Deep, carbonate-rich melts are key constituents of kimberlites and are crucial for understanding the cycle of volatile elements in the mantle. On the Siberian craton, the Udachnaya-East kimberlite hosts extremely well-preserved nodules composed of chlorides + carbonates + sulfates, that do not present any relict sedimentary textures. These salty nodules display textures that are commonly observed in quenched liquids and may thus represent the very last stage liquid of the kimberlite. Alternatively, they could represent assimilated sedimentary material, or even post-magmatic hydrothermal alteration, because kimberlites are known to ascend through the lithosphere while assimilating material from their wall rocks.

Here we focus specifically on those chloride-carbonate nodules, which are composed of 70% chloride + 30% alkali-carbonate and sulfate, and used two radiogenic systems (Rb-Sr, Sm-Nd) and the isotopic composition of sulfur, in addition to their major and trace element compositions (n=3). We then compared the results with the same geochemical data on host kimberlites (n=4), sedimentary cover (n=3) and hydrothermal veins (n=3).

Taken together, our results show that the nodules are not the product of a contamination by the Cambrian sedimentary cover. Trace element patterns of the nodules display extreme enrichments in the same elements that are relatively depleted in the host kimberlite but also in kimberlites worldwide (K, Rb, Sr, Pb), suggesting that chloride-carbonate nodules are snapshots of the latest stage liquid present in the kimberlite system. Their isotopic compositions (Rb-Sr, Sm-Nd and δ³⁴S) are consistent with a common magmatic source with their host kimberlite. We propose that chloride-carbonate nodules record a missing compositional endmember, which could explain the trend towards more radiogenic Sr isotope ratios at nearly constant Nd signatures observed in their host kimberlite, as well as in other kimberlites worldwide. This observed trend suggests the presence of a recycled component with high Rb/Sr (such as salts or terrigenous sediments) in the mantle sampled by some kimberlites, either in the lithosphere or the asthenosphere. This study highlights that the role of alkalies and halogens may have been underestimated in the genesis of kimberlites at depths where diamonds are stable, as well as in more evolved magmatic stages. Segregations of chlorides and carbonates occur specifically in sulfate-bearing kimberlites, which may thus sample a mantle domain in which sulfates with δ³⁴S > 0‰ are dominant. The existence of such a reservoir could explain the apparent imbalance observed between the chondritic value (δ³⁴S of 0‰) and the negative S isotopic compositions of mantle sulfides (MORB and peridotites).

富含碳酸盐的深层熔体是金伯利岩的关键成分，对于了解地幔中挥发性元素的循环至关重要。在西伯利亚克拉通上，Udachnaya-East 金伯利岩拥有保存极其完好的结核，由氯化物 + 碳酸盐 + 硫酸盐组成，不存在任何残存的沉积结构。这些咸结核显示出通常在淬火液体中观察到的纹理，因此可能代表金伯利岩的最后阶段液体。或者，它们可以代表同化的沉积物质，甚至是岩浆后热液蚀变，因为已知金伯利岩在同化围岩中的物质时会上升穿过岩石圈。

Here we focus specifically on those chloride-carbonate nodules, which are composed of 70% chloride + 30% alkali-carbonate and sulfate, and used two radiogenic systems (Rb-Sr, Sm-Nd) and the isotopic composition of sulfur, in addition to their major and trace element compositions (n=3). We then compared the results with the same geochemical data on host kimberlites (n=4), sedimentary cover (n=3) and hydrothermal veins (n=3).

总之，我们的结果表明结核不是寒武纪沉积盖层污染的产物。结核的微量元素模式显示出在寄主金伯利岩和全球金伯利岩（K、Rb、Sr、Pb）中相对贫化的相同元素中极度富集，这表明氯化物-碳酸盐结核是最新阶段液体存在的快照在金伯利岩系。它们的同位素组成（Rb-Sr、Sm-Nd 和 δ ³⁴S) 与其寄主金伯利岩的共同岩浆源一致。我们提出，氯化物-碳酸盐结核记录了一个缺失的组成端元，这可以解释在其寄主金伯利岩以及全球其他金伯利岩中观察到的几乎恒定的 Nd 特征下具有更多放射成因 Sr 同位素比率的趋势。这种观察到的趋势表明，在岩石圈或软流圈中的一些金伯利岩采样的地幔中存在具有高 Rb/Sr 的再循环组分（如盐或陆源沉积物）。这项研究强调，在钻石稳定的深度以及更进化的岩浆阶段，可能低估了碱金属和卤素在金伯利岩成因中的作用。氯化物和碳酸盐的偏析特别发生在含硫酸盐的金伯利岩中，³⁴ S > 0‰ 占优势。这种储层的存在可以解释在球粒陨石值（δ ³⁴ S 为 0‰）和地幔硫化物（MORB 和橄榄岩）的负 S 同位素组成之间观察到的明显不平衡。