Home – Home – Jornals – Russian Geology and Geophysics 2026 number 7

This special issue of the journal Russian Geology and Geophysics comprises studies presented in the wake of the scientific conference “Processes of mineral formation at high pressures: the origin of diamond and mantle magmas”, dedicated to the 90th anniversary of the birth of the outstanding researcher of diamond formation processes, Academician Nikolai Vladimirovich Sobolev. The conference was held on June 17-19, 2025, at the V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, in Novosibirsk Akademgorodok. The range of topics addressed, from detailed studies of crystal lattice defects in individual diamond crystals and their inclusions to the regimes and timing of the plate tectonics on the early Earth, fully reflects N.V. Sobolev’s contribution to the formation of our modern understanding of the evolution of the Earth’s mantle composition and the formation settings of kimberlites and diamonds.

The main process that changes Earth’s silicate composition after core segregation is the formation and recycling of continental crust. These processes are closely related to the tectonic regimes that operated at different times during Earth’s history. This review combines recent geochemical data and geodynamic models of how continental crust formed throughout Earth’s history, especially during the Hadean and Archean eons. Continental crust cannot form by direct melting of the dry ultramafic mantle. It requires water, mafic protolith, and minerals compatible with high-field-strength elements (Ti, Nb, Ta, Zr, Hf), such as amphibole, rutile, ilmenite, or jadeite pyroxene. For the early Earth, the most likely model involves two stages: first, basaltic or picritic (oceanic) crust is extracted from the mantle, leaving behind a refractory harzburgitic residue. Then, after hydration, the oceanic crust subducts, melts or releases water to flux melting in the mantle, creating continental-crust magmas. Meanwhile, the remaining refractory residue mixes with refractory mantle material, producing a depleted mantle reservoir. Canonical Nb/U and Ce/Pb ratios are unaffected by mantle melting under dry conditions but change during melt generation when amphibole and high-Ti phases are present. Therefore, these ratios are useful indicators of continental crust formation. Geochemical tracers such as: 1) Sr isotope compositions of komatiite melts and plagioclase in anorthosites; 2) element ratios in komatiite melts; 3) trace element contents and Hf isotopic compositions of zircon, and 4) decay products of short-lived Sm and Hf isotopes in rocks either support or do not contradict the operation of active continental crust formation and mantle depletion during the Hadean. Production and recycling of continental crust in this period likely involved episodic, short-lived subduction triggered by plumes. Overall, these findings suggest that tectonic regimes in the Hadean (4.4-4.0 Ga after magma-ocean solidification) and in the Eoarchean (4.0-3.6 Ga) were more dynamic and varied in time and space than previously thought. However, the development of global plate tectonics requiring a connected network of subduction zones, mid-ocean ridges, and transform faults could only start later, during the Archean. The causes of the onset of plate tectonics throughout Earth’s history are still debated, and new ideas (such as those involving surface processes, such as the erosion of continents) are being proposed and require further testing.

This paper presents new evidence for the fluorine enrichment of the diamond-bearing mantle of the Siberian craton. For the first time, we have discovered inclusions containing calcium and aluminum fluorides in placer diamonds from the northeastern Siberian craton. In contrast to microinclusions in fibrous diamonds and coated diamonds, the studied inclusions can be interpreted as fluoride melts. In diamond ISTD-119, inclusions are oriented parallel to the octahedron faces and can be considered syngenetic, whereas in diamond ISTD-124, an elongated inclusion is oriented parallel to slip lines. This suggests that the formation of the inclusion was synchronous with the stage of diamond deformation. In variety V diamonds XLS-147 and ISTD-193, the morphology of inclusions located in the central parts of the crystals allows them to be interpreted as healed cracks. Fluorine is present in the inclusions, as calcium compounds with stoichiometry close to fluorite. The inclusions comprise zones consisting of calcium fluorides and zones consisting of carbonates and/or iron oxides. In variety V diamond XLS-147, aluminosilicate and aluminosilicate-fluoride inclusions have been identified. In contrast to the inclusions in diamonds ISTD-119, ISTD-193, and ISTD-124, the fluoride component of these inclusions is aluminum fluoride. There is evidence for the growth of variety V diamonds in subduction zone. The compositional variations of the studied inclusions indicate immiscibility of fluoride, aluminosilicate, and carbonatite melts. These inclusions testify to the fluorine-enriched diamond-forming mantle of the Siberian craton at the time of the diamond formation.

Olivine is the main rock-forming mineral in kimberlites, and the results of numerous studies of kimberlitic olivine are widely used in most models of kimberlite petrogenesis. Kimberlitic olivine investigations also provide constraints for models of the composition, structure, and evolution of the lithospheric mantle beneath ancient cratons. However, a persistent problem is the identification of magmatic olivine in kimberlites and olivine derived from disaggregated mantle xenoliths, as both may display very similar morphologies. In this study, we demonstrate that although the morphology of these olivines can be similar, they can be distinguished using compositional and internal structural criteria. The study is based on an extensive analytical dataset (more than 500 olivine grains up to 5 mm in size were analyzed from a single volcaniclastic kimberlite phase of the Udachnaya-East pipe, Siberian craton) and on a summary of the previously obtained data on the compositions of olivine from 158 xenoliths of sheared peridotites from the same kimberlite pipe. New data on the composition of olivine from 20 sheared peridotite microxenoliths (≤0.5 cm) are also presented in this work. Microxenoliths of sheared peridotites can be divided into two groups according to the Mg# of their olivine: 89.5-92.5 (50% of the samples) and 84.5-87.5 (45% of the samples). In contrast, olivine from sheared peridotite xenoliths (≥5 cm) shows a unimodal Mg# distribution, with most olivine compositions (80% of the samples) falling within the Mg# range 89.5-92.5. The paper presents a series of well-documented examples where olivine neoblasts derived from disaggregated sheared peridotite xenoliths acted as nuclei for the growth of magmatic olivine. In addition, a unique example of the disaggregation of a microxenolith of sheared peridotite in situ during kimberlite emplacement is provided. This example is the compact occurrence of nonresorbed and unzoned euhedral neoblasts within a 1 mm field of view in the kimberlite rock. The largest portion of true magmatic kimberlitic olivine occurs in the groundmass size fraction (<0.25 mm) of unaltered rocks; however, olivine of this size fraction in kimberlites worldwide remains de facto unstudied.

Studying the composition and crystallization conditions of olivine in kimberlites is crucial for understanding their petrogenesis and assessing their diamond potential. Our investigation focuses on the origin of the least studied generation of this mineral: late high-Mg olivine. The research material comprises samples of unaltered kimberlites from the Udachnaya-East pipe, where all olivine generations are fully preserved. Scanning electron microscopy and microprobe analysis reveal that high-Mg olivine exhibits the following compositional variations: Mg# (Mg/(Mg+Fe²⁺)×100, mol.%) 93.3-98.7, 0.01-0.05 wt.% NiO, 0.12-1.88 wt.% CaO, and 0.18-0.94 wt.% MnO. This olivine forms a paragenetic association with late magmatic minerals of kimberlites: magnetite, perovskite, apatite, monticellite, sodalite, phlogopite, djerfisherite, and calcite. High-Mg olivine occurs as individual grains (the first such finding in kimberlites), rinds, daughter phases within melt inclusions, phases in embayments and fractures within earlier olivine generations, and phases in the interstices of microxenoliths. Its crystallization is interpreted to have occurred from alkaline-carbonate-chloride melts. The temperatures and oxygen fugacity values for the crystallization of high-Mg olivine are semi-quantitatively estimated at 670-780 °C and +3.6 to +7.4 lg units ΔQFM. These findings indicate that such olivine crystallized from evolved kimberlitic melts, which contradicts earlier models proposing its formation from fluids or during kimberlite serpentinization.

Natural diamonds are polygenic and form over a very wide range of P-T parameters, crystallization medium compositions, and oxygen fugacity. As has been demonstrated in recent years, the genesis of some diamonds is directly related to their crystallization from metal-carbon melts. Since natural mineral formation is assumed to include various components typical of the mantle media, we consider it relevant to analyze the experimental results of the effect of C-O-H-N-S fluids on the crystallization features and indicator properties of diamond. The experimental data presented in this review show that increasing the concentration of fluid components (N, O, S, H₂O, CH₄-H₂) at constant P-T parameters inhibits diamond crystallization in metal-carbon melts and eventually leads to the formation of metastable graphite instead of diamond. Increasing P and T decreases the inhibitory effect of impurities and expands the diamond crystallization region. We have revealed the basic patterns of specific changes in the morphology, defect-impurity composition, and internal structure of diamond crystals, depending on the type and concentration of impurity in the crystallization medium. We have established that impurity-induced specific changes in diamond morphology and trends in nitrogen concentration in diamond are indicative of crystallization conditions and provide a basis for reconstructing diamond formation under reducing conditions in the metal-bearing mantle.

Experiments are conducted on pelite doped with 3 wt.% CaCO₃ over a pressure range of 3.0-7.8 GPa and temperatures of 750-1090 °C. The goal is that the total carbonate content in initial samples should reach 7 wt.%, consistent with that of the global subducting sediment model (GLOSS). Dehydration and decarbonation of the pelite under the thermal regimes of subduction zones produce a residue composed of garnet, clinopyroxene, phengite, coesite, ± Mg-Fe carbonate, and kyanite, along with accessory rutile, monazite, and zircon. In addition, a mobile phase forms that evolves significantly with increasing P-T conditions along the average subduction geotherm. At 3.0 GPa and 750 °C, a melt with a granite-like composition forms. This melt is enriched in SiO₂ and Al₂O₃, has a K₂O/Na₂O ratio of 1.2, and contains up to 19 wt.% H₂O + CO₂. At 5.5-7.8 GPa and 850-940 °C, a supercritical fluid-melt forms (H₂O + CO₂ ≈ 40 wt.%) that is enriched in SiO₂ and K₂O, poor in Al₂O₃, and has a K₂O/Na₂O ratio reaching 9.5. The supercritical fluid-melt generated in carbonate-bearing pelite can efficiently transport large-ion lithophile elements (LILE) and light rare earth elements (LREE), including trace element markers of both dilute aqueous fluids (Ba and U) and granite-like melts (Sr, LREE, and Th). Host minerals play an important role in trace element fractionation: phengite (LILE), monazite (LREE), and rutile (high field strength elements, HFSE). An increase in carbonate concentration in the pelite leads to a slight decrease in the partition coefficients of the most incompatible elements due to a higher proportion of the mobile phase and an increased CO₂ concentration within it. The supercritical fluid-melt equilibrated with rutile-bearing residue retains the negative Nb anomaly characteristic of marine sediments and can transfer this anomaly to arc magmas if it participates in their generation.

The paper presents results of investigation of a natural Ib-IaA diamond containing Y-defects from Yubileinaya kimberlite pipe, Yakutia. Analysis of spatial distribution of nitrogen-related A- and C-centers and intensity of infrared (IR) absorption at Raman frequency (1332 cm^-1) reveals anticorrelation between these defects. Transmission electron microscopy of a zone with abundant Y-defects shows presence of dislocations in various configurations and numerous clusters of point defects generated by non-conservative dislocation movement. Extended defects with shape resembling thin (1-3 nm) rhombic plates with the largest dimension up to 5-20 nm are observed. Analysis of contrast of these defects shows that they represent nanosized voids (vacancy clusters). It is suggested that the defects were formed by annihilation of dislocation dipoles with subsequent growth by consumption of vacancies produced by non-conservative motion of dislocations. Upon excitation by 787 nm laser, numerous narrow photoluminescence lines are observed between 800-900 nm, their intensity and position show pronounced spatial heterogeneity on scale of few microns. Qualitatively similar behavior was earlier noted for hydrogenated nanodiamonds. It is suggested that the unusual behavior of the luminescence lines may be explained by recombination processes on internal walls of the discovered nanovoids.

Round diamonds of Variety V according to Yu.L. Orlov are endemic representatives of placers of the northeast of the Siberian Platform. Their primary source has not yet been identified which makes it difficult to estimate the age of these crystals. Geochemical features (δ¹³С from -17 to -25‰, N_ppm from 1200 to 2000) indicate a contribution of subducted materials in their formation. Two diamond crystals that constitute the intergrowth studied in this work have δ¹³C of -20.8 and -20.7‰ and nitrogen impurity concentrations of 1693-2036 and 1568-1953 at. ppm, respectively. Zircon inclusions with δ¹⁸O = 7.9-8.8‰ found in one of these crystals have a U-Pb age of 262.5 ± 2.7 and 232.6 ± 3.0 Ma. The zircon inclusion with an older age may reflect early stages of the intergrowth formation or its constituent diamond crystal fragments, or it could be protogenetic in origin. We argue that the crystallization substrate and source material for this sample originated from a fragment of subducted oceanic crust containing organic sediments and hydrothermally altered basalts. The studied diamond was transported to the surface no earlier than the Triassic episode of kimberlite magmatism on the Siberian craton.

The paper is concerned with experimental studies aimed at assessing the possibility of diamond crystallization during interactions of alkaline-earth and transition metal carbonates with sulfur and at characterizing the obtained diamond crystals and inclusions in them. The experiments were carried out on a multi-anvil high-pressure “split sphere” apparatus (BARS) at constant P - T - t parameters: 6.3 GPa, 1550 °C, and duration of 20 h, in the Mg,Ca-carbonate-sulfur system with different CaO/MgO ratios and in the Fe,Mg,Ca-carbonate-sulfur system with variable FeO/CaO/MgO proportions. It has been experimentally established that in the alkaline-earth carbonate-sulfur systems, a Ca,Mg-carbonate melt with dissolved sulfur (1.2-6.0 wt.%) forms, which is a diamond growth medium. An octahedron is the stable growth form of diamond crystals, and an increase in the Ca# value of the system is accompanied by an increase in the growth rate of the {111} faces. In the Fe,Mg,Ca-carbonate-sulfur system, Fe,Mg,Ca-carbonate melt with dissolved sulfur is shown to be the crystallization medium and the source of diamond carbon, and diamond growth occurs as a result of the redox interaction of the sulfide and carbonate melts. An increase in the Fe# value of the system from 0.41 to 0.78 is accompanied by: (1) an increase in the growth rate of the {100} faces; (2) a change in the predominant elements of the octahedron face microrelief from triangular growth layers to hexagonal vicinals; and (3) an increase in the number and size of inclusions and the evolution of their phase composition. Inclusions in diamond form as a result of the melt preservation on the {100} faces during the transformation of a cuboctahedron into an octahedron. The inclusions contain quenched carbonate melt (quenching phases are carbonate, magnetite, and graphite), sulfur melt, and Fe-S-O melt. The revealed carbonate + magnetite + graphite assemblage in the inclusions testifies to a siderite redox dissociation reaction at the P - T parameters of the lithospheric mantle. These results shed light on the possible origin of magnetite inclusions in upper-mantle diamonds and suggest that the presence of magnetite in syngenetic inclusions is an indicator of the participation of iron-bearing carbonates in natural diamond formation processes.