Reconstructing the original composition of kimberlite melts in the mantle and delineating the processes that modify them during magmatic ascent and emplacement in the crust remains a significant challenge in kimberlite petrology. One of the most significant processes commonly cited to drive initial kimberlite melts towards more Si-Mg-rich compositions and decrease the solubility of CO₂ is the assimilation of mantle orthopyroxene. However, there is limited direct evidence to show the types of reactions that may occur between mantle orthopyroxene and the host kimberlite melt.

To provide new constraints on the interaction between orthopyroxene and parental kimberlite melts, we examined a fresh (i.e. unmodified by secondary/post-magmatic alteration) orthopyroxenite xenolith, which was recovered from the serpentine-free units of the Udachnaya-East kimberlite (Siberian Craton, Russia). This xenolith is composed largely of orthopyroxene (~ 90%), along with lesser olivine and clinopyroxene and rare aluminous magnesian chromite. We can show that this xenolith was invaded by the host kimberlite melt along grain interstices and fractures, where it partially reacted with orthopyroxene along the grain boundaries and replaced it with aggregates of compositionally distinct clinopyroxene, olivine and phlogopite, along with subordinate Fe-Cr-Mg spinel, FeNi sulphides and djerfisherite (K₆(Fe,Ni,Cu)₂₅S₂₆Cl).

Primary melt inclusions in clinopyroxene replacing xenolith-forming orthopyroxene, as well as secondary melt inclusion trails in xenolith orthopyroxene, clinopyroxene and olivine are composed of similar daughter mineral assemblages that consist largely of: NaK chlorides, along with varying proportions of phlogopite, Fe-Cu-Ni sulphides, djerfisherite, rasvumite (KFe₂S₃), Cr-Fe-Mg spinel, nepheline and apatite, and rare rutile, sodalite, barite, olivine, Ca-K-Na carbonates and NaK sulphates. The melt entrapped by these inclusions likely represent the hybrid products produced by the invading kimberlite melt reacting with orthopyroxene in the xenolith.

The mechanism that could explain the partial replacement of orthopyroxene in this xenolith by clinopyroxene, olivine and phlogopite could be attributed to the following reaction:

Orthopyroxene + Carbonatitic (melt) ➔ Olivine + Clinopyroxene + Phlogopite + CO₂.

This reaction is supported by theoretical and experimental studies that advocate the dissolution of mantle orthopyroxene within an initially silica-poor and carbonate-rich kimberlite melt. The mineral assemblages replacing orthopyroxene in the xenolith, together with hosted melt inclusions, suggests that the kimberlitic melt prior to reaction with orthopyroxene was likely carbonate-rich and Na-K-Cl-S bearing. The paucity of carbonate in the reaction zones around orthopyroxene and in melt inclusions in clinopyroxene replacing xenolith-forming orthopyroxene and xenolith minerals (orthopyroxene, clinopyroxene and olivine) is attributed to the consumption of carbonates and subsequent exsolution of CO₂ by the proposed decarbonation reaction.

Concluding, we propose that this orthopyroxenite xenolith provides a rare example of the types of reactions that can occur between mantle orthopyroxene and the host kimberlite melt. The preservation of this xenolith and zones around orthopyroxene present new insights into the composition and evolution of parental kimberlite melts and CO₂ exsolution.

重建地幔中金伯利岩熔体的原始成分并描绘在岩浆上升和进入地壳期间改变它们的过程仍然是金伯利岩岩石学的重大挑战。推动初始金伯利岩熔体向更富含 Si-Mg 的成分并降低 CO ₂溶解度的最重要过程之一是地幔斜方辉石的同化。然而，表明地幔斜方辉石和主体金伯利岩熔体之间可能发生的反应类型的直接证据有限。

为了对斜方辉石和母体金伯利岩熔体之间的相互作用提供新的限制，我们检查了一种新鲜的（即未经二次/岩浆蚀变改性的）正方辉石包体，它是从 Udachnaya-East 金伯利岩（西伯利亚克拉通）的无蛇纹岩单元中回收的， 俄罗斯）。这种捕虏体主要由正辉石 (~ 90%) 以及较少的橄榄石和单斜辉石以及稀有的铝镁铬铁矿组成。我们可以证明，这种捕虏体被主体金伯利岩熔体沿着晶粒间隙和裂缝侵入，在那里它部分地与沿晶界的斜方辉石反应，并用成分不同的单斜辉石、橄榄石和金云母以及次要的 Fe-Cr- Mg 尖晶石、Fe Ni 硫化物和 djerfisherite (K ₆(Fe、Ni、Cu) ₂₅ S ₂₆ Cl)。

单斜辉石中的初级熔体包裹体取代了形成捕虏体的正辉石，以及捕体正方辉石、单斜辉石和橄榄石中的次级熔体包裹体轨迹由类似的子矿物组合组成，主要包括：Na K氯化物，以及不同比例的金云母、Fe- Cu-Ni 硫化物、djerfisherite、rasvumite (KFe ₂ S ₃ )、Cr-Fe-Mg 尖晶石、霞石和磷灰石，以及稀有的金红石、方钠石、重晶石、橄榄石、Ca-K-Na 碳酸盐和 Na K 硫酸盐。被这些包裹体包裹的熔体可能代表了入侵的金伯利岩熔体与捕虏体中的斜方辉石反应产生的混合产物。

可以解释单斜辉石、橄榄石和金云母部分替代该捕虏体中的正辉石的机制可归因于以下反应：

正辉石 + 碳酸岩（熔体） ➔ 橄榄石 + 单斜辉石 + 金云母 + CO ₂。

这一反应得到了理论和实验研究的支持，这些研究主张地幔斜方辉石在最初贫硅且富含碳酸盐的金伯利岩熔体中溶解。包封岩中取代斜方辉石的矿物组合以及寄宿的熔体包裹体表明，与斜方辉石反应之前的金伯利岩熔体可能富含碳酸盐并含有 Na-K-Cl-S。邻辉石周围反应区和单斜辉石熔体包裹体中碳酸盐的缺乏，取代了形成捕虏体的斜辉石和捕虏体矿物（斜辉石、单斜辉石和橄榄石），这归因于碳酸盐的消耗和随后通过提议的脱碳反应排出的 CO ₂。

最后，我们认为这种正辉石捕虏体提供了一个罕见的例子，说明地幔正辉石和主体金伯利岩熔体之间可能发生的反应类型。这种捕虏体和邻辉石周围区域的保存提供了对母体金伯利岩熔体和 CO ₂出溶的组成和演化的新见解。