴辉岩比来自最低 Kaapvaal 岩石圈的榴辉岩具有更多的还原氧化还原成分 (ΔlogfO2 = FMQ-5.3 - FMQ-3.3)。较浅地幔榴辉岩的成分叠印类似于玄武岩而不是金伯利岩/碳酸盐岩交代作用，这也得到它们相对还原的氧化还原状态的支持。来自中岩石圈榴辉岩的贱金属硫化物的 HSE 丰度和分布类似于来自南部非洲的卡鲁泛滥玄武岩，表明已确定的 Kaapvaal 克拉通岩石圈浅层地幔交代作用与中生代 Karoo 大型火成岩事件之间存在联系。与来自更深岩石圈榴辉岩的类似物相比，中岩石圈榴辉岩的硫化物铂族元素丰度更高，这与其对比的氧化态相结合，可能意味着在大陆地幔富硫交代过程中氧化还原控制的 HSE 迁移岩石圈。