The petrogenesis and relationship of diamondite to well-studied monocrystalline and fibrous diamonds are poorly understood yet would potentially reveal new aspects of how diamond-forming fluids are transported through the lithosphere and equilibrate with surrounding silicates. Of 22 silicate- and oxide-bearing diamondites investigated, most yielded garnet intergrowths (n = 15) with major element geochemistry (i.e. Ca–Cr) classifying these samples as low-Ca websteritic or eclogitic. The garnet REE patterns fit an equilibrium model suggesting the diamond-forming fluid shares an affinity with high-density fluids (HDF) observed in fibrous diamonds, specifically on the join between the saline–carbonate end-members. The δ13C values for the diamonds range from − 5.27 to − 22.48‰ (V-PDB) with δ18O values for websteritic garnets ranging from + 7.6 to + 5.9‰ (V-SMOW). The combined C–O stable isotope data support a model for a hydrothermally altered and organic carbon-bearing subducted crustal source(s) for the diamond- and garnet-forming media. The nitrogen aggregation states of the diamonds require that diamondite-formation event(s) pre-dates fibrous diamond-formation and post-dates most of the gem monocrystalline diamond-formation events at Orapa. The modelled fluid compositions responsible for the precipitation of diamondites match the fluid-poor and fluid-rich (fibrous) monocrystalline diamonds, where all grow from HDFs within the saline-silicic-carbonatitic ternary system. However, while the nature of the parental fluid(s) share a common lithophile element geochemical affinity, the origin(s) of the saline, silicic, and/or carbonatitic components of these HDFs do not always share a common origin. Therefore, it is wholly conceivable that the diamondites are evidence of a distinct and temporally unconstrained tectono-thermal diamond-forming event beneath the Kaapvaal craton.

中文翻译：

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钻石岩：Kaapvaal 克拉通下方明显的构造热钻石形成事件的证据

人们对金刚石岩的成因及其与研究充分的单晶和纤维状金刚石的关系知之甚少，但可能会揭示形成金刚石的流体如何穿过岩石圈并与周围的硅酸盐平衡的新方面。在研究的 22 种含硅酸盐和氧化物的金刚石岩中，大多数产生了石榴石共生体（n = 15），主要元素地球化学（即 Ca-Cr）将这些样品归类为低钙 websteritic 或榴辉岩。石榴石 REE 模式符合平衡模型，表明形成钻石的流体与在纤维状钻石中观察到的高密度流体 (HDF) 具有亲和力，特别是在盐酸盐-碳酸盐末端成员之间的连接处。钻石的 δ13C 值范围为 - 5.27 至 - 22.48‰ (V-PDB)，而韦氏石榴石的 δ18O 值范围为 + 7.6 至 + 5。9‰（V-SMOW）。组合的 C-O 稳定同位素数据支持了钻石和石榴石形成介质的热液蚀变和含有机碳的俯冲地壳源模型。钻石的氮聚集状态要求金刚石形成事件发生在纤维状金刚石形成之前，而在 Orapa 的大部分宝石单晶金刚石形成事件发生之后。导致金刚石沉淀的模拟流体成分与贫流体和富流体（纤维状）单晶金刚石相匹配，它们都是从盐-硅-碳酸盐三元体系内的 HDF 中生长出来的。然而，虽然母液的性质具有共同的亲石元素地球化学亲和力，但盐水、硅质、这些 HDF 的和/或碳酸盐成分并不总是具有共同的来源。因此，完全可以想象，金刚石岩是 Kaapvaal 克拉通下方独特且不受时间限制的构造热金刚石形成事件的证据。