Composition of pyroxenes from kimberlite and eclogite xenoliths of Catoca kimberlite pipe (Angola)

Microprobe analysis (JSM 6510 LA/JET-2200) of eclogite pyroxenes from xenoliths (Nikitina et al., 2014) and pyroxenes of Сatoca pipe (reveal the absence of grain zonation ) (Fig.1). Two major groups are in Na₂O–Al₂O₃ and Cr₂O₃–Al₂O₃ diagrams (Sobolev, 1974): high alumina (Hi-Al₂O₃), low magnesian (LMgO), high magnesian (Hi-MgO) (Nikitina et al., 2014). Most sodium-rich pyroxenes are Hi-Al₂O₃, and chrome-Hi-MgO eclogites. Pyroxene grains from magnesian eclogites are enriched with sodium and depleted in chromium in center.  Pyroxene grains from Hi-Al2O3 eclogites are not zonal. Pyroxenes from kimberlites are more diverse in composition, lower in Al₂O₃ higher in Cr₂O₃. In the classification diagrams, the fields of their compositions corresponding to eclogite pyroxenes are well identified, while the field of Hi-Al₂O₃ eclogites is absent. To determine the genetic affiliation of pyroxenes from kimberlites of facies and 3 allows the method of identifying chemical-genetic groups (Garanin et al., 1991) on the basis of cluster analysis (Ivanov, 2017). Only 21% of pyroxenes from kimberlites belong to weakly diamondiferous eclogites, the most numerous – 52% – pyroxenes of weakly diamondiferous lherzolites, 10% – weakly diamondiferous ilmenite peridotites and pyroxenites, 15% – weakly diamondiferous lherzolites and websterites, 2% – intergrowths with diamonds.  This indicates multistage diamond generation in eclogites of  Catoca  kimberlites  (Korolev et al., 2013; Nikitina et al., 2014), and in peridotites, pyroxenites, lherzolites, and websterites (Fig.2)

 

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