Unusual Inclusions And Genesis Of Microdiamonds From The Zelenyi Yar Neogene Placer (The Ukrainian Shield): A Tem Study

doi:10.21203/rs.3.rs-4691358/v1

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Unusual Inclusions And Genesis Of Microdiamonds From The Zelenyi Yar Neogene Placer (The Ukrainian Shield): A Tem Study

https://doi.org/10.21203/rs.3.rs-4691358/v1

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16 individual diamond microcrystals from the Neogene placer of Zeleny Yar on the Ukrainian Shield have been studied using TEM methods. Placer sediments are fine-grained sands of coastal marine origin redeposited in a river valley. The placer heavy minerals include ilmenite, rutile, zircon, kyanite, sillimanite, staurolite, tourmaline, mantle and impact type diamonds. Undisputed minerals genetically related with mantle diamond were not found in the placer. The studied mantle diamonds have different morphologies: octahedron, octahedroid, octahedron-rhombic dodecahedron transition crystal, cube, cuboid, crystals of {100} + {111} + {110} combinational form, fragments of octahedra. The size of the crystals does not exceed 0.5 mm. They differ in color - from colorless to brown and gray, and growth mechanisms - tangential for octahedra and transitional crystals and fibrous for cubic crystals. Some optically clear diamond crystals contain distinct dark inclusions already visible under a binocular microscope. 21 TEM foils 15 × 10 × 0.150 µm in size each, prepared from typical crystals, have been studied. Two groups of diamonds are distinguished based on inclusions: 2 diamonds with sets of inclusions typical for diamonds from ophiolites and the remaining 13 crystals with sets of multiphase inclusions and structural defects typical for diamonds from kimberlites, lamproites and lamprophyres. Crystals of the first group are cube-octahedron and a fragment of an octahedron. Their inclusion are: Mn-olivine, MnNi alloy with minor Si content, MnNiO₃ oxide, Mn₅C₂ carbide, rhodonite (Mn(SiO₃) and amorphous silica. MnNiO₃ oxide is associated with silica and chlorine. Most of the second group crystals are characterized by curved dislocations, carbon platelets were found in several crystals and nitrogen platelets in one crystal. Inclusions in these diamonds are: dolomite, ilmenite, magnetite, apatite, mica, chlorite and orthopyroxene. One crystal contains a Fe-rich forsterite inclusion. They belong to the typical eclogite assemblages of mantle diamond from kimberlites. Crystals of this group experienced significant annealing. Possible original sources for crystals of the first group are proposed serpentenites, for second group - kimberlites or lamproites or lamprophyres.

Diamond

Morphology

Inclusions

Microstructure

Platelets Eclogite paragenesis

Neogene placer

Ukrainian Shield

Four eras of diamond-bearing placer formation have been recognized within the territory of Ukraine: three on the Ukrainian Shield - Proterozoic, Neogene and Quaternary, and two on the folded Donbas - Carboniferous-Permian and Quaternary. The source rocks of diamonds from all these placers are currently unknown. The most powerful one is the Neogene era with the formation of numerous diamond-bearing placers throughout the territory of the Ukrainian Shield (USh) and the Dnipro-Donetsk Rift.

Currently, several Neogene diamondiferous placers are known on the USh. The most known of them are Samotkan and Zeleny Yar (southeastern and central part of the Ush, respectively). They are located at a considerable distance from each other (Fig. 1), but diamonds from them reveal similar characteristics.

Diamonds are common in the so-called Poltava titano-zirconium sands and are characterized by a number of specific features, which do not allow complete identification as diamonds from kimberlites (lamproites) or metamorphic rocks. They are characterized by their small size - up to 0.6 mm, the predominance of cubic shapes among the main habits of crystals, an increased content of colored crystals - brown, purple, brown, yellow, gray, green, etc., "light" carbon isotopic composition for many and an unusual set and ratio of impurity and structural defects for the majority of the crystals. Another characteristics of diamonds from Neogene sands is a significant number of nitrogen-free crystals and diamonds of spectral type Ib and Iab, as well as a relatively high content in crystals of OH groups, hydrocarbons (CH₂, CH₃ groups and bonds > C = CH₂) and C = O, N-H, CO₃, NO₃ groups. Such set of impurities in diamonds suggests crystallization in an environment enriched in volatile components. These features of diamond microcrystal’s from the Neogene placers of the USh indicate their specific and yet unknown transport of diamond into the earth's crust, i.e. possible new unusual original sources of diamond, for example, certain types of lamprophyres and komatiites.

The purpose of this study is to determine the defects of diamond crystals from the Zelenyi Yar placer, to study their mineral and multiphase fluid inclusions to unveil their genetic type and to reconstruct their crystallization environment and thermal history.

The Neogene placer Zeleny Yar is located in the central part of the USh - in the southwestern part of the Ros-Tikych domain, 12 km northeast of the city Orativ, and according to various authors it has an alluvial or coastal marine origin (Tsymbal and Bondar 1966). It is composed of fine- and medium-grained quartz sands and sandstones of the upper horizon of the Poltava series of the Novopetrivska suite. These sands fill a northwestern buried river valley, traced for 40 km in length and 1–3 km in width and of 2–5 to 10–15 m in thickness. Crystals of heavy minerals (zircon, rutile, ilmenite, monazite, tourmaline, staurolite) in the placer reveal a high degree of mechanical wear (rounding), indicating their distant original sources and a long common sedimentation history. The placer contains diamonds of both mantle and impact origin. The latter make up approximately 3% of the mined placer microdiamonds and are paramorphoses of graphite. In general, the diamond content in the placer is 0.06 carats per 1 m³ of ore sand (Polkanov 2009). Potential associated minerals of mantle diamond in the placer are Cr-spinelides and ilmenites with moderate Mg concentration.

Sixteen crystals of different morphologies - octahedron, octahedroid, octahedron-rhombic dodecahedron transition crystal, cube, cuboid, crystals of the {100} + {111} + {110} combinational form and octahedron fragments - were selected from the collection of Zelenyi Yar diamonds for the TEM study. The size of the crystals did not exceed 0.5 mm. They differ in color - from colorless to brown and gray, and growth mechanisms - tangential for octahedron and transitional crystals and fibrous for cubic crystals. Some optically clear diamond crystals contain distinct dark inclusions visible under a binocular microscope.

The internal features of selected diamonds were investigated at Potsdam Imaging and Spectral Analysis Facility (PISA), (GFZ Potsdam, Germany). Eelectron-transparent foils (15x10x0.15 µm) were prepared for TEM investigation using a focused ion beam system (FIB) FEI - Helios G4 Dual Beam - FIB-SEM (Wirth 2004, 2009). TEM research was carried out using a Tecnai F20 G2 X-Twin FEG-TEM. The TEM is equipped with a Gatan Imaging Filter (GIF), a Fishione high-angle annular dark-field system (HAADF), and an EDAX X-ray analyzer with ultrathin window. The TEM was operated at 200 kV with a field emission gun as an electron source. Conventional bright-field (BF) and dark-field (DF) imaging and electron diffraction was applied. BF, DF, and high-resolution lattice fringe images were acquired as energy-filtered images, applying a 20 eV window to the zero-loss peak. Simultaneously, EDX spectra were acquired from the studied phase using a very small spot size (< 10 nm, depending on the size of the object investigated), thus focusing on the object of interest. The chemical composition provides basic information about the phase studied. Diffraction patterns calculated from high-resolution lattice fringe images applying a fast fourier transformation (FFT) were used to identify phases. The acquisition time for high-resolution lattice fringe images (HREM) can be as short as 0.2 sec. thus minimizing irradiation damage. The measured length of the different diffraction vectors (reciprocal space) is transformed into d_hkl lattice spacing in real space. Additionally, the angles between the different vectors are measured. The observed lattice fringe d-spacing and the angles between adjacent planes are compared with calculated d-spacing and angles based on unit cell parameters (data from literature) for the expected phase. The observed angles between adjacent lattice planes should match the angles calculated based on structural data from literature within an error of ± 1.5°. Finally, the diffraction pattern will be indexed, and together with the chemical composition from EDS the phase is fully identified.

Previous study

Morphology

The mantle type microdiamonds of Zelenyi Yar are the largest among those from other Neogene titanium-zirconium placers of the Ukrainian Shield and reach a size of 0.6 mm. They are represented by cubes, crystals of the combinational form {111} + {110} + {100}, cube-octahedrons, octahedrons and crystals of the transitional form {111} + {110}, rhombic dodecahedrons are rare. 48% of the mined samples are worn crystals and chips, 18% are partially worn, and only 34% of the grains are well preserved crystals. The best preserved are cubes and crystals of combinational form.

Color and photoluminescence

Up to 20% of Zelenyi Yar microdiamonds have a low intensity yellow, green, pink-purple, brown or gray color. About 70% of Zelenyi Yar microdiamonds have blue, green, yellow or orange UV luminescence. Octahedrons usually have green, orange, and yellow and only a small part of them has blue photoluminescence (PL). A significant part of the cubes and crystals of the combinational form {111} + {110} + {100} has an orange PL, the rest - yellow and green. The PL of microdiamonds from the Zelenyi Yar placer is caused by various defect centers — mainly nitrogen ones: blue (N3), greenish-blue (N3 + H3 + H4), green (H3), yellow-green (N3 + H3; N3 + H3 + 575 nm; H3 + 575 nm), red and orange (409 nm + 575 nm).

IR-spectroscopy

A FTIR study of the Zelenyi Yar diamonds reveals that a significant part (> 50%) of them is free of- or relatively low in nitrogen. The content of nitrogen impurity in the crystals ranges from 57 to 612 ppm (the average nitrogen content is 224 ppm). Based on the infrared characteristics the Zelenyi Yar diamonds are subdivided into type IIa — 50%, type Ia — 45% (IaA — 29%, IaAB — 11%, Iаb — 5%) and type Ib — 5%. The temperature of residence of nitrogen bearing diamonds (type IaAB) in the mantle for 2 billion years is in the range of 1090–1190 ºС, calculated according to the method of W.R. Taylor and H.J. Milledge (Taylor and Milledge 1995).

Raman spectroscopy

All the microdiamonds from Zelenyi Yar studied by Raman spectroscopy are similar in terms of the position of the main line in the Raman spectrum, while in terms of full width at half height – (FWHM), they are different. Pigmented crystals and cubes have a higher FWHM value, so they can be considered as more defective. Based on Raman spectroscopy data, some inclusions in the studied diamonds might be magnetite.

Carbon and helium isotopes

The carbon isotope composition of the Zelenyi Yar microdiamonds is heterogeneous – δ¹³С-values ranging from − 26.74 to − 3.55‰ VPDB. No relationship between shape, color and isotopic composition of carbon has been found for the studied microdiamonds. Crystals of tangential ({111} + {110}, {111} + {110} + {100}, dodecahedron) and fibrous (cubes) growth show both light and heavy carbon. For one sample (colorless octahedron) from Zelenyi Yar, the content of helium isotopes was determined: ³He = 73.42 × 10^–12 cm³/g and ⁴He = 0.265 × 10^–6 cm³/g. The content of helium isotopes, as well as a ³He/⁴He ratio (27.67 × 10^–6), for this crystal is within a range of such values for kimberlite and lamproite diamonds from native sources and placers worldwide, and differs from those for microdiamonds from metamorphic rocks of the Kokchetav massif in Kazakhstan, which have extremely high concentrations of helium (Lavrova et al 1999).

The morphological, photoluminescence, spectroscopic and isotopic characteristics of Zelenyi Yar microdiamonds are similar to those of mantle diamonds from kimberlites, lamproites or lamprophyres (Kvasnytsya 2021). However, they only permit tentatively to predict their possible mantle environment of crystallization: eclogitic or peridotitic (harzburgitic, lherzolitic). If we take into account the wide range of δ¹³С carbon isotopic values, the increased nitrogen content (the average nitrogen content is 224 ppm) and predominantly cubic shape of the crystals, the mantle environment was most likely eclogitic. It is also difficult to predict both the probable age of microdiamonds (Archaean - Lower Proterozoic?) and a possible age of diamondiferous source rocks (Middle-Upper Proterozoic?). As a result, it is difficult to reproduce the actual history of Zelenyi Yar diamonds from the currently available data. A TEM and AEM study of mineral and fluid inclusions in them becomes very important, to provide some new insights.

Only one microdiamond (smooth faceted cuboid, ZY-3), out of 16 studied with TEM, was found to be free of common internal features, e.g. inclusions, structural defects or platelets. At least five different types, based on the presence of the above-mentioned characteristics, can be distinguished among the other fifteen crystals (Table 1):

Table 1

Inclusions and structural defects in Zelenyi Yar microdiamonds
Sample number, shape of microdiamond	Micro-nanoinclusions, рlatelets and dislocations	Character of microdiamonds and their nanoinclusions
Metallic and other inclusions
ZY-5 Cube-octahedron (Fig. 2–4)	Mn-olivine (up to 1.8 µm nm in size), MnNi alloy (up to 300 nm in size) with low Si content, MnNiO₃ oxide (up to 110 nm in size), Mn₅C₂ carbide (up to 100 nm in size), amorphous silica (up to 325 nm in size)	Crystal of tangential (spiral or two-dimensional nucleation) growth, dissolution pits on {100} faces of microdiamond, crystallographic orientation of inclusions in matrix of diamond, rounded polyhedral shape of inclusions. Large pore indicating the presence of a fluid or gas, which excludes synthetic origin of diamond.
ZY-2 Fragment of octahedron (Fig. 5)	Rhodonite (up to 150 nm in size)	Crystal of tangential growth
Multiphase mineral inclusions
ZY-7 Fragment of octahedron (Fig. 6, 7, 8)	Dolomite (up to 800 nm in size), ilmenite (up to 600 nm in size), magnetite (up to 120 nm in size), apatite (up to 150 nm in size), orthopyroxene (up to 500 nm in size), plus porosity indicating the presence of a fluid and chlorite (up to 2 µm in size) - indicating presence of water	Crystal of tangential growth, vacuoles are filled with multiphase inclusions, apatite inclusion has a hexagonal outline
ZY-4 Cuboid (Fig. 9)	Fe-rich olivine (up to 110 nm in size)	Crystal of fibrous growth, smooth crystal faces, high density of curved dislocations along (111)
ZY-12 Aggregate of cubes (Figs. 10, 11)	Phlogopite (up to100 nm in size), apatite (up to 250 nm in size), unidentified silicate (up to 300 nm in size)	Crystals of fibrous growth, crystallographic orientation of inclusions in matric of diamond, stacking faults in diamond, straight dislocation lines indicating dislocation glide
Рlatelets of carbon and nitrogen
ZY-6 Octahedron (Fig. 12a)	Low density of carbon platelets (up to 50 nm in size)	Crystal of tangential growth, very thin dissolution pits on {111} faces
ZY-15 Contact twin of crystals of transitional {111} + {110} form (Fig. 12b)	Very high density of carbon platelets (up to 50 nm in size)	Crystals of tangential growth
ZY-11 Crystal of combinational form {111} + {110} + {100} (Fig. 13)	Rare platelets of nitrogen (up to 60 nm in size) and dislocations along (111) documented with N-K edge in EEL spectrum, long straight dsilocation lines	Crystal of tangential growth, the cube + octahedron faces are habitus.
Dislocations
ZY-1 Cuboid (Fig. 14a, b)	Very high density of strongly curved dislocations	Crystal of fibrous growth, smooth crystal faces
ZY-14 Octahedroid (Fig. 14c, d)	Very high density of curved dislocations indicating dislocation climb a thermally activated process	Crystal of tangential growth, smooth crystal faces, developed microfaces {hkl}
ZY-16 Crystal of combinational form {111} + {110} + {100} (Fig. 14e, f)	Rare straight and curved dislocations	Crystal of tangential growth, the cube faces are habitus.
ZY-9 Fragment of octahedron (Fig. 14g, h)	Very high density of straight and strongly curved dislocations indicating dislocation glide and climb	Crystal of tangential growth, elements of polycentric layering
ZY-8 Fragment of octahedron (Fig. 14i, j)	Low density of straight and curved dislocations	Crystal of tangential growth, elements of polycentric layering
ZY-10 Fragment of octahedron (Fig. 14k, l)	Rare density of curved dislocations	Crystal of tangential growth
Polysynthetic twinning
ZY-13 Fragment of impact apographite diamond (Fig. 15)	Intense polysynthetic twinning along (11\(\:\overline{2}\)1) of graphite	Crystal of martensitic growth (recrystallization), blurred twinning pictures

Diamonds with metal alloys and other solid Mn phase nano-inclusions;
Diamonds with multiphase fluid and mineral inclusions;
Diamonds with carbon or nitrogen platelets;
Diamonds with dislocations;
Impact diamond.

Sample ZY-5 with metal alloy and other solid Mn phase inclusions is a cubo-octahedron in which the flat and smooth faces of the octahedron significantly outweigh the pitted faces of the cube by size (Fig. 2). The cube faces are covered with quadrangular pits. In three foils from this crystal, inclusions of Mn-olivine, MnNi alloy with low concentration of Si, MnNiO₃ oxide, Mn₅C₂ carbide, and amorphous silica were found (Fig. 3, 4). Some of them are oriented in the (111) planes of diamond. The shape of many inclusions is close to polyhedral, but the crystal faces are often rounded. Mn-olivine and MnNi alloy are intergrown. A MnNiО₃ phase partially fills a cavity together with silica and chlorine. Amorphous silica also occurs as isolated inclusions. The chemical composition and structural data of the above mentioned manganese phases is given in the Table 2 and Table 4 Suppl. Nickel is present as an impurity in all Mn-olivine inclusions.

Table 2

Chemical composition of Mn inclusions in the ZY-5 and ZY-2 diamonds, wt. %
Oxides/ elements	Composition	Impurities	Crystal system, unit cell parameters, literature data
Mn-olivine (сrystal ZY-5)
MnO	1–71.93; 2–66.29	Ni	Orthorhombic, а_о = 4.9023 Å, b_о = 10.5964 Å, c_о = 6.2567 Å for tephroite Mn₂SiO₄ (Fujino et al 1981)
CaO	1–0.58; 2–0.66
SiO₂	1–27.49; 2–32.45
MnNi alloy (сrystal ZY-5)
Mn	1–84,13; 2–57.73; 3–35.62	Fe, Cr, Si	Tetragonal, а_о =2.639 Å, b_о =2.639Å, c_о =3.62 Å for synthetic compound Mn,Ni (Buschow et al 1983)
Ni	1–15,87; 2–38.23; 3–60.08
Si	1–0; 2–4.04; 3–4.30
MnNiO₃ (сrystal ZY-5)
MnO	53.51 (theoretical 48,73)		Orthorhombic, а_о=2.8241 Å, b_о=9.4365 Å, c_о=6.9161 Å for synthetic compound MnNiO₃ (Griesemer et al 2021)
NiO	46.49 (51,27)
Mn₅C₂ (сrystal ZY-5)
Mn, Ni, Cr	Mn – 89.17 (theoretical 91,97), Ni – 10.08, Cr – 0,75	Si	Monoclinic, а_о =11.6724 Å, b_о =4.5864 Å, c_о =5.0969 Å for synthetic compound Mn₅C₂ (Karen et al 1991)
C	(theoretical 8,03)	Si
Mn(SiO₃) (сrystal ZY-2)
MnO	57.23	Ni	Triclinic, а_о = 7.682 Å, b_о = 11.818 Å, c_о =6.707 Å (Narita et al 1977)
SiO₂	42.77	Ni

Table 3

Composition (wt. %) of MnNi inclusions in ophiolite and basalt diamonds in different sources
Location/Sourse/ Host rock/ Reference	Mn	Ni	Co	Impurities
Ray-Iz, Polar Ural / Chr, Per/ (Yang et al 2015)	17.8–19.3	72.2–75.6	5.0-5.6	Fe, Si
Luobusa, Tibet / Chr / (Xu et al 2009)	19.06	75.34	5.6
Pozanti-Karsanti, Turkey / Chr / (Lian et al 2018)	22.02 24.47	73.68 71.0	4.30 4.53
Hegenshan, Inner Mongolia / Chr / (Yang et al 2015)	25.71	68.01	6.27
Mirdita, Albania / Chr / (Wu et al 2019)	27.3	68.3	4.4
Tolbachik, Kamchatka / Basalt / (Litasov et al 2019)	50–60	20–25		Fe, Cr
Zelenyі Yar Neogene placer, Ukrainian Shield / Sands	1–84,13; 2–57.73; 3–35.62	1–15,87; 2–38.23; 3–60.08		Fe, Cr, Si, Cl, O
Chr - chromitite, Per - peridotite.

In sample ZY-2, which is a fragment of octahedron, only inclusions of a MnNiO phase were detected (Fig. 5). As it can be seen from the EDS spectrum the ratio Si, Mn is approximately 1:1. Diffraction data from high-resolution images could be indexed as rhodonite a triclinic Mn-silicate. The chemical composition and structural data of rhodonite is given in the Table 2 and Table 4 Suppl.

Diamond ZY-7, which is a fragment of octahedron, contains single- and multiphase inclusions of dolomite, ilmenite, magnetite, apatite, chlorite and orthopyroxene (Figs. 6, 7, 8). Dolomite, ilmenite (Fe_1.04Ti_0.96O₃) and orthopyroxene are found together within the vacuole. The apatite crystal has an imperfect polyhedral shape; inclusions of other minerals are xenomorphic crystals. Ilmenite was determined from chemical composition and the according difraction pattern (FFT) (Fig. 7b). Orthopyroxene was determined from diffraction data and chemical composition (wt. %): SiO₂ – 47.01, Al₂O₃ – 4.42, FeO – 9.20, MgO – 38.53, K₂O – 0.84. K is probably due to an amorphous quench phase hit by the electron beam. From Fig. 6b it is obvious the porosity was orginally filled with a fluid that could have escaped during FIB milling into the vacuum. Carbon in the EDX spectrum is due to the daimond host. Structural data of orthopyroxene are given in the Table 4 Suppl.

In two samples ZY-4 and ZY-12 fluid multiphase inclusions were also found. Due to their very small size, identification of some mineral phases in these inclusions is absent or not precise. In cuboid (Fig. 9) numerous curved dislocations and a single inclusion with hexagonal contours were found. The latter was identified as Fe-rich olivine - forsterite (Table 4 Suppl). Chemical composition of forsterite (wt. %): SiO₂ – 39.48, FeO – 29.22, MgO – 31.30.

Phlogopite, apatite and an unidentified silicate of Si-Ti-Al-Fe-Mg-K-O composition, as well as curved dislocations were found in the cubes aggregate with a rough surface (Fig. 10, 11). Extended dislocations are developed mainly within the rectilinear boundaries that separate crystal on blocks of different sizes. Inclusions are placed directly on the rectangular contacts between the blocks. Apatite fills the cavities, often together with unidentified silicate.

Diamonds ZY-6, ZY-15 with carbon platelets are a perfect octahedron and a contact twin of crystals of transitional form {111} + {110} (Fig. 12). Carbon platelets in these diamonds, with a size of 10 to 50 nm, are developed along (020). In ZY-15 short curved dislocations also observed.

Sample ZY-11 with nitrogen platelets up to 50 nm documented by EELS (Fig. 13d) and curved dislocations is a crystal of combinational form {111} + {110} + {100} (Fig. 13).

Diamonds with dislocations are crystals of different morphology (Fig. 14). They are represented by cuboid (ZY-1), octahedroid (ZY-14), crystal of the combinational form {111}+ {100}+{110} (ZY-16) and fragments of octahedra (ZY-8, -9, -10). Dislocations in the near-surface part of these crystals are mainly curved short linear defects. Curved dislocation lines are indicative for annealing deformed diamond because they represent dislocation climb, which is a thermally activated process. It should be noted that these defects are developed in both tangential and fibrous grown crystals. An annealing process is alos documented in Fig. 14b and h where dislocation walls surround small areas (< 500 nm) with no dislocation inside. The dislocation walls have formed by dislocation migration during thermal annealing.

Sample ZY-13 is a fragment of impact diamond (Fig. 15). In its TEM foil the typical features of an impact apographitic diamond – polycrystalline structure and polysynthetic twinning in the plane (11\(\:\overline{2}\)1) – is clearly visible. Such diamonds are composed of two (diamond, lonsdaleite) or three crystalline phases (diamond, lonsdaleite, graphite). Their crystals are characterized by polysynthetic (11\(\:\overline{2}\)1) twinning and polycrystalline structure of the twins themselves, which are typical features of apographitic impact diamond from meteorite structures (Kvasnytsya and Wirth 2013). Obviously, due to the predominance of the diamond phase in this crystal, the twinning pattern of the graphite precursor is something blurred.

The discovery of MnNi alloy with admixture of Si and other manganese compounds such as MnNiO₃, Mn(SiO₃), Mn₅C₂ and Mn-olivine in Zelenyi Yar microdiamond inclusions was unexpected. So far, Mn-rich inclusions in diamod have been reported from microdiamonds from ophiolite rocks (chromitites and peridotites) and occasionally from eruptions of modern volcanoes (Lian and Yang 2019, Litasov et al 2019, Dobrzhinetskaya et al 2022). Mn-Ni-Co alloys with different composition have been found frequently in ophiolite and basalt microdiamonds (Table 3). However, in contrast to inclusions in ophiolite microdiamonds, this type of inclusions in Zelenyi Yar samples do not contain cobalt. We also emphasize the association of most of these inclusions with a fluid, silica and chlorine, and porosity, which excludes synthetic origin of these diamonds. Manganese carbide (Mn₅C₂) has not been reported as an inclusion in both mantle diamonds (Shirey et al 2013, Shirey et al 2024, Stachel et al 2022, Nestola et al 2023, Pamato et al 2023) or ophiolitic microdiamonds and among a wide range of minerals in ophiolitic rocks in general (Jang et al 2015, Dobrzhinetskaya et al 2022). We have not found any reliable information about natural manganese carbide, as well as about natural oxide MnNiO₃.

Thus, at the moment we do not have a conclusive explanation for the presence of Mn₅C₂, MnNiО₃ and rhodonite in the Zelenyi Yar microdiamonds. Synthetic Mn₅C₂ is stable at temperatures of 300–1100 K (30-830^oC) (Karen et al 1991), and 880-1000^oC according to (Gasik, Lyakishev 2008). Regarding the MnNi alloy, Wu with co-authors (Wu et al 2019), taking into account the literature data on the analysis of the Mn–Ni system at different PT parameters (Guo and Du 2005; Gupta 1999), suggests that its melting point can reach ~ 1328°C. This may correspond to mantle depths over than 150–200 km.

But carbides are known as inclusions in diamonds. So, moissanite inclusions are known in lithospheric diamonds (Moore et al 1986a, Moore et al 1986b, Jaques et al 1986, Leung 1990, Stachel et al 2022), while in ultradeep diamonds in addition to moissanite iron and nickel carbides are also found (Wilding e tal 1991, Kaminsky, Wirth 2011, Smith et al 2016, Kaminsky 2017, Smith et al 2018, Tschauner et al 2018, Shirey et al 2024). Moissanite inclusions in diamonds and its associations directly in the rock are reported from ophiolitic peridotites (Yang et al 2021, Liu et al 2021). These findings indicate that silicon and iron carbides are commonly present in deep parts of the mantle. Moissanite is classified as inclusions in diamonds of eclogite association (Stachel et al 2022).

From the observations above we conclude that two Zelenyi Yar microdiamonds with Mn-rich inclusions originated from the upper mantle. Their potential original sources can be serpentinite bodies that occur in the Berdychiv block and its outskirts, to the west-northwest of the Zelenyi Yar placer. Dozens bodies of such ultramafic rocks were there discovered by drill cores. However, they are insufficiently studied. There are no geological signs of the ophiolite rocks to be present in this part of the Ukrainian Shield, as well as there is no ophiolite belt on it in general. Significantly distant from the Zeleny Yar placer are also bodies of Vendian basalts, the powerful manifestations of which are located far to the northwest - in Western Volyn.

The Zelenyi Yar microdiamonds with Mn-rich inclusions (ZY-5, ZY-2) have a flat-faced octahedral and cubo-octahedral shape. The same shape is characteristic to microdiamonds from ophiolites and basalts, and synthetic HPHT diamond crystals. On synthetic diamond crystals, synthesized in the metal-carbon system, not only the face of the octahedron, but also the face of the cube can behave as a smooth surface, which is due to the surface reconstruction of the {100} faces - the transition of the rough face of the cube K into the smooth face F, according to the terminology of P Hartman (Hartman 1965). According to I. Sunagawa (Sunagawa 2005), during diamond crystallization, this surface reconstruction occurs in such an synthetic melt, in which the metal element as a solvent has a small ionic radius, while this does not occur in a natural silicate solvent with a large ionic radius. Therefore, it is possible to predict the determining role of metals in the formation of such a form of the above-mentioned diamond types and the crystallization of similar Zelenyi Yar microdiamonds from a manganese-enriched melt-fluid. It can be assumed that transition metals as Fe, Ni, Mn, Co, Cr are the reason for the layered growth of the cube faces both on microdiamonds from ophiolites and basalts, and on synthetic HPHT diamond crystals. Layer (spiral or two-dimensional nucleation) growth determines the appearance of flat and smooth faces of the cube. Therefore, diamond crystals of these genetic types have almost the same morphology.

Most of the Zelenyi Yar microdiamonds – with exception of those two containing Mn-rich nanoinclusions - with multiphase mineral inclusions are comparable with diamonds from kimberlites, lamproites, and lamprophyres. The set of mineral inclusions in the Zelenyi Yar microdiamonds (dolomite, ilmenite, magnetite, apatite, phlogopite, chlorite) is characteristic for lithospheric diamonds (Stachel et al 2022). Previously, we recorded the same polyphase mineral inclusions in samples from the Neogene Samotkan placer on the Ukrainian Shield (Kvasnytsya, Wirth 2013). According to (Stachel et al 2022), inclusions of ilmenite, magnetite, phlogopite and dolomite belong to an eclogite association. Therefore, the association of mineral inclusions in the Zelenyi Yar microdiamonds corresponds to the eclogite mantle environment during crystallization. If we also take into account the wide range of δ¹³C carbon isotopic values (from − 26.74 to − 3.55‰), the high nitrogen content (average is 224 ppm) and the widespread cubic shape of the crystals, then we can conclude an eclogite association. It was shown statistically that kimberlite diamonds of the eclogite association, compared to peridotite ones, have a wider range of carbon isotopic composition values and increased nitrogen content (Stachel et al 2009, Shirey et al 2013). In addition, cubic diamond crystals were not observed in xenoliths of peridotites from kimberlites, but only in diamond-bearing xenoliths of various types of eclogites. In other words, the microdiamonds of the Zelenyi Yar placer are more comparable to kimberlite diamonds of an eclogite association than of peridotite association. Multiphase nanoinclusions reflect the composition of the fluid from which these diamonds have originated. They testify that the crystallization environment of the Zelenyi Yar microdiamonds contained carbonate, alkali ions, and chlorine. Therefore, based on the composition of these multiphase assemblages, it can be assumed that the Zelenyi Yar microdiamonds have grown from carbonate to low-silicon fluid melts enriched in alkaline and volatile components.

To the group of crystals with multiphase inclusions we also include the Zelenyi Yar microdiamonds with dislocations, nitrogen and carbon platelets. The latter were first directly observed using a transmission electron microscopy (Evans, Phaal 1962) and an EELS method (Berger, Pennycook 1982) in diamond of Ia spectral type. Carbon platelets are long or short planar defects that occur in the (100) planes of diamond crystals, containing aggregated forms of nitrogen and are one of the most common defects in mantle diamonds (Harris 2018). In addition to TEM studies, the presence of carbon platelets in many of the Zelenyi Yar microdiamonds is indicated by FTIR spectra (the so-called B2 center, lines 1358–1380 cm^–1, 1430 and 330 cm^–1). The content of B2 center in the Zelenyi Yar microdiamonds is quite significant - from 0.04 to 3.18 relative units with an average of 0.54 (Kvasnytsya 2021). The relationship between platelet size and the B` infrared peak has been studied in detail in a recent paper by Speich et al. (Speich et al., 2017).

Platelets in diamond are extended planar defects that are thought to be generated during the nitrogen aggregation process in type Ia diamonds (Speich et al., 2017). According to (Woods, 1986), platelets are defects in the structure of a diamond crystal several atom layers thick, which are formed during the deposition of interstitial carbon during the growth of B1 centers. In other words, these are lamella of an interstitial character, which are caused by displacements of the diamond lattice and are mainly represented by carbon with a certain amount of nitrogen atoms. They arise during the formation of B1 nitrogen centers and testify the annealing of diamonds during their residence in the mantle. This influence is also clearly indicated by curved dislocations that are characteristic for microdiamonds of the Zelenyi Yar. Curved dislocation lines indicate dislocation climb, which reqires annealing of the diamond at elevated temperatures because dislocation climb is a thermally activated process. Therefore, it can be concluded that the Zelenyi Yar microdiamonds were significantly annealed during their long-term residence in the mantle. Calculated from FTIR spectroscopy data, the possible annealing temperatures in the mantle for the Zelenyi Yar microdiamonds are 1090–1190 ºС for 2 billion years.

Therefore, we conclude that the Neogene Zelenyi Yar placer diamonds can have originated from at least three genetically different primary sources: a) kimberlites or lamproites or lamprophyres; b) ultrabasic rocks - serpentinites; c) impactites. The location of these potential source rocks is predicted to the west — northwest of the Zelenyi Yar placer — on the Berdychiv block of the Podilsky domain. One of the clearest indicators of exactly this direction of terrigenous material transportation to the Neogene deposits of the Zelenyi Yar placer is impact diamond (Fig. 13). It can be predicted that they trace this path of transportation into the placer for mantle diamond as well. Potential original sources for the Zelenyi Yar impact diamond are the Illintsi and Bilylivka meteorite structures, which are located to the southwest up to 38 km and northwest up to 65 km from the Zelenyi Yar placer, respectively. The most likely source for the Zelenyi Yar impact diamond is the Illintsi crater. The age of the latter is 310 to 370 million years, approximately close to the Silurian-Devonian boundary (Valter et al 1998). Its present diameter is about 3.2 km (its initial size was 6.5 km), so the crater is strongly eroded. The diamond content of these crater impactites was estimated by A.A. Walter to 10⁴-10⁵ carats. Palaeographic conditions in the Neogene, obtained lithofacial and mineralogical data for this area of the USh (Pavlyuk, Dovgan 2004, Tsymbal 2014) also indicate that the main primary sources of terrigenous material for the formation of the Zelenyi Yar placer were Precambrian rocks and their weathered crusts on the Berdychiv block and its surroundings.

Thus, various minerals of solid and multiphase fluid inclusions were identified in the Zelenyi Yar microdiamonds, and the number of defects in the crystals was determined. This allows us to i) reconstruct the environment of diamond crystallysation; ii) establish their belonging predominantly to the eclogite association; iii) estimate PT-conditions of diamond crystallization and thermal history of their assumed residence in the mantle; iv) potential geological and genetic type of their parent rocks. As a result of that research, there are new perspectives for the search for primary sources of diamonds on the Berdychiv block of the Ukrainian Shield. The finding of microdiamonds with inclusions characteristic to ophiolitic microdiamonds on the Precambrian crystalline shield raises the question of the existence of new primary sources of such diamonds within ancient stable platform structures.

Acknowledgements We are thankful to

Author contribution VK and RW conceived, designed and carried out the research. RW conducted the TEM study of the samples. OV and AS participated in the preparation and examination of diamond samples. VP took part in selecting diamond samples from the collection and presented geological materials. VK drafted the manuscript. All authors contributed to data interpretation, discussion, and revision of the manuscript.

Funding This study has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101005611 for Transnational Access conducted at Helmholtz-Zentrum Potsdam Deutsches Geoforschungs Zentrum (GFZ)”; Project ID: EXCITE_TNA_C4_2023_19.

Availability of data and materials All data presented in the text of the article are fully available without restriction from authors upon request. Code availability is not applicable.

Competing interests The authors declare that they have no competing interests.

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Unusual Inclusions And Genesis Of Microdiamonds From The Zelenyi Yar Neogene Placer (The Ukrainian Shield): A Tem Study

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