Abstract Diamonds containing fluid inclusions provide invaluable samples of upper mantle fluids, the study of which illuminates not only diamond formation but also the long-term evolution of the subcratonic, lithospheric mantle. The very large range of inclusion compositions worldwide has been interpreted to represent four end-member fluids: saline (rich in Na + K + Cl); silicic (rich in Si + Al); and carbonatitic (rich in Ca + Mg + Fe, with low-Mg and high-Mg end members). However, the sources and evolution of these fluids and the processes involved in diamond formation are still unclear. We used an unusual study of diamonds from the Panda kimberlite (Ekati Mine, Northwest Territories, Canada) in which both mineral and fluid inclusions in the diamonds were analyzed (Tomlinson et al., 2006) to develop models of the saline, silicic, and low-Mg carbonatitic fluids present in the Panda fluid inclusions. The models used aqueous speciation and solubility calculations to link the solid and fluid inclusion chemistry with model upper mantle rock types. We used the extended Deep Earth Water model to calculate equilibrium constants previously calibrated with experimental rock solubilities referring to upper mantle temperatures and pressures ( Huang and Sverjensky, 2019 ). Our results at 950 °C and 4.5 GPa suggest that the saline fluid could originate from peridotite, the silicic fluid from eclogite, and the low-Mg carbonatitic fluid from carbonated dunite. The fluid models were then used to predict the irreversible, chemical mass transfer when the carbonatitic fluid infiltrated a harzburgite containing a saline fluid. Simultaneous reduction of formate and bicarbonate in the carbonatitic fluid and oxidation of aqueous hydrocarbons from the peridotitic fluid during mixing and reaction with harzburgite resulted in the formation of diamond, olivine, garnet, and clinopyroxene, and increases in the logf O 2 and pH . Olivine was predicted to become more Fe-rich and garnet more Ca and Fe-rich with reaction progress, in agreement with reported temporal trends (core-to-rim) in the Panda mineral inclusions. The fluid at the site of diamond formation became more saline with reaction progress and the predicted aqueous phase concentrations of all elements changed consistent with trends in Panda fluid inclusions. In contrast, a prediction for a saline fluid infiltrating a harzburgite containing a carbonatitic fluid resulted in trends of the silicate minerals and the salinity with reaction progress that were in the opposite direction to data from the Panda diamonds. Overall, our study strongly supports the notion that fluids from subducting slabs could mix and precipitate diamonds containing carbon from both oxidized and reduced sources, while adding Ca and Fe to the sub-lithospheric cratonic mantle through metasomatic reactions.

中文翻译：

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熊猫钻石形成过程中碳酸岩与盐水的混合

摘要 含有流体包裹体的钻石提供了上地幔流体的宝贵样本，其研究不仅阐明了钻石的形成，而且阐明了亚克拉通岩石圈地幔的长期演化。世界范围内非常广泛的包裹体成分已被解释为代表四种终端流体：盐水（富含 Na + K + Cl）；硅质（富含Si+Al）；和碳酸岩（富含 Ca + Mg + Fe，具有低镁和高镁端元）。然而，这些流体的来源和演变以及钻石形成所涉及的过程仍不清楚。我们对来自 Panda 金伯利岩（加拿大西北地区的 Ekati 矿）的钻石进行了一项不同寻常的研究，其中分析了钻石中的矿物和流体包裹体（Tomlinson 等人，2006 年），以开发含盐、硅、和存在于 Panda 流体包裹体中的低镁碳酸岩流体。这些模型使用水相形态和溶解度计算将固体和流体包裹体化学与模型上地幔岩石类型联系起来。我们使用扩展的深地球水模型来计算平衡常数，之前用实验岩石溶解度参考上地幔温度和压力（Huang and Sverjensky，2019）。我们在 950 °C 和 4.5 GPa 下的结果表明，含盐流体可能来自橄榄岩，硅质流体来自榴辉岩，低镁碳酸质流体来自碳酸化纯长岩。然后使用流体模型来预测当碳酸岩流体渗入含有盐水流体的菱镁矿时发生的不可逆的化学传质。在碳酸岩流体中甲酸盐和碳酸氢盐的同时还原以及橄榄岩流体中的含水烃在与方辉石混合和反应过程中的氧化导致金刚石、橄榄石、石榴石和斜辉石的形成，并且 logf O 2 和 pH 值增加。随着反应的进行，预计橄榄石会变得更富含铁，而石榴石会变得更富含钙和铁，这与熊猫矿物包裹体中报道的时间趋势（核心到边缘）一致。随着反应的进行，金刚石形成地点的流体变得更加咸，所有元素的预测水相浓度发生变化，与熊猫流体包裹体的趋势一致。相比之下，对盐水渗入含有碳酸岩流体的斜方辉石的预测导致硅酸盐矿物和盐度随反应进程的趋势与熊猫钻石的数据相反。总的来说，我们的研究强烈支持这样一种观点，即俯冲板块中的流体可以混合和沉淀来自氧化和还原源的含碳金刚石，同时通过交代反应将 Ca 和 Fe 添加到次岩石圈克拉通地幔中。