Abstract The trace element and radiogenic isotope systematics of clinopyroxene have frequently been used to characterise mantle metasomatic processes, because it is the main host of most lithophile elements in the lithospheric mantle. To further our understanding of mantle metasomatism, both solution-mode Sr-Nd-Hf-Pb and in situ trace element and Sr isotopic data have been acquired for clinopyroxene grains from a suite of peridotite (lherzolites and wehrlites), MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside), and PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks from the Kimberley kimberlites (South Africa). The studied mantle samples can be divided into two groups on the basis of their clinopyroxene trace element compositions, and this subdivision is reinforced by their isotopic ratios. Type 1 clinopyroxene, which comprises PIC, wehrlite, and some sheared lherzolite samples, is characterised by low Sr (~100–200 ppm) and LREE concentrations, moderate HFSE contents (e.g., ~40–75 ppm Zr; La/Zr ~ 0.05. The isotopic compositions of Type 2 clinopyroxenes are more variable and extend from compositions resembling the “enriched mantle” towards those of Type 1 rocks (e.g., eNdi = −12.7 to −4.4). To constrain the source of these variations, in situ Sr isotope analyses of clinopyroxene were undertaken, including zoned grains in Type 2 samples. MARID and lherzolite clinopyroxene cores display generally radiogenic but variable 87Sr/86Sri values (0.70526–0.71177), which are correlated with Sr contents and La/Zr ratios, and which might be explained by the interaction between peridotite and melts from different enriched sources within the lithospheric mantle. Most notably, the rims of these Type 2 clinopyroxenes trend towards compositions similar to those of the host kimberlite and Type 1 clinopyroxene from PIC and wehrlites. These results are interpreted to represent clinopyroxene overgrowth during late-stage (shortly before/during entrainment) metasomatism by kimberlite magmas. Our study shows that a pervasive, alkaline metasomatic event caused MARID to be generated and harzburgites to be converted to lherzolite in the lithospheric mantle beneath the Kimberley area, which was followed by kimberlite metasomatism during Cretaceous magmatism. This latter event is the time at which discrete PIC, wehrlite, and sheared lherzolite lithologies were formed, and MARID and granular lherzolites were partly modified.

中文翻译：

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单斜辉石的同位素分析证明了金伯利岩熔体交代作用对岩石圈地幔的影响

摘要 单斜辉石的微量元素和放射性同位素系统学常被用来表征地幔交代过程，因为它是岩石圈地幔中大多数亲石元素的主要宿主。为了进一步了解地幔交代作用，从一组橄榄岩（锂辉石和纬长岩）、MARID（云母角闪石）中获得了单斜辉石颗粒的溶液模式 Sr-Nd-Hf-Pb 和原位微量元素和 Sr 同位素数据-金伯利金伯利岩（南非）的金红石-钛铁矿-透辉石）和 PIC（金云母-钛铁矿-斜辉石）岩石。所研究的地幔样品可以根据其单斜辉石微量元素组成分为两组，并且这种细分通过它们的同位素比得到加强。1 型单斜辉石，包括 PIC、wehrlite、这些 2 型单斜辉石的边缘趋向于类似于主体金伯利岩和来自 PIC 和 Wehrlite 的 1 型单斜辉石的成分。这些结果被解释为代表金伯利岩岩浆交代后期（不久前/夹带期间）的单斜辉石过度生长。我们的研究表明，在金伯利地区下方的岩石圈地幔中，普遍存在的碱性交代事件导致 MARID 生成，并且斜方辉石转化为锂辉石，随后在白垩纪岩浆作用期间发生金伯利岩交代。后一事件是形成离散的 PIC、纬长岩和剪切的二长橄榄岩岩性并且 MARID 和粒状二长橄榄岩部分改性的时间。