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首页> 外文期刊>Contributions to Mineralogy and Petrology >Enrichment processes at the base of the Archean lithospheric mantle: observations from trace element characteristics of pyropic garnet inclusions in diamonds
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Enrichment processes at the base of the Archean lithospheric mantle: observations from trace element characteristics of pyropic garnet inclusions in diamonds

机译：太古代岩石圈地幔底部的富集过程：从金刚石中的石榴石石榴石包裹体的微量元素特征观察

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摘要

Trace element concentrations of peridotitic garnet inclusions in diamonds from two Chinese kimberlite pipes were determined using the ion microprobe. Garnet xenocrysts from the same two kimberlite pipes were also analyzed for comparison. In contrast to their extremely refractory major element compositions, all harzburgitic garnets showed enrichment in light rare earth elements (REE) relative to chondrite, resulting in sinuous REE patterns. Both normal and sinuous REE patterns were observed from the lherzolitic garnets. Concentrations of REE in garnets changed significantly from diamond to diamond and no specific correlations were observed with their major element compositions. Analyses of randomly selected two to three points within every grain of a large number of garnet inclusions by the ion microprobe demonstrated that there was no evident compositional heterogeneity, and multiple grains of one phase from a single diamond host also exhibit very similar compositions. This implies that the trace element heterogeneity within one grain or among multiple inclusions from the same diamond host, as reported from Siberian diamonds, is not a common feature for these Chinese diamonds. Concentrations of Na, Ti, and Zr tend to decrease when garnets become more refractory, but variations of Sr and Li are more complex. Compositions rich in light REE and relatively poor in high field strength elements (HFSE) of the harzburgitic garnet inclusions in diamonds are generally consistent with metasomatism by carbonatite melts. The trace element features observed from the garnet inclusions in Chinese diamonds may be caused by carbonatite melt infiltration and partial melt extraction. Spatial and temporal gradients in melt/rock ratio and temperature are the main reasons for the large variations of REE patterns and other trace element concentrations.
机译：使用离子微探针测定了两条中国金伯利岩管中钻石中的橄榄石石榴石包裹体中的痕量元素浓度。还分析了来自相同的两个金伯利岩管的石榴石异晶。与它们的极难熔的主要元素组成相比，所有的长方石榴石都显示出相对于球粒陨石富含轻稀土元素（REE），从而形成弯曲的REE模式。从锂沸石石榴石观察到正常和弯曲的REE模式。石榴石中的REE浓度在钻石之间变化显着，并且未观察到其主要元素组成之间的特定相关性。通过离子微探针对大量石榴石夹杂物的每个晶粒内随机选择的两到三个点进行分析，结果表明没有明显的成分异质性，并且来自单个金刚石主体的一个相的多个晶粒也显示出非常相似的组成。这意味着，根据西伯利亚钻石的报道，一个晶粒内或同一钻石主体的多个夹杂物中的痕量元素异质性并不是这些中国钻石的共同特征。石榴石变得更难熔时，Na，Ti和Zr的浓度趋于降低，但Sr和Li的变化更为复杂。钻石中的轻质石榴石夹杂物富含轻质稀土元素，而相对缺乏高场强元素（HFSE），通常与碳酸盐熔岩的交代作用相一致。从中国钻石中的石榴石夹杂物观察到的微量元素特征可能是由碳酸盐岩熔体渗透和部分熔体萃取引起的。熔体/岩石比率和温度的时空梯度是REE模式和其他痕量元素浓度变化很大的主要原因。

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