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Article

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

by
Felix V. Kaminsky
1,*,
Dmitry A. Zedgenizov
2,3,
Vyacheslav S. Sevastyanov
1 and
Olga V. Kuznetsova
1
1
Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 119991 Moscow, Russia
2
Zavaritsky Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences, 620016 Ekaterinburg, Russia
3
V. S. Sobolev Institute of Geology and Mineralogy, Siberian Branch Russian Academy of Sciences, 630090 Novosibirsk, Russia
*
Author to whom correspondence should be addressed.
Minerals 2023, 13(9), 1217; https://doi.org/10.3390/min13091217
Submission received: 9 August 2023 / Revised: 12 September 2023 / Accepted: 13 September 2023 / Published: 15 September 2023
(This article belongs to the Special Issue Kimberlites and Related Rocks: New Insight into Petrogenesis and Diamond Potential of Deeply-Derived Mantle Magmas)
Browse Figures
Versions Notes

Abstract

:
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.

1. Introduction

In recent decades, lower-mantle mineral inclusions have been identified in superdeep diamonds from Brazil, Guinea, Canada, South Africa, and Australia [1,2,3,4,5,6,7,8,9]. These inclusions provide a unique opportunity to establish the actual composition of lower-mantle mineral phases and reveal compositional and structural features of the lower mantle where host diamonds crystallize.
Detailed studies of lower-mantle mineral inclusions in deep-seated diamonds demonstrated the significant features of the lower mantle: its enrichment in Si, the presence of a carbonatitic association, and compositional stratification within the lower mantle ([8] and references therein). However, there are few available samples of diamonds with lower-mantle mineral inclusions, much fewer than Lunar samples, for instance. Each new discovery of lower-mantle material is, therefore, of great importance.
One of the major sources of superdeep diamonds with lower-mantle mineral inclusions is the Juina area in Mato Grosso State, Brazil. A set of oxide mineral inclusions, ferropericlase, CaSiO3 (former CaSi-perovskite), MgSiO3 (former bridgmanite), SiO2, and others in diamonds from this area, were inferred to originate from depths below 1700 km in [2]. A suggestion on MgSiO3 as bridgmanite and CaSiO3 as former CaSi-perovskite was based on their specific compositions (low Al2O3 in bridgmanite, for example) and their associations, characteristic for the lower mantle [4]. A few kilometres west of the central Juina area, is the Rio Sorriso (the right tributary of the Rio Aripuana) area, rich in alluvial diamonds. The primary diamond sources in this area are the Pandrea kimberlites, discovered a few kilometres east of the Rio Sorriso. The diamond collection of the Rio Sorriso diamonds was studied earlier by [10], but needed to be completed.
In our previous publications ([8] and references therein), we presented a series of images with intergrowths of these minerals with each other, demonstrating their equilibrium at the time they were formed and incorporated into growing diamonds. The major problem for ferropericlase inclusions in superdeep diamonds is their wide compositional variations from periclase with the magnesium index mg# = Mg/(Mg + Fe)at at 0.90–0.92 to magnesiowustite with mg# = 0.36 [8], in some cases even to 0.16 [7], which is close to the end-member wüstite stoichiometry. (For simplicity, we use below the term “ferropericlase” (fPer) for all varieties of the (Fe,Mg)O compositions.) This contradicts experimental data in pyrolitic systems in the pressure range 25–60 GPa, according to which the magnesium index of ferropericlase, in lower-mantle material with mg# = 0.80–0.95, should be 0.73–0.88 [11,12]. In experiments at pressures up to 100 GPa, for the lower mantle with the most likely magnesium index of mg# = 0.89–0.92, the magnesium index of coexisting ferropericlase and bridgmanite should be, respectively, at 0.83 and 0.93 just beyond the 660 km discontinuity, and 0.85 and 0.92 at the core–mantle boundary [13]. Further experiments demonstrated that iron-rich ferropericlase (not associated with bridgmanite) forms under lithospheric conditions [14,15,16]. According to experimental data, CaSiO3 as breyite may also form under upper mantle conditions [17,18,19]. That may be the case for single mineral inclusions. However, the association of CaSiO3 and other minerals with high-pressure minerals like bridgmanite (including touching associations) is good evidence for their lower-mantle origin.
In this work, we presented new data on mineral inclusions in diamonds from the Rio Sorriso placer deposit and offered our interpretation of their origin. For the first time, we also studied the carbon-isotopic compositions of the Rio Sorriso diamonds. Our conclusions on their origin may be applied to not only the Rio Sorriso and Juina areas but also may be crucial for understanding the complex structure of the deep mantle.

2. Materials and Methods

Among the 52 diamonds from the Rio Sorriso studied here, 24 contained 51 mineral inclusions, 29 of which formed non-touching associations in nine samples (Table 1). The diamond crystals were rounded rhombic dodecahedra, as is typical for Brazilian diamonds. The grain sizes varied from 0.1 to 1 mm.
The samples were polished to expose the inclusions. Exposed inclusions were identified in the electron-backscattering mode (BSE). Energy-dispersive spectrometry (EDS) was applied to estimate the composition of inclusions using a focused electron beam (15 kV, 10 nA) and an acquisition time of 30–60 s.
The major elements of minerals were analyzed using a JEOL JXA 8100 electron microprobe. The entire protocol of the X-ray electron probe microanalysis (precision, accuracy, and reference materials) is described in detail in [20]. Quantitative electron probe microanalyses (EPMA) were performed at an accelerating voltage of 15 kV, a sample current of 20 nA, and a beam diameter of 2 μm.
Raman spectra were collected using a Horiba Jobin Yvon LabRAM HR800 Raman microspectrometer equipped with a 532 nm Nd:YAG laser and an Olympus BX41 microscope at ×50 magnification. Spectra were recorded at room temperature in a backscattering geometry in the range of 100 to 1200 cm−1 with a spectral resolution of approximately 1 cm−1. The spectra were calibrated using the 520.6 cm−1 line of a silicon wafer. The wavenumbers were accurate to ±1 cm−1.
The carbon-isotopic ratios were measured using a Flash EA 1112 (Thermo Fisher Scientific, Waltham, MA, USA) coupled to a Finnigan Delta Plus XP isotope-ratio mass spectrometer. The diamonds were crushed in an agate mortar, and diamond fragments of approximately 50–100 μm were inserted into Sn capsules and dropped into the combustion reactor (1020 °C). The temperature of the reduction reactor was maintained at 650 °C. All carbon-isotopic compositions of the samples are reported in standard δ notation in the Vienna Pee Dee Belemnite Reference standard (VPDB) scale (δ13CVPDB). Two to nine fragments of each sample were analyzed to calculate the average and deviation (1σ) values.
To prevent bias and to minimize error, the accuracy of the isotopic data was evaluated by analyzing the certified reference material NBS 22 (oil with δ13C = −30.03 ± 0.04‰) obtained from the International Atomic Energy Agency, Vienna, Austria). High-purity helium (>99.9999%; from NIIKM) and CO2 (≥99.999%; Voessen, Moscow, Russia) were used as carrier gas or working standard gas, respectively. The maximum standard deviation (STD) for δ13C analysis of reference material was 0.15‰.

3. Results

Most of the inclusions were ferropericlase, including 15 single grains and 12 grains in association with MgSiO3 (which we suggest as retrogressed bridgmanite, see below), CaSi- and Ca(Si,Ti)-perovskites, magnesioferrite, magnesite, and merrillite. Ferropericlase formed octahedral and cuboctahedral, sometimes elongated and flattened crystals (Figure 1).
Magnesite and merrillite were identified in three diamonds. This was the first time these minerals were found in superdeep diamonds in association with ferropericlase. Merrillite was identified in the terrestrial environment for the first time [21]. In addition, eclogitic garnet and two grains of diopside composition were identified in three diamonds. The composition of the mineral inclusions identified in the studied diamonds is presented in Table 2 and Table S1.

3.1. Inclusions of Ferropericlase

Like elsewhere in Brazil and also in Guinea and Canada ([1,3,4,7], and others), ferropericlase inclusions in the Rio Sorriso diamonds have very variable iron content, from 24.6 to 62.9 wt.% FeO, and mg# values of 0.477–0.842 (Figure 2). The great variation in the composition of ferropericlase, with mg# values between 0.36 and 0.92 (that is even more variable than the mg# values of the ferropericlase grains studied here), was established in earlier works ([8,10] and references therein). In the studied ferropericlase samples, the mg# values demonstrated a bimodal distribution with two maxima: at 0.596 (variation from 0.477 to 0.718) and 0.790 (variation from 0.756 to 0.842) (Figure 2).
The two ferropericlase groups differed not only in their mg# values but also in their Ni contents. Ferropericlase of the first group (low-Fe, high-Ni) contained 8270–10,660 ppm Ni, while that of the second group (high-Fe, low-Ni) contained 600–3050 ppm Ni (Figure 3). In one of the samples (#1.2.1), low-Fe, high-Ni (mg# = 0.769) and high-Fe, Ni-poor (mg# = 0.566–0.567) ferropericlase inclusions coexisted (Table S1).
In several samples, ferropericlase associates with MgSiO3 (which we suggest former bridgmanite; see below in Section 3.2), CaSiO3 and Ca(Si,Ti)O3, magnesioferrite, magnesite, and merrillite. MgSiO3 and CaSiO3 associate with low-Fe, high-Ni ferropericlase (the first group), magnesite, and merrillite associate with high-Fe, Ni-poor ferropericlase (the second group). In sample #1.2.2, where CaSiO3 associates with Ca(Si,Ti)O3, ferropericlase was also high-Fe, Ni-poor.

3.2. Other Mineral Inclusions

The analyzed grains of MgSiO3 (samples #1.5.2a and #3.1.3b; the latter associated with high-Ni–low Fe ferropericlase; Table S1) have high Mg index values (mg# = 0.911–0.928), low Al (Al2O3 = 1.26–2.25 wt.%), and very low Ni (0–0.01 wt.%) contents, characteristic of the lower-mantle ultramafic association [8]. The very low Ni contents in the MgSiO3 phase are specific for former bridgmanite. Ferropericlase, with which MgSiO3 associates in sample #3.1.3, was high-Ni–low-Fe variety with mg# = 0.767–0.841. These Mg indices correspond precisely to the mg# values in coexisting ferropericlase and bridgmanite crystallized in experiments modelling the lower-mantle conditions beyond the 660 km discontinuity [12,13]. This gives the reason to consider the observed MgSiO3 inclusions as former bridgmanite.
The identification of CaSiO3 inclusions is more complex. Based on their low Al content (Al2O3 = 0.06–0.59 wt.%), all studied CaSiO3 grains belong to the ultramafic association [8]. In the absence of structural data, they may be CaSi-perovskite (“davemaoite” [22,23,24]) or low-pressure breyite, which crystallizes at lithospheric conditions or forms as a retrograde phase of CaSi-perovskite at 9–10 GPa (e.g., [25]). In our samples, CaSiO3 grains associate with both types of ferropericlase, low-Fe and high-Ni (the first group, sample #2.2.3) and high-Fe and Ni-poor ferropericlase (the second group, sample #1.2.2) (Table S1). In the latter sample, high-Fe and Ni-poor ferropericlase associated not only with CaSiO3 but also with Ca(Si,Ti)O3. We suggest that both CaSiO3 mineral phases occur in the studied diamonds, former CaSi-perovskite (sample #2.2.3) and breyite (sample #1.2.2).
The grain of Ca(Si,Ti)O3-perovskite with 40.28 wt.% TiO2, 12.92 wt.% SiO2, and minor Al2O3 (1.83 wt.%), Cr2O3 (1.33 wt.%) and FeO (0.51 wt.%) (Table S1, sample #1.2.2b2) occurred in association with low-Ni and high-Fe ferropericlase (sample #1.2.2a) and suggests lithospheric origin. Like CaSiO3 in sample #2.2.3, it also had low Al (0.05 wt.% Al2O3) and may be attributed to the ultramafic association; the association with low-Ni ferropericlase suggests a possible breyite.
CaSiO3 + Ca(Si,Ti)O3 inclusions were previously identified in diamonds from the Juina-5 kimberlite pipe [26] and Sao Luis alluvial deposits, also in the Juina area [7,27]. Two of these grains contained a significant amount of Si, like in our sample (11.00–12.05 wt.% SiO2). One of the Ca(Si,Ti)O3 inclusions with relatively low Ti (6.65 wt.% TiO2; #13 in [7]) associates with former bridgmanite and may have attributed to the lower mantle.
Magnesite, as a member of the deep-seated natrocarbonatitic association, was identified earlier as inclusion in diamond in association with other carbonates (dolomite, eitelite), halides (halite, sylvite), sulfides (pentlandite, violarite), native Fe0, and an unnamed phosphate Na4Mg3(PO4)2(P2O7) [8]. In the studied sample set, magnesite, in touching association with low-Ni ferropericlase in diamond #1.2.4 (Figure 4a), was confirmed by Raman spectroscopy (Figure 4b). It was almost pure MgCO3 with minor admixtures of Fe (2.76 wt.% FeO), Mn (0.12 wt.% MnO), Ca (1.29 wt.% CaO), and Na (0.06 wt.% Na2O). It had the empirical formula (Mg0.94Fe0.04Ca0.02)CO3. This was the first find of magnesite as intergrowth with ferropericlase. The admixture of Na in this sample points to its natrocarbonatitic association.
Merrillite Ca18Na2Mg2(PO4)14, earlier described as β-tricalcium phosphate Ca3(PO4)2 [21], was identified in two Rio Sorriso samples: #3.6.2 and #3.10.2. In sample #3.6.2, merrillite occurred as a single elongate inclusion, approximately 40 μm in size. In sample #3.10.2, merrillite formed a chain of five tabular, elongate inclusions, each 15–50 μm in size (Figure 5), in association with ferropericlase with mg = Mg/(Fe + Mg)at = 0.477 and low-Ni (NiO = 0.10 wt.%). The composition of merrillite from sample #3.10.2 was (from four analyses, in wt.%): 0.02–0.05 SiO2, 0–0.08 TiO2, 0.02–0.10 Al2O3, 0–0.02 Cr2O3, 1.19–2.07 FeO, 0.01 NiO, 0.08–0.12 MnO, 0–2.95 MgO, 46.4–48.4 CaO, 1.61 SrO, 0.08–1.92 Na2O, 0.04–0.63 K2O, 43.9–44.2 P2O5, 2.03 SO3 [21]. This gives the empirical formula of the found merrillite as Ca18.85Sr0.36Na1.44K0.32Fe0.39P14.07S0.57O56.00 (average of four analyses) on the basis of 56 oxygen atoms per formula. This composition differs from the known Lunar samples, in which Mg >> Fe [28], although in sample #3.6.2 MgO = 2.95 wt.%, and is closer to Martian ferromerrillite [29]. The Raman spectra of these merrillite grains are presented in Figure 6. There were several vibrations with modes at 407, 960, 973, and 1080 cm−1 from sample no. 3.6.2, and 217, 404, 443, 473, 607, 965, 970, and 1065 cm−1 from sample no. 3.10.2. These mode energies correspond almost exactly to the Raman shifts of merrillite from the Suizhou meteorite [30]. Based on these data, the structure of the found merrillite was trigonal with the R3–m space group [21]. Like other phosphates previously identified in diamonds from the Juina area, merrillite may be attributed to the carbonatitic association alongside magnesite.
Three diamonds from the studied ones contained upper-mantle mineral grains. Eclogitic-type garnet was identified in sample #F2, and two grains of CaMgSi2O6, compositionally corresponding to diopside, were identified in samples #2.11.2 and #4.18.1. It is thus possible that, like the Guinean [31] and Canadian [5] deposits, the Rio Sorriso placer contains upper-mantle diamonds alongside lower-mantle ones.

3.3. Carbon-Isotope Composition

The carbon-isotope values (δ13C) of 53 analyzed Rio Sorriso diamonds ranged from −3.64 to −8.94‰ VPDB (Table 3 and Table S2). Intra-sample variations were small, with Δ = 1–3‰ VPDB and deviations less or slightly exceeding 1‰ VPDB, which is characteristic of most natural diamonds. Still, in the 1980s, we demonstrated that most diamonds have intra-sample δ13C variations at ± 2‰ PDB and some at 5–10‰ PDB [32]. Since then, dozens of SIMS analyses have shown intra-sample variations caused by diamond zonation up to 10–14‰. Only one sample (#6.3.1) had δ13C = −11.41 ± 2.10‰ VPDB. In this sample, δ13C values varied from −9.58‰ to −15.03‰ VPDB (Δ = 5.45‰ VPDB) between various diamond fragments. This is caused, most likely, by the zonation of the crystal and varying carbon isotopic compositions in different crystal zones during the diamond growth. Diamond #6.3.1 may not belong to the deep mantle population. All the other diamonds in the sample set have a more homogeneous carbon-isotopic composition (Δ = 1–3‰ VPDB).
Of particular interest were carbon-isotopic compositions of diamonds enclosing distinct, high-Ni (low-Fe) and low-Ni (high-Fe) ferropericlases. The diamond hosts of each of these ferropericlase groups had different δ13CVPDB values: −5.52 ± 0.75‰ for diamonds with high-Ni, low-Fe ferropericlase; and −7.07 ± 1.23‰ for those with low-Ni, high-Fe ferropericlase (Table 3 and Table S2, Figure 7). The results of Student’s t-test, performed with the use of the SigmaPlot12 software, demonstrated that for the two diamond groups with 13 degrees of freedom and a 95% coefficient confidence interval, the difference of means was 0.451 to 2.643. The difference in the mean values of the two groups (−5.52‰ and −7.07‰) was greater than would be expected by chance; there was a statistically significant difference between the two groups (P = 0.009). A non-parametric Mann–Whitney U-test also confirmed the statistical difference between δ13C values for the two diamond groups with a 95% confidence: the U-value for them was 4, while a critical value at P < 0.05 was 8, and the z-score was 2.5103 with the p-value 0.01208.

4. Discussion

The distinguished two separate groups of ferropericlase have different compositions, different carbon-isotope characteristics of the host diamonds, and different mineral associations. The first group, high-Mg (low-Fe), contained 8270–10,660 ppm Ni, while the second group, low-Mg (high-Fe) ferropericlase, contained significantly less, 600–3050 ppm Ni (Figure 3). The high-Ni and low-Fe ferropericlases of the first group associated with former bridgmanite and former CaSi-perovskite (“davemaoite”) from the lower mantle (Table 1), which means its lower-mantle origin as well. As established by Frost et al. [33], the Ni content in ferropericlase was an indicator of the metal Fe phase(s) in the magmatic system of the deep mantle. An increase in the fraction of the metallic phase led to a decrease in the Ni concentration in the lower-mantle material and, consequently, to a decrease in the Ni content in ferropericlase. According to experimental data [34], the concentration of Ni in the lower mantle before the release of a metallic alloy is estimated to be 1 wt.% (10,000 ppm), similar to the Ni content of Mg-rich (low-Fe) ferropericlase in our sample set (Figure 3). Thus, a high-Ni and low- Fe and Mg-rich ferropericlase was formed in a medium that contained little metallic alloy, probably within the uppermost lower mantle.
It was suggested that Mg-rich ferropericlases mainly represent ambient lower mantle trapped as protogenetic inclusions, while Fe-rich ferropericlases, syngenetic with host diamonds, indicate redox growth in the upper mantle [35]. Earlier, based on the Fe partitioning between bridgmanite and ferropericlase, we proposed that the Fe content increases with depth (pressure), reflecting a higher Fe content in the deeper part of the lower mantle [36]. However, in that correlation, all ferropericlases associated with bridgmanite were high-Ni, high-Mg, and low-Fe varieties, and that conclusion may be applied only to the first group ferropericlases.
Fe-rich, Ni-poor ferropericlase inclusions of the second group are too iron-rich to have equilibrated as part of an assemblage associated with primitive mantle peridotite or harzburgite [9]. Several experimental works demonstrated that they might originate within shallower Earth’s layers, upper mantle, and or transition zone [16,17,18]. The analysis of crystallographic orientation relationships between natural ferropericlase inclusions and host diamonds confirmed these data. It demonstrated that Mg-rich ferropericlase grains were trapped by growing diamonds within the lower mantle, and Fe-rich ferropericlases are syngenetic with their host diamonds and form under redox growth in the upper mantle [37]. The low-Ni and high-Fe ferropericlases do not associate with former bridgmanite and former CaSi-perovskite (“davemaoite”) but with Ca(Si,Ti)O3, magnesite, and merrillite.
These facts indicate that, in one locality, two distinct groups of ferropericlase inclusions and, possibly, two generations of the host diamonds may occur. Ferropericlase grains with high mg# (0.756–0.842) and high Ni content (8270–10,660 ppm), associated with low-Al MgSiO3 (which we suggest former bridgmanite) (Al2O3 = 1.26 wt.%; sample #3.1.3) and CaSiO3 (which we suggest former CaSi-perovskite) (samples #2.2.1 and 2.2.3), originated from the upper parts of the lower mantle. Low-mg# (0.477–0.718) and low-Ni (600–3050 ppm) ferropericlases originated from the upper mantle and or transition zone. In sample #1.2.1, one low-Fe, high-Ni and two high-Fe, Ni-poor ferropericlase inclusions were found to coexist. To our knowledge, such an association was never observed before in natural samples and was never obtained in experiments. The coexistence of compositionally different ferropericlase grains in the same diamond, as well as by variable carbon isotopic composition in diamond #1.2 (from −5.53 ± 0.38‰ to −8.94 ± 1.34‰ Table S2), suggests that diamond #1.2 grew under changing conditions. This does not necessarily mean that these diamonds formed at great depths, ascending through the mantle in a plume and then grew again at shallower depths through different processes and under different conditions. The observed chemical disequilibrium may rather reflect progressive chemical mass transfer during a reaction between a fluid released by a deeply subducted mafic slab with ambient peridotite [35,38].
Magnesioferrite was found as a single grain #F3 in association with ferropericlase. Earlier, magnesioferrite was identified as microcrystals in ferropericlase formed as an exsolution phase in Ni-rich ferropericlase grains [35]. It was suggested that the appearance of magnesioferrite in such a position results by the oxidation of ferropericlase caused by the intrusion of subducted material into the sublithospheric mantle [35]. To the best of our knowledge, magnesioferrite in this study is the first find as a single grain.
Magnesite was found, in sample #1.2.4, in association with Ni-poor and Fe-rich ferropericlase (Figure 4). In this case, ferropericlase might also crystallize as a product of redox reactions involving oxidized carbonate or carbonated melt and reducing peridotite at the upper mantle, the transition zone or the lower mantle conditions [9].
Merrillite was known earlier only in meteorites and Lunar rocks. Recently, a Na-poor analogue of merrillite, keplerite Ca9(Ca0.5 0.5)Mg(PO4)7 with an R3c space group was described from the Marjialahati meteorite; it was suggested as an indicator of high-temperature environments characterized by extreme depletion of Na [39]. Among the studied Rio Sorriso diamonds, we found two samples with merrillite inclusions [21]. In these diamonds, the δ13CVPDB values were −5.56 ± 0.54‰ (sample #3.6.2) and −5.59 ± 0.45‰ (sample #3.10.2),—characteristic to deep juvenile carbon composition. In sample #3.10.1, merrillite associates with low-Ni and high-Fe ferropericlase and, like magnesite, may be attributed to the carbonatitic association, along with other phosphates identified as inclusions in diamond. We suggest that merrillite from the studied diamonds may be a retrograde phase of tuite, a high-pressure modification of tricalcium phosphate (γ-Ca3(PO4)2) [21].
Identifying both minerals, magnesite and merrillite, as inclusion in diamonds in association with ferropericlase is additional evidence for the existence of a carbonatitic association in the deep mantle and the participation of carbonates in the formation of diamonds [40]. Experiments demonstrated that Fe-rich, Ni-poor ferropericlase might form by reacting mantle peridotite with carbonate melt at pressure conditions, corresponding to the deep upper mantle and transition zone [16,17]. These experiments explain why Fe-rich, Ni-poor ferropericlase is not associated with bridgmanite but often with carbonatitic minerals.
All studied diamonds have carbon isotopic composition δ13CVPDB between −3.64 and −8.94‰ (except for #6.3.1 with δ13CVPDB = −11.41 ± 2.10‰). This corresponds to the δ13CVPDB “juvenile” values of diamonds of the ultramafic association [41]. The δ13CVPDB values in diamonds with inclusions support the distinction between the two groups of ferropericlase. Diamonds with high-Ni, low-Fe ferropericlase inclusions from the lower mantle have δ13CVPDB = −5.52 ± 0.75‰—notably heavier than the δ13C values in diamonds with low-Ni (high-Fe) ferropericlase inclusions from the upper mantle or the transition zone with δ13CVPDB = −7.07 ± 1.23‰ (Supplementary Table S2, Figure 5). The difference between the two groups is statistically significant with 95% coefficient confidence interval, according to the Student t-test and the Mann–Whitney U-test. This difference in δ13C values reflects a possible stratification in the carbon-isotopic composition of the lower mantle. Accepting the origin of high-Ni, low-Fe ferropericlase in the lower mantle, it may be suggested that, in the lower mantle, the carbon-isotopic composition tends to become isotopically heavier (less depleted in 13C) than in the lithospheric diamonds. This conclusion is preliminary substantiated only for the Juina area so far and needs to be checked for other regions. More data are needed to substantiate the proposed hypothesis.

5. Conclusions

New finds of mineral inclusions in lower-mantle diamonds from the Rio Sorriso, Juina area, Brazil, allow the following conclusions to be drawn.
  • In the Rio Sorriso diamonds, the ferropericlase inclusions are resolved into two distinct genetic and compositional groups: (1) a protogenetic, high-Ni and low-Fe group, and (2) a syngenetic, low-Ni and high-Fe group. 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 within the upper mantle or the Earth’s transition zone.
  • Mineral inclusions in the Rio Sorriso diamonds 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. Minerals of the carbonatitic association as inclusions in diamonds (magnesite and merrillite) also occur.
  • The diamond hosts of each ferropericlase group have different carbon-isotopic compositions, indicating that diamonds become isotopically heavier with depth (δ13CVPDB = −5.52 ± 1.23‰ in the lower mantle versus −7.07 ± 0.75‰ in the upper mantle or transition zone).
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. Subsequent studies of ferropericlase inclusions in super-deep diamonds should be performed.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min13091217/s1, Table S1: Chemical compositions of oxide, carbonate and silicate(?) inclusions in the studied lower-mantle diamonds from Rio Soriso, Table S2: Isotopic composition of carbon in diamonds from Rio Soriso.

Author Contributions

Conceptualization, F.V.K. and D.A.Z.; Methodology, D.A.Z. and O.V.K.; Investigation, F.V.K., D.A.Z. and V.S.S.; Writing—Original Draft Preparation, F.V.K.; Writing—Review and Editing, F.V.K. and D.A.Z.; Visualization, F.V.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the State Assignment of Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences; FMUS-2019-0013.

Data Availability Statement

Analytical data will be provided upon reasonable request to the corresponding author.

Acknowledgments

We appreciate the useful comments of three anonymous reviewers and the Academic Editor, which helped us to improve the manuscript. EMP and EDS analyses were performed at the Analytical Center for multi-elemental and isotope research, V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences. Carbon-isotopic analysis was carried out at the V.I. Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences. We thank these institutions for providing analytical capabilities.

Conflicts of Interest

The authors declare no conflict of interest.

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Minerals 13 01217 g001
Figure 1. Crystals of ferropericlase included in the Rio Sorriso diamonds. (a) Sample #2.2.1. (b) Sample #3.1.3b. (c) Sample #3.1.3c (all high-Ni and low-Fe). (d) Sample #3.6.3a (low-Ni and high-Fe).
Figure 1. Crystals of ferropericlase included in the Rio Sorriso diamonds. (a) Sample #2.2.1. (b) Sample #3.1.3b. (c) Sample #3.1.3c (all high-Ni and low-Fe). (d) Sample #3.6.3a (low-Ni and high-Fe).
Minerals 13 01217 g001
Minerals 13 01217 g002
Figure 2. Distribution of the Mg index (mg#) in ferropericlase grains. The arrows indicate the positions of average mg# values for two groups of ferropericlase.
Figure 2. Distribution of the Mg index (mg#) in ferropericlase grains. The arrows indicate the positions of average mg# values for two groups of ferropericlase.
Minerals 13 01217 g002
Minerals 13 01217 g003
Figure 3. Ni content vs. Mg index (mg#) in ferropericlase grains.
Figure 3. Ni content vs. Mg index (mg#) in ferropericlase grains.
Minerals 13 01217 g003
Minerals 13 01217 g004
Figure 4. Magnesite in touching association with ferropericlase included in Rio Sorriso diamond #1.2.4. (a)—Electronic image. (b)—Its Raman spectrum.
Figure 4. Magnesite in touching association with ferropericlase included in Rio Sorriso diamond #1.2.4. (a)—Electronic image. (b)—Its Raman spectrum.
Minerals 13 01217 g004
Minerals 13 01217 g005
Figure 5. Inclusions of merrillite in diamond #3.10.2 from the Rio Sorriso area. From [21], with permission from the Mineralogical Society of America.
Figure 5. Inclusions of merrillite in diamond #3.10.2 from the Rio Sorriso area. From [21], with permission from the Mineralogical Society of America.
Minerals 13 01217 g005
Minerals 13 01217 g006
Figure 6. Raman spectra of merrillite. (a) From sample 3.6.2. (b) From sample 3.10.2. From [21], with permission from the Mineralogical Society of America.
Figure 6. Raman spectra of merrillite. (a) From sample 3.6.2. (b) From sample 3.10.2. From [21], with permission from the Mineralogical Society of America.
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Minerals 13 01217 g007
Figure 7. Carbon isotopic composition in Rio Sorriso diamonds. (a) With low-Ni and high-Fe ferropericlase inclusions. (b) With high-Ni and low-Fe ferropericlase inclusions. (c) All diamonds studied.
Figure 7. Carbon isotopic composition in Rio Sorriso diamonds. (a) With low-Ni and high-Fe ferropericlase inclusions. (b) With high-Ni and low-Fe ferropericlase inclusions. (c) All diamonds studied.
Minerals 13 01217 g007
Table 1. Inclusions recovered from Rio Sorriso diamonds.
Table 1. Inclusions recovered from Rio Sorriso diamonds.
Minerals and AssociationsNo of DiamondsNo of Analyzed InclusionsSample # *Association Type
Single grains
High-Ni ferropericlase (Ni-fPer)451.2.1a, 2.2.1, 2.2.4, 2.7.2a, 3.9.3b Ultramafic lower-mantle
Low-Ni ferropericlase (Fe-fPer)5101.2.1c, 1.2.1d, 1.2.3b, 1.2.3c1, 1.2.3c2, 2.8.2, 2.8.3, 3.6.3a, 3.8.1d, 3.10.1bUltramafic
lithospheric
Bridgmanite (Brd)331.5.2a, F.1, Q.3Ultramafic lower-mantle
CaSiO3112.2.2aUltramafic lower-mantle,
Former CaSiPvk
Merrillite113.6.2cCarbonatitic
Diopside? (Di?)222.11.2a, 4.18.1Lithospheric
Subtotal1622
Associations
Ni-fPer + Brd163.1.3b1, 3.1.3b2, 3.1.3b3, 3.1.b4, 3.1.3b5, 3.1.3bUltramafic lower-mantle
fPer + Brd + unidentified13N3
Ni-fPer + CaSiO3132.2.3a, 2.2.3b, 2.2.3cUltramafic lower-mantle,
Former CaSiPvk
Fe-fPer + CaSO3 + Ca(Si,Ti)O3131.2.2a, 1.2.2b1, 1.2.2b2Ultramafic lithospheric
CaSiPrv + Ca(Si,Ti)O312Q2Lithospheric
fPer + Magnesioferrite13F3Ultramafic
Fe-fPer + Magnesite131.2.4b, 1.2.4d, 1.2.4cCarbonatitic
Fe-fPer + Merrillite163.10.1aCarbonatitic
Subtotal829
Total2451
Note: * The samples indicated in italics are partly analyzed.
Table 2. Representative chemical analyses of oxide inclusions in studied diamonds from the Rio Sorriso (wt.%).
Table 2. Representative chemical analyses of oxide inclusions in studied diamonds from the Rio Sorriso (wt.%).
Sample3.1.3b12.2.3a1.2.2a3.10.1a3.1.3b2.2.3a1.2.2b
MineralNi-fPerNi-fPerFe-fPerFe-fPerBrdCaSiO3CaSiTiO3
AssociationNi-fPer+
Brd		Ni-fPer+
CaSiO3		Fe-fPer+
CaSiO3
+CaSiTiO3		Fe-fPer+
Merrill		Ni-fPer+
Brd		Ni-fPer+
CaSiPrv		Fe-fPer+
CaSiO3+
CaSiTiO3
SiO20.020.060.050.0156.3251.5812.92
TiO20.010.010.020.010.120.0440.28
Al2O30.150.040.040.121.260.091.83
Cr2O30.750.530.161.210.080.011.33
FeO33.7435.8859.0662.864.910.280.51
NiO1.111.180.220.100.010.02n.a.
MnO0.490.290.371.470.110.03n.a.
MgO62.2960.5238.4432.1935.32n.a.n.a.
CaO0.00n.a.n.a.0.010.1047.4438.06
Na2O0.610.210.091.430.070.06n.a.
K2O0.000.01n.a.n.a.0.020.02n.a.
Total99.1898.7298.4599.4198.3199.5694.93
fe#0.2330.2490.4630.5230.072––
mg#0.7670.7510.5370.4770.928––
Note: n.a.—not analyzed.
Table 3. Carbon-isotopic composition in Rio Sorriso diamonds with ferropericlase inclusions.
Table 3. Carbon-isotopic composition in Rio Sorriso diamonds with ferropericlase inclusions.
SampleNo of Anal.δ13 CVPDB, ‰ Ni in fPer, ppm
AverageSt. Deviation
With high-Ni ferropericlase inclusions
1.2.13−5.530.388275
2.2.15−5.600.599053
2.2.34−5.610.749288
2.2.45−5.200.149438
2.7.23−5.931.089595
3.2.29−6.440.819862
3.5.28−5.500.8310,023
3.9.35−6.150.279980
3.10.25−5.590.459037
N5−3.640.719172
Average −5.520.75
With low-Ni ferropericlase inclusions
1.2.23−5.760.901537
1.2.36−8.941.341709
2.8.33−6.370.601737
3.6.32−7.560.871784
3.10.13−6.700.171466
Average −7.071.23
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Kaminsky, F.V.; Zedgenizov, D.A.; Sevastyanov, V.S.; Kuznetsova, O.V. Distinct Groups of Low- and High-Fe Ferropericlase Inclusions in Super-Deep Diamonds: An Example from the Juina Area, Brazil. Minerals 2023, 13, 1217. https://doi.org/10.3390/min13091217

AMA Style

Kaminsky FV, Zedgenizov DA, Sevastyanov VS, Kuznetsova OV. Distinct Groups of Low- and High-Fe Ferropericlase Inclusions in Super-Deep Diamonds: An Example from the Juina Area, Brazil. Minerals. 2023; 13(9):1217. https://doi.org/10.3390/min13091217

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Kaminsky, Felix V., Dmitry A. Zedgenizov, Vyacheslav S. Sevastyanov, and Olga V. Kuznetsova. 2023. "Distinct Groups of Low- and High-Fe Ferropericlase Inclusions in Super-Deep Diamonds: An Example from the Juina Area, Brazil" Minerals 13, no. 9: 1217. https://doi.org/10.3390/min13091217

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