Polycrystalline diamond aggregates (PDAs) are small rocks composed mostly of diamond grains, but often containing also peridotitic, eclogitic and/or websteritic minerals as accessory phases. PDAs are formed rapidly in Earth’s mantle, and the diamonds preserve heterogeneity not often seen in monocrystalline diamond. Here, diamond grains from forty-three PDAs from the Venetia diamond mine (RSA) with grain-sizes < 1 mm are presented. They have heterogeneous and complex cathodoluminescence signatures that are best explained by multiple, separate diamond growth episodes from compositionally distinct COH fluids/melts. The diamonds show a large range of nitrogen concentrations (0.5 to 2,891 at. ppm), δ15N (−4.3 to + 16.8 ‰) and δ13C values (−27.8 to −7.6 ‰). The positive δ15N median of + 6.4 ‰ and negative δ13C median of −21.2 ‰ indicate derivation of the diamond-forming fluid from organic materials in subducted oceanic crust and lithosphere. Two PDAs have δ13C and δ15N values typical for Earth’s mantle. Thirty-three PDAs contain websteritic garnets, or peridotitic garnets and clinopyroxenes and/or micas. Unradiogenic εNdi values in the garnets (−15.9 to −29.7) and clinopyroxene (−8.3) and δ18O values of 6.49 to 8.09 ‰ in websteritic garnets are consistent with an origin from subducted altered oceanic crust and support the findings from N and C systematics in the diamonds. Nitrogen aggregation data for the diamonds range from 25 % to 100 %B and vary by as much as 60 % within some individual PDAs. We explain the geochemical and isotopic heterogeneity of diamonds and silicates as well as the complex cathodoluminescence features with a model of episodic melt/fluid −rock interaction involving a reducing asthenospheric melt in the cratonic roots or the thermal boundary layer. We suggest that large volumes of PDA are formed in the cratonic roots and thermal boundary layer by this mechanism, making them an important reservoir for carbon storage, which is corroborated with their locally high abundance (ca. 20 %) in some kimberlites.

中文翻译：

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多晶金刚石聚集体及其在地球深部碳循环中的作用


多晶金刚石骨料 （PDA） 是主要由金刚石颗粒组成的小岩石，但通常也包含橄榄岩、榴辉岩和/或腹板矿物作为辅助相。PDA 在地幔中迅速形成，钻石保留了单晶金刚石中不常见的非均质性。这里展示了来自 Venetia 钻石矿 （RSA） 的 43 个 PDA 的金刚石颗粒，粒度为 < 1 mm。它们具有异质和复杂的阴极发光特征，最好通过成分不同的 COH 流体/熔体的多个独立钻石生长事件来解释。钻石的氮浓度范围很广（ppm 为 0.5 至 2,891）、δ15N（-4.3 至 + 16.8 ‰）和 δ13C 值（-27.8 至 -7.6 ‰）。正 δ15N 中位数 + 6.4 ‰ 和负 δ13C 中位数 −21.2 ‰ 表示金刚石形成液是从俯冲洋壳和岩石圈中的有机物质衍生而来的。两个 PDA 具有地球地幔的典型 δ13C 和 δ15N 值。33 个 PDA 包含腹板石榴石，或橄榄石柘榴石和斜辉石和/或云母。石榴石中的非放射性 εNdi 值（-15.9 至 -29.7）和斜辉石 （-8.3） 和 δ18O 值为 6.49 至 8.09 ‰，与俯冲蚀变海洋地壳的起源一致，并支持钻石中 N 和 C 系统学的发现。钻石的氮聚集数据范围为 25 % 至 100 %B，在某些单独的 PDA 中差异高达 60%。我们通过偶发性熔体/流体-岩石相互作用模型解释了金刚石和硅酸盐的地球化学和同位素非均质性，以及复杂的阴极发光特征，该模型涉及克拉通根或热边界层中的减少软流圈熔体。 我们认为，通过这种机制，在克拉通根和热边界层中形成了大量的 PDA，使其成为重要的碳储存库，这在一些金伯利岩中的局部高丰度 （约 20%） 得到了证实。