Carbonatitic inclusions in deep mantle diamond from Juina, Brazil: new minerals in the carbonate-halide association

Eleven new minerals were identified in deep mantle primary carbonatitic association as inclusions in diamond from the Juina area, Mato Grosso, Brazil. Specifically, two carbonates [magnesite and eitelite Na₂Mg(CO₃)₂], two phosphates [mixed-anion phosphate Na₄Mg₃(PO₄)₂(P₂O₇), Fe-diphosphate Fe₂Fe₅(P₂O₇)₄], two fluorides [oskarssonite AlF₃ and Ba-rich fluoride (Ba,Sr)AlF₅], three sulfides [pentlandite (Fe,Ni)₉S₈, violarite FeNi₂S₄, and millerite NiS], hematite, and metallic Ni-iron were detected; the two phosphates and Ba-rich fluoride are observed in the natural environment for the first time. The mineral compositions of the analyzed inclusions are variable, even at a nanometer scale, which indicates variability in the source media during the formation of diamond. Volatiles, represented in the form of porosity, played a significant role in this process. Most mineral phases contain volatile elements, as well. Carbonatitic inclusions most likely originated from high-density fluid (HDF) microinclusions encapsulated in diamond during its growth. During the ascent of diamond, HDF inclusions underwent disintegration in composition and crystallized as polymineralic inclusions. Formation of diamond in the studied case took place in a carbonatitic, carbonate-halide-phosphate-fluoride medium, which was enriched in volatiles and acted as an open system during diamond formation.