Kimberlites are pipe-like igneous bodies, consisting of a pyroclastic crater and diatreme, commonly underlain by coherent root-zone rocks, and with associated dyke/sill complexes. The processes that control the different modes of coherent kimberlite emplacement remain uncertain. In addition, late evolution of kimberlite melts during emplacement into the upper crust remains poorly constrained. Therefore, it is unclear whether there is a link between melt composition/evolution and the emplacement mechanism of coherent kimberlites (i.e. planar dykes/sills vs. irregular bodies in the root zone). An absence of comparative studies on late-stage magmatic phases across the different emplacement modes of coherent kimberlite from the same locality hamper resolution of these issues. Therefore, we report petrographic and mineral chemical data for groundmass apatite in samples of dyke, sill, and root-zone kimberlites from the Kimberley cluster (South Africa). Early crystallised phases (olivine, spinel, Mg-ilmenite) in dyke/sill and root-zone kimberlites have indistinguishable compositions, and hence crystallised from similar primitive melts. Conversely, apatite compositions are generally distinct in dyke/sill (low Sr, high and variable Si) and root-zone kimberlites (high and variable Sr, low Si). The Si enrichment of apatite in dykes/sills is attributed to the coupled incorporation of CO 3 2− and SiO 4 4− for PO 4 3− , reflecting higher CO 2 contents in their parental melts, and potentially higher Si contents due to the preferential crystallisation of carbonates over mica/monticellite. The low Sr contents of apatite in dyke/sill kimberlites reflect equilibrium with a (kimberlite) melt (i.e. D Sr is close to unity for carbonate and silicate melts), whereas the higher Sr contents of apatite in root-zone kimberlites require crystallisation from, or overprinting by a H 2 O ± CO 2 fluid (significantly higher D Sr ). The relative enrichment of CO 2 in kimberlite dykes/sills is evident from the abundance of carbonates, the presence of mesostasis dolomite and calcite phenocrysts in some samples, and concomitant reduced proportions of other groundmass phases (e.g. serpentine, mica, monticellite). During late alteration of kimberlite dykes/sills, monticellite is typically replaced by carbonates, whereas olivine and pleonaste are relatively stable, indicating the melts which form dykes/sills evolve to higher CO 2 /H 2 O ratios. It is unlikely that these two distinct evolutionary paths were caused by crustal contamination before or during near surface magma emplacement, because crustal assimilation is not recorded in the O and Sr isotopic composition of late crystallising olivine rinds or carbonates, respectively. We suggest that higher concentrations of CO 2 are retained in kimberlite dykes/sills due to higher confining pressures (i.e. lack of breakthrough to the surface). In contrast, exsolution of CO 2 from root-zone kimberlites increased melt H 2 O/CO 2 ratios and promoted the crystallisation of mica and monticellite at the expense of dolomite and calcite. Apatite compositions have the potential to aid in the discrimination of kimberlites from lamproites (higher LREE, Sr, F, and S, lower Si contents) and carbonatites (higher LREE, F, Cl and S, lower Fe contents). However, the compositions of kimberlitic apatite overlap those from aillikites, probably due to similar late-stage melt compositions.

中文翻译：

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磷灰石成分和基质矿物学记录了由不同侵位机制引起的相干金伯利岩中不同的熔体/流体演化轨迹

金伯利岩是管状火成岩体，由火山碎屑火山口和火山口组成，通常由连贯的根区岩石和相关的岩脉/窗台复合体组成。控制相干金伯利岩侵位不同模式的过程仍然不确定。此外，金伯利岩熔体在上地壳侵位过程中的晚期演化仍然缺乏约束。因此，目前还不清楚熔体成分/演化与连贯金伯利岩的侵位机制（即平面岩脉/窗台与根区的不规则体）之间是否存在联系。缺乏对来自同一地点的相干金伯利岩不同侵位模式的晚期岩浆相的比较研究阻碍了这些问题的解决。所以，我们报告了来自金伯利群（南非）的岩脉、窗台和根区金伯利岩样品中基质磷灰石的岩相学和矿物化学数据。岩脉/基岩和根区金伯利岩中的早期结晶相（橄榄石、尖晶石、镁钛铁矿）具有难以区分的成分，因此从相似的原始熔体中结晶。相反，磷灰石成分在岩脉/窗台（低 Sr、高和可变 Si）和根区金伯利岩（高和可变 Sr、低 Si）中通常是不同的。岩壁/窗台中磷灰石的 Si 富集归因于 CO 3 2- 和 SiO 4 4- 对 PO 4 3- 的耦合掺入，反映了其母熔体中较高的 CO 2 含量，并且由于优先云母/蒙脱石上的碳酸盐结晶。岩脉/窗台金伯利岩中磷灰石的低 Sr 含量反映了与（金伯利岩）熔体的平衡（即碳酸盐和硅酸盐熔体的 D Sr 接近统一），而根区金伯利岩中磷灰石的较高 Sr 含量需要从以下结晶，或通过 H 2 O ± CO 2 流体套印（显着更高的 D Sr ）。CO 2 在金伯利岩脉/窗台中的相对富集从碳酸盐的丰度、某些样品中介稳白云岩和方解石斑晶的存在以及伴随的其他基体相（例如蛇纹石、云母、蒙脱石）的比例减少而明显看出。在金伯利岩脉/基岩的晚期蚀变期间，蒙脱石通常被碳酸盐取代，而橄榄石和pleonaste相对稳定，表明形成岩脉/基岩的熔体演化为更高的CO 2 /H 2 O比率。这两条不同的演化路径不太可能是由近地表岩浆侵位之前或期间的地壳污染引起的，因为在晚期结晶橄榄石外皮或碳酸盐的 O 和 Sr 同位素组成中没有分别记录地壳同化作用。我们认为，由于较高的围压（即没有突破到地表），较高浓度的 CO 2 保留在金伯利岩脉/岩床中。相比之下，根区金伯利岩中CO 2 的溶出增加了熔体H 2 O/CO 2 比，并以白云石和方解石为代价促进了云母和蒙脱石的结晶。磷灰石成分有可能有助于区分金伯利岩与菱镁矿（较高的轻稀土、锶、氟和硫，较低的硅含量）和碳酸岩（较高的轻稀土、氟、氯和硫，较低的铁含量）。