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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.

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.

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

Comprehensive studies of diamond crystals with a low nitrogen concentration from the placer deposits and kimberlite pipes of Yakutia have facilitated the detection of three titanium-nitrogen-related centers: OK1/S1, N3/440.3 nm and NU1/485 nm in the EPR and photoluminescence spectra. Additional proof of titanium presence in the structure of EPR N3 center was obtained from the hyperfine structure of titanium magnetic isotopes 47 Ti and 49 Ti. Titanium-nitrogen-related centers in the neighboring substitutional sites (N3/440.3 nm) and in the structure of the interstitial (NU1/485 nm) are not found in cubic habit crystals, which are characterized by the presence of the OK1/S1 center with a double semi-vacancy structure. All three types of nitrogen-titanium-related centers are present in cuboctahedral habit diamonds. It is assumed that these habit-specific distributions of defects in diamond are a result of the peculiarities of incorporating nitrogen and titanium impurities into the different growth sector of crystals and aggregation thereof at high pressure and temperature.

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.

Diamond microinclusions provide the unique opportunity to study the composition of the mineral-forming medium. The paper presents the first data on the composition of cloudy microinclusions in the cores of octahedral diamonds from the Internatsional'naya pipe. These zones are of cuboid shape and have a fibrous internal structure. Here, the microinclusions form a continuous trend from chloride–carbonate to carbonate composition. Their composition slightly overlaps that of the microinclusions in cuboids from the same pipe, but in a carbonate-enriched zone. Also, data on N aggregation suggest that the early cubic zones which formed before octahedra crystallized at higher temperature or spent considerably more time in the mantle than cubic crystals and fibrous rims of variety IV diamonds.

The analysis of the electron paramagnetic resonance (EPR) line shape of ultra-dispersed diamond (UDD) obtained by conversion of trotyl and hexogen mixture and purified from other phases and metal compounds is carried out. The observed wide line with g = 2.0028 and a line width of 8.7 G is shown to be formed by superposition of three lines with line widths of 15.3, 8.5 and 3 G and with a ratio of integral intensities of 70:30:1. The procedure of decomposition and subtraction of wide lines has revealed the resolved hyperfine structure (HFS) from donor nitrogen with parameters A = 40.8 G and B = 29.2 G. Experimentally obtained dependence of the line width of the exchange line on the concentration of donor nitrogen in synthetic diamonds assumed that variations in line widths of the EPR spectrum components are caused by different local concentration of donor nitrogen due to distribution of nitrogen impurity during crystallization of diamond nanoparticles. EPR spectra of UDD after annealing in vacuum and at high pressures in the range of diamond phase stability are also discussed. At high-pressure annealing above 973 K, the areas with high concentration of defects are graphitized and a narrow Dyson-shape line from conductivity electrons and a resolved HFS from donor nitrogen can be observed without additional treatment of the EPR spectrum.

A new type of nitrogen-related center in natural diamond labeled NU1 is identified as a 〈100〉 split interstitial configuration by means of electron paramagnetic resonance (EPR) and photoluminescence (PL) techniques. In PL this center is seen as an electron-vibration system with a zero-phonon line at 485 nm in the crystals containing S1/OK1 and 440.3 nm/N3 centers. NU1 EPR spectra are described by a spin-Hamiltonian with parameters: S = ½, I = 1 and A xx (N) = 22.5 G, A yy (N) = 19.5 G, A zz (N) = 20.55 G, g xx = 2.0043, g yy = 2.0032, g zz = 2.0020. Directions of A zz and g zz coincide and correspond to [001]. The directions of A xx and A yy coincide with those of g xx and g yy and correspond to [110] and [−110], respectively. Analysis of the phonon structure of the NU1 center suggested that titanium can be the second atom together with nitrogen in the structure of a split interstitial.

article i nfo The world's largest diamondiferous eclogite (8.8 kg, UD-111/02) from the Udachnaya kimberlite, Siberia, is the subject of this investigation. High-resolution X-ray computed tomography (HRXCT), chemistry of host minerals and diamond inclusions, stratigraphy of the diamonds (cathodoluminescence zoning), and carbon isotopes have been obtained. HRXCT images, 3-d models, and subsequent detailed dissection of this Group B eclogite revealed linear distributions of diamonds along alteration zones. Both unusual multiple-diamond clusters with dodecahedral morphologies, and distinctive internal zoning recorded by cathodoluminescence analyses are indicative of multiple stages of diamond growth and resorption in UD-111-02. Carbon-isotope ratios (δ 13 C) of 16 diamonds have a total range of ~7‰ (−2.0 to −9.2‰), with the majority at −5.4‰.

A representative sample of microdiamonds in calc-silicate and garnet-pyroxene-quartz rocks and gneisses from the cross section of an adit driven at the Kumdy-Kol’ deposit (Northern Kazakhstan) has been analyzed. Microdiamonds from these rocks were studied by Fourier-transform infrared spectroscopy for the first time. It has been established that nitrogen impurity content (300–3000 ppm) and nitrogen aggregation degree (14–75%) vary widely and do not correlate with each other. The variation is probably due to the uneven distribution of nitrogen in crystals and to their specific internal structures. The results of the study show that in most diamondiferous rocks, diamonds crystallized from a fluid/melt of composition varying between aqueous-carbonate and aqueous-silicate end-members. Spectroscopy studies partly disagree with literature data on individual nanoinclusions in diamonds. The cause of this discrepancy may be the evolution of the fluid/melt during diamond crystallization.

The well-known diamond electron paramagnetic resonance (EPR) OK1 and N3 defects are discussed in the context of incorporation of heavy metal, supposedly Ti, in the double semivacancy and substitutional structures, respectively. X-ray phase analysis of inclusions of the growth medium, eclogite, in the diamonds under study shows an atypically high concentration of titanium compounds. To support proposed models, the hyperfine structure (HFS) from 13C in the OK1 center has been analyzed more precisely. The data obtained are consistent with attributing additional satellites around the main EPR lines to HFS from 47Ti and 49Ti. A double semivacancy model describes the directions of g and A tensor components of the OK1 center. The features of infrared and photoluminescence spectra related to the OK1/S1 and N3/440.3 nm centers are presented.

In this paper, we have presented a set of experiments on the high-pressure–high-temperature treatment of synthetic diamonds grown in a Fe–Ni–C system with additional titanium as a nitrogen getter. The defect responsible for the 440.3 nm photoluminescence system related to the electron paramagnetic resonance N3 center has been first created. This finding is an evidence in favor of an earlier assumption of incorporating the titanium ion in the structure of the N3/440.3 nm centers.

X-ray topography (XRT) images were obtained using a double-crystal topographic method, with a magnification system in the Laue case. The experiment was performed at the beamline station “Microscopy and tomography” of the VEPP-3 synchrotron radiation source. Two asymmetrically cut crystals in the Kirkpatrick-Baez scheme were used to increase the spatial resolution. The usage of an asymmetrically cut crystal with a magnification factor of 20 allows us to improve spatial resolution in registered images. A series of slabs of natural diamonds from kimberlite pipes and placers of the Siberian platform was examined. This study confirmed the high efficiency of the XRT method to recover information on the internal structure of natural diamonds. Together with other mineralogical data, the features of internal structure determined by this way are important to deduce the origin and history of growth of natural diamonds.

Article history:Received 23 January 2008Accepted 29 June 2008Available online 10 July 2008Keywords:SHRIMP dataU/PbZirconKokchetavUHP metamorphismpartial melting The Kokchetav Massif of northern Kazakhstan is the best-known metamorphic diamond locality amongnumerous ultrahigh-pressure (UHP) terranes in the world. At the Kumdy-Kol deposit, diamondiferous rocksare interbedded with granitic gneisses, and biotite gneisses; some have been migmatized. Some granitegneisses and migmatites were formed by partial melting of diamondiferous rocks. To verify such suggestion,sensitive high resolution ion microprobe (SHRIMP) U–Pb dating of zoned zircons from migmatites at theKumdy-Kol regionwas performed to constrain the age of partial melting of the Kokchetav UHP metamorphicrocks. Most age data from core and rim domains of zircon separates are concordant. The apparent 206 Pb/ 238 Uages for core and rim domains of zirconsare nearly identical within analytical error. All SHRIMP analyses ofzircons from four samples fall in the range 508–538 Mawith the weighted mean age for all zircon domains at526±2.1 (MSWD=1.7). Our data show that migmatization of UHP pelites occurred later than the peakmetamorphism (537±9 Ma) and the decompression partial melting took place during exhumation ofdiamondiferous rocks from mantle depths to amphibolite-facies conditions at mid-crustal levels (507±8 Ma).© 2008 Elsevier B.V. All rights reserved.

We present the results of a detailed investigation of large (18 cm long, 6.9 kg) diamondiferous eclogite xenolith from the Udachnaya kimberlite pipe. We have studied the spatial distribution of diamonds, their relationship with the matrix minerals, mineral inclusions and their major- and trace-element compositions. The xenolith is composed of pale green clinopyroxene, round garnet grains and phlogopite, and is characterized by irregular distribution of minerals and development of zones enriched in phlogopite and garnet. Minerals in diamondiferous xenoliths show major-element compositions transitional between Udachnaya eclogites and those from peridotites. The host clinopyroxene are enriched relative to pyroxenes from Group A eclogites and websterites in respect K 2 O and LREE. We believe that such peculiarities reflect the interaction between mantle peridotites and LREE-enriched melts. The sources of the melts are uncertain. The distribution of diamonds (1 to 5 mm in size) in the xenolith is extremely irregular: they are more abundant in the garnet- and phlogopite-enriched zones. Diamonds often make up veins and schlieren. Nitrogen aggregation characteristics suggest that the xenolith contains diamonds of at least two generations. High degrees of aggregation in colorless diamonds of the earlier generation indicates that they formed at least 1 Ga before kimberlite eruption, whereas yellow diamonds of the second generation (type IaA) were formed only a short time before kimberlite emplacement. The presence of K-Clinopyroxene, coesite, CaCO 3 , and high-silica mica inclusions in diamonds of the second generation, and their internal and external morphology, suggest that these diamonds crystallized during interaction of the eclogitic substrate with high-density fluids compositionally intermediate between carbonatitic and hydrous-silicic end members rich in Si, K, Ti.

The first studies of diamond-hosted inclusions in xenoliths have demonstrated that the compositions of inclusions and minerals in xenoliths may differ substantially [1]. It was noted that a higher K content in clinopyroxene as compared with that in the matrix is among the most important differences [2]. The Mir Pipe yielded diamonds with numerous garnet and pyroxene inclusions with highly variable compositions [3, 4]. Some authors explain these compositional variations in inclusions by fractionation from the silicate melt [3].

A unique xenolith of eclogite, 23×17×11 cm in size and 8 kg in weight, was found in the Udachnaya kimberlite pipe. One hundred twenty-four diamond crystals recovered from it were analyzed by a number of methods. The diamonds differ in morphology, internal structure, color, size, and composition of defects and impurities. The xenolith contains diamonds of octahedral and cubooctahedral habits. In cathodoluminescence, the octahedral crystals have a brightly glowing core with octahedral zones of growth and a weakly glowing rim. In the cores of these crystals the N impurity is mostly present in the B1 form (30 to 60%). At the same time, N in the rim is chiefly in the A form. The cubooctahedral crystals show a weak luminescence. The content of nitrogen and degree of its aggregation are close to those in the rim of octahedral crystals. The diversity of morphology and impurity composition of diamonds from the xenolith can be explained by their formation in two stages. At the first stage, the diamonds formed which became the cores of octahedra. After a long-time interruption, at the second stage of diamond formation crystals of cubooctahedral habit appeared and the octahedral crystals were overgrown. Wide variations in nitrogen contents in the xenolith crystals allowed their use to estimate the kinetics of aggregated nitrogen. The data obtained show that the aggregation of A centers into B1 centers in the diamonds is described by a kinetic reaction of an order of 1.5.

Analysis of data on micro- and nano-inclusions in mantle-derived and metamorphic diamonds shows that, to a first approximation, diamond-forming medium can be considered as a specific ultrapotassic, carbonate/chloride/silicate/water fluid. In the present work, the processes and mechanisms of diamond crystallization were experimentally studied at 7.5 GPa, within the temperature range of 1,400–1,800°C, with different compositions of melts and fluids in the KCl/K 2 CO 3 /H 2 O/C system. It has been established that, at constant pressure, temperature, and run duration, the mechanisms of diamond nucleation, degree of graphite-to-diamond transformation, and formation of metastable graphite are governed chiefly by the composition of the fluids and melts. The experimental data suggest that the evolution of the composition of deep-seated ultrapotassic fluids/melts is a crucial factor of diamond formation in mantle and ultrahigh-pressure metamorphic processes.

High-resolution X-ray computed tomography (HRXCT) is a technology ideally applicable to a wide range of geological investigations. It is an express non-destructive method to produce images corresponding to series of slice projections through a sample. In the present study, HRXCT was applied to rock samples with the use of synchrotron radiation from the VEPP-3 storage ring, at the “X-ray microscopy and tomography” station. The method was calibrated and preliminary measurements were carried out on the Low-Dose Digital Radiographic Device “Siberia”. The data obtained have determined the internal structure of rock samples with a spatial resolution of 100 μm. HRXCT has been applied to the xenoliths of diamondiferous eclogites from the Udachnaya kimberlitic pipe located in Yakutia, Russia. It has allowed determination of the distribution of rock-forming (garnet and clinopyroxene) and accessory (diamond, rutile, and sulfide) minerals of different X-ray absorption. This is important to find out the genetic relationship of diamonds with associated minerals and the sequence of crystallization.

The bulk major- and trace-element compositions of micro-inclusions in 24 cuboid diamonds from the Udachnaya kimberlite pipe (Siberia, Russia) have been quantitatively analyzed by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LAM-ICPMS). Micro-inclusions in the studied diamonds represent a bulk sample of the fluids from which the diamonds crystallized; they define a continuous range between magnesian and calcic carbonate-rich compositions at relatively constant Fe contents (Ca/(Ca + Mg) = 0.31–0.68; Mg/(Mg + Fe) = 0.66–0.78). In general the major- and trace-element patterns of the trapped fluids in Udachnaya cuboid diamonds are similar to those of kimberlites and carbonatites. However, some important differences are observed: (1) the fluids in diamonds are enriched in K, Na, Fe and depleted in Al relative to the host kimberlite; (2) the REE patterns of the fluids are steeper than those of kimberlites; (3) many of the fluids show strong depletion in Ti, Zr and Y. The observed geochemical features are consistent with a genetic link between the diamond-forming fluids and ephemeral carbonatitic fluids/melts which may be precursors of the host kimberlite. These fluids/melts may originate either from the metasomatic and influx of carbonatitic agents or from partial melting of previously carbonated eclogites and peridotites. Some elemental variations may be explained by fractional crystallization of such fluid/melts, or mixing between fluids with different compositions.

In this communication we present an estimation of the concentration of single substitutional nitrogen atoms (P1 centre) from EPR data in Kokchetav diamonds. We first performed an analysis of local P1 centre concentrations using van Wyk’s treatment (van Wyk et al ., 1997) of the line width of the resolved EPR spectrum. Also by analysing the dependence of the line-width of the broad exchange line on the P1 concentration in range higher than 1000 ppm in synthetic diamonds, we obtained a calibration curve allowing the estimation of the local P1 centre concentration higher than 1000 ppm and the local volumes occupied by substitutional nitrogen atoms without taking measurements of microcrystalline volume. In addition to the development of a method for calculating high local nitrogen concentrations, the aggregation process in P,T-treated Kokchetav diamonds has been given considerable attention and it was shown that the rate of nitrogen aggregation in these diamonds is consistent with data given in Evans (Evans & Qi, 1982).

Water-related features were studied using infrared absorption spectroscopy in fibrous diamonds with micro-inclusions. Both OH-stretching and HOH-bending vibrations were observed. The lack of correlation between the intensities of HOH and OH bands in different samples and the complexity of the OH-stretching band indicate that a large fraction of water is present as hydroxyl groups in minerals.Heating and cooling experiments were performed to elucidate the properties of fluids in micro-inclusions in diamonds. The results of experiments at various temperatures support the presence of several water-related components in individual diamonds. These spectroscopic investigations of fibrous diamonds revealed two or more interrelated water-related components preserved in micro-inclusions. The existence of several water phases or water solutions with different salinity and solutes within one diamond crystal is possible.

The recently obtained data on the superimposed character of diamonds in eclogite xenoliths have compelled us to consider the problem of diamond formation in the upper mantle from a new standpoint [1]. In [2], we reported the first results of the study of a unique diamond-bearing xenolith (length18 cm, weight 6900 g) from the Udachnaya kimberlite pipe (Yakutia). The results testify to the superimposed character of diamonds (particularly, the chemistry of minerals) and suggest that diamond crystallized as a product of interaction between the high-K and C-bearing fluid with the ultramafic substrate [2]. The study of diamond-hosted inclusions from xenoliths and the comparison of their chemical composition with minerals of the xenolith groundmass would be of special interest for reconstructing conditions of formation, composition, and evolution of diamond. In this communication, we present results of the study of inclusions along with defects, impurities, and the internal structure of diamond from this xenolith. We selected from the xenolith 44 diamond crystals 1‐5 mm in size. The crystals with crystalline inclusions were polished along axes (110) and (100) until the exposure of inclusions. These inclusions were examined with a Camebax-micro electron microprobe. The internal structure of diamond crystals was investigated with cathode luminescence on a LEO 1430VP scanning electron microscope. The IR absorption spectra were recorded with a Bruker VERTEX 70 IR Fourier spectrometer to study the impurities and to estimate concentration of structural defects in diamonds from this xenolith. The diamonds in xenoliths occur largely in the intergranular space and as inclusions in pyroxene and mica. They have the shape of differently distorted octahedra. Cathode luminescent images distinctly show two zones (core and shell). As a rule, the core is characterized by a weak luminescence, while the shell reveals a more intense luminescence (Fig. 1). The nonuniform zoning and distorted growth layers confirm the previous conclusion on the growth of the xenolith-hosted diamonds in a confined space. According to the results of IR Fourier spectroscopy, diamonds from this xenoliths pertain to the IaA/B type, widespread among natural diamonds; i.e., they contain impurity centers A (a pair of nitrogen atoms that isomorphically replace carbon), B1 (four nitrogen atoms around a vacancy), and in some cases B2 (sheetlike defects oriented parallel to (001)) with linear dimensions varying from a few nanometers to a few micrometers [3]. The total content of nitrogen admixture in the crystals ranges from 40 to 800 ppm. Besides the absorption bands related to the main nitrogen impurity centers, additional lines 3310, 3237, 3150, 3107, 1405

A progression from cuboid to octahedral growth has been observed in 16 natural diamonds from Yakutian kimberlites. X-ray and cathodoluminescence topography have revealed that the change in morphology of diamonds with cloudy cuboid cores may occur without mixed-habit growth but via generation of numerous octahedral apices on cuboid surfaces and subsequent gradual transformation into regular octahedral morphology. Nitrogen aggregation in both cuboid and octahedral domains of such diamonds suggests that they have had a long residence time under mantle conditions. Micro-inclusions in the cuboid domains of the diamonds testify to the nucleation and growth of cuboid cores from a hydrous-carbonatitic (oxidized) fluid. The transition from cuboid hummocky growth rich in inclusions to octahedral growth without inclusions may be linked to decreasing supersaturation in the parent fluid. Measurements of δ13C and Nppm by ion microprobe show that the chemical variations observed between inner cuboid domains and outer octahedral zones commonly have a systematic character and as such they are probably not due to purely kinetic effects. The peripheral octahedral zones are always enriched in 13C in comparison with inner cuboid ones, and the total nitrogen content decreases with the change from cuboid to octahedral growth. The octahedral outer zones show a gradual progressive increase in δ13C, with an overall change of up to 5‰ from the cuboid core (δ13C usually between −8 and −6‰) to the diamond margin (δ13C usually between −4 and −2‰). Decreases in δ13C of this magnitude with a gradual increase in 13C may be attributed to the Rayleigh fractionation operating on a single parent fluid of close to normal mantle δ13C composition with diamond precipitating by the reduction of carbonatitic fluid in a closed system. However, one sample shows a variation of δ13C of approximately −17 to −6‰ and therefore suggests a possible change of fluid source composition from one containing subducted crustal organic carbon to one with common mantle carbon.

Based upon detailed studies of diamondiferous metamorphic rocks, many authors share the opinion that diamonds crystallize in the field of their thermodynamic stability. Nevertheless, some problems remain, and the most important questions are as follows: (1) What is the pressure under which diamond crystallized? (2) Does the composition of diamondiferous rocks correspond to the medium of diamond crystallization? (3) Why are microdiamonds irregularly distributed in dolomite marbles and garnet-pyroxene rocks? (4) What is the role of carbonates in diamond genesis? To answer these questions, we carried out petrographic and mineralogical studies and experimentally modeled the process of microdiamond crystallization in diamondiferous garnet-pyroxene rocks and dolomite marbles. Diamondiferous marbles and garnet-pyroxene rocks occur as layers and lenses in biotite gneisses of the Kumdy-Kol microdiamond deposit, northern Kazakhstan. Mineralogical and petrographical data demonstrate that pyroxene of diamondiferous r...

A highly potassic COH fluid, with dissolved phosphate, chloride and sulfate/sulfide, was identified as nanometer-size inclusions in metamorphic microdiamonds of Kokchetav dolomite marble and garnet–quartz–pyroxene rock using an analytical electron microscope (AEM) coupled with an energy-dispersive X-ray (EDX) spectrometer. This provides a direct piece of evidence for the formation of metamorphic microdiamonds from such a fluid. Although this crust-derived potassic fluid and the associated solid precipitates are in some respects similar to the fluid inclusions in fibrous/cloudy mantle diamonds, distinct chemical differences exist. This potassic COH fluid, in addition, has the potential to trigger high to ultrahigh-K magmatic and K metasomatic processes in mantle.

The exhumation of ultrahigh-pressure (UHP) metamorphic units from depths more than 100–120 km is one of the most intriguing questions in modern petrology and geodynamics. We use the diamondiferous Kumdy-Kol domain in the Kokchetav Massif to show that exhumation models should take into consideration initially high uplift velocities (from 20 down to 6 cm/year) and the absence of the deformation of UHP assemblages. The high rate of exhumation are indicated by ion microprobe (SHRIMP) dating of zircons from diamondiferous rocks and supported by the low degree of nitrogen aggregation in metamorphic diamonds. Diamondiferous rocks in the Kumdy-Kol domain occur as steeply dipping (60°–80°) thin slices (few hundred metres) within granite–gneiss. Using geological, petrological and isotopic–geochemical data, we show that partial melting of diamondiferous metamorphic rocks occurred; a very important factor which has not been taken into account in previous models. Deformation of diamondiferous rocks at Kumdy-Kol is insignificant; diamond inclusions in garnet are often intergrown with mica crystals carrying no traces of deformation. All these facts could be explained by partial melting of metapelites and granitic rocks in the Kumdy-Kol domain. The presence of melt is responsible for an essential reduction of viscosity and a density difference (Δρ) between crustal rocks and mantle material and reduced friction between the upwelling crustal block, the subducting and overriding plates. Besides Δρ, the exhumation rate seems to depend on internal pressure in the subducting continental crustal block which can be regarded as a viscous layer between subducting continental lithosphere and surrounding mantle. We construct different models for the three stages of exhumation: a model similar to “corner flow” for the first superfast exhumation stage, an intermediate stage of extension (most important from structural point of view) and a very low rate of exhumation in final diapir+erosional uplift.

Kokchetavite, a new polymorph of K-feldspar (KAlSi3O8), has been identified as micrometer-size inclusions in clinopyroxene and garnet in a garnet-pyroxene rock from the Kokchetav ultrahigh-pressure terrane, Kazakhstan. Kokchetavite has a hexagonal structure with a =5.27(1) A, c=7.82(1) A, V=188.09 A3, Z=1, and is found to be associated with phengite + α/β-cristobalite (or quartz) + siliceous glass ± phlogopite/titanite/calcite/zircon, occurring as multi-phase inclusions in clinopyroxene and garnet. It is concluded that kokchetavite could not be an exsolution phase in host minerals. Instead, it might be metastably precipitated from an infiltrated K-rich melt during rock exhumation. Alternatively, although less likely, kokchetavite might be derived from dehydration of K-cymrite, which, in turn, was formed at high pressures. In either case, kokchetavite is a metastable polymorph of K-feldspar.

Thin sections of ultrahigh pressure (UHP) metamorphic rocks from the Dora Maira Massif (Italy) and the Kokchetav Massif (Kazakhstan) were investigated using the hot cathode cathodoluminescence (CL) technique. Coloured images of important, but otherwise invisible growth features could be easily identified with this tool within seconds. These features are in excellent correlation with chemical variations of minerals revealed by electron microprobe (EMP). Generally, CL is induced by activator-elements (e.g. Mn and REE) and lattice defects whereas so-called quencher-elements like Fe may reduce or even extinct luminescence. Since X-ray-intensity mapping images (MAPS) of minerals can take up to 50 hours, the CL-method represents an ideal and rapid approach prior to chemical characterization. In addition to typical carbonates such as calcite, Mg-bearing calcite and dolomite, a number of rock forming and accessory minerals including Mg- and Mg-Ca-garnets, diopsidic and jadeitic pyroxenes, kyanite, K-feldspar, quartz, coesite, diamond, zircon, apatite, and bearthite were examined. Features observed in garnets include small-scale oscillatory zoning patterns, changes in morphology during growth as well as different crack generations which were partly annealed. SiO2 phases (coesite, quartz, chalcedony) as well as exsolution textures of dolomite and Mg-bearing calcite are easy to distinguish due to their different CL-colours. Pyroxene displays complex zonation patterns and -to some extent- exsolution-textures of K-feldspar. Kyanite reveals distinct growth zones; in combination with mineral inclusion studies it is possible to discriminate between different kyanite-forming reactions. The different crystallographical orientation of twinned kyanite crystals leads to various luminescence colours, thus, the suture of the twin plane is well defined. Prior to SHRIMP analyses, knowledge of the internal structures of zircon is indispensable. Even very tiny coesite crystals are easy to distinguish from quartz or chalcedony by their disparate luminescence colours. Accessory luminescent minerals like diamond, apatite, bearthite are easy to identify in thin section even if they occur in very small abundance within the matrix or as inclusions. The CL method presented here for UHP-metamorphic rocks is recommended as a pathfinder for the discovery of internal structures of minerals prior to their chemical characterization using EMP.

Micro-inclusions (1 –10 μm) in 55 diamonds of cubic habit from the Udachnaya kimberlite pipe have been studied using vibrational spectroscopy. This has revealed a multiphase assemblage in cuboid diamonds from the Udachnaya kimberlite pipe. This assemblage includes carbonates, olivine, apatite, graphite, water and silicate glasses. The micro-inclusions preserve the high internal pressure and give confidence that the original materials were trapped during growth of the host diamond. The internal pressures, extrapolated to mantle temperatures, lie within the stability field of diamond and the relatively low temperatures are typical for the formation of cuboid diamonds. In contrast to previously reported data for African diamonds, the micro-inclusions in the cuboids from Udachnaya are extremely carbonatitic in composition (H2O/(H2O+CO2) ≈5 –20%) with the observed assemblage of microinclusions similar to some types of carbonatites. The low water and silica content testify that the material in the micro-inclusions of the Udachnaya diamonds was near-solidus carbonatitic melt. Vibrational spectroscopy has provided the evidence of carbonatitic melts in cuboid diamonds.

The Kokchetav Massif of northern Kazakhstan is a typelocality of occurrence of microdiamonds in ultrahighpressure metamorphic" rocks. It records continental subduction and exhumation from depths greater than 120 km and provides unparalleled insights into mantle dynamics, global material recycling and the processes responsible for the genesis and destruction of continent. Through the support of a US Civilian Development and Research Fund, a group of Russian, US, Taiwanese and Kazakhstan geologists visited the reconstructed 800-m underground gallery to conduct field survey and sample collections. This gallery provides a useful geologic laboratory for international community to undertake similar geotectonic or different ecological and environmental studies.

Electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopies have been used to study Brazilian and Central African carbonados. We report the first observation of the N 2 V + and N + 2 nitrogen defect centers in carbonados. The fact that Brazilian carbonados contain the N 3 V (previously reported), N 2 V + , and N + 2 defects, the formation of which requires complicated aggregation of nitrogen, indicates histories at elevated temperature and pressure. The observation of both the radiation-induced Al defect, which has an annealing temperature of 700 K, and radiation defects containing two and/or five vacancies, suggest a continuous process of the formation and transformation of radiation defects in the carbonado under variable pressure and temperature conditions while in the upper mantle. The temperature at which these defects are formed varies from 300 to 1500 K. In addition we report a hydrogen-containing defect labeled H1, previously only observed in synthetic diamonds prepared by chemical vapor deposition. Together with the previously reported light carbon isotope ratio, the results of this study confirm the hypothesis that carbonados are derived from hydrocarbon source material.

Clinozoisite gneisses were studied from the Barchi-Kol area, located 17 km to the west of the Kumdy-Kol microdiamond deposit at Kokchetav massif (Northern Kazakhstan). As distinct from the deposit, the studied rocks are characterized by predominance of diamonds of octahedral habit. Ultrahigh-pressure mineral assemblages were investigated mainly as inclusions in zircons from these rocks. It is established that nucleation and growth of some zircon grains began at the peak of metamorphism (T = 950–1000°C and P > 40 kbar) and continued while temperature and pressure decreased to T = 650–750°C and P = 10–12 kbar, Ultrahigh-pressure metamorphic conditions for clinozoisite gneisses from the Barchi-Kol area are comparable with those for the Kumdy-Kol microdiamond deposit and correspond to T = 900–1000°C and P > 40 kbar. High P-T of metamorphism and bulkrock composition did not affect the morphology of the diamond crystals. The abundance of fluid or melt is proposed to be responsible for the variable extent of the completeness of cuboid reshape reaction, resulting in the formation of octahedral diamond crystals. The preservation of coesite as inclusions within zircon grains, as well as within garnets, suggests the rapid cooling and fast exhumation of the studied rocks. Based on concentration profiles in garnets from diamondiferous clinozoisite rocks, the duration of retrograde metamorphism is estimated to be less than 0.1 Ma.

This study reports δ13C, δ15N and N-content values for microdiamonds from ultrahigh-pressure metamorphic rocks of the Kokchetav massif in Kazakhstan. Both alluvial diamonds and in-situ diamonds from a garnet–clinopyroxene rock and a marble (i.e. a garnet–pyroxene dolomitic rock) were investigated. In-situ diamonds were analysed in batches, because of their small size (average 40 μm), whereas the larger alluvial diamonds were analysed individually. The latter group has δ13C-values ranging from −15.92‰ to −10.57‰, δ15N from −1.8‰ to +1.1‰ and N-contents from 2300 to 3650 ppm. Diamonds from the garnet–clinopyroxene rock yield mean values of −10.50‰ for δ13C, +5.9‰ for δ15N and a high average nitrogen content of 11,150 ppm. Values for diamonds in marble are −10.19‰, +8.5‰ and 2650 ppm, respectively. For diamonds from garnet–clinopyroxene rock and marble, there is more nitrogen released by bulk combustion than estimated by infrared (IR) spectroscopy, the differences being of about 7000 and 1500 ppm, respectively. These differences suggest that a significant quantity of nitrogen is IR-inactive and may be present as fluid inclusions. Their carbon and nitrogen isotopic compositions are compatible with an in-situ crystallisation of diamond from dominantly metasedimentary sources, suggesting that sedimentary nitrogen can be subducted to very high pressures. Carbon isotopic fractionation between coexisting carbonate and diamond suggests crystallisation temperatures before the peak of metamorphism at temperatures probably below 700°C and deduced pressures of 3 GPa. Relative to the isotopic data reported for sediments, metasediments and in-situ diamonds, the slightly 15N-depleted compositions of alluvial diamonds is striking. These values suggest that the contribution of any metasedimentary source is unlikely and may point toward a mafic/ultramafic protolith.

Isotopic and geochemical data of the Zerenda series metamorphic rocks from the Kokchetav massif are reported. Some of these rocks contain microdiamond inclusions in garnets and other indicators of ultrahigh pressure metamorphism (P > 40 kbar, T = 900–1000 °C). The diamond-bearing rocks exhibit distinctive geochemical characteristics compared to typical crustal rocks. The REE patterns range from LREE depleted to slightly LREE enriched [chondrite normalized (La/Yb)N– 0.1–5.4] with a negative Eu anomaly. They are depleted in incompatible elements (e.g. Sr, Ba, U, Th) with respect to the upper crust. In contrast non-diamondiferous rocks of the Zerenda series exhibit normal crustal geochemistry. All rocks of the Zerenda series have very radiogenic lead isotopes. The measured μ values (238U/204Pb) compared with those calculated for the interval between crust formation and ultrahigh pressure (UHP) metamorphism suggest a decrease by factors of up to 200 during the UHP metamorphism. The Sm-Nd mineral isochrons from the diamond-bearing rocks and other rock types of the Zerenda series give a Middle Cambrian (524–535 Ma) age of metamorphism. The Nd model ages show that crust formation occurred about 2.3 Ga ago. Significant fractionation of Sm and Nd and loss of incompatible elements may be due to partial melting of the protoliths. The Ar-Ar age determinations of secondary biotite and muscovite from the diamond-bearing rocks yield an age of 517 ± 5 Ma. This cooling age requires a short time interval between UHP metamorphism and uplift to a crustal level. Ultrahigh pressure metamorphism might be a significant source of Pb for the mantle. We propose that the radiogenic Pb of the oceanic array is the contamination traces of numerous UHP events. Beside the geological aspect we demonstrate a method of dating a high grade metamorphic terrain using Nd isotopes. We compare whole rock isochrons and mineral isochrons and in this way get some insight into the behaviour of the Sm-Nd system during very high grade metamorphic events.

Microdiamonds from garnet clinopyroxenites of the Kokchetav massif (northern Kazakhstan) and associated alluvial diamonds have been investigated using infrared spectroscopy. All diamonds have high nitrogen contents (from 747 up to 4488 ± 20% at. ppm) and are of type Ib-IaA. Nitrogen data are consistent with a diamond formation over a narrow temperature range. Microdiamonds from garnet clinopyroxenites are characterized by water and carbonate inclusions, similar to those ocurring in fibrous coated diamonds from kimberlite, which suggest that diamonds grew from a C-H-O fluid. In contrast, alluvial microdiamonds do not contain any carbonate inclusions and H2O is absent or present in only minor amounts. Alluvial diamonds are considerably more N-rich but have a similar range of nitrogen aggregation states. It is suggested that alluvial diamonds belong to a separate diamond population compared with the microdiamonds from garnet clinopyroxenites. The Kokchetav microdiamonds are distinct from kimberlitic fibrous diamonds in that they contain abundant Ib centres, have high nitrogen contents and show different populations depending on host rock type. These characteristics support a metamorphic origin for the microdiamonds from the UHPM rocks of the Kokchetav massif.

Representative diamond-bearing gneisses and dolomitic marble, eclogite and Ti-clinohumite-bearing garnet peridotite from Unit I at Kumdy Kol and whiteschist from Unit II at Kulet, eastern Kokchetav Massif, northern Kazakhstan, were studied. Diamond-bearing gneisses contain variable assemblages, including Grt+Bt+Qtz±Pl±Kfs±Zo±Chl±Tur±Cal and minor Ap, Rt and Zrn; abundant inclusions of diamond, graphite+chlorite (or calcite), phengite, clinopyroxene, K-feldspar, biotite, rutile, titanite, calcite and zircon occur in garnet. Diamond-bearing dolomitic marbles consist of Dol+Di±Grt+Phl; inclusions of diamond, dolomite±graphite, biotite, and clinopyroxene were identified in garnet. Whiteschists carry the assemblage Ky+Tlc+Grt+Rt; garnet shows compositional zoning, and contains abundant inclusions of talc, kyanite and rutile with minor phlogopite, chlorite, margarite and zoisite. Inclusions and zoning patterns of garnet delineate the prograde P–T path. Inclusions of quartz pseudomorphs after coesite were identified in garnet from both eclogite and gneiss. Other ultrahigh-pressure (UHP) indicators include Na-bearing garnet (up to 0.14 wt% Na2O) with omphacitic Cpx in eclogite, occurrence of high-K diopside (up to 1.56 wt% K2O) and phlogopite in diamond-bearing dolomitic marble, and Cr-bearing kyanite in whiteschist. These UHP rocks exhibit at least three stages of metamorphic recrystallization. The Fe-Mg partitioning between clinopyroxene and garnet yields a peak temperature of 800–1000 °C at P >40 kbar for diamond-bearing rocks, and about 740–780 °C at >28–35 kbar for eclogite, whiteschist and Ti-bearing garnet peridotite. The formation of symplectitic plagioclase+amphibole after clinopyroxene, and replacement of garnet by biotite, amphibole, or plagioclase mark retrograde amphibolite facies recrystallization at 650–680 °C and pressure less than about 10 kbar. The exsolution of calcite from dolomite, and development of matrix chlorite and actinolite imply an even lower grade greenschist facies overprint at c. 420 °C and 2–3 kbar. A clockwise P–T path suggests that supracrustal sediments together with basaltic and ultramafic lenses apparently were subjected to UHP subduction-zone metamorphism within the diamond stability field. Tectonic mixing may have occurred prior to UHP metamorphism at mantle depths. During subsequent exhumation and juxtaposition of many other tectonic units, intense deformation chaotically mixed and mylonitized these lithotectonic assemblages.

The Maksyutov metamorphic complex is the first locality where coesite pseudomorphs in garnet were described. The importance of this discovery was not understood until ultrahigh-pressure (UHP) metamorphism was independently recognized in the Dora Maira Massif of the western Alps and the Western Gneiss Region of Norway. The coesite pseudomorphs are significant because they suggest that the lower unit of the Maksyutov complex probably underwent UHP metamorphism at depths greater than 80 km in a paleosubduction zone. The Maksyutov complex, situated in the southern Ural Mountains of Russia, forms an elongate N-S belt along the boundary between the European and Russian plates. The complex contains two superimposed tectonic unitsa lower eclogite-bearing schist unit that underwent high-pressure (HP) to UHP metamorphism and an upper meta-ophiolite unit subjected to blueschist/greenschist-facies metamorphism. The lower unit lithologies range from quartzofeldspathic, to graphite-rich, to mafic-ultramafic composition...

The Kokchetav Complex is a tectonic mega-melange consisting of seven pre-Ordovician units (units I-VII) of contrasting lithologies and P–T conditions of metamorphism, overlain and/or intruded by four post-recrystallization entities. Most of the constituent rock types display affinities with continental crust; paraschists and paragneisses, which carry biogenically produced carbon, clearly were laid down near the surface of the Earth. Microdiamond (and rare coesite) inclusions are contained in strong, refractory garnet, zircon, clinopyroxene, and kyanite, some of the constituent neoblastic phases of this metasedimentary unit. Systematic mineral parageneses and textural relationships support the hypothesis that the metamorphic assemblages represent a close approach to chemical equilibrium at the time of formation. Metamorphism of diamond-bearing paragneisses and schists transpired at 535 ± 5 Ma; physical conditions included minimum pressures of 40 kbar and temperatures exceeding 900 °C. Other associated units contain mineralogic evidence of somewhat lower to considerably lower pressures and temperatures: observed magnesite + diopside pairs, coesite, grossular-pyropic garnet, potassic clinopyroxene, Si-rich phengite, barroisite-crossite(?), aluminous titanite and/or Al-rutile, and the assemblage talc + kyanite + garnet all testify to relatively elevated pressures of formation. The metamorphosed lithotectonic units represent individual, discrete stages in what initially may have been a continuous P-T series, but intense post-metamorphic dislocation has resulted in the preservation of a chaotically mixed sequence rather than an unbroken gradation in preserved conditions of metamorphism. Only units I-III, and probably VIb may represent portions of a dismembered subduction zone lithologie assemblage. The uplift to mid-crustal levels and cooling of the mega-melange took place by about 515–517 Ma, at which time the complex was stabilized as a part of the Kazakhstan microcontinental collage. An hypothesized Late Vendian-Early Cambrian subduction of the Kazakhstan-North Tianshan(?) microcontinental salient to depths exceeding 125 km, followed by decoupling from the descending oceanic crust-capped lithospheric plate is held responsible for the ultrahigh-pressure metamorphism of the Kokchetav Complex. Inasmuch as vestiges of a calc-alkaline volcanic/plutonic arc of approximately Early Cambrian age are preserved as only scattered relics in the general region, the plate-tectonic setting may have involved an intra-oceanic, Marianas-type, incipient arc which was subsequently removed through transform faulting or erosion.

The Kokchetav and Dabie Shan complexes are typical examples of ultrahigh-pressure metamorphic complexes (UHPM) and are important units of the largest suture zones within the Eurasian continent. The...

Garnet crystals from diamondiferous high-pressure metamorphic rocks of the Kokchetav Massiv, Kazakhstan, were studied by electron microprobe and single crystal microscope-spectrometry in the UV/VIS and IR spectral ranges. The crystals are quaternary solid solutions Gross 48 Pyr 33 Alm 16 Spess 2 , (I.) from garnet-pyroxene-carbonate rock K7, and Gross 21 Pyr 24 Alm 49 Spess 5 , (II.) from garnet-biotite-gneiss, both with small TiO 2 -contents (0.3 wt%). Their yellow (I.) or deep-red colour (II.) is caused by the position of the UV absorption edge and a broad Fe 2+[8] Ti 4+[6] charge-transfer band centered at 21 500 cm −1 . For comparison, a high-pressure garnet Py26, Py 70 Alm 17 Uv 13 (III.), from Liaoning-50 kimberlite, NE China, was included in the IR and microprobe study. All crystals studied contained defect OH − -groups, corresponding to those in synthetic OH − -bearing pyropes, giving rise to a single, relatively sharp absorption band (Δv 1/2 ≃ 100 cm −1 ) in the range 3560 to 3610 cm −1 . The exact energy position depends on the type and site fractions of ions in the OH − -coordinating metal sites, A [8] and B [6] . This hydroxyl «water» seems to be not uniformly distributed in the garnet crystals and ranges from 0.02 to 0.25 wt% (wt% calculated from integral intensities using e i,Gross ). In addition to ν OH , the IR spectra show a broad band at 3400 cm −1 , characteristic of (H 2 O) n -clusters. There seems to be a constant ratio α i,OH− /(α i,OH− + α i,H2O ) for the various spots on the different crystals I, II, and III