Kimberlite Research Articles

The first results of the study of large diamonds from industrial deposits of Yakutia

A representative amount of diamonds larger than 10,8 carats extracted from deposits of Yakutia during the separate ore processing of each kimberlite pipe has been studied for the first time. It is shown that according to such typomorphic characteristics of a diamond as habitus, nitrogen content and aggregation, hydrogen concentration in a diamond, it is possible to carry out a preliminary evaluation of the deposit to predict the presence of large and giant diamonds. It has been established that large diamonds from the deposits of the Daldyn-Alakitsky area have a wide variations of nitrogen impurity and its aggregations in comparison with diamonds from the Malobotuobinsky, Srednemarkhinsky areas. It is determined that the content of large diamonds in the pipes of Yakutia is inversely proportional to the number of rounded dodecahedroids. The most promising deposits for finding of large diamonds are those in which the majority of diamonds belong to one population – average nitrogen low-aggregated diamonds that were formed at a temperature of ~1100оC. According to the study of geological collections of diamonds, it is shown that in kimberlite pipes with an increased nitrogen content in diamonds, a raising in the proportion of large diamonds in the deposit is noted. On the contrary, according to the aggregation of nitrogen in diamonds, as a parameter of the post-growth history, there is a negative correlation with the content of large diamonds. Increased value concentrations of hydrogen in diamonds not only negatively affect the total diamond content of the deposit, but also generally control the decrease in the content of large diamonds.

PREDICTION OF THE PRIMARY SOURCES OF GIANT CLIPPIR DIAMONDS IN THE NORTH OF YAKUTIA BY THE METHOD OF MINERALOGICAL AND GEOCHEMICAL ANALOGY (METHOD 5E DIAGRAMS)

The finds of giant CLIPPIR-type diamonds in the placers hosted by the Ebelyakh River basin in the north of Yakutia suggest that similar diamonds can be found in the kimberlites of the Anabar region and the nearest northern deposits located within the collisional Khapchan and Daldyn terranes. To predict the discovery of such diamonds, the authors use the 5E diagram method based on the principle of similarity of the compositions of five oxides of satellite minerals (Grt, Cpx, Chr and Ilm) of diamond (DSM) with the reference diagrams for the Karowe pipe (K-6, Botswana) with the composition of those for any other pipe. It was previously shown that the convergence of the DSM compositions of the Karowe and V. Grib (Arkhangelsk diamond province) pipes is 74 %, which might indicate possible presence of CLIPPIR diamonds in the predicted pipe. The application of this technique to two kimberlite pipes of the Anabar region demonstrated that the probability of detecting such diamonds in the Leningrad pipe (Lower Devonian, medium diamond content potential) is 74 %, and in the Malokuonamskaya pipe (Lower Triassic, medium diamond content) is 20 %. A comparison of diagrams 5E and additional PTС – fO2 diagrams of reconstructed sections of the lithospheric mantle showed their efficiency in predicting favorable conditions of CLIPPIR-type diamond crystallization. It is assumed that formation of such diamonds may occur within the proto-kimberlite magmatic chamber located near the boundary of the lithosphere and associated with the asthenospheric source. It should be surrounded by low-oxidized carbon-rich mantle eclogites and dunites with high pressure and temperature, as well as magnesium-rich ilmenite-chromite metasomatites. The use of the 5E diagram method, as the approach to predict giant CLIPPIR diamonds in poorly diamondiferous kimberlites, might attract investing for exploration to audit the industrial potential of a series of such kimberlites in the Anabar region. Its implementation would significantly increase the efficiency of geological exploration and assessment of the potential of poorly diamondiferous pipes that have been suspended as subeconomic.

AMPHIBOLE-PHLOGOPITE XENOLITHS FROM KIMBERLITES AS EVIDENCE OF METASOMATIC PROCESSES IN THE LITHOSPHERIC MANTLE BENEATH KUOIKA FIELD

This paper devoted to the mantle xenoliths with evidence of modal metasomatic alteration from the Obnazhennaya kimberlite pipe (Kuoika kimberlite field). The first data presents for trace elements content in minerals from such rocks, and they are compared with xenoliths from this pipe having no evidence of modal metasomatism. Based on the trace elements content in clinopyroxene and amphibole, as well as 40Ar/39Ar dating of phlogopite, it was concluded that enriched in Nb+Ta and REE melt-fluids were formed beneath the Kuoika field in the Lower Proterozoic, which lead to the phlogopite-amphibole formation from pyroxene.

Heterogeneous water distribution in between peridotite xenoliths from Kaapvaal Craton kimberlites: Constraints on diamond barren nature of kimberlites

Nominally anhydrous mantle minerals (olivine, pyroxenes, garnets, etc.) in 11 peridotite xenoliths from four different uneconomic and economic Kaapvaal Craton kimberlite pipes (Matsoku, Thaba Putsoa, Pipe 200 and Bultfontein) have been investigated using Fourier transform infrared spectroscopy (FTIR). All xenoliths contain accessories of garnet, diopside, chromite, and phlogopite. High orthopyroxene content (>30 mol vol.%) in most xenoliths from all kimberlites and its interconnected channel-like nature hint towards hydrous siliceous fluid metasomatism. Peridotite xenoliths from uneconomic kimberlites show development of phlogopite and clinopyroxene (± chromite) forming veins and in garnet rims suggesting metasomatism by alkaline silico-carbonatite (possibly kimberlite-related) melt. The xenoliths contain significant H2O in olivine (17–62 ppm), orthopyroxene (21–230 ppm), and clinopyroxene (87–833 ppm), whereas garnets are dry and only show IR absorbance bands at > 3,670 cm−1 for contamination of hydrous minerals. Compared to the economic kimberlites in the Kaapvaal Craton, the uneconomic kimberlite xenoliths from this study have lower orthopyroxene and olivine H2O content. In the xenoliths affected by garnet breakdown metasomatism, the H2O content of orthopyroxene and olivine is higher and lower, respectively. The structural hydroxyl distribution profile across olivine and higher inter-mineral water partition coefficient, suggest diffusion of hydrogen and possible re-equilibration. Statistical analysis of the olivine spectra suggests that hydrogen bands at 3540, 3624, 3638, and 3672 cm−1 are a good discriminant of economic and uneconomic kimberlites and in literature, they are associated with metasomatism, weathering-associated processes, high water activity, and oxygen fugacity. The lower water concentration in xenoliths from uneconomic kimberlite from the margin of the craton than the economic kimberlites from the interior of the Kaapvaal Craton and identified metasomatism hints towards dehydration of xenoliths by water-poor and CO2-rich melts in tectonized cross-lithospheric zones causing diamond resorption and may be responsible for the diamond-poor nature of uneconomic kimberlites in northern Lesotho.

A 187Re-187Os and highly siderophile element study of diamondiferous kimberlite melt-mantle interactions and the inferred age of continental lithosphere

The ∼ 1.15-billion-year-old (Ga) Premier kimberlite pipe (Cullinan diamond mine), South Africa, is composed of several distinct kimberlite facies (Grey, Brown, Pale Piebald, Dark Piebald, Black Coherent [Type 3C], Blue/Brown Transitional, and Fawn). We report bulk rock Re-Os isotope data for Premier kimberlite facies, as well as for a suite of entrained peridotite and mafic xenoliths. These data are complemented by bulk rock highly siderophile element (HSE: Re, Pd, Pt, Ru, Ir, Os), major- and trace-element abundances. Measured 187Os/188Os for the kimberlite facies range from 0.1223 to 0.1672 (ΥOsi of −2.5 to + 17.4), peridotite xenoliths range from 0.1096 to 0.1244 (ΥOsi of −13.3 to −1.1), and pyroxenite xenoliths range from 0.1796 to 0.938 (ΥOsi of + 27 to + 419). A single measured amphibolite xenolith has the most radiogenic measured 187Os/188Os of 2.86 (ΥOsi of + 43). Harzburgite xenoliths yield time of rhenium depletion model ages (TRD) of ∼ 1.5 to 2.8 Ga, consistent with average TRD ages for Premier peridotites (2.4 ± 0.4 Ga). With these and published data, we considered the relationships between kimberlite and mantle xenoliths, compare estimates of relative peridotite incorporation to sampled diamond grade, and explore recratonization versus refertilization arguments with regards to TRD model ages. Kimberlite melt infiltration into Premier peridotite xenoliths is evident from melt veins accounting for ∼ 2 and ∼ 14 modal % of samples, and has led to incompatible element enrichment, including elevated Re. In turn, kimberlites show geochemical evidence for addition of peridotite xenolith fragments, with Type 3C having > 30 % more peridotite contribution than the Brown volcaniclastic facies. Kimberlites and peridotites plot on a 187Re/188Os versus 187Os/188Os mixing line (R2 = 0.92), with kimberlites having older apparent ages than the true age of crystallization. This mixing line provides estimates of lithospheric incorporation into the kimberlites, where the units with higher peridotite incorporation do not correlate with diamond grade. This is likely due to lithological and post-emplacement alteration heterogeneity within the kimberlite units, perhaps also reflecting the eclogitic paragenesis of many Premier diamonds. The peridotites provide evidence for the nature of the lithosphere beneath Premier prior to ∼ 1.15 Ga. Metasomatism of the peridotites is possibly linked to the Bushveld Igneous Event at ∼ 2 Ga, as well as to other magmatic events that affected the Kaapvaal craton from the Archean to the Mesoproterozoic. Premier peridotites do not suggest that the cratonic lithosphere beneath the region was completely replaced. Samples with Proterozoic TRD eruption model ages may represent Archean lithosphere that experienced alteration by metasomatism and modification, such as during the Bushveld Igneous Event.

Thermal State and Thickness of the Lithospheric Mantle Beneath the Northern East-European Platform: Evidence from Clinopyroxene Xenocrysts in Kimberlite Pipes from the Arkhangelsk Region (NW Russia) and Its Applications in Diamond Exploration

This paper presents the reconstruction of the architecture of the lithospheric mantle, including its thermal state and thickness, as well as the scale and efficiency of its sampling by four kimberlites from the Arkhangelsk diamondiferous province: Arkhangelskaya, Lomonosovskaya, V. Grib, and TSNIGRI-Arkhangelskaya. These kimberlites differ in terms of their composition, diamond content, and location. Data presented include the major-element composition of clinopyroxene xenocrysts (>2000 grains), P–T calculations from compositionally filtered Cr-diopside grains, and the reconstruction of local paleogeotherms. Additionally, we used available data on Ni content in peridotitic garnet xenocrysts to calculate their T values and project them onto local Cr-diopside-derived geotherms to reconstruct the vertical distribution of mantle xenocrysts and assess the efficiency of lithospheric mantle sampling by different kimberlites. We identified the presence of a >200 km-thick lithospheric mantle beneath the region at the time of kimberlite emplacement. We also found that the diamond content of the studied pipes was, to some extent, dependent on the following set of factors: (1) the thermal state of the lithospheric mantle; (2) the width of the real “diamond window” marked by mantle xenocrysts, especially by diamond-associated garnets; and (3) the efficiency of lithospheric mantle sampling by kimberlite. The results of this study can be used to inform diamond exploration programs within the region.

Detection of low-contrast objects by electrical prospecting methods

Many geological problems cannot be solved by electrical prospecting methods. Often the reason for this is a slight difference in the electrical resistivity of target objects from that of the host medium. At the same time, the use of the transverse magnetic (TM) type field makes it possible to identify low-contrast objects and delineate their boundaries with high accuracy. TM-type prospecting is more efficient since its signal is much more strongly affected by changes in resistivity and other electrodynamic parameters. The article examines one of the difficult cases – a search for kimberlite pipes in Yakutia. Such objects differ very little from the host medium in terms of the horizontal resistivity. Exploration works to search for kimberlite pipes in Yakutia, carried out by conventional electrical prospecting methods, have many problems. The article considers the results of successful TM-type prospecting field surveys where kimberlite pipes stood out very prominently. Also presented are the results of modeling an even more complex situation in which the pipes are located at depths of over 140 m and overlain by traps; there are many objects in the area that create additional anomalies.

Magnetic surface geometry inversion of Kimberlites in Botswana

AbstractSurface geometry inversion of geophysical data has recently been introduced as an effective approach for generating surface‐based geological models. The models obtained through surface geometry inversion clearly delineate the contacts between distinct rock units, making them easily interpretable by geologists. Surface geometry inversion has shown promising preliminary results in other works, but the practical application of surface geometry inversion on real geophysical data has not been thoroughly investigated. To move towards a better understanding of the practicalities involved, we applied surface geometry inversion to a real magnetic dataset acquired over two kimberlite pipes located in north‐central Botswana. The objective was to assess the effectiveness and limitations of the surface geometry inversion approach in accurately characterizing the subsurface geometry and identifying the boundaries of the kimberlite pipes. We first perform an anomaly separation approach to isolate the magnetic anomalies associated with the kimberlite pipes. A surface geometry inversion algorithm was applied to the original and separated datasets using various initial models and other control parameters. Several tests were performed to investigate the effects that data processing, initial models, and other parameter choices have on the surface geometry inversion results. We successfully recover the geometry, extension and dip of the two kimberlite pipes. We discuss the results of our various tests and provide advice for practitioners interested in applying surface geometry inversion methods to their data. Our work indicates that surface geometry inversion can be used as a complementary approach to voxel inversion, and we propose an iterative surface geometry inversion algorithm as a possible alternative approach to voxel inversion for simple geological scenarios. This work provides valuable insights into the appropriate application of surface geometry inversion on real geophysical datasets.

Trace element geochemistry of mantle xenoliths from Zarnitsa kimberlite pipe, Daldyn field, Yakutia: Complex history of melts interactions with lithospheric mantle

Mantle xenoliths from Zarnitsa pipe studied in gray eruptive breccias (early) brown autholitic breccia (BAB) and black macrocrystic kimberlites (last dike; BMK), include garnet and spinel dunites-harzburgites, pyroxenites, eclogites, glimmerites and megacrysts. PT reconstructions using xenoliths reveal sharply layered structure (8 levels), estimated with the single grain mineral estimates mark hot (Cpx, Ilm) and cold (OPx, Gar) inflected geotherm. The interaction with plume melts is found at the lithosphere asthenosphere boundary (LAB), pyroxenite layer (3-4 GPa), Gar-Sp transition and Moho. Eclogites reveal Fe# growth from LAB to middle pyroxenites layer. Clinopyroxenes and ilmenite estimates marks melt refertilisations in interlayers between coupled subduction slabs. Source of capture from first to third stage deepening to LAB and Cr- rich garnets (to 19.5 Cr2O3) are below the LAB.The grey erupted breccia (GEB) includes mainly depleted and deformed peridotites. The later BAB includes pyroxenites, eclogites, and refertilised, deformed, and veined peridotites in BMK. Geochemistry of minerals changes from primary mid ocean ridge basalts (MORB) and back arc peridotites with low REE and large ion lithophile elements (LILE) and deeps in high field strength elements (HFSE) to metasomatized by alkaline (high Na and LILE) and adakitic melts (high Al, Na, Sr, and elevated HFSE) varieties and refertilised lherzolites due to plume at last stage. Mantle column metasomatized with scattered phlogopites in early grey eruptive breccia to amphibole-phlogopite ilmenite veins at last stages. Amphiboles trace mantle from lithosphere – asthenosphere boundary to Moho.Growth of the diamond grade from early eruptive breccia to later kimberlite phases refer to decreasing of the crust material and deepening of the xenoliths capture level. Metasomatism dissolve the diamonds but growth of megacrystic diamond crystals increase diamond grade.

Petrophysical taxa of diamond deposit of Komsomolskaya kimberlite pipe (Yakutsk diamondiferous province)

Petrophysical taxa of diamond deposit of Komsomolskaya kimberlite pipe (Yakutsk diamondiferous province)

Mineralogy of garnets from the Jubileinaya kimberlite pipe and Ottorzhenets kimberlite body

The article presents results of a study of the chemical composition of garnet xenocry-stals from a concentrate of the heavy fraction of kimberlites from the Yubileinaya pipe and Ottorzhenets kimberlite body. Samples from porphyry kimberlites and autolithic kimberlite breccias were taken for analysis from the Yubileinaya pipe. Significant differences are noted in the studied samples of peridotite garnets from the Yubileinaya kimberlite pipe and the Ottorzhenets body. Compared to the Yubileinaya pipe, the Ottorzhenets body exhibits a decreased amount of garnets of the harzburgite-dunite paragenesis in the composition range of Cr2O3 > 8.5 wt. % and CaO < 3 wt. %. On this basis, we assume that the Ottorzhenets body is not a displaced fragment of the Yubileinaya pipe, but belongs to a new unknown kimberlite body. We also assume that the diamond-bearing potential of the primary Ottorzhenets body is reduced compared to the Yubileinaya pipe, which is consistent with the information on the diamond con-tent, according to the data of AK ALROSA (PJSC)

Petrological and geochemical features of Middle Paleozoic basites in the Muna-Tyung interfluve region

The discovery of kimberlite pipes containing diamonds in the Viluy-Markha dyke belt led to a surge in interest in studying the products of Middle Paleozoic basic magmatism associated with these pipes in terms of their spatial and temporal characteristics. A distinct group of high-titanium rocks linked to diamond-bearing kimberlites was identified among the basic intrusions, serving as a key indicator for locating kimberlite pipes. This discovery prompted further research on basites and potential areas, such as the northeastern region of the dyke belt within the Muno-Tunga interfluve region, where alluvial diamonds have been found but their original sources remain unidentified. The presence ofdiamond varieties typical of kimberlites with high diamond concentrations in the placer diamonds of this area suggests the possibility of a new kimberlite field. The study of Middle Paleozoic basites in the Muno-Tunga interfluve region revealed high-titanium basites with similar petrochemical characteristics to those associated with diamond-bearing kimberlites. These basites are situated on the flanks of the dyke belt marked by compensating compression, making them a promising indicator for kimberlite prospecting. This study has demonstrated that areas with preferential compression are highly prospective for identifying primary diamond deposits.

Composition of Secondary Melt Inclusions in Magnesiochromite of a Mantle Lherzolite Xenolith from the V. Grib Kimberlite Pipe (East European Craton) as an Indicator of Low H2O Content of the Kimberlite Melt

Composition of Secondary Melt Inclusions in Magnesiochromite of a Mantle Lherzolite Xenolith from the V. Grib Kimberlite Pipe (East European Craton) as an Indicator of Low H2O Content of the Kimberlite Melt

Confocal Raman spectroscopic study of melt inclusions from peridotite xenoliths in economic and barren kimberlites from Kaapvaal Craton

AbstractMelt inclusions trapped in minerals inside xenoliths from kimberlites can help to examine the composition of kimberlite melt and/or metasomatic processes in the subcratonic lithospheric mantle as well as shed more light on the role of these melts in diamond destruction. In this study, confocal Raman spectroscopy of secondary melt inclusions in olivine from xenoliths in five different Kaapvaal Craton kimberlites was used for testing any compositional differences between melt inclusions from economically productive (Bultfontein and Frank Smit) and uneconomic diamond barren (Matsoku, Thaba Putsoa, and Pipe 200) kimberlite pipes. The xenoliths represent a range of pressures (37–45 kbar) and temperatures (1000–1300°C). The 26 daughter minerals identified within melt inclusions include Ca–Mg (±Na, K, P, Cl)‐bearing carbonates, alkali (±Ca, Ba, Cl, F, H2O, CO2)‐bearing sulfates, phosphates, oxides, silicates, and a rare nitrate. The mineral assemblages in melt inclusions are similar in both economic and barren kimberlite pipes from the interior and the edge of the craton, indicating the similar composition of the entrapped melts in all studied samples. However, the petrographic study revealed different metasomatic processes recorded by xenoliths from barren and economic kimberlites. Metasomatism by a melt enriched in K, Ca, and H2O could be instrumental in diamond destruction and the low diamond grade of the three barren kimberlites from Lesotho. Our study revealed no effect of kimberlite melt composition on diamond preservation in the studied kimberlites: instead, diamond grade is most likely affected by diamond destruction in the mantle source prior to kimberlite emplacement with kimberlite ascent.

Geochemistry of forty‐one eclogitic and pyroxenitic mantle xenoliths from the Central Slave Craton, Canada (Ekati Diamond Mine)

AbstractThis article describes a novel dataset on non‐diamondiferous eclogite and garnet pyroxenite xenoliths from four kimberlite pipes of the Ekati Diamond Mine (Central Slave Craton, Canada). Xenoliths brought to the surface by kimberlite eruptions are direct sources of information on the composition and evolution of the Earth's mantle. Eclogite and garnet pyroxenite xenoliths, specifically, are testimony of subduction into, and metasomatism of, the mantle beneath cratons. Furthermore, these rocks are major hosts for diamond and thus an important part of the deep carbon cycle. The sample suite consists of 41 small xenoliths (2–5 cm) recovered from drill cores. The dataset includes major and trace element concentrations for garnet, clinopyroxene and ilmenite, as well as stable oxygen isotope compositions of garnets. Strontium and neodymium isotopic compositions are reported for garnet and clinopyroxene for four samples which were large enough to allow for analysis. Overall, this dataset significantly expands and complements existing datasets on diamondiferous and non‐diamondiferous xenoliths from the Slave Craton in Canada, furthering our understanding of the composition of the Slave subcratonic lithosphere. The dataset includes several samples with rare mineral assemblages, including an olivine‐bearing eclogite as well as ilmenite and apatite‐bearing garnet‐pyroxenites, and thus provides data shedding light on rarely reported compositional nuances in xenolith suites found in kimberlites.

Correction to: Ferric Iron in Eclogitic Garnet and Clinopyroxene from the V. Grib Kimberlite Pipe (NW Russia): Evidence of a Highly Oxidized Subducted Slab

Correction to: Ferric Iron in Eclogitic Garnet and Clinopyroxene from the V. Grib Kimberlite Pipe (NW Russia): Evidence of a Highly Oxidized Subducted Slab

Ferric Iron in Eclogitic Garnet and Clinopyroxene from the V. Grib Kimberlite Pipe (NW Russia): Evidence of a Highly Oxidized Subducted Slab

Abstract Estimates of oxygen fugacity of eclogitic rocks are linked to the redox evolution of the oceanic protolith during subduction and its residence in the lithospheric mantle, and, based on knowledge of pressures and temperatures, allow modelling of the speciation of volatile elements and diamond (or graphite) versus carbonate stability. To date, the oxygen fugacity of mantle eclogites has been shown to vary between −6 (Kasai, Congo and Udachnaya, Siberia) and −0.1 (Udachnaya, Siberia) log units (relative to the fayalite–magnetite–quartz buffer, FMQ), linked to the low Fe3+ contents of garnets. In this study, we investigated the Fe oxidation state of coexisting garnet and clinopyroxene hand-picked out of 17 diamond-free high-MgO and low-MgO mantle eclogites (dated at 2.84 Ga) from the Grib kimberlite pipe (East-European platform). Measured Fe3+/∑Fe values range between 0.03 and 0.19 for garnet and 0.18–0.38 for clinopyroxene, the former being higher than what was measured previously in garnets equilibrated at mantle conditions. The Fe3+/∑Fe of the reconstructed bulk rock ranges between 0.10 and 0.15 for high-MgO eclogites and 0.10 and 0.24 for low-MgO eclogites (with uncertainties of ± 0.02 and ± 0.03 in both cases). Thermobarometric calculations result in equilibration pressures and temperatures of 3.0–5.2 (± 0.4) GPa and 720–1050 (± 60) °C for both high-MgO and low-MgO eclogites, slightly lower than previous P–T estimates of mantle eclogites from the Udachnaya kimberlite pipe (Siberian craton). At these conditions, ∆logfo2 (FMQ) calculated using the available oxythermobarometric model varies from −1.7 to −0.6 log units for high-MgO eclogites and from −2.9 to 0.9 log units for low-MgO eclogites. Samples recording ∆logfo2 (FMQ) ≤ −1 log units overlap with North Slave, West Africa and Udachnaya eclogites, with no difference among eclogite types. The average values of −1.2 (± 0.4) log units for high-MgO and −0.6 (± 1.1) log units for low-MgO eclogites suggest different redox conditions of basaltic protoliths during subduction worldwide. Previous geochemical studies on the same rock samples reported evidence of cryptic metasomatism in both garnet and clinopyroxene that we demonstrate being not recorded by their major elements, while modal metasomatism evidenced by the presence of phlogopite as a product of interaction with a kimberlitic melt only affects the MgO of the bulk rock. Therefore, we suggest that high Fe3+/∑Fe ratios for garnet (> 0.10) and for reconstructed bulk rocks in the case of both low-MgO and high-MgO samples cannot be due to metasomatic interaction with an oxidized fluid, but rather are the consequence of Fe3+ redistribution in an unusually oxidized mafic protolith upon metamorphism. Our results highlight the redox variability of eclogites of Archaean age at conditions more oxidized than present-day mid-ocean ridge basalts (MORBs) and imply an oxidizing nature of the convective mantle source where magma was formed with consequent speciation of C in the form of carbonate fluid explaining, therefore, the lack of eclogitic diamonds in V. Grib kimberlite pipe.

Determination of the internal structural heterogeneity of natural diamond: Methodological aspects of using confocal Raman spectroscopy with polarization analysis

Aim. To describe a technique for studying the internal structural heterogeneity of natural diamond crystals, based on confocal Raman spectroscopy with polarization analysis, including angular resolution, at high spectral (0.5–0.6 cm–1) and spatial (1 μm) resolution. Results. The parameters of the F2g vibrational mode in diamond (position, width, intensity, shape, including the Gaussian and Lorentzian contributions to the broadening) are determined by the superposition influence of a number of factors, including the type and content of structural stresses, deformations, various types of defects, as well as orientation of crystallographic axes of the crystal relative to the directions of incident and scattered rays and the directions of their electric polarization vectors. The proposed analytical technique includes: (1) analysis of the crystallographic orientation of the sample in the spectrometer coordinate system and possible misorientations of its fragments with an error of ≈8–15°; (2) visualization of the distribution of structural stresses, deformations, twins, impurity defects and their associates based on sample surface mapping by spectral parameters of the F2g vibration mode; (3) obtaining statistical characteristics of the internal structural heterogeneity of the samples based on diagrams of spectral parameter frequency with a statistically significant number (≈103): unimodality (uni-, bimodal distributions) and distribution dispersion (from ≈0.1 to ≈0.6 cm–1 for width and from ≈0.04 to ≈0.6 cm–1 for line position). The procedure was tested using two synthetic CVD diamond single crystals doped with nitrogen and boron. The possibility of typification of natural samples by statistical characteristics of internal heterogeneity is considered using the example of samples from kimberlite pipes of Yakutia and placers of the Western Cis-Urals. Conclusions. A method for determining the internal structural heterogeneity of natural diamond crystals based on confocal Raman spectroscopy with polarization analysis is proposed. The possibility of using statistical characteristics of heterogeneity as a typomorphic feature of the original diamond source is demonstrated. The proposed diagrams are promising for sample comparison and typification.

Petromagnetism and paleomagnetism of kimberlite pipesof the Verkhnemunskoe deposit (Yakutsk diamondiferous province)

The purpose of the studies of kimberlite pipes of the Verkhnemunskoe diamond field is to calculate the Middle Paleozoic paleomagnetic pole to clarify the trajectory of the apparent migration of the pole and reconstructions of the paleogeographic position of the Siberian platform at the time of the manifestation of active tectono-magmatic processes. The Verkhnemunskoye deposit is located within the Verkhnemunskoye kimberlite field of the Yakutsk diamondiferous province and includes five kimberlite pipes (Deimos, Zapolyarnaya, Komsomolskaya-Magnitnaya, Novinka and Poiskovaya), the age of which according to geological and isotopic data is estimated as Late Devonian-Early Carboniferous (372–347 Ma). For the first time scalar and vector physical parameters of kimberlites and captured xenoliths from different structural-material complexes of the Earth’s crust, as well as the host terrigenous sedimentary rocks of the Early Paleozoic were obtained, which are necessary for the development of physical-geological models of the Verkhnemunskoe field deposits. A relatively deep level of erosional shearing of the field has been established. The primary (synchronous with the formation of the field) natural residual magnetization was preserved in the kimberlite cohesive mass. The main carrier minerals of natural remanent magnetization vectors of kimberlites are unaltered magnesioferrite and magnetite, which indicates their thermo-sufficient nature. The natural remanent magnetization vectors of captured xenoliths indicate that the influence of hypergenic processes did not strongly affect the NRM vectors of kimberlites. Firing test is positive. The paleomagnetic pole with coordinates Φ = 26.5°N, Λ = 142.2°E, dp/dm = 6.2/7.8° was calculated from the obtained clusters of N = 10 vectors of the primary natural remanent magnetization of kimberlite pipes. On its basis, we reconstructed the paleogeographic position of the Siberian Platform, which at the time of the kimberlite intrusion was located in the middle latitudes of the northern hemisphere and was facing north with its southern edge.

Two Types of Symplectites in a Lower Crust Granulite Xenolith from the Zarnitsa Kimberlite (Yakutia): A Record of Si-Metasomatism and Decompression

Abstract ––The paper presents data on a granulite xenolith from the Zarnitsa kimberlite pipe (Yakutia, Russia), which stores a record of two metasomatic events in addition to the main stage of metamorphism. The granulitic mineral assemblage consists of garnet, clinopyroxene, and plagioclase as main phases. The granulite xenolith contains kyanite–clinopyroxene and later orthopyroxene–plagioclase symplectites. Kyanite–clinopyroxene symplectites appear as short veins inside or between grains of rock-forming minerals. Orthopyroxene–plagioclase symplectites form kelyphite rims in all garnets or occur as veins in garnet grains. The P–T conditions for granulite in the lower crust reconstructed by Perple_X phase equilibrium modeling are 700–750 ℃ and 1.2–1.3 GPa. According to TWQ thermodynamic calculations, the kyanite–clinopyroxene symplectites were produced by Si-metasomatism at invariable Р–Т parameters. The growth of orthopyroxene–plagioclase symplectites after garnet was maintained by Ca inputs upon heating and decompression (200 ℃ temperature increase and 0.6 GPa pressure decrease) while the xenolith was transported by ascending kimberlite melt.