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.

All major, rock-forming lower-mantle minerals (bridgmanite, CaSi-perovskite, ferropericlase and stishovite) are “nominally anhydrous minerals” (NAMs), in which hydrogen comprises less than 1...

The concentrations of Fe, Al, and Ni and their distributions were determined for all known natural assemblages of ferropericlase (fPer) and bridgmanite (Bridg), coexisting as inclusions in deep-mantle diamonds from Brazil, Canada, Guinea, and South Australia. Based upon these data, it is likely that some areas within the deep lower mantle are iron-rich and differ markedly from a pyrolitic composition. In the lowermost lower mantle, Bridg is Al-rich and fPer is Ni-poor, witnessing the presence of a free metallic phase in the mineral-forming environment. The iron partitioning in the Bridg + fPer association $[K_{\mathrm{D}}^{\text{Bridg-fPer}}=({[\mathrm{Fe}/\mathrm{Mg}]}^{\text{Bridg}})/{({[\mathrm{Fe}/\mathrm{Mg}]}^{\text{fPer}})}_{\text{at}}$ in juvenile diamond inclusions is as low as 0.1–0.2. During ascent of the diamonds with their inclusions to the surface, the $K_{\mathrm{D}}^{\text{Bridg-fPer}}$ eventually increases to values of 0.4–0.5 and even as high as 0.7. The details of the element partitioning between natural Bridg and fPer in the lower mantle are as follows: iron in Bridg is ferrous Fe²⁺ in the A site, substituting for Mg²⁺; almost all iron in fPer is ferrous Fe²⁺; the share of ferric Fe³⁺ iron in fPer is Fe³⁺/ΣFe = 8–12 at%; iron concentrations in both Bridg and fPer increase with depth (pressure), reflecting the increasing Fe content in the lower part of the lower mantle, different from that of a pyrolitic model. Al in Bridg is mainly in the cation site B and partly in the cation site A, in both cases substituting for Si, Mg, and Fe with vacancy formation; and in the case of Al positioning into both B and A sites, a charge-balanced reaction occurs. The natural samples show very diverse $K_{\mathrm{D}}^{\text{Bridg-fPer}}$ values and elemental distribution that cannot be simply explained by our current understanding on alumina dissolution in Bridg and the spin transition of Fe²⁺ in fPer. These major differences between experimental results and observations in natural samples demonstrate the complex, inhomogeneous iron speciation and chemistry in the lower mantle.

Diamonds from the Rio Sorriso placer in the Juina area, Mato Grosso State, Brazil, contain mineral inclusions of ferropericlase associated with MgSiO₃, CaSiO₃, magnesite, merrillite, and other minerals. The ferropericlase inclusions in Rio Sorriso diamonds are resolved into two distinct genetic and compositional groups: (1) protogenetic, high-Ni and low-Fe (Ni = 8270–10,660 ppm; mg# = 0.756–0.842) ferropericlases, and (2) syngenetic, low-Ni and high-Fe (Ni = 600–3050 ppm; mg# = 0.477–0.718) ferropericlases. Based on the crystallographic orientation relationships between natural ferropericlase inclusions and host diamonds, high-Ni and low-Fe ferropericlases originate in the upper part of the lower mantle, while low-Ni and high-Fe ferropericlases, most likely, originate in the lithosphere. Mineral inclusions form the ultramafic lower-mantle (MgSiO₃, which we suggest as bridgmanite, CaSiO₃, which we suggest as CaSi-perovskite, and high-Ni and low-Fe ferropericlase) and lithospheric (CaSiO₃, which we suggest as breyite, Ca(Si,Ti)O₃, and low-Ni and high-Fe ferropericlase) associations. The presence of magnesite and merrillite inclusions in association with ferropericlase confirmed the existence of a deep-seated carbonatitic association. Diamonds hosting high-Ni and low-Ni ferropericlase have different carbon-isotopic compositions (δ¹³C = −5.52 ± 0.75‰ versus −7.07 ± 1.23‰ VPDB, respectively). It implies the carbon-isotopic stratification of the mantle: in the lower mantle, the carbon-isotopic composition tends to become isotopically heavier (less depleted in ¹³C) than in lithospheric diamonds. These regularities may characterize deep-seated diamonds and ferropericlases not only in the Juina area of Brazil but also in other parts of the world.