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)