Ca, Mn, Na admixtures in pyropes as indicator of the diamond grade in kimberlites

 Triangular diagrams of pyrope compositions for minor elements – Mn, Na, Ti in wt%  are proposed to assess the diamond grade  of kimberlite pipes.

. Basing on group cluster analysis, diagrams are constructed for pyropes from high-diamondiferous (>1 ct/t) (Fig.1), poor and non-diamondiferous kimberlites (<0.1 ct/t) (Fig.2) of Yakutia and Angola. Oval areas correspond to peridotite, eclogitic, and websterite mantle paragenesis (Sobolev, 1971), the diameter of the analytical point is proportional to the CaO content. In highly diamondiferous kimberlites (Fig. 1), pyropes of peridotite dominating and eclogitic and websterite associations are equally present. Trends in their compositions overlap in the ratios of Na2O, MnO and TiO₂ in all CG

Pyropes of diamond-poorly kimberlites (Fig. 2) do not show such overlap, and for each of their CG they are isolated or have a predominance of one paragenesis over another.

Discussion. The presence of pyrope CGs from different deep sources indicates the hybridization of the proto-kimberlite melt, which assimilated the rocks of the peridotite, eclogite, and websterite "layers" of the mantle. This explains the high diamond content of kimberlites, which assimilated diamonds from these three deep sources. In poor diamondiferous kimberlites, pyropes of various paragenesis are extremely unevenly represented, which is due to the weak interaction of the melt with the "layers" of diamondiferous eclogites and websterites. The catalytic role of Ti, Na, and Mn in the process of diamond formation was pointed out by V.A. Milashev (Milashev, 1994) and J. Gurney (Gurney et al., 1994). Pyropes of highly diamondiferous kimberlites are characterized by a short TiO2 trend and a fairly long Na₂O trend (Fig. 1). Pyropes of poorly diamondiferous kimberlites, on the contrary, are distinguished by a long or unexpressed TiO₂ trend and a limited Na₂O (Fig. 2)

Conclusions. The revealed regularities in the distribution of impurity elements in pyropes from kimberlites of different degrees of diamond content are confirmed by diagrams with an MgO-MnO "diamond window", where most of the pyrope grains of highly diamondiferous pipes fall (Figs. 1 and 2). They can be used to assess the diamondiferous potential of kimberlites based on mineralogical and geochemical criteria.

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