Kokchetav Massif Research Articles

US-Russian Civilian Research and Development Fund Project: An origin of microdiamonds in metamorphic rocks of the Kokchetav Massif, Northern Kazakhstan

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.

A comparison of the evolution of diamondiferous quartz-rich rocks from the Saxonian Erzgebirge and the Kokchetav Massif: are so-called diamondiferous gneisses magmatic rocks?

The petrography and chemical composition of minerals of quartz-rich diamondiferous rocks from the Kokchetav Massif, especially the zonation of garnet, were studied and compared with diamondiferous quartzofeldspathic rocks from the Saxonian Erzgebirge. Many compositional and textural features were found to be similar. For instance, microdiamonds are enclosed systematically in a specific intermediate growth zone of garnet in these rocks. On the basis of experimental data, a magmatic scenario was constructed to check if the quartz-rich diamondiferous rocks are of magmatic origin. By this, the P– T paths, derived here for the Kokchetav rocks, and the textural observations it is concluded that the minerals of the diamondiferous rocks have crystallized from silicate melts. These melts originated by anatexis of deeply submerged metasediments (Erzgebirge: at T as high as 1200°C, Kokchetav Massif: at 50–100°C lower T) and ascended from at least 200 km depth. Relics of the pre-anatectic evolution are still present, for instance, as garnet cores. After ascent and emplacement of the magma in deep portions of thickened continental crust (Kokchetav Massif: 45–50 km close to 800°C, Erzgebirge: 55–60 km at 30–50°C lower T) considerable quantities of (white and/or dark) micas formed by peritectic reactions from melt. For instance, garnets could be resorbed at this stage and biotite grew instead. After the magmatic stage, retrogression took place much stronger in the Kokchetav Massif. This was accompanied by deformation transforming broadly the magmatic texture of quartz-rich diamondiferous rocks from the Kokchetav Massif to a gneissic texture.

Ultrarapid exhumation of ultrahigh-pressure diamond-bearing metasedimentary rocks of the Kokchetav Massif, Kazakhstan?

The diamond-bearing, ultrahigh-pressure Kokchetav Massif recrystallized at eclogite-facies conditions deep in the mantle at 180-km depth at 535±3 Ma, and yet new 40Ar/ 39Ar ages suggest that it may have been exhumed to crustal depths (as indicated by closure of mica to Ar loss) by ∼529 Ma. These data indicate a possible exhumation rate of tens of kilometers per million years, i.e., a rate that is comparable to rates of horizontal plate motion and subduction.

Oxygen, carbon, and strontium isotope geochemistry of diamond-bearing carbonate rocks from Kumdy-Kol, Kokchetav Massif, Kazakhstan

Diamond-bearing carbonate rocks from Kumdy-Kol, Kokchetav massif, Kazakhstan, were strongly altered by fluids flowing through fractures and infiltrating along grain boundaries during exhumation. Alteration includes retrogradation of high-grade silicate assemblages by hydrous minerals, replacement of diamond by graphite and of dolomite by calcite. Diamond-bearing carbonate rocks are among the most intensely altered isotopically with δ 18O VSMOW values as low as +9‰, δ 13C VPDB=−9‰, and 87Sr/ 86Sr as high as 0.8050. Evidence of isotopic equilibration between coexisting dolomite and high-Mg calcite during ultrahigh-pressure metamorphism (UHPM) is preserved only rarely in samples isolated from infiltrating fluids by distance from fractures. Isotopic heterogeneity and isotopic disequilibrium are widespread on a hand-specimen scale. Because of this lack of homogeneity, bulk analyses cannot provide definitive measurements of 13C/ 12C fractionation between coexisting diamond and carbonate. Our study adequately documents alteration on a scale commensurate with observed vein structures. But, testing the hypothesis of metamorphic origin of microdiamonds has not fully succeeded because our analytical spatial resolution, limited to 0.5 mm, is not small enough to measure individual dolomite inclusions or individual diamond crystals.

Two stage growth of microdiamond in UHP dolomite marble from Kokchetav Massif, Kazakhstan

AbstractThe abundance and morphology of microdiamond in dolomite marble from Kumdy‐kol in the Kokchetav Massif, are unusual; a previous study estimated the maximum content of diamonds in dolomite marble to be about 2700 carat ton−1. Microdiamond is included primarily in garnet, and occasionally in diopside and phlogopite pseudomorphs after garnet. They are classified into three types on the basis of their morphology: (1) S‐type: star‐shaped diamond consisting of translucent cores and transparent subhedral to euhedral very fine‐grained outer parts; (2) R‐type: translucent crystals with rugged surfaces; and (3) T‐type: transparent, very fine‐grained crystals. The S‐type is the most abundant.Micro‐Laue diffraction using a 1.6‐µm X‐ray beam‐size demonstrated that the cores of the star‐shaped microdiamond represent single crystals. In contrast, the most fine‐grained outer parts usually have different orientations compared to the core. Laser–Raman studies indicate that the FWHM (Full Width at Half Maximum) of the Raman band of the core of the S‐type diamond is slightly larger than that for the outer parts. Differences in morphology, crystal orientations, and in the FWHM of the Raman band between the core and the fine‐grained outer‐parts of S‐type microdiamond suggest that the star‐shaped microdiamond was formed discontinuously in two distinct stages.

Low δ18O eclogites from the Kokchetav massif, northern Kazakhstan

AbstractOxygen isotopic compositions of silicates in eclogites and whiteschists from the Kokchetav massif were analyzed by whole‐grain CO2‐laser fluorination methods. Systematic analyses yield extremely low δ18O for eclogites, as low as −3.9‰ for garnet; these values are comparable with those reported for the Dabie‐Sulu UHP eclogites. Oxygen isotopic compositions are heterogeneous in samples of eclogite, even on an outcrop scale. Schists have rather uniform oxygen isotope values compared to eclogites, and low δ18O is not observed. Isotope thermometry indicates that both eclogites and schists achieved high‐temperature isotopic equilibration at 500–800 °C. This implies that retrograde metamorphic recrystallization barely modified the peak‐metamorphic oxygen isotopic signatures. A possible geological environment to account for the low‐δ18O basaltic protolith is a continental rift, most likely subjected to the conditions of a cold climate. After the basalt interacted with low δ18O meteoric water, it was tectonically inserted into the surrounding sedimentary units prior to, or during subduction and UHP metamorphism.

Metal-sulfur-COH-silicate fluid mediated diamond nucleation in Kokchetav ultrahigh-pressure gneiss

Submicron metal sulfides, randomly oriented microdiamonds and phlogopite have been identified as multiphase inclusions in garnet of a garnet-clinopyroxene-quartz crustal rock from the Kokchetav Massif in Kazakhstan. Analytical electron microscopy shows that metal sulfides are entrapped among diamond aggregates away from infiltration cracks and do not exhibit any specific crystallographic relationship with associated microdiamonds, thus implying a possible syngenetic nucleation of microdiamonds from a precursive fluid containing dissolved metal sulfides. The diamonds increased in size via a spiral growth mechanism, as manifested by dislocations radiating from nuclei toward faceted surface outcrops.

Crystallization environment of Kazakhstan microdiamond: evidence from nanometric inclusions and mineral associations

AbstractNanometric solid inclusions in diamond incorporated in garnet and zircon from felsic gneiss of the Kokchetav massif, Kazakhstan, have been examined utilizing electron microscopy and focused ion beam techniques. Host garnet and zircon contain numerous pockets of multiple inclusions, which consist of 1–3 diamond crystals intergrown with quartz, phengite, phlogopite, albite, K‐feldspar, rutile, apatite, titanite, biotite, chlorite and graphite in various combinations. Recalculation of the average chemical composition of the entrapped fluid represented by multiple inclusion pockets indicates that such fluid contained a low wt% of SiO2, suggesting a relatively low‐temperature fluid rather than a melt. Transmission electron microscopy revealed that the diamond contains abundant nanocrystalline inclusions of oxides, rare carbonates and silicates. Within the 15 diamond crystals studied, abundant inclusions were found of SiO2, TiO2, FexOy, Cr2O3, ZrSiO4, and single grains of ThxOy, BaSO4, MgCO3, FeCr2O4 and a stoichiometric Fe‐rich pyroxene. The diversity of trace elements within inclusions of essentially the same stoichiometry suggests that the Kokchetav diamond crystallized from a fluid containing variable amounts of Si, Fe, Ti, Cr, Zr, Ba, Mg and Th and other minor components such as K, Na, P, S, Pb, Zn, Nb, Al, Ca, Cl. Most of the components in crystals included in diamond appear to have their origin in the subducted metasediments, but some of them probably originate from the mantle. It is concluded that Kokchetav diamond most likely crystallized from a COH‐rich multicomponent supercritical fluid at a relatively low temperature (hence the apparently low content of rock‐forming elements), and that the diversity of major and minor components suggests interactions between subducted metasediments and mantle components.

The Diamond-Bearing Kokchetav Massif, Kazakhstan: Petrochemistry and Tectonic Evolution of a Unique Ultrahigh-Pressure Metamorphic Terrain.: C.D. Parkinson, I. Katayama, J. Liou & S. Maruyama, eds. Frontiers Science Series No. 38, Universal Academy Press Inc., Tokyo, Japan. US$70. ISBN 4-946443-78-9.

It may surprise many geoscientists that some of the metamorphic rocks comprising parts of the Kokchetav Massif in Kazakhstan are, in terms carats/tonne, the most diamondiferous rocks yet recognized. Unfortunately, although the diamond crystals are abundant, they are exceedingly small, typically 5–

Reply to comments by Y. Zhu: K-feldspar in clinopyroxene from Grt-Cpx silicate rocks of the Kokchetav Massif

Reply to comments by Y. Zhu: K-feldspar in clinopyroxene from Grt-Cpx silicate rocks of the Kokchetav Massif

Carbon recycled into deep Earth: Evidence from dolomite dissociation in subduction-zone rocks: Comment and Reply

[Zhu and Ogasawara (2002)][1] interpret the textures and mineralogy of dolomitic marbles from the Kokchetav massif in terms of pressure-induced dissociation of dolomite (dol) to aragonite (ara) + magnesite (mag). On the basis of this assumption they postulate a minimum metamorphic pressure of 7.85

Mineral inclusions in zircon from diamond-bearing marble in the Kokchetav massif, northern Kazakhstan

Mineral inclusions in zircon from diamond-bearing marble in the Kokchetav massif, northern Kazakhstan

Mineral inclusions in zircon from diamond-bearing marble in the Kokchetav massif, northern Kazakhstan

In order to constrain the metamorphic evolution of deeply subducted impure marble from the Kokchetav massif, we investigated mineral inclusions in zircon, which is known to protect ultrahigh-pressure (UHP) phases from late-stage overprinting. Consequently, diamond, coesite, diopside, garnet, phlogopite, calcite, dolomite, graphite and apatite were identified as inclusion in the zircons. Silica phases were absent in the matrix, indicating they were completely consumed by the following prograde reaction: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(Dolomite {+} 2 SiO\_{2} = Diopside {+} 2 CO\_{2}\) \end{document}. The relict of coesite inclusions in zircon indicates that prograde P-T trajectory crossed cut the quartz-coesite transition before the decarbonate reaction. Matrix diopsides contain abundant exsolved phengite lamellae, whereas the exsolution is absent in the inclusions in zircon. The diopside inclusions contain higher amounts of K2O (up to 0.56 wt%) and CaEskola component (up to 3.5 mol%) than those of the matrix (0.14 wt% and 2.1 mol%, respectively). Thus, phengite exsolution occurs in matrix pyroxene with low K2O and CaEskola components compared to inclusions in zircon. This observation indicates that decreasing K2O and CaEskola components during decompression resulted in the phengite exsolution in diopside. A trace of hydroxyl in phengite needles was perhaps initially incorporated within the precursor clinopyroxene under high-pressure conditions. The peak metamorphic P-T conditions of the impure marble are estimated to 60-80 kbar and 960-1050°C, derived from K2O solubility in diopside buffered by phlogopite and from the garnet-clinopyroxene geothermometer.

Garnet-biotite-clinozoisite gneiss: a new type of diamondiferous metamorphic rock from the Kokchetav Massif

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.

Phlogopite and Coesite Exsolution from Super-Silicic Clinopyroxene

We document the exsolution of phlogopite and coesite/quartz from pre-existing super-silicic clinopyroxene in dolomite marble from the Kokchetav massif, northern Kazakhstan. The exsolution texture was formed by clinopyroxene decomposition through the reaction (3enstatite + 2KalSi2O6)cpx = phlogopite + 4coesite. Phlogopite exsolution must have occurred at pressures less than 8.0 GPa (at 1000°C), where the garnet + super-silicic clinopyroxene + phlogopite assemblage was stable, based on experimentally defined phase relations. Observations described in this report suggest that the precursor clinopyroxene was stable at pressures higher than 8 GPa (>1000°C), implying that the dolomite marble was subducted to mantle depths greater than 240 km. Such pro-found subduction could be an important mechanism to transport abundant H2O and potassium into the deep Earth.

The mineralogy of the Kokchetav ‘lamproite’: implications for the magma evolution

Kokchetav ‘lamproite’ occurs in the east end of Kokchetav massif and consists of phenocryst (mainly clinopyroxene) and matrix (mainly feldspar). The compositions of clinopyroxene, magnetite and biotite phenocryst were determined using wavelength dispersive spectrometry on a JEOL Super-probe 8900 electron microprobe for the purpose of revealing the process of magma evolution. Analyses revealed a core–rim variation, which is consistent with three stages of magmatic evolution: Mg-rich clinopyroxene cores (diopside) and biotite cores (phlogopite) crystallized in a deep magma chamber (stage I); Fe-rich clinopyroxene rim (salite) and biotite rim crystallized at low pressure in a shallow magma chamber (stage II); Magnetite phenocryst core also crystallized in a shallow magma chamber, and co-exists with Fe-rich clinopyroxene rim and biotite rim. The magnetite rims probably formed during magma eruption at the same time when groundmass crystallized (stage III). The calculated temperatures for ilmenite–magnetite pair range from 679 to 887°C, log fO 2 values range from −11.1 to −14.9 log units. These values represent the latest conditions of magma as ilmenite exsolution in magnetite probably occurred during magma eruption from the shallow chamber to surface.

Coesite exsolution from supersilicic titanite in UHP marble from the Kokchetav Massif, northern Kazakhstan

Coesite exsolved from supersilicic titanite was discovered in an impure calcite marble at Kumdykol in the Kokchetav UHP (ultrahigh-pressure) metamorphic terrane, northern Kazakhstan. This impure marble consists mainly of calcite, K-feldspar, diopside, and symplectites of diopside + zoisite, with minor amount of titanite, phengite, and garnet. No diamond was found in the marble. Coesite and quartz, which have needle or platy shapes measuring about 20-60 mm in length, occur as major exsolved phases in the cores and mantles of titanite crystals with minor calcite and apatite. The strongest Raman band for the coesite needles and plates was confirmed at about 524 cm -1 with a weak band at about 271 cm -1 . To estimate the initial composition of the titanite before coesite exsolution, exsolved phases were reintegrated by measuring their area fractions on digital images. The highest excess Si in titanite was thus determined to be 0.145 atoms per formula unit (apfu). This composition requires a pressure higher than 6 GPa on the basis of phase relations in the system CaTiSiO 5 -CaSi 2 O 5 . This pressure is consistent with other evidence of high pressure in the same marble, such as 1.4-1.8 wt% K 2 O and over 1000 ppm H 2 O in diopside. Supersilicic titanite and coesite exsolution also indicate that SiO 2 exsolution occurred in the coesite stability field during exhumation of the UHP metamorphic unit.

The origin and formation of metamorphic microdiamonds from the Kokchetav massif, Kazakhstan: a nitrogen and carbon isotopic study

This study reports δ 13C, δ 15N 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 δ 13C-values ranging from −15.92‰ to −10.57‰, δ 15N 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 δ 13C, +5.9‰ for δ 15N 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.

Ion micro-probe U–Pb zircon geochronology of peak and retrograde stages of ultrahigh-pressure metamorphic rocks from the Kokchetav massif, northern Kazakhstan

Sensitive high resolution ion micro-probe (SHRIMP) U–Pb analyses of zoned zircons were performed to constrain the age and geodynamic significance of the Kokchetav ultrahigh-pressure (UHP) metamorphic rocks, which recrystallized at P>60 kbar and T>1000°C. Cathodoluminescence images were correlated with micro-assemblages of mineral inclusions in zircons from diamond-bearing gneisses. These revealed UHP mineral-bearing cores and low-pressure mineral-bearing rims, some zircons also retain inherited cores. Our SHRIMP data identified four discrete stages: Middle Proterozoic protolith age, 537±9 Ma for UHP metamorphic conditions, 507±8 Ma for the late-stage amphibolite facies overprint, and 456–461 Ma for post-orogenic thermal events. Thus, we conclude that Middle Proterozoic supracrustal protoliths of the Kokchetav UHP–HP rocks were subducted to mantle depths in the Middle Cambrian, and exhumed to mid-crustal levels in the Late Cambrian. The exhumation rate calculated from the SHRIMP data and relative P– T conditions is approximately 5 km/Ma. The rapid exhumation rate for the massif is in the same range as other UHP metamorphic terranes, and suggests that slab decoupling may be a driving force for exhumation of UHP metamorphic rocks from mantle depths.

Multiple zircon growth during fast exhumation of diamondiferous, deeply subducted continental crust (Kokchetav Massif, Kazakhstan)

Diamondiferous rocks from the Kokchetav Massif, Kazakhstan, represent deeply subducted continental crust. In order to constrain the age of ultra high pressure (UHP) metamorphism and subsequent retrogression during exhumation, zircons from diamondiferous gneisses and metacarbonates have been investigated by a combined petrological and isotopic study. Four different zircon domains were distinguished on the basis of transmitted light microscopy, cathodoluminescence, trace element contents and mineral inclusions. Mineral inclusions and trace element characteristics of the zircon domains permit us to relate zircon growth to metamorphic conditions. Domain 1 consists of rounded cores and lacks evidence of UHP metamorphism. Domain 2 contains diamond, coesite, omphacite and titanian phengite inclusions providing evidence that it formed at UHP metamorphic conditions (P>43 kbar; T~950 °C). Domain 3 is characterised by low-pressure mineral inclusions such as garnet, biotite and plagioclase, which are common minerals in the granulite-facies overprint of the gneisses (P~10 kbar; T~800 °C). This multi-stage zircon growth during cooling and exhumation of the diamondiferous rocks can be best explained by zircon growth from Zr-saturated partial melts present in the gneisses. Domain 4 forms idiomorphic overgrowths and the rare earth element pattern indicates that it formed without coexisting garnet, most probably at amphibolite-facies conditions (P~5 kbar; T~600 °C). The metamorphic zircon domains were dated by SHRIMP ion microprobe and yielded ages of 527±5, 528±8 and 526±5 Ma for domains 2, 3 and 4 respectively. These indistinguishable ages provide evidence for a fast exhumation beyond the resolution of SHRIMP dating. The mean age of all zircons formed between UHP metamorphic conditions and granulite-facies metamorphism is 528±3 Ma, indicating that decompression took place in less than 6 Ma. Hence, the deeply subducted continental crust was exhumed from mantle depth to the base of the crust at rates higher than 1.8 cm/year. We propose a two-stage exhumation model to explain the obtained P–T–t path. Fast exhumation on top of the subducted slab from depth >140 to ~35 km was driven by buoyancy and facilitated by the presence of partial melts. A period of near isobaric cooling was followed by a second decompression event probably related to extension in a late stage of continental collision.