The paper presents data on garnets from serpentinized peridotite xenoliths in the Nyurba and Botuoba kimberlite pipes of the Nakyn kimberlite field. The major and trace-element compositions of the garnets were analyzed to determine their compositional specifics and genesis. Based on the REE content and chondrite-normalized distribution patterns, the garnets are divided into two types with sinusoidal ((Sm/Er)_n > 1) and normal ((Sm/Er)_n < 1) REE distribution patterns. In terms of the Y, Zr, Ti, and Eu relations, and the shape of REE distribution pattern, all the garnets correspond to garnets of metasomatized peridotites, except for one sample falling into the field of depleted garnets of harzburgite–dunite paragenesis. The geochemical characteristics of the garnets record two types of metasomatic agents: carbonatite/fluid for type 1 garnets and silicate/melt for type 2 garnets. The carbonatite metasomatic agent produced harzburgitic garnet and its further transformation into lherzolitic garnet. Silicate metasomatism, which led to the formation of the REE pattern of type 2 garnets, likely overprinted two different types of garnets and, respectively, gave two evolutionary trends. These are depleted residual garnets and type 1 garnets previously subjected to carbonatite metasomatism. The low Y and Th contents in combination with the low Ti/Eu ratios in garnets suggest a moderate reworking of lithospheric peridotites by silicate melts, which is consistent with the high diamond grade of the Nakyn kimberlite field.

本文提供了来自 Nakyn 金伯利岩油田 Nyurba 和 Botuoba 金伯利岩管中蛇纹石化橄榄岩捕虏体的石榴石数据。分析了石榴石的主要和微量元素成分，以确定它们的成分特性和成因。根据稀土元素含量和球粒陨石归一化分布模式，石榴石分为正弦（(Sm/Er) _n > 1）和正常（(Sm/Er) _n< 1) REE 分布模式。从 Y、Zr、Ti、Eu 关系和 REE 分布格局的形状来看，所有石榴石都对应于交代橄榄岩的石榴石，除了 1 个样品属于贫化石榴石-纯英岩共生领域。石榴石的地球化学特征记录了两种类型的交代物质：1 型石榴石的碳酸盐岩/流体和 2 型石榴石的硅酸盐/熔体。碳酸岩交代剂产生了菱镁质石榴石，并进一步转化为红橄榄石石榴石。硅酸盐交代作用导致了 2 型石榴石 REE 模式的形成，可能叠加了两种不同类型的石榴石，并分别给出了两种进化趋势。这些是耗尽的残余石榴石和先前经历过碳酸盐岩交代作用的 1 型石榴石。石榴石中较低的 Y 和 Th 含量以及较低的 Ti/Eu 比表明硅酸盐熔体对岩石圈橄榄岩进行了适度的再加工，这与 Nakyn 金伯利岩矿区的高金刚石品位一致。