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Water in the lower crust plays a critical role in rheological layering of the continental lithosphere. Sixteen granulite xenoliths were collected from the Late Devonian Udachnaya and Komsom...

—The isotope-geochemical features of diamondiferous metamorphic rocks of the Kokchetav subduction–collision zone (KSCZ) show that both the basement rocks and the sediments of the Kokchetav massif were their protoliths. A whole-rock Sm–Nd isochron from the diamondiferous calc-silicate, garnet–pyroxene rocks and migmatized granite-gneisses of the western block of the KSCZ yielded an age of 1116 ± 14 Ma, while an age of 1.2–1.1 Ga was obtained by U–Pb dating of zircons from the granite-gneiss basement of the Kokchetav microcontinent. Based on these data, we assume that the protoliths of the calc-silicate, garnet–pyroxene rocks and the granite-gneisses of the KSCZ were the basement rocks sharing an initially single Nd source, which was not influenced by high- to ultrahigh-pressure metamorphism (~530 Ma). Therefore, their geochemical features are probably not directly related to ultrahigh-pressure metamorphism. The corresponding rock associations lack isotope-geochemical evidence of partial melting that would occur during ultrahigh-pressure metamorphism, which suggesting that they were metamorphosed under granulite-facies conditions. At the same time, the high-alumina diamondiferous rocks of the Barchi area (garnet–kyanite–mica schists and granofelses), which were depleted to different degrees in light rare-earth elements (REE) and K, have yielded a Sm–Nd whole-rock isochron age of 507 ± 10 Ma indicating partial melting of these rocks during their exhumation stage. The close ɛNd (1100) values of the basement rocks and garnet–kyanite–mica schist with geochemical characteristics arguing against its depletion during high-pressure metamorphism indicate that the basement rocks were a crustal source for high-alumina sediments.

Zircon xenocrysts from two diamond-barren kimberlite pipes (Leningrad and Ruslovaya) in the West Ukukit kimberlite field opened a ‘window’ to the buried crustal basement in the northern Siberian cr...

New mineralogical and isotope–geochemical data for zircon megacrysts (n = 48) from alluvium of Kholomolokh Creek (a tributary of the Ebelakh River) are reported. Using the geochemical classification schemes, the presence of zircons of kimberlitic and carbonatitic genesis was shown. The U–Pb dating of zircons revealed two major age populations: the Triassic (258–221 Ma, n = 18) and Jurassic (192–154 Ma, n = 30). Weighted mean 206Pb/238U ages allowed us to distinguish the following age stages: 155 ± 3, 161 ± 2, 177 ± 1.5, 183 ± 1.5, 190 ± 2, 233 ± 2.5, and 252 ± 4 Ma. It is suggested that the Ebelyakh diamonds could have been transported from the mantle depths by kimberlite, as well as by other related rocks, such as carbonatite, lamprophyre, lamproite, olivine melilitite, etc. Diamonds from the Ebelyakh placers most likely have polygenic native sources and may be associated with polychronous and multistage Middle Paleozoic and Mesozoic kimberlite and alkaline–ultrabasic magmatism in the eastern slope of the Anabar Shield (the Ebelyakh, Mayat, and Billyakh river basins).

The data available indicate the complex evolution of deformed peridotites of mantle xenoliths, the P–T parameters of which indicate that they are fragments of the metasomatized lower part of the cratonic lithosphere. The zoning established in garnets from xenoliths in kimberlite pipes is interpreted as a result of metasomatism that occurred shortly before xenoliths reached the surface. Metasomatic alterations in xenoliths of deformed harzburgites were manifested not only in the development of zoning of minerals. The study results show that there is a discrepancy between the data calculated based on the contents of incompatible elements in minerals of xenoliths and those obtained due to direct measurements of the bulk composition of xenoliths. To determine the balance of incompatible elements, we have carried out experiments on leaching xenoliths of deformed lherzolites from the Udachnaya kimberlite pipe. It was established that a significant part of LREEs in the studied xenoliths occurs in the intergranular space. The distribution pattern of incompatible elements and, in particular, the presence of a positive Eu anomaly indicate that the appearance of the intergranular component is not associated with contamination of xenoliths by the kimberlite melt. Quite a few xenoliths demonstrate a positive Eu anomaly, which indicates the influence of the subducted crustal component at one of the modification stages of xenoliths.

The eclogites of the North Muya complex (Eastern Siberia) are located within the Early Neoproterozoic metasedimentary and felsic rocks of the Baikal-Muya Fold Belt (BMFB). The eclogites show subduction-related affinity, with large-ion lithophile (LILE) and light rare-earth element (LREE) enrichment and high field-strength element (HFSE) depletion signatures, similar to the exposed plutonic and volcanic rocks of the Early Neoproterozoic (Early Baikalian) subduction setting in the BMFB. Coupled Nd (eNd(T) of +6 to −1.4) and Sr (87Sr/86Sr ratio of 0.705–0.708), along with key trace-element indicators, imply progressive crustal recycling (up to 5–10%) from the Early Precambrian continental rocks to a depleted mantle source or equivalent crustal contribution via intracrustal contamination. Mineral δ18O data (+3.9 − +11.5) indicate that the contaminant or recycled crustal substrate might be represented by rocks altered at both low and high-temperature, or result from variable fluid-rock interaction in the subduction channel. Pseudosection modelling of eclogites, coupled with zircon U Pb geochronology (~630 Ma) suggest that the Ediacarian high-pressure metamorphic event for different rocks shared a maximum depth corresponding to 2.5–2.7 GPa with variable temperature range (560–760 °C), reflecting their potential relation to distinct slices of the subducted crust. The estimated metamorphic conditions for both the burial and exhumation of rocks indicate a continental subduction setting, but with a relatively cold geotherm (~20–25 °C/kbar). These conditions resulted from the continental subduction of the Baikal-Muya composite structure beneath the relatively thin and immature overlying arc lithosphere of southern Siberia. Some carbonate-bearing eclogites and garnet-pyroxene rocks, metamorphosed under T below 700 °C and a minimum P up to 1.4 GPa, exhibit LREE-enriched patterns and low eNd(T) values of −7 to −16. These rocks have Paleoproterozoic to Archean model ages and may support the existence of a Paleoproterozoic or older lithosphere in the Baikal-Muya Fold Belt, but their subduction history and origin remain uncertain due to geochemical and isotopic signatures probably overprinted by carbonate metasomatism.

The paper describes mineralogical characteristics of SiO2 inclusions in sublithospheric diamonds, which typically have complicated growth histories showing alternating episodes of growth, dissolution, and postgrowth deformation and crushing processes. Nitrogen contents in all of the crystals do not exceed 71 ppm, and nitrogen is detected exclusively as B-defects. The carbon isotope composition of the diamonds varies from δ13С = –26.5 to –6.7‰. The SiO2 inclusions occur in association with omphacitic clinopyroxenes, majoritic garnets, CaSiO3, jeffbenite, and ferropericlase. All SiO2 inclusions are coesite, which is often associated with micro-blocks of kyanite in the same inclusions. It was suggested that these phases have been produced by the retrograde dissolution of primary Al-stishovite, which is also evidenced by the significant internal stresses in the inclusions and by deformations around them. The oxygen isotope composition of SiO2 inclusions in sublithospheric diamonds (δ18O up to 12.9‰) indicates a crustal origin of the protoliths. The negative correlation between the δ18O of the SiO2 inclusions and the δ13C of their host diamonds reflects interaction processes between slab-derived melts and reduced mantle rocks at depths greater than 270 km.

Although the continental lower crust is often assumed to be dry and strong, water in nominally anhydrous minerals can significantly decrease viscosity of granulites and affect the mechanical coupling between the crust and upper mantle. Here we measured water content and fabrics of 16 granulite xenoliths from the Udachnaya and Komsomolskaya kimberlites in the central Siberian craton, which were erupted in the Late Silurian. The equilibrium pressure and temperature of the granulite samples are in the range of 0.9–1.3 GPa and 683–822 ºC. The mean water contents in clinopyroxene, garnet and plagioclase are 744±272 ppm, 100±64 ppm, 423±245 ppm, respectively, suggesting the water-rich lower crust. The bulk water contents in granulites are independent on pressure and composition, but show a negative correlation with temperature. Compared with previous studies on granulite xenoliths and terrane granulites, our granulite samples have much higher bulk water contents. The lattice-preferred orientation of clinopyroxene is characterized by activation of the dominant slip system (100)[001], whereas garnet is randomly orientated. Plagioclase developed two dominant slip systems (001)[010] and (001)[100]. Calculated seismic anisotropy indicates that the weak fabric strength of these granulite samples will result in weak seismic anisotropy of the lower crust beneath the Siberian craton. We propose that during eruption of the kimberlite pipes in the Late Silurian, the lower crust of the Siberian craton, at least beneath the kimberlite fields, had high water contents, relatively low strength, weak seismic anisotropy, and high electrical conductivity. Such status may be representative for the lower crust beneath a stable craton. The following Siberian Traps in the end of Permian was associated with the magma underplating, which probably dehydrated and strengthened the lower crust of the Siberian carton.

This paper reports isotopic and geochemical studies of eclogites from the western ultrahigh pressure (UHP) and eastern high-pressure (HP) blocks of the Kokchetav subduction–collision zone. These HP and UHP eclogites exhumed in two stages: (1) The rocks of the western block metamorphosed within the field of diamond stability (e.g., Kumdy-Kol and Barchy); (2) In contrast, the metamorphic evolution of the eastern block reached the pressure peak within the stability field of coesite (e.g., Kulet, Chaglinka, Sulu-Tyube, Daulet, and Borovoe). The eclogites vary widely in the ratios of incompatible elements and in the isotope ratios of Nd (143Nd/144Nd = 0.51137–0.513180) and Sr (87Sr/86Sr = 0.70393–0.78447). The Sulu-Tyube eclogites display isotope-geochemical features close to N-MORB, while those from the other sites are compositionally similar to E-type MORB or island arc basalts (IAB). The model ages TNd(DM) of eclogites vary between 1.95 and 0.67 Ga. The Sulu-Tyube eclogite yields the youngest age; it has the values of εNd(T) (7.2) and 87Sr/86Sr (0.70393) close to the depleted mantle values. The crustal input to the protolith of the Kokchetav eclogites is evident on the εNd(T)–86Sr/87Sr and εNd(T)–T plots. The eclogites make up a trend from DM to country rocks. Some eclogites from the Kulet, Kumdy-Kol, and Barchy localities display signs of partial melting, such as high Sm/Nd (0.65–0.51) and low (La/Sm)N (0.34–0.58) values. The equilibrium temperatures of these eclogites are higher than 850 °C. The geochemical features of eclogites testify to the possibility of the eclogite protolith formation in the tectonic setting of passive continental rift margin subducted to depths over 120 km.

We present the detailed mineralogical, geochemical (major and trace element), bulk Nd and Sr and mineral O isotope data for eclogites, associated orthogneisses and metasedimentary rocks from the recently discovered eclogite-bearing complex of the Zamtyn-Nuruu range (SW Mongolia). Trace element studies reveal the enrichment of eclogites with LILE and LREE relative to typical mid-ocean ridge basalts at similar levels of HREE and HFSE but without a clear arc-derived Nb minimum. The eclogites have relatively narrow range of mostly radiogenic eNd(T) values (+2.3 to +3.7 as back-calculated for 550 Ma) and model age TDM of 1.47–1.77 Ga at a wide range of initial 87Sr/86Sr ratio due to variable LILE mobilization in pre- or synmetamorphic processes. The geochemical and isotope data indicate the variably enriched MORB-like protolith for eclogites that have their variable composition through differentiation of precursor melts, slightly enriched mantle source and/or variable degree of crustal contamination. The mostly siliciclastic eclogite-hosting metasediments are likely derived from the intra-continental riftogenic basin, whereas the medium-pressure metapelites of the Maykhan Tsakhir formation originate from deeper passive margin sedimentation. Their Mesoproterozoic two-stage model age TDM−2ST of 1.49–1.63 Ga and crustal eNd(T) values (−3.5 to −5.3) significantly differ from the Zamtyn Nuruu complex rocks and indicate a mixed Nd source likely resembled from the juvenile Neoproteorozoic magmatism with depleted Nd signatures and the older Meso- or Paleoproterozoic crustal substrate. Contrastingly, eclogite-hosting gneisses exhibit the eNd(T) of the ancient crust (−12.9) and Paleoproterozoic model age TDM−2ST of 2.22 Ga. Both eclogites and metasediments evidently exhibit an input from the ancient crustal source that may be represented by orthogneisses of the Alag Khadny complex. Oxygen isotope data (δ18O of garnets mostly within +5.5 to +6.6) at high Fe3+/ΣFe ratio of bulk rocks (0.15–0.21) indicate variable but mostly limited interaction of precursor rocks with oceanic water, degassing at shallow crustal level or at the surface or relatively more oxidized (back-arc or subarc) mantle source. Considering the limited trace element evidence for the input from continental crust, generally elevated Fe3+/ΣFe for samples showing different retrograde alteration degrees, and no or limited δ18O evidence of pre-metamorphic interaction with oceanic water, we may suggest that primitive low-K tholeitic basalts close to T-MORB tholeites derived from a heterogeneously enriched mantle source of a rifted continental margin as precursor rocks for the studied eclogites. As a consequence of the geological structure and geochemical evidence, connection of rifting to the Rodinia break-up is proposed.

Mosaic diamonds from the Zarnitsa kimberlite (Daldyn field, Yakutian diamondiferous province) are morphologicaly and structurally similar to dark gray mosaic diamonds of varieties V and VII found frequently in placers of the northeastern Siberian craton. However, although being similar in microstructure, the two groups of diamonds differ in formation mechanism: splitting of crystals in the case of placer diamonds (V and VII) and growth by geometric selection in the Zarnitsa kimberlite diamonds. Selective growth on originally polycrystalline substrates in the latter has produced radial microstructures with grains coarsening rimward from distinctly polycrystalline cores. Besides the formation mechanisms, diamonds of the two groups differ in origin of mineral inclusions, distribution of defects and nitrogen impurity, and carbon isotope composition. Unlike the placer diamonds of varieties V and VII, the analyzed crystals from the Zarnitsa kimberlite enclose peridotitic minerals (olivines and subcalcic Cr-bearing pyropes) and have total nitrogen contents common to natural kimberlitic diamonds (0 to 1761 ppm) and typical mantle carbon isotope compositions (–1.9 to –6.2‰ δ13C; –4.2‰ on average). The distribution of defect centers in the Zarnitsa diamond samples fits the annealing model implying that nitrogen aggregation decreases from core to rim.

In the present paper, the results of our isotope–geochemical studies on eclogites of the ultrahighpressure metamorphic complex of the Kokchetav massif are reported. The fact that the distribution of nonmobile elements in most of the samples was close to that of E-type MORB basalts is shown by using geochemical multielement diagrams normalized to N-MORB. Six samples were found to have a negative anomaly over niobium that may have resulted from contamination with crustal material. For eclogites of the Kokchetav massif, the 147Sm/144Nd ratio was found to range widely from 0.143 to 0.367. The eNd-values calculated for the age of the highly barometric stage of metamorphism (530 million years) varied from–10.3 to +8.1. Eclogites show a dispersion of model ages from 1.95 billion years to 670 million years. On the graphs in the eNd(T)–87Sr/86Sr and eNd(T)–T coordinates, eclogites were shown to form trends that can be interpreted as a result of contamination of the eclogite protolith by the host rocks. Based on the data obtained, it is proposed that the basalts of rift zones that may have geochemical characteristics of N-MORB basalts and at the same time may be contaminated by the continental crust may have served as proxies for eclogite protoliths of the Kokchetav massif.

According to present views, the crustal terranes of the Anabar province of the Siberian craton were initially independent blocks, separated from the convecting mantle at 3.1 (Daldyn terrane), 2.9 (Magan terrane) and 2.5 Ga (Markha terrane) (Rosen, 2003, 2004; Rosen et al., 1994, 2005, 2009). Previous studies of zircons in a suite of crustal xenoliths from kimberlite pipes of the Markha terrane concluded that the evolution of the crust of the Markha terrane is very similar to that of the Daldyn terrane. To test this conclusion we present results of U-Pb and Hf-isotope studies on zircons in crustal xenoliths from the Zapolyarnaya kimberlite pipe (Upper Muna kimberlite field), located within the Daldyn terrane, and the Botuobinskaya pipe (Nakyn kimberlite field) in the center of the Markha terrane. The data on xenoliths from the Botuobinskaya kimberlite pipe record tectonothermal events at 2.94, 2.8, 2.7 and 2 Ga. The event at 2 Ga caused Pb loss in zircons from a mafic granulite. U-Pb dating of zircons from the Zapolyarnaya pipe gives an age of 2.7 Ga. All zircons from the studied crustal xenoliths have Archean Hf model ages ranging from 3.65 to 3.11 Ga. This relatively narrow range suggests that reworking of the ancient crust beneath the Nakyn and Upper Muna kimberlite fields was minor, compared with the Daldyn and Alakit-Markha fields (Shatsky et al., 2016). This study, when combined with dating of detrital zircons, implies that tectonic-thermal events at 2.9–2.85, 2.75–2.7 and 2.0–1.95 Ga occurred everywhere on the Anabar tectonic province, and could reflect the upwelling of superplumes at 2.9, 2.7 and 2 Ga. The presence of the same tectonic-thermal events in the Daldyn and Markha terranes (Rosen et al., 2006a,b) supports the conclusion that the identification of the Markha terrane as a separate unit is not valid.

Mineralogical, petrographic, and geochemical studies of mafic granulites of the South Muya Block (Central Asian Orogenic Belt) have been carried out. The granulite protoliths were olivine- and plagioclase- rich cumulates of ultramafic–mafic magmas with geochemical affinities of suprasubduction rocks. The isotope–geochemical characteristics of the granulites indicate the enriched nature of their source, associated with recycling into the mantle of either ancient crust or oceanic sediments, or intracrustal contamination of melts at the basement of the ensialic arc. Formation of garnet-bearing parageneses has occurred during high-pressure granulite metamorphism associated with accretion in the eastern part of the Baikal–Muya composite terrane.

The paper presents new data on the internal structure of super-deep (sublithospheric) diamonds from Sa&otilde, Luiz river placers (Brazil) and from alluvial placers of the northeastern Siberian platform (Yakutia). The sublithospheric origin of these diamonds is supported by the presence of mineral inclusions corresponding to associations of the transition zone and lower mantle. The features of morphology and internal structure have been studied by optical and scanning electron microscopy (SEM), cathodoluminescence topography (CL), and electron backscatter diffraction (EBSD) techniques. Diamonds typically have complicated growth histories displaying alternating episodes of growth, dissolution, and post-growth deformation and crushing processes. Most crystals have endured both plastic and brittle deformation during the growth history. Abundant deformation and resorption/growth features suggest a highly dynamic growth environment for super-deep diamonds. High temperatures expected in the transition zone and lower mantle could explain the plastic deformations of super-deep diamonds with low nitrogen content.

New findings of silicate-melt inclusions in two alluvial diamonds (from the Kholomolokh placer, northeastern Siberian Platform) are reported. Both diamonds exhibit a high degree of N aggregation state (60&ndash, 70% B) suggesting their long residence in the mantle. Raman spectral analysis revealed that the composite inclusions consist of clinopyroxene and silicate glass. Hopper crystals of clinopyroxene were observed using scanning electron microscopy and energy-dispersive spectroscopic analyses, these are different in composition from the omphacite inclusions that co-exist in the same diamonds. The glasses in these inclusions contain relatively high SiO2, Al2O3, Na2O and, K2O. These composite inclusions are primary melt that partially crystallised at the cooling stage. Hopper crystals of clinopyroxene imply rapid cooling rates, likely related to the uplift of crystals in the kimberlite melt. The reconstructed composition of such primary melts suggests that they were formed as the product of metasomatised mantle. One of the most likely source of melts/fluids metasomatising the mantle could be a subducted slab.

It is widely accepted that granulite xenoliths from kimberlites provide a record of granulite facies metamorphism at the basement of cratons worldwide. However, application of the phase equilibria modeling for seven representative samples of mafic granulites from xenoliths of the Udachnaya kimberlite pipe, Yakutia, revealed that a granulitic garnet + clinopyroxene + plagioclase ± orthopyroxene ± amphibole ± scapolite mineral assemblage was likely formed in the middle crust under amphibolite facies conditions (600–650 °C and 0.8–1.0 GPa) in a deficiency of fluid. Clinopyroxene in the rocks is characterized by elevated aegirine content (up to 10 mol.%) both in the earlier magmatic cores and in the later metamorphic rim zones of the grains. Nevertheless, the phase equilibrium modeling for all samples indicates surprisingly reduced conditions, i.e. oxygen fugacity 1.6–3.3 log units below the FMQ (Fayalite-Magnetite-Quartz) buffer. In contrast, the coexistence of Fe-Ti oxides indicates temperatures of 850–990 °C and oxygen fugacity about lg(FMQ) ± 0.5, conditions which correspond to earlier stages of rock evolution. Reduction of oxygen fugacity during cooling is discussed in the context of the evolution of a complex fluid. The reconstructed P-T conditions for the final equilibration in the mafic granulites indicate that temperatures were ~250 °C higher than those extrapolated from the continental conductive geotherm of 35–40 µW/m2 deduced from peridotite xenoliths of the Udachnaya pipe. Although the granulites resided in the crust for a period for at least 1.4 Ga, they did not re-equilibrate to the temperatures of the geotherm, likely due to the blocking of mineral reactions under relatively low temperatures and fluid-deficient conditions

Polycrystalline diamond aggregates (boart, framesites, diamondites) have been widely studied but their origin is poorly understood. We report the results of a study in situ of two polished fragments of fine-grained (40–400 μm size of individual diamond grains) dense polycrystalline diamond aggregates from the Mir pipe containing visible multiple interstitial garnet inclusions. They were analyzed for major and trace elements of inclusions and one of them — for δ 13 C and N abundance and isotopic composition of host diamonds. These aggregates are classified as variety IX by Orlov (1977). No cavities were observed in these samples. Sixty two irregular garnet grains and one clinopyroxene inclusion were detected and analyzed in sample Mr 832. Garnets are homogeneous within single grains but variable in Mg# [100Mg/(Mg + Fe)] from 60 up to 87 and CaO contents (3.3–5.3 wt.%) among grains with a trend to negative correlation. Low Cr (550–640 ppm) confirms eclogitic (E-type) paragenesis. High Na 2 O contents (5.2 wt.%) of a single pyroxene inclusion are additional evidence of eclogitic nature of this sample. Wide variations in trace elements (ppm) are characteristic for garnet grains: Sr (2.7–25.6), Y (9.7–14.1), Zr (15.6–38.7) and positive Eu anomaly is present. The δ 13 C of diamonds within studied sample is variable (− 6.4 ÷− 9.8 ‰) as well as N abundance (75–1150 ppm) and δ 15 N − 27, − 38, − 58 ‰. The second peridotitic (U/P-type) sample Mr 838 contains eight inclusions of Mg-rich Cr-pyropes (Mg# ~ 85, Cr 2 O 3 3.2–3.4 wt.%) and magnesite inclusion with 4.35 wt.% FeO and 1.73 wt.% CaO. Trace element content in pyropes is relatively uniform (ppm): Sr (0.4–1.6), Y (13.2–13.4) and Zr (13.0). We conclude that heterogeneous distribution of the trace elements among garnet grains in Mr 832 and magnesite presence in Mr 838 are indicative of the effects of mantle metasomatism and rapid crystallization shortly before the eruption of the kimberlite.

The staged high-pressure annealing of natural cubic diamonds with numerous melt microinclusions from the Internatsional’naya kimberlite pipe was studied experimentally. The results mainly show that the carbonate phases, the daughter phases in partially crystallized microinclusions in diamonds, may undergo phase transformations under the mantle P–T conditions. Most likely, partial melting and further dissolution of dolomite in the carbonate–silicate melt (homogenization of inclusions) occur in inclusions. The experimental data on the staged high-pressure annealing of diamonds with melt microinclusions allow us to estimate the temperature of their homogenization as 1400–1500°C. Thus, cubic diamonds from the Internatsional’naya pipe could have been formed under quite high temperatures corresponding to the lithosphere/asthenosphere boundary. However, it should be noted that the effect of selective capture of inclusions with partial loss of volatiles in relation to the composition of the crystallization medium is not excluded during the growth. This may increase the temperature of their homogenization significantly between 1400 and 1500°C.

U–Pb age and Hf-isotope data were collected on zircons from crustal xenoliths from the Late Devonian kimberlite pipes of Markha terrane. The xenoliths include mafic garnet granulites (Gt+Pl+Cpx±Opx±Amp±Bt±Scp) and garnet–biotite gneisses (Grt+Bt+Pl+Kfs+Qtz±Scp). The data from this study demonstrate that the crust of the Markha terrane experienced several tectonothermal events. Zircons with Archean Hf model ages (TDM = 3.13–2.5 Ga) are predominant in all xenoliths with one exception. These data imply that most of the lower and middle crust beneath the Markha terrane was produced in the Archean time. Later this Paleoarchean crust was significantly reworked in several tectonothermal events, including a Neoarchean stage (2.9–2.5 Ga), and several Paleoproterozoic metamorphic stages (1.98, 1.9 and 1.8 Ga). There is very little evidence for generation of juvenile crust associated with these metamorphic events. Importantly, crustal xenoliths from kimberlite pipes of the Markha terrane record all the same events that occurred in the adjacent Daldyn and Magan terranes.

The Baikal–Muya Foldbelt is one of the oldest and the most enigmatic terrain among the Central Asian Orogenic Belt. During the Early (1.0–0.8 Ga) and Late (0.8–0.6 Ga) Neoproterozoic orogenic cycles a notable amount of juvenile crust has been formed in subduction–collision settings along with the reworking of the existing Early Precambrian continental crust. The latter is perfectly presented within the North Muya metamorphic block (Anamakit–Muya zone) including the oldest high-pressure subduction-related rocks in the Central Asia. In order to evaluate the process of the juvenile crust formation in the Precambrian continental unit, whole-rock trace elements along with zircon U–Pb ages and Hf-isotopic composition were analyzed in granites of unknown age that intrude the Precambrian volcanic-sedimentary rocks of the Parama series in the southern North Muya Block. Combined geochemical and zircons isotopic studies indicate the formation of granites with the main pulse at ∼810 Ma due to partial melting of a juvenile mafic crust accompanied by reworking of the older crustal material from the continental arc basement. The composition of the granites was controlled by processes of intracrustal differentiation together with a probable contamination by older crustal substrate. Geochemistry and age of the studied granites are comparable to the granites of the Muya complex observed in the volcano–plutonic Kelyana–Irokinda zone. It is assumed that the Muya block and the Kelyana–Irokinda zone (terrane) had possibly been parts of the large continental-arc system beyond the Siberia margins.

Despite the violent eruption of the Siberian Traps at ~250 Ma, the Siberian craton has an extremely low heat flow (18–25 mW/m2) and a very thick lithosphere (300–350 km), which makes it an ideal place to study the influence of mantle plumes on the long-term stability of cratons. Compared with seismic velocities of rocks, the lower crust of the Siberian craton is composed mainly of mafic granulites and could be rather heterogeneous in composition. The very high Vp (> 7.2 km/s) in the lowermost crust can be fit by a mixture of garnet granulites, two-pyroxene granulites, and garnet gabbro due to magma underplating. The high-velocity anomaly in the upper mantle (Vp = 8.3–8.6 km/s) can be interpreted by a mixture of eclogites and garnet peridotites. Combined with the study of lower crustal and mantle xenoliths, we recognized multistage magma underplating at the crust–mantle boundary beneath the Siberian craton, including the Neoarchean growth and Paleoproterozoic assembly of the Siberian craton beneath the Markha terrane, the Proterozoic collision along the Sayan–Taimyr suture zone, and the Triassic Siberian Trap event beneath the central Tunguska basin. The Moho becomes a metamorphism boundary of mafic rocks between granulite facies and eclogite facies rather than a chemical boundary that separates the mafic lower crust from the ultramafic upper mantle. Therefore, multistage magma underplating since the Neoarchean will result in a seismic Moho shallower than the petrologic Moho. Such magmatism-induced compositional change and dehydration will increase viscosity of the lithospheric mantle, and finally trigger lithospheric thickening after mantle plume activity. Hence, mantle plumes are not the key factor for craton destruction.

Diamonds of eclogitic assemblages are dominant in the placer diamond deposits of the northeastern Siberian platform. In this study we present new trace elements and stable isotopes (δ13C and δ18O) data for alluvial diamonds and their garnet inclusions from this locality. Cr-rich garnets of peridotitic affinity in the studied diamonds have a narrow range of δ18O values from 5.7‰ to 6.2‰, which is largely overlapping with the accepted mantle range. This narrow range suggests that the garnet inclusions showing different REE patterns and little variations in oxygen isotopes may have formed by different processes involving fluid/melts that, however, were in oxygen isotopic equilibrium with the mantle. The trace element composition of the eclogitic garnet inclusions supports a crustal origin for at least the high-Ca garnets, which show flat HREE patterns and in some cases a positive Eu-anomaly. High-Ca eclogitic garnets generally show heavier oxygen isotope compositions (δ18O 6.5–9.6‰) than what is observed in low-Ca garnets (δ18O 5.7–7.4‰). The variability in oxygen isotopes and trace elements is suggested to be inherited from contrasting crustal protoliths. The relationship between the high δ18O values of inclusions and the low δ13C values of the host diamonds implies that the high-Ca garnet inclusions were derived from intensely hydrated (e.g., δ18O > 7‰) and typically oxidised basaltic rock close to the seawater interface, and that the carbon for diamonds was closely associated with this protolith.

High-pressure mafic granulites and garnet pyroxenites occur within the South Muya block as boudins or lenses among metamorphic rocks of the Kindikan Group. Their primary minerals crystallized at 670-750 °C and 9.5–12.0 kbar. Granulite metamorphism peaked at 630 Ma, according to LA-ICP-MS U–Pb zircon ages. Judging by their major- and trace-element compositions and Hf isotope ratios in zircons, the South Muya granulites were derived from differentiated within-plate basalts, which, in turn, resulted from melting of juvenile mantle source and Meso- or Paleoproterozoic crust. The events of granulite and eclogite metamorphism in the South and North Muya blocks, respectively, were coeval and the two blocks were spatially close to each other at the onset of Late Baikalian subduction and collision events.

We demonstrate for the first time the presence of iron carbides in placer diamonds from the northeastern region of the Siberian craton. It was found that the inclusions are polycrystalline aggregates, and iron carbides filling the fissures in the diamonds, thus providing clear evidence that the iron melts were captured first. Iron carbides were identified in diamonds containing mineral inclusions of eclogitic (Kfs, sulfide) and peridotitc (olivine) paragenesis. Iron carbides with minor amounts of admixed nickel were detected in a diamond sample containing an olivine inclusion (0.3 wt% Ni), indicating that the iron melt was not in equilibrium with the mantle peridotite.The low nickel contents of the iron carbides provide the best evidence that the subducted crust is a likely source of the iron melt. Diamonds containing carbide inclusions are characterised by a relatively low nitrogen aggregation state (5–35%), which is not consistent with the high temperature of the transition zone. Therefore, we have reason to assume that the studied diamonds are from the lower regions of the lithosphere. Considering all factors, the model for the interaction of the ascending asthenospheric mantle with the subducting slab seems to be more realistic.

A xenolith of bimineralic eclogite from the Udachnaya kimberlite pipe provides a snapshot of interaction between mantle rocks and diamond-forming fluids/melts. The major-element composition of the eclogite is similar to that of N-MORB and/or oceanic gabbros, but its trace-element pattern shows the effects of mantle metasomatism, which resulted in diamond formation. The diamonds are clustered in alteration veins that crosscut primary garnet and clinopyroxene. The diamonds contain microinclusions of a fluid/melt dominated by carbonate and KCl. Compared to the worldwide dataset, the microinclusions in these diamonds fall in middle of the range between saline fluids and low-Mg carbonatitic melts. The fluid/melt acted as a metasomatic agent that percolated through ancient eclogitic rocks stored in the mantle. This interaction is consistent with calculated partition coefficients between the rock-forming minerals and diamond-forming fluid/melt, which are similar to experimentally-determined values. Some differences between the calculated and experimental values may be due to the low contents of water and silicates in the chloride-carbonate melt observed in this study, and in particular its high contents of K and LILE. The lack of nitrogen aggregation in the diamonds implies that the diamond-forming metasomatism took place shortly before the eruption of the kimberlite, and that the microinclusions thus represent saline carbonate-rich fluids circulating in the basement of lithospheric mantle (150–170 km depth).

The results of the investigations of the transformation of impurity defects in natural diamonds of various habits at the stage of high-temperature annealing at P = 6 GPa and T = 2200°C are presented. The studies conducted allowed us to ascertain that the transformations of Aand B-defects in diamonds of octahedral and cubic habits follow general regularities. This fact shows that most of the diamonds of cubic habit with low degree of aggregation of nitrogen centers were not really annealed over a long-term interval. Unlike octahedral diamonds, those of cubic habit are characterized by a pronounced increase in the peak of H-containing defects (3107 cm–1) after annealing.

Evidence for the involvement of a subduction component in diamond formation is analyzed based on literature data and our studies. Examination of xenoliths of diamondiferous eclogites, including X-ray tomography analysis, testifies to the superposed character of most diamonds. Diamond generation is accompanied by the serious modification of eclogite substratum. Isotope–geochemical data show that the eclogites originated from oceanic-crust rocks. The oxygen isotope compositions of garnets and clinopyroxenes from websterite xenoliths are similar to the mantle average (5.3–5.6‰). The eclogite minerals vary considerably in oxygen isotope composition (δ18O of 5.3 to 12.4‰). Diamonds of eclogitic paragenesis predominate dramatically in the placers of the northeastern Yakutian diamondiferous province. In placer eclogitic diamonds, δ13C varies from − 27.2 to − 3‰ (n = 28). In diamonds of ultrabasic paragenesis, the range of δ13C values is much narrower (from − 7.1 to − 0.5‰). All diamonds of variety V have a lighter carbon isotope composition (from − 24.1 to − 17.4‰). In a wide range of crystals with a contrasting carbon isotope composition, the isotope composition of the rim tends toward the average mantle value. This suggests that the eclogitic diamonds grew first with the participation of carbon from subducted continental crust and finally with the involvement of mantle carbon.

Headless placer diamond deposits occur in Cenozoic alluvium within the Paleoproterozoic Khapchan fold belt of the Olenek province in the northeastern part of the Siberian Platform, yet the known kimberlite pipes from this region are low-grade or non-diamondiferous. To characterise the subcontinental diamondiferous lithospheric mantle of this region, the mineral inclusions in 265 diamonds were exposed and analysed. Inclusions of the eclogite suite are predominant (> 72%): garnet, omphacitic clinopyroxene, coesite, K-feldspar, rutile and corundum. The garnet and Cpx inclusions are within the range of eclogitic inclusions worldwide. Diamonds of the peridotitic contain olivine, Сr-pyrope garnet, ortopyroxene and chromite. The olivines have Fo contents between 89.7 and 93.8 mol. % (average – 92.4). Majoritic garnets of both peridotitic and eclogitic parageneses were identified in four diamonds. Most eclogitic diamonds display positive Eu anomalies. High-Ca garnets are LREE-depleted, show strong positive Eu (up to 4.25) and Sr anomalies and have HREE contents lower than those of the lowand intermediate-Ca garnets. The presence of majorite inclusions indicates that some of the diamonds may have sublithospheric origins. Although diamonds with peridotitic-suite garnets constitute a minor percentage of the volume, the composition of these diamonds indicates a depleted composition for the peridotitic mantle. Diamonds of harzburgitic paragenesis compose 57% of the peridotitic suite, which is close to the world average. For an assumed pressure of 5 GPa, eclogitic garnet and clinopyroxene gave temperatures in the range of 1028–1290 oC. The composition of mineral inclusions of the peridotitic suite and the equilibrium temperatures of the eclogitic suite suggest that the lithospheric mantle of the Olenek province had a composition and thermal regime that was similar to those of the Archean cratonic mantle at the time the diamonds were brought to the surface.

We studied clastics of high-alumina garnet–kyanite–mica schists and garnet–kyanite–quartz granofelses, including diamond-bearing ones, found in the eluvial sediments near Lake Barchi. In contents of major elements the studied rocks correspond to argillaceous shales. The garnet–kyanite–quartz granofelses are poorer in K (0.49–1.35 wt.% K2O) than the garnet–kyanite–mica schists (4.9–2.2 wt.% K2O) but have the same contents of other major components. The REE patterns of most of the garnet–kyanite–phengite schists are similar to those of the Post-Archean Australian Shale (PAAS) (x¯La/Yb=13). All garnet–kyanite–quartz rocks are much stronger depleted in LREE (x¯La/Yb=1.4) and other incompatible elements. Our studies show that allanite and monazite are the main concentrators of LREE and Th in the garnet–kyanite–phengite rocks of the Barchi site. Monazite, occurring as inclusions in garnet, contains not only LREE but also Th, U, and Pb. Rutile of the nondepleted rocks is enriched in Fe and Nb impurities only. The garnet–kyanite–quartz granofelses bear rutile, apatite, and xenotime as accessory phases. Rutile of the depleted rocks shows wide variations in contents of Nb, Ta, and V impurities. In places, the contents of Nb and Ta reach 10.5 and 2.3 wt.%, respectively. The rutile decomposes into rutile with Nb (1.4 wt.%) and Fe (0.87 wt.%) impurities and titanium oxide rich in Fe (6.61 wt.%), Nb (up to 20.8 wt.%), and Ta (up to 2.81%) impurities. Based on the measured contents of incompatible elements in differently depleted high-alumina rocks, the following series of element mobility during UHP metamorphism has been established: Th > Ce > La > Pr > Nd > K > Ba > Rb > Cs > Sm > Eu. The contents of U, P, and Zr in the depleted rocks are similar to those in the nondepleted rocks. The studies have shown that metapelites subducted to the depths with diamond stability conditions can be depleted to different degrees. This might be either due to their exhumation from different depths of the subduction zone or to the presence of an external source of water controlling the temperature of dissolution of phengite and the formation of supercritical fluid/melt.

Plates made of diamonds from the São Luiz province (Brazil) were investigated by confocal scanning luminescence microscopy. The samples have many macroinhomogeneities (cracks and inclusions), but there is a quasi-uniform distribution of luminescence centers in the bulk. At all investigated points of the crystals, the same group of centers was observed: N3, H4, 575, and a red band with a maximum at 690–700 nm. The visible nonuniformities in the distribution of luminescence over the area of the plates are determined by relatively small fluctuations in the ratio of the intensities of individual bands in the spectra. Nitrogen centers of different degrees of aggregation (H4, N3, and 575 nm, with four, three, and one nitrogen atom, respectively) coexist in these crystals. In the same zones of the samples, the distribution of blue luminescence (N3 centers) is diffuse (uniform), but the distribution of yellow-green luminescence is characterized by layering on (111). This might be a consequence of the tangential growth of octahedron faces or a result of plastic deformation of the crystals and dislocations along (111).

U–Pb dating and Hf-isotope analysis of zircons and whole-rock Nd-isotope analyses were carried out on country rocks of the eclogite–gneiss complex of the North Muya dome in the Anamakit-Muya zone of the Baikal Muya accretionary fold belt. Zircons from garnet–biotite gneisses (Qtz + Kfsp + Pl + Bt + Grt) and garnet–biotite–muscovite schist (Pl + Kfsp + Bt + Mu + Grt + Qtz) were dated using the LA-ICP-MS technique. Based on U–Pb isotope data and CL images zircon grains were divided into three groups: detrital, magmatic and metamorphic zircons. Metamorphic zircons display no zoning or the cloudy zoning. The grains morphology together with the well-developed oscillatory zoning clearly identifies the igneous origin of magmatic zircons. The metamorphic zircons (ages 576–680 Ma) have Th/U ratios varying from 0.271 to 0.004, whereas the ratio in magmatic zircons ranges from 0.779 to 0.11. Magmatic zircons from granite-gneisses of the North Muya dome exhibit a relatively narrow spread in the crystallization age with the major peak at ca 764 Ma. Younger ages are interpreted as due to the partial resetting of U–Pb system during the subsequent metamorphic evolution. Detrital zircons from two-mica schist sample Mu-93-10 give ages of 1.88–2.66 Ga. The oldest detrital zircon from this sample plots near concordia and has a 207 Pb/ 206 Pb age of 3.2 Ga. Zircons from this sample are characterized by the widest scatter of ɛ Hf ( t ) values (from +13.9 to −15.3) and T DM C model ages (0.82–3.86 Ga). Zircons from other samples have a much narrower ranges of ɛ Hf ( t ) (+11.6 to −0.7) and T DM C (0.85–1.52 Ga). The involvement of older crustal material is also evident from the whole-rock Nd isotopic compositions. The gneisses and schists exhibit a range of Nd isotopic compositions with ɛ Nd ( t ) values ranging from −3.5 to +3.6 and t Nd(DM) from 1.64 to 1.09 Ga. The integration of the Hf-isotope data with the age spectra provides with the first evidence for the existence of Mesoarchean crust in the Baykal-Muya sector of the Central Asian Orogenic Belt.

Evidence for phase transitions in mineral inclusions in superdeep diamonds of alluvial placers in the Sao Luiz River deposits (Brazil) is obtained by the electron backscatter diffraction technique. It has been shown that the crystal structure of superdeep diamonds is significantly deformed around inclusions of MgSi-, CaSi-, and CaTiSi-perovskites, SiO2 (stishovite?), and Mg2SiO4 (ringwoodite?). On the contrary, significant deformations around inclusions of olivine, ferropericlase, and majoritic garnet are not detected. The absence of deformation near these minerals reveals the lack of phase transitions with dramatic volume changes. The present study suggests that the formation of superdeep diamonds proceeds at different levels of the sublithospheric mantle, transition zone, and lower mantle. © 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

We present new data on the main and additional optically active defects in diamonds with cloudy microinclusions from the Mir kimberlite pipe. It has been found that reshaping might have occurred either in a closed system with nitrogen and hydrogen depletion or owing to new portions of a diamond-forming fluid/melt. The internal structure and the distribution of optically active defects suggest both continuous growth of such diamonds and a multistage scenario with a series of postcrystallizational transformations, including resorption, high-temperature annealing, and degradation of nickel-nitrogen complexes. © 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

Distribution of water among the main rock-forming nominally anhydrous minerals of mantle xenoliths of peridotitic and eclogitic parageneses from the Udachnaya kimberlite pipe, Yakutia, has been studied by IR spectroscopy. The spectra of all minerals exhibit vibrations attributed to hydroxyl structural defects. The content of H2O (ppm) in minerals of peridotites is as follows: 23–75 in olivine, 52–317 in orthopyroxene, 29–126 in clinopyroxene, and 0–95 in garnet. In eclogites, garnet contains up to 833 ppm H2O, and clinopyroxene, up to 1898 ppm (~ 0.19 wt.%). The obtained data and the results of previous studies of minerals of mantle xenoliths show wide variations in H2O contents both within different kimberlite provinces and within the Udachnaya kimberlite pipe. Judging from the volume ratios of mineral phases in the studied xenoliths, the water content varies over narrow ranges of values, 38–126 ppm. At the same time, the water content in the studied eclogite xenoliths is much higher and varies widely, 391–1112 ppm.

Diamonds from placer deposits in the northeastern Siberian platform were examined for variations in the isotopic composition of carbon and concentrations of nitrogen. The diamonds display large variability in N concentrations, ranging from below detection to 3500 ppm. Nitrogen levels in diamonds with eclogitic inclusions are generally high (average of 950 ppm) compared to diamonds of the ultramafic suite (average of 513 ppm). Diamonds belonging to variety V (classification scheme of Orlov, 1977) have relatively high N levels (from 1500 to 3500 ppm, average of 2549 ppm). The average nitrogen level is five times higher than the worldwide average. Values of δ 13 C in the diamonds range from −27.2 to −3‰ ( n = 28) in eclogitic diamonds and from −7.1 to −0.5‰ ( n = 16) in peridotitic diamonds. Diamonds of variety V range in δ 13 C from −24.1 to −17.4‰. The distributions of nitrogen and its aggregation state in some diamond crystals imply the occurrence of multiple growth events. This is especially evident in the nitrogen content, which decreases and then increases from core to rim. The results indicate that there is no correlation between local variations in the isotopic composition of carbon and either the content or degree of aggregation of nitrogen. The diamonds of variety V differ from most diamonds worldwide in their high N content and light C-isotope composition. The large ranges of δ 13 C and the lack of correlation with nitrogen levels are inconsistent with the open-system Rayleigh isotopic fractionation of carbon species. The nature of the variations in the carbon isotope composition and the nitrogen concentrations indicate that the diamond growth medium had at least two sources of fluids/melts (mantle and recycled Earth crust via subduction). Mantle carbon was involved in the process of diamond formation during the final stages of diamond growth.

The variety of morphology and properties of natural diamonds reflects variations in the conditions of their formation in different mantle environments. This study presents new data on the distribution of impurity centers in diamond type Ib-IaA from xenolith of bimineral eclogite from the Udachnaya kimberlite pipe. The high content of non-aggregated nitrogen C defects in the studied diamonds indicates their formation shortly before the stage of transportation to the surface by the kimberlite melt. The observed sectorial heterogeneity of the distribution of C- and A-defects indicates that aggregation of nitrogen in the octahedral sectors occurs faster than in the cuboid sectors.

To clarify the tectonic-thermal evolution of the Anabar tectonic province in the central Siberian Craton, we performed an isotope-geochemical study of 20 xenoliths from the Udachnaya, Zarnitsa, and Komsomolskaya kimberlite pipes to represent different crustal levels. Most mafic granulites have Proterozoic Nd model ages and geochemical characteristics close to those of intraplate basalts, whereas some mafic and intermediate granulites with Archean model ages exhibit geochemical features of supra-subduction ophiolitic basalts. Analysis of U-Pb ages and hafnium isotopic composition of zircon indicates that the main tectonic-thermal events modified the crust at 2.7 and 1.9–1.8 Ga, which is consistent with ages of mantle depletion events from previous studies. All zircons have Archean Hf model ages ( T DM C 3.6 - 3.2 Ga). Overall, thermal events with ages of 2.9–2.8, 2.7, 2.4, 1.97 and 1.8 Ga have remarkable influence on the studied zircons. Tectono-thermal events at 2.4 1.97, 1.9 and 1.8 Ga with no addition of juvenile material are recorded by zircons from xenoliths of mafic and intermediate granulites and metadiorites. A compilation of isotope-geochemical data demonstrates that instead of age-stratified, the crust of the Anabar tectonic province consists of variably reworked Paleoarchean rocks and juvenile Proterozoic rocks at all crustal levels. Hence the crust and mantle of the Siberian Craton has been coupled since the Paleoarchean.

Diamonds from Juina province, Brazil, and some others localities reveal the existence of a deep, Ca-rich carbonate-silicate source different from ultramafic and eclogite compositions. In this study, we describe the first observation of merwinite (Ca 2.85 Mg 0.96 Fe 0.11 Si 2.04 O 8 ) in a diamond; it occurs as an inclusion in the central growth domain of a diamond from the Sao Luiz river alluvial deposits (Juina, Brazil). In addition, the diamond contains inclusions of walstromite-structured CaSiO 3 in the core and (Mg 0.86 Fe 0.14 ) 2 SiO 4 olivine in the rim. According to available experimental data, under mantle conditions, merwinite can only be formed in a specific Ca-rich and Mg- and Si-depleted enviroment that differs from any known mantle lithology (peridotitic or eclogitic). We suggest that such chemical conditions can occur during the interaction of subduction-derived calcium carbonatite melt with peridotitic mantle. The partial reduction of the melt could cause the simultaneous crystallization of Ca-rich silicates (CaSiO 3 and merwinite) and diamond at an early stage, and (Mg 0.86 Fe 0.14 ) 2 SiO 4 olivine and diamond at a later stage, after the Ca-Mg exchange between carbonatite melt and peridotite has ceased. This scenario is supported by the presence of calcite microinclusions within merwinite.

Taking advantage of the common occurrence of superdeep mineral inclusion assemblages, we examined core-to-rim primary zonation in 69 diamonds from the alluvial deposits of the Sao-Luis River (Juina, Brazil). Syngenetic inclusions were represented by phases of superdeep paragenesis. The dominant inclusions are majoritic garnets, ferropericlases and CaSi- and CaSiTi-perovskites. Rare inclusions of MgSi-perovskites, olivines, clinopyroxenes, TAPP, phases of SiO2, kyanites, AlSi-phases, KFsp (K-hollandite?), CF, NAL, grossular, merwinite, native iron, Fe-sulphides, magnesite and CaCO3 + CaMgSi2O6 (composite inclusions) were also found. The diamonds from Sao-Luis display wide variations in carbon isotopic compositions (δ13C), from +2.7 to −25.3‰. The diamonds with inclusions of ferropericlase have a very narrow range of δ13C values, from −2.1 to −7.7‰, which are close to the “normal” mantle values. In many cases, diamonds with inclusions of calcic majoritic garnet and CaSi- and CaSiTi-perovskites display marked differences from this range. Low δ13C values (−10 to −25 ‰) were observed exclusively in a series of diamonds with majorite garnets, Ca-silicates, aluminous silicates and SiO2. The δ13C values from the cores to rims in certain individual crystals varied substantially, indicating multiple stages of growth. The highly negative δ13C values in the cores (−20 to −25 ‰) potentially represent organic matter in metasediments of altered oceanic crust, and the lower δ13C values may represent mixing trends towards “normal” mantle compositions. In this study, we also found a set of diamonds that display an opposite trend of change of the carbon source, from primordial mantle to subducted/crust (either biotic or abiotic carbon).

We present the first results of studying the major- and trace-element composition of microinclusions in the coats of type IV diamonds from the Sytykanskaya pipe. These microinclusions are of silicate–carbonate composition. Similar compositions are reported for diamonds from the placers of the northeastern Siberian Platform and cuboids from the Internatsional’naya pipe. The microinclusions studied are close to kimberlites and carbonatites in trace-element composition but depleted in HFSE, Mg, and transition metals and enriched in K and LILE. The distribution of incompatible elements in the microinclusions studied is similar to the “table” pattern, which was observed for high-density hydrous-silicic fluids.

Diamond from metaultramafic rocks of the Mesoarchean (2.96–3.0 Ga) Olondo greenstone belt, located in the western Aldan–Stanovoy shield, has been studied. Diamonds occur in lenses of olivine–serpentine–talc rocks within metaultramafic rocks of intrusive habit, whose composition corresponds to peridotite komatiites. All diamonds from the metaultramafic rocks are crystal fragments 0.3 to 0.5 mm in size. Morphological examination has revealed laminar octahedra, their transitional forms to dodecahedroids, crystals with polycentric faces, and spinel twins. The crystals vary in photoluminescence color: dark blue, green, yellow, red, or albescent. Characteristic absorption bands in crystals point to nitrogen impurities in the form of A and B1 defects and tabular B2 defects. The crystals studied belong to the IaA/B type, common among natural diamonds. The overall nitrogen content varies from

Results of study of eclogite–gneiss complex of the Muya Block (East Siberia) are presented. Several structural types of the studied eclogites have been recognized. Kyanitic eclogite has been found for the first time. The host granite-gneisses are two-mica and biotite varieties, mainly garnet-bearing. The exposure of eclogites from different depths of the subducted plate at the present-day denudation level might be the reason for the wide range of the equilibrium temperatures of the Muya block eclogites (590–740 °C). The Sm–Nd dating of the eclogites and host gneisses showed the Neoproterozoic age of high-pressure metamorphism (∼630 Ma). The model age (TDM) of the eclogites (720 Ma) differs considerably from the model age of the host gneisses (>1.3 Ga). The geochemical features of the eclogites point to the mobility of LILE (Rb, Cs, Ba, K) and LREE during their interaction with fluids, whereas the gneisses in the same process showed the mobility of LILE only. The oxygen isotope composition of minerals in the eclogites varies over a narrow range (δ18O = 5.5–3.9) and is close to the average mantle value, which evidences a negligible interaction between the eclogite protoliths and meteoric or sea water. The study of fluid inclusions in quartz from the eclogites and host gneisses showed a predominance of liquid-nitrogen inclusions in the former and carbon dioxide inclusions in the latter.

The Kulet eclogite in the Kokchetav Massif, northern Kazakhstan, is identified as recording a prograde transformation from the amphibolite facies through transitional coronal eclogite to fully recrystallized eclogite (normal eclogite). In addition to minor bodies of normal eclogite with an assemblage of Grt + Omp + Qz + Rt ± Ph and fine-grained granoblastic texture (type A), most are pale greyish green bodies consisting of both coronal and normal eclogites (type B). The coronal eclogite is characterized by coarse-grained amphibole and zoisite of amphibolite facies, and the growth of garnet corona along phase boundaries between amphibole and other minerals as well as the presence of eclogitic domains. The Kulet eclogites experienced a four-stage metamorphic evolution: (I) pre-eclogite stage, (II) transition from amphibolite to eclogite, (III) a peak eclogite stage with prograde transformation from coronal eclogite to UHP eclogite and (IV) retrograde metamorphism. Previous studies made no mention of the presence of amphibole or zoisite in either the pre-eclogite stage or coronal eclogite, and so did not identify the four-stage evolution recognized here. P–T estimates using thermobarometry and Xprp and Xgrs isopleths of eclogitic garnet yield a clockwise P–T path and peak conditions of 27–33 kbar and 610–720 °C, and 27–35 kbar and 560–720 °C, respectively. P–T pseudosection calculations indicate that the coexistence of coronal and normal eclogites in a single body is chiefly due to different bulk compositions of eclogite. All eclogites have tholeiitic composition, and show flat or slightly LREE-enriched patterns [(La/Lu)N = 1.1–9.6] and negative Ba, Sr and Sc and positive Th, U and Ti anomalies. However, normal eclogite has higher TiO2 (1.35–2.65 wt%) and FeO (12.11–16.72 wt%) and REE contents than those of coronal eclogite (TiO2

The first data are presented on the compositions of microinclusions in fibrous diamonds from the Ebelyakh placers, northeastern Siberian Platform. Their fluid/melt microinclusions are of silicate or carbonate compositions. In general, the trace element patterns for the microinclusions correspond to kimberlites and carbonatites. The major-element composition differs significantly; for example, the microinclusions are considerably enriched in K and Na. In two of the studied diamonds, the microinclusion compositions differ considerably in the cores and rims. In one of them, the composition of the medium changes from chloride–carbonate to predominantly carbonate (sample HI-90); in the other one, from carbonate to silicate (sample HI-98). Similar carbon isotope characteristics of diamonds with microinclusions of two contrasting media might suggest their crystallization from a mantle reservoir with the same carbon isotope characteristics. The geochemical features of the microinclusions in the placer diamonds revealed their relationship with protokimberlitic carbonate–silicate fluids. Such fluids might result from the metasomatic interaction of volatiles and/or the low-degree partial melting of peridotite and eclogite substrates.