Abstract Mineral major and trace elements combined with Sr isotopes of clinopyroxene are used to unravel the origins and evolution of mantle eclogite and pyroxenite xenoliths from the Doornkloof-Sover orangeite dike (western Kaapvaal craton), and to investigate the generation and destruction of diamond in these rocks. Two different eclogite types are present: (1) MgO-poor eclogites (MgO = 7.3 to 14.5 wt%; n = 43) with accessory diamond ± corundum and kyanite, high garnet grossular content (median Ca# = 0.25) and clinopyroxene jadeite content (0.39). Reconstructed bulk rocks are LREE-depleted (median La 0.29 ppm) and have low median Cr2O3 (0.06 wt%) and incompatible trace-element contents (e.g. Sr, Zr, Ba, Pb, Th), and high Li and transition metal abundances. Some are characterised by stepped REE patterns or steep slopes in the MREE, similar to eclogites affected by interaction with dehydration fluids generated in subduction zones. These fluids may also have deposited diamond in typically reducing eclogite assemblages at diamond-stable pressures. (2) MgO-rich eclogites and pyroxenites (MgO = 14.0 to 20.0 wt%; n = 29), which are barren and enriched in LREE (median La 1.39 ppm), Cr2O3 (0.25 wt%) and incompatible trace elements, with lower Li and transition metal abundances than the MgO-poor group. These are typical signatures of carbonated ultramafic melt metasomatism in the mantle lithosphere. Strontium isotopic compositions vary widely in both groups, but high Cr2O3 and Ba contents are dominantly associated with 87Sr/86Sr > 0.7055. This reflects interaction with metasomatic agents remobilised from ancient lithospheric metasomes, which eventually gave rise to regional orangeite magmatism. The presence of strong positive Eu anomalies in both groups, including two pyroxenites, requires low-pressure igneous protoliths, presumably derived from a ca. 3 Ga spreading ridge, as reported for other eclogite materials from the western Kaapvaal craton. Based on the proportions of MgO-poor and –rich eclogites and pyroxenites, approximately 40% of the diamond inventory were destroyed by mantle metasomatism centred at ~135 ± 15 km depth, overlapping a low-velocity anomaly (mid-lithospheric discontinuity). Two diamondiferous orangeites ≤20 km from Doornkloof-Sover contain significantly different eclogite xenolith populations: At Newlands, MgO-poor diamondiferous eclogites are present in addition to barren MgO-rich ones and pyroxenites, suggesting that the host orangeite sampled a source region equally affected by diamond-destructive mantle metasomatism, whereas at Bellsbank, all eclogites are MgO-poor and LREE-depleted. This may explain higher diamond grades reported for this locality compared to Newlands or Doornkloof-Sover.

中文翻译：

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Kaapvaal 克拉通西部的含金刚石和贫瘠榴辉岩和辉石分别记录了俯冲过程和地幔交代作用

摘要 矿物主要和微量元素与单斜辉石 Sr 同位素结合，用于解开 Doornkloof-Sover 橙岩脉（西 Kaapvaal 克拉通）地幔榴辉岩和辉石岩捕虏体的起源和演化，并研究金刚石在这些岩脉中的生成和破坏。岩石。存在两种不同的榴辉岩类型：(1) 贫 MgO 榴辉岩（MgO = 7.3 至 14.5 wt%；n = 43），带有辅助金刚石 ± 刚玉和蓝晶石，高石榴石总钙含量（中值 Ca# = 0.25）和单斜辉石硬玉含量(0.39)。重建的大块岩石是 LREE 贫化（La 中值 0.29 ppm），Cr2O3 中值低（0.06 wt%）和不相容的微量元素含量（例如 Sr、Zr、Ba、Pb、Th），以及高 Li 和过渡金属丰度。一些以阶梯状 REE 模式或 MREE 中的陡坡为特征，类似于受俯冲带产生的脱水流体相互作用影响的榴辉岩。这些流体也可能在钻石稳定压力下以典型的还原榴辉岩组合形式沉积钻石。(2) 富 MgO 榴辉岩和辉石岩（MgO = 14.0 至 20.0 wt%；n = 29），贫瘠且富含轻稀土（中值 La 1.39 ppm）、Cr2O3（0.25 wt%）和不相容微量元素，具有较低的Li 和过渡金属丰度高于 MgO 贫乏组。这些是地幔岩石圈中碳酸化超基性熔体交代作用的典型特征。两组的锶同位素组成差异很大，但高 Cr2O3 和 Ba 含量主要与 87Sr/86Sr > 0.7055 相关。这反映了与从古代岩石圈交代体中重新动员的交代因子的相互作用，最终引起了区域性橙色岩浆作用。两组中都存在强烈的正 Eu 异常，包括两个辉石岩，需要低压火成岩原岩，推测来自约 3 Ga 扩张脊，如来自 Kaapvaal 克拉通西部的其他榴辉岩材料的报道。根据贫镁和富镁榴辉岩和辉石岩的比例，大约 40% 的钻石库存被以约 135 ± 15 公里深度为中心的地幔交代作用破坏，与低速异常（中岩石圈不连续性）重叠。距离 Doornkloof-Sover ≤ 20 公里的两个含金刚石橙岩包含显着不同的榴辉岩包体：在 Newlands，除了贫瘠的富含 MgO 的榴辉岩和辉石岩之外，还存在贫 MgO 的含金刚石榴辉岩，表明寄主橙岩采样的源区同样受到破坏钻石的地幔交代作用的影响，而在 Bellsbank，所有榴辉岩都缺乏 MgO 和 LREE。这可能解释了与 Newlands 或 Doornkloof-Sover 相比，该地区报告的钻石等级更高。