Juina Area Research Articles

Distinct Groups of Low- and High-Fe Ferropericlase Inclusions in Super-Deep Diamonds: An Example from the Juina Area, Brazil

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.

Low- and High-Fe Ferropericlase Inclusions in Super-Deep Diamonds and Their Depth of Origin: An Example from the Juina Area, Brazil,

Low- and High-Fe Ferropericlase Inclusions in Super-Deep Diamonds and Their Depth of Origin: An Example from the Juina Area, Brazil,

Ferropericlase inclusions in ultradeep diamonds from Sao Luiz (Brazil): high Li abundances and diverse Li-isotope and trace element compositions suggest an origin from a subduction mélange

The most remarkable feature of the inclusion suite in ultradeep alluvial and kimberlitic diamonds from Sao Luiz (Juina area in Brazil) is the enormous range in Mg# [100xMg/(Mg + Fe)] of the ferropericlases (fper). The Mg-richer ferropericlases are from the boundary to the lower mantle or from the lower mantle itself when they coexist with ringwoodite or Mg- perovskite (bridgmanite). This, however, is not an explanation for the more Fe-rich members and a lowermost mantle or a “D” layer origin has been proposed for them. Such a suggested ultra-deep origin separates the Fe-rich fper-bearing diamonds from the rest of the Sao Luiz ultradeep diamond inclusion suite, which also contains Ca-rich phases. These are now thought to have an origin in the uppermost lower mantle and in the transition zone and to belong either to a peridotitic or mafic (subducted oceanic crust) protolith lithology. We analysed a new set of more Fe-rich ferropericlase inclusions from 10 Sao Luiz ultradeep alluvial diamonds for their Li isotope composition by solution MC-ICP-MS (multi collector inductively coupled plasma mass spectrometry), their major and minor elements by EPMA (electron probe micro-analyser) and their Li-contents by SIMS (secondary ion mass spectrometry), with the aim to understand the origin of the ferropericlase protoliths. Our new data confirm the wide range of ferropericlase Mg# that were reported before and augment the known lack of correlation between major and minor elements. Four pooled ferropericlase inclusions from four diamonds provided sufficient material to determine for the first time their Li isotope composition, which ranges from δ7Li + 9.6 ‰ to −3.9 ‰. This wide Li isotopic range encompasses that of serpentinized ocean floor peridotites including rodingites and ophicarbonates, fresh and altered MORB (mid ocean ridge basalt), seafloor sediments and of eclogites. This large range in Li isotopic composition, up to 5 times higher than ‘primitive upper mantle’ Li-abundances, and an extremely large and incoherent range in Mg# and Cr, Ni, Mn, Na contents in the ferropericlase inclusions suggests that their protoliths were members of the above lithologies. This melange of altered rocks originally contained a variety of carbonates (calcite, magnesite, dolomite, siderite) and brucite as the secondary products in veins and as patches and Ca-rich members like rodingites and ophicarbonates. Dehydration and redox reactions during or after deep subduction into the transition zone and the upper parts of the lower mantle led to the formation of diamond and ferropericlase inclusions with variable compositions and a predominance of the Ca-rich, high-pressure silicate inclusions. We suggest that the latter originated from peridotites, mafic rocks and sedimentary rocks as redox products between calcite and SiO2.

Nitrogen nanoinclusions in milky diamonds from Juina area, Mato Grosso State, Brazil

A unique set of diamonds with a ‘milky’ optical appearance from the Rio Soriso placer deposit in the Juina area, Mato Grosso, Brazil was studied by combined transmission electron microscopy (TEM) and fourier transform infrared (FTIR) spectroscopy. The main characteristics of the studied samples are large numbers of randomly distributed {111}-faceted octahedral defect nanostructures. The dislocation densities of the focused ion beam (FIB) foils are generally low. Dislocation loops are observed only around larger inclusions. The inclusion size shows a bimodal distribution and spreads around values of 20 and 200nm. Electron energy-loss spectroscopy (EELS) and energy-dispersive X-ray (EDX) spectroscopy mapping of both subsets yield high nitrogen contents for all sealed inclusions. In cases where the nanoinclusions touch the surface of the FIB section no nitrogen signal could be detected, indicating the loss of a fluid or gas phase as the carrier of nitrogen. FTIR mapping of the same regions showed a strong correlation between structurally bound nitrogen, hydrogen and the abundance of nanoinclusions. We propose that the most likely phase included in these nanoinclusions is NH3. These nanoinclusions could be the result of a high-temperature episode or of long residence times at shallower depths and lower temperatures. Thus they might represent the last stage of the nitrogen aggregation, or they may be syngenetic trapped NH-bearing source fluids.

STEM EDX Nitrogen Mapping of Nanoinclusions in Milky Diamonds from Juina, Brazil, Using a Windowless Silicon Drift Detector System

Energy-dispersive X-ray spectroscopy (EDX) performed using scanning transmission electron microscopy (STEM) in combination with a windowless detector setup allows high-resolution imaging and chemical composition mapping even of light elements present in low concentrations. The used TEM-system combines a field emission electron source with four silicon drift detectors allowing for high detection sensitivity. We used this enhanced system to investigate 20 to 200 nm sized inclusions in milky diamonds from Rio Soriso, Juina area, Brazil. The diamonds act as a chemical inert container and therefore protect their inclusions from further chemical reactions with their surroundings. We visualize the presence and distribution of nitrogen within focused ion beam (FIB) slices containing these nanoinclusions. The investigation of these specific diamonds may open a new window to deeper parts of the Earth (>660 km) as they represent pristine material of this deep mantle environment.

Isotopic fractionation of oxygen and carbon in decomposed lower-mantle inclusions in diamond

Two carbonatitic mineral assemblages, calcite + wollastonite and calcite + monticellite, which are encapsulated in two diamond grains from the Rio Soriso basin in the Juina area, Mato Grosso State, Brazil, were studied utilizing the NanoSIMS technique. The assemblages were formed as the result of the decomposition of the lower-mantle assemblage calcite + CaSi-perovskite + volatile during the course of the diamond ascent under pressure conditions from 15 to less than 0.8 GPa. The oxygen and carbon isotopic compositions of the studied minerals are inhomogeneous. They fractionated during the process of the decomposition of primary minerals to very varying values: δ18O from −3.3 to +15.4 ‰ SMOW and δ13C from −2.8 to +9.3 ‰ VPDB. These values significantly extend the mantle values for these elements in both isotopically-light and isotopically-heavy areas.

Oxidation potential in the Earth's lower mantle as recorded by ferropericlase inclusions in diamond

Ferropericlase (fPer) inclusions from kimberlitic lower-mantle diamonds recovered in the Juina area, Mato Grosso State, Brazil were analyzed with transmission electron microscopy, electron energy-loss spectroscopy and the flank method. The presence of exsolved non-stoichiometric Fe3+-enriched clusters, varying in size from 1–2 nm to 10–15 nm and comprising ∼3.64 vol.% of fPer was established. The oxidation conditions necessary for fPer formation within the uppermost lower mantle (P=25 GPa, T=1960 K) vary over a wide range: Δlog⁡fO2 (IW) from 1.58 to 7.76 (Δ=6.2), reaching the fayalite–magnetite–quartz (FMQ) oxygen buffer position. This agrees with the identification of carbonates and free silica among inclusions within lower-mantle Juina diamonds. On the other hand, at the base of the lower mantle Δlog⁡fO2 values may lie at and below the iron–wüstite (IW) oxygen buffer. Hence, the variations of Δlog⁡fO2 values within the entire sequence of the lower mantle may reach ten logarithmic units, varying from the IW buffer to the FMQ buffer values. The similarity between lower- and upper-mantle redox conditions supports whole mantle convection, as already suggested on the basis of nitrogen and carbon isotopic compositions in lower- and upper-mantle diamonds. The mechanisms responsible for redox differentiation in the lower mantle may include subduction of oxidized crustal material, mechanical separation of metallic phase(s) and silicate–oxide mineral assemblages enriched in ferric iron, as well as transfer of fused silicate–oxide material presumably also enriched in ferric iron through the mantle.

A primary natrocarbonatitic association in the Deep Earth

In addition to ultramafic and mafic associations, a primary natrocarbonatitic association occurs in the lower mantle. To date, it was identified as inclusions in diamonds from the Juina area, Mato Grosso State, Brazil. It comprises almost 50 mineral species: carbonates, halides, fluorides, phosphates, sulfates, oxides, silicates, sulfides and native elements. In addition, volatiles are present in this association. Among oxides, coexisting periclase and wustite were identified, pointing to the formation of the natrocarbonatitic association at a depth greater than 2000 km. Some iron-rich (Mg,Fe)O inclusions in diamond are attributed to the lowermost mantle. The initial lower-mantle carbonatitic melt formed as a result of low-fraction partial melting of carbon-containing lower-mantle material, rich in P, F, Cl and other volatile elements, at the core-mantle boundary. During ascent to the surface, the initial carbonatitic melt dissociated into two immiscible parts, a carbonate-silicate and a chloride-carbonate melt. The latter melt is parental to the natrocarbonatitic lower-mantle association. Diamonds with carbonatitic inclusions were formed in carbonatitic melts or high-density fluids.

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 Na 2 Mg(CO 3 ) 2 ], two phosphates [mixed-anion phosphate Na 4 Mg 3 (PO 4 ) 2 (P 2 O 7 ), Fe-diphosphate Fe 2 Fe 5 (P 2 O 7 ) 4 ], two fluorides [oskarssonite AlF 3 and Ba-rich fluoride (Ba,Sr)AlF 5 ], three sulfides [pentlandite (Fe,Ni) 9 S 8 , violarite FeNi 2 S 4 , 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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Three-Dimensional Fe Speciation of an Inclusion Cloud within an Ultradeep Diamond by Confocal μ-X-ray Absorption Near Edge Structure: Evidence for Late Stage Overprint

A stream of 1-20 μm sized mineral inclusions having the negative crystal shape of its host within an "ultra-deep" diamond from Rio Soriso (Juina area, Mato Grosso State, Brazil) has been studied with confocal μ-X-ray absorption near edge structure (μXANES) at the Fe K and Mn K edges. This technique allows the three-dimensional nondestructive speciation of the Fe and Mn containing minerals within the inclusion cloud. The observed Fe-rich inclusions were identified to be ferropericlase (Fe,Mg)O, hematite and a mixture of these two minerals. Confocal μ-X-ray fluorescence (μXRF) further showed that Ca-rich inclusions were present as well, which are spatially separated from or in close contact with the Fe-rich inclusions. The inclusions are aligned along a plane, which most likely represents a primary growth zone. In the close vicinity of the inclusions, carbon coated planar features are visible. The three-dimensional distribution indicates a likely fluid overprint along an open crack. Our results imply that an imposed negative diamond shape of an inclusion alone does not exclude epigenetic formation or intense late stage overprint.

IRON CARBIDE INCLUSIONS IN LOWER-MANTLE DIAMOND FROM JUINA, BRAZIL

Iron carbides in association with native iron, graphite, and magnetite were identified in a crystal of diamond from the Juina area, Brazil, that contains a series of other, deep-mantle mineral inclusions. Among the iron carbides, Fe 3 C, Fe 2 C (“chalypite”), and Fe 23 C 6 (haxonite) are present; the two latter phases are identified in the terrestrial environment for the first time. Some of the analyzed iron carbide grains contain 7.3–9.1 at.% N and are, in fact, nitrocarbide. We suggest, on the basis of the high-pressure mineral parageneses previously observed in the diamond and experimental data on the system Fe–C, that “chalypite” crystallized within a pressure interval of 50–130 GPa from an iron–carbon melt rich in nitrogen. Following crystallization, iron carbides and native iron were partially oxidized to magnetite, and encapsulated in diamond along with other high-pressure minerals. The finds of various iron carbides, some of which are rich in nitrogen, in lower-mantle diamond confirm a significant role of carbides and nitrogen in the Earth’s interior.

In situ X-ray diffraction investigations of a multiphase inclusion in a diamond from the Juina district, Brazil

Abstract An inclusion in a diamond from the Juina area, Brazil was investigated in situ by single-crystal X-ray diffraction. It consists of six different phases: garnet, orthopyroxene and four individuals of diopsidic clinopyroxene. The application of the ALIGN-program helped to obtain accurate crystallographic data sets.

Nyerereite and nahcolite inclusions in diamond: evidence for lower-mantle carbonatitic magmas

AbstractNyerereite and nahcolite have been identified as micro- and nano-inclusions in diamond from the Juina area, Brazil. Alongside them are Sr- and Ba-bearing calcite minerals from the periclase-wiistite series, wollastonite II (high), Ca-rich garnet, spinels, olivine, phlogopite and apatite. Minerals of the periclase- wustite series belong to two separate groups: wustite and Mg-wustite with Mg# = 1.9—15.3, and Fe- periclase and periclase with Mg# = 84.9—92.1. Wollastonite-II (high, with Ca:Si = 0.992) has a triclinic structure. Two types of spinel were distinguished among mineral inclusions in diamond: zoned magnesioferrite (with Mg# varying from 13.5—90.8, core to rim) and Fe spinel (magnetite). Olivine (Mg# = 93.6), intergrown with nyerereite, forms an elongate, lath-shaped crystal and most likely represents a retrograde transformation of ringwoodite or wadsleyite. All inclusions are composed of poly-mineralic solid mineral phases. Together with previously found halides, sulphates and other mineral inclusions in diamond from Juina, they form a carbonatitic-type mineral paragenesis in diamond which may have originated in the lower mantle and/or transition zone. Wustite inclusions with Mg# = 1.9—3.4, according to experimental data, may have formed in the lowermost mantle. The source for the observed carbonatitic-type mineral association in diamond is lower-mantle natrocarbonatitic magma. This magma may represent a juvenile mantle melt, or be the result of low-degree partial melting of deeply-subducted carbonated oceanic crust. This magma was rich in volatiles, such as Cl, F and H, which played an important role in the formation of diamond.

Unusual micro- and nano-inclusions in diamonds from the Juina Area, Brazil

A series of uncommon micro- and nano-inclusions has been identified in diamonds from the Rio Soriso placer deposit in Mato Grosso State, Brazil. The micro-inclusions are variable in size, from 1 to 300 µm. Usually, they are polymineralic, being formed predominantly by intergrowths of carbonates, silicates and other minerals. Carbonates are represented mostly by dolomite and occasionally by calcite. Silicates found are coesite, wollastonite-II, cuspidine and monticellite. Sulphides and ilmenite form micro-inclusions as well. Nano-inclusions are different from micro-inclusions not only in size (not exceeding 200 nm); they are usually included in micro-inclusions. Among nano-inclusions, halides (NaCl, KCl, CaCl 2 and PbCl 2), anhydrite, spinel, phlogopite, PbO 2, TiO 2 with an α-PbO 2 structure, native Fe, and other phases are identified. All these minerals are of the eclogitic association: they are either associated with coesite or are included in a diamond with a light, ‘organic’ carbon isotopic composition (with δ 13C from − 14 to − 25‰ PDB). This confirms our earlier conclusion that diamonds from the Juina area may have formed as a result of subduction of the crustal material to depths of at least the lower transition zone or even the lower mantle. The pressure estimates for the investigated diamonds vary in a range from 3 to 10 GPa. An interesting assemblage of calcite + cuspidine + wollastonite + monticellite + fluid identified in one of the studied diamonds is considered as a product of a reaction of wollastonite + fluid forming cuspidine + monticellite. The presence of numerous pores, cavities and bubbles in mineral inclusions, and identification of an association of volatile-containing mineral inclusions, such as halides (NaCl, KCl, CaCl 2, and PbCl 2), fluorine-containing silicate cuspidine, and phlogopite emphasize the important role of volatiles, particularly chlorine and fluorine in the formation of the diamonds.

Kimberlitic sources of super-deep diamonds in the Juina area, Mato Grosso State, Brazil

The Juina diamond field, in the 1970–80s, was producing up to 5–6 million carats per year from rich placer deposits, but no economic primary deposits had been found in the area. In 2006–2007, Diagem Inc. discovered a group of diamondiferous kimberlitic pipes within the Chapadão Plateau (Chapadão, or Pandrea cluster), at the head of a drainage system which has produced most of the alluvial diamonds mined in the Juina area. Diamonds from placer deposits and newly discovered kimberlites are identical; they have super-deep origins from the upper-mantle and transition zone. Field observations and petrographic studies have identified crater-facies kimberlitic material at seven separate localities. Kimberlitic material is represented by tuffs, tuffisites and various epiclastic sediments containing chrome spinel, picroilmenite, manganoan ilmenite, zircon and diamond. The diamond grade varies from 0.2–1.8 ct/m 3. Chrome spinel has 30–61 wt.% Cr 2O 3. Picroilmenite contains 6–14 wt.% MgO and 0.2–4 wt.% Cr 2O 3. Manganoan ilmenite has less than 3 wt.% MgO and 0.38–1.41 wt.% MnO. The 176Hf/ 177Hf ratio in kimberlitic zircons is 0.028288–0.28295 with ε Hf = 5.9–8.3, and lies on the average kimberlite trend between depleted mantle and CHUR. The previously known barren and weakly diamondiferous kimberlites in the Juina area have ages of 79–80 Ma. In contrast, zircons from the newly discovered Chapadão kimberlites have a mean 206Pb/ 238U age of 93.6 ± 0.4 Ma, corresponding to a time of magmatic activity related to the opening of the southern part of the Atlantic Ocean. The most likely mechanism of the origin of kimberlitic magma is super-deep subduction process that initiated partial melting of zones in lower mantle with subsequent ascent of proto-kimberlitic magma.

Super-deep diamonds from kimberlites in the Juina area, Mato Grosso State, Brazil

One thousand three hundred sixty-five diamonds from seven newly discovered kimberlitic pipes in the Juina area were comprehensively studied. These diamonds, like the ones from previously studied Juina placer deposits, are very homogeneous in their morphology and optical properties. Two diamond populations exist in the Pandrea pipes: the major population with a highly aggregated nitrogen impurity (% N B = 75–100%), and a secondary population with a moderately aggregated nitrogen impurity (% N B = 20–65%), while only one major population is present in the diamonds from placers. The diamonds from the pipes have a permanent, relatively high hydrogen impurity concentration. Among the mineral inclusions in diamonds from the Juina pipes, ferropericlase is predominant; chrome spinel, picroilmenite, Mn-ilmenite, MgCaSi-‘perovskite’ phase, rutile, sulphide, native iron, and iron-oxides were also identified. Most of the inclusions belong to the lower-mantle paragenesis; some (rutile and sulphide) are of eclogitic paragenesis. Mineral inclusions in diamonds from kimberlitic pipes are different in composition from the same minerals in placer diamonds. Both kimberlitic and placer diamonds belong to the same carbon isotopic population, but have differences in the δ 13C distribution and were probably formed from different local carbon sources. These data indicate that diamonds from both groups, kimberlites and placer deposits, in the Juina area, belong to the same genetic population with most of the stones originating within the super-deep conditions. However, there are differences between these two groups, which indicate that besides the known Pandrea pipes which may have partly supplied diamonds to the placer deposits, there may be other, still unknown primary sources of diamonds in the Juina area.

Lower mantle diamonds from Rio Soriso (Juina area, Mato Grosso, Brazil)

The morphology, colour, fluorescence, cathodoluminescence, nitrogen content and aggregation state, internal structure and mineral inclusions have been studied for 69 alluvial diamonds from the Rio Soriso (Juina area, Mato Grosso State, Brazil). Nitrogen in most diamonds (53%) is fully aggregated as B centres, but there is also a large proportion of N-free stones (38%). A strong positive correlation between nitrogen and IR-active hydrogen concentrations is observed. The diamonds contain (in order of decreasing abundance) ferropericlase, CaSi-perovskite, magnetite, MgSi-perovskite, pyrrhotite, ‘olivine’, SiO2, perovskite, tetragonal almandine-pyrope phase and some other minerals represented by single grains. The Rio Soriso diamond suite is subdivided into several subpopulations that originated in upper and lower mantle of ultramafic and mafic compositions, with the largest subgroup forming in the ultramafic lower mantle. Analysed ferropericlase grains are enriched in Fe (Mg#=0.43–0.89), which is ascribed to their origin in the lowermost mantle. The Juina kimberlites may be unique in sampling the material from depths below 1,700 km that ascended in a plume formed at the core–mantle boundary.

Superdeep diamonds from the Juina area, Mato Grosso State, Brazil

Alluvial diamonds from the Juina area in Mato Grosso, Brazil, have been characterized in terms of their morphology, syngenetic mineral inclusions, carbon isotopes and nitrogen contents. Morphologically, they are similar to other Brazilian diamonds, showing a strong predominance of rounded dodecahedral crystals. However, other characteristics of the Juina diamonds make them unique. The inclusion parageneses of Juina diamonds are dominated by ultra-high-pressure ("superdeep") phases that differ both from "traditional" syngenetic minerals associated with diamonds and, in detail, from most other superdeep assemblages. Ferropericlase is the dominant inclusion in the Juina diamonds. It coexists with ilmenite, Cr-Ti spinel, a phase with the major-element composition of olivine, and SiO2. CaSi-perovskite inclusions coexist with titanite (sphene), "olivine" and native Ni. MgSi-perovskite coexists with TAPP (tetragonal almandine-pyrope phase). Majoritic garnet occurs in one diamond, associated with CaTi-perovskite, Mn-ilmenite and an unidentified Si-Mg phase. Neither Cr-pyrope nor Mg-chromite was found as inclusions. The spinel inclusions are low in Cr and Mg, and high in Ti (Cr2O3 10 wt%). Most ilmenite inclusions have low MgO contents, and some have very high (up to 11.5 wt%) MnO contents. The rare "olivine" inclusions coexisting with ferropericlase have low Mg# (87–89), and higher Ca, Cr and Zn contents than typical diamond-inclusion olivines. They are interpreted as inverted from spinel-structured (Mg, Fe)2Si2O4. This suite of inclusions is consistent with derivation of most of the diamonds from depths near 670 km, and adds ilmenite and relatively low-Cr, high-Ti spinel to the known phases of the superdeep paragenesis. Diamonds from the Juina area are characterized by a narrow range of carbon isotopic composition (δ13C=–7.8 to –2.5‰), except for the one majorite-bearing diamond (δ13C=–11.4‰). There are high proportions of nitrogen-free and low-nitrogen diamonds, and the aggregated B center is predominant in nitrogen-containing diamonds. These observations have practical consequences for diamond exploration: Low-Mg olivine, low-Mg and high-Mn ilmenite, and low-Cr spinel should be included in the list of diamond indicator minerals, and the role of high-Cr, low-Ti spinel as the only spinel associated with diamond, and hence as a criterion of diamond grade in kimberlites, should be reconsidered.