Abstract

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.

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