N. P. Pokhilenko

Trace elements in garnets and chromites: Diamond formation in the Siberian lithosphere

Proton-microprobe analyses of trace elements in garnet and chromite inclusions in diamonds (DI) from the Mir, Udachnaya, Aikhal and Sytykanskaya kimberlites in Yakutia, CIS, provide new insights into the processes that form diamond. Equivalent data on garnet and chromite concentrates from these pipes yield information on the thermal state and chemical stratification of the Siberian lithosphere. Peridotite-suite diamonds from Yakutia have formed over a temperature interval of ca. 600°C, as measured by Ni and Zn thermometry on garnet and chromite inclusions in diamonds. Individual diamonds contain inclusions recording temperature intervals of >400°C; ranges of >100°C are common. Diamond formation followed a severe depletion event(s), and a separate enrichment in Sr. Comparison of temperatures on DI garnet and spinel with temperatures derived from diamondiferous harzburgites, exposed inclusions in boart and concentrate minerals suggests that the diamond-containing part of the lithosphere has cooled significantly since the Siberian diamonds crystallized. The peridotite-suite diamonds probably formed mainly in response to one or more relatively short-lived thermal events, related to magmatic intrusion. The northern part of the Daldyn-Alakit district may have had a typical cratonic geotherm at the time of diamond formation, and during kimberlite intrusion. The southern part of the district, and the Malo-Botuobiya kimberlite field, probably had a relatively low geotherm (ca. 35 mW/m2). The vertical distribution of garnet and chromite types indicates that the mantle above 120 km depth is dominated by lherzolites, whereas the deeper parts of the lithosphere are a mixture of lherzolites and more depleted harzburgites and dunites.

Melt inclusions in olivine phenocrysts in unaltered kimberlites from the Udachnaya-East pipe, Yakutia: Some aspects of kimberlite magma evolution during late crystallization stages

The results of a complex study of melt inclusions in olivine phenocrysts contained in unaltered kimberlites from the Udachnaya-East pipe indicate that the inclusions were captured late during the magmatic stage, perhaps, under a pressure of <1 kbar and a temperature of ≤800°C. The inclusions consist of fine crystalline aggregates (carbonates + sulfates + chlorides) + gas ± crystalline phases. Minerals identified among the transparent daughter phases of the inclusions are silicates (tetraferriphlogopite, olivine, humite or clinohumite, diopside, and monticellite), carbonates (calcite, dolomite, siderite, northupite, and Na-Ca carbonates), Na and K chlorides, and alkali sulfates. The ore phases are magnetite, djerfisherite, and monosulfide solid solution. The inclusions are derivatives of the kimberlite melt. The complex silicate-carbonate-salt composition of the secondary melt inclusions in olivine from the kimberlite suggests that the composition of the kimberlite melt near the surface differed from that of the initial melt composition in having higher contents of CaO, FeO, alkalis, and volatiles (CO2, H2O, F, Cl, and S) at lower concentrations of SiO2, MgO, Al2O3, Cr2O3, and TiO2. Hence, when crystallizing, the kimberlite melt evolved toward carbonatite compositions. The last derivatives of the kimberlite melt had an alkaline carbonatite composition.

Jadeite in Chelyabinsk meteorite and the nature of an impact event on its parent body

The Chelyabinsk asteroid impact is the second largest asteroid airburst in our recorded history. To prepare for a potential threat from asteroid impacts, it is important to understand the nature and formational history of Near-Earth Objects (NEOs) like Chelyabinsk asteroid. In orbital evolution of an asteroid, collision with other asteroids is a key process. Here, we show the existence of a high-pressure mineral jadeite in shock-melt veins of Chelyabinsk meteorite. Based on the mineral assemblage and calculated solidification time of the shock-melt veins, the equilibrium shock pressure and its duration were estimated to be at least 3–12 GPa and longer than 70 ms, respectively. This suggests that an impactor larger than 0.15–0.19 km in diameter collided with the Chelyabinsk parent body at a speed of at least 0.4–1.5 km/s. This impact might have separated the Chelyabinsk asteroid from its parent body and delivered it to the Earth.

Melting of kimberlite of the Udachnaya-East pipe: Experimental study at 3–6.5 GPa and 900–1500°C

200 Kimberlites are the products of crystallization of the deepest magmas generated in the Earth’s mantle (below 150 km) and are the main sources of natural diamonds. In spite of significant progress in under� standing kimberlite petrogenesis, many problems, such as reconstruction of the compositions of primary kimberlite melts and their evolution during intrusion, are still debatable. The main problem is that the com� position of kimberlites does not correspond to the composition of parental melts. First, kimberlites are contaminated by a significant portion of xenogenic material represented by xenoliths and their fragments (xenocrysts). Second, most kimberlites worldwide underwent postmagmatic alterations to various

Diamond crystallization in the Fe-Co-S-C and Fe-Ni-S-C systems and the role of sulfide-metal melts in the genesis of diamond

Diamond crystals 0.1–0.8 carats were synthesized in experiments conducted in a BARS split-sphere multianvil high-pressure apparatus in the systems Fe-Co-S-C and Fe-Ni-S-C at a pressure of 5.5 GPa and temperature of 1300°C. The microtextures of the samples and the phases accompanying diamond (carbides, graphite, monoslufide solid solution, pentlandite, and taenite) are examined in much detail, the properties of metal-sulfide-carbon alloys are discussed, and issues related to the genesis of sulfide inclusions in diamonds and graphite crystallization in the diamond stability field are considered. The experiments demonstrate that diamonds can be synthesized and grow in pre-eutectic metal-sulfide melts with up to 14 wt % sulfur at relatively low P-T parameters, which correspond to the probable temperatures and pressures of natural diamond-forming processes at depths of approximately 150 km in the Earth’s upper mantle.

Paleozoic U-Pb age of rutile inclusions in diamonds of the V–VII variety from placers of the northeast Siberian platform

1151 Diamonds attributed according to the classifica tion of Yu.L. Orlov to the V and VII varieties are wide spread in rich placers of the northeastern part of the Siberian Platform [6]. Mineralogical investigation (certification) of these diamonds showed that they represent a unified genetical type subdivided by Orlov according to the following formal characteristics: dia monds of the V variety are represented by monocrys tals, and diamonds of the VII variety are represented by intergrowths. Thus, we unite them into one variety V–VII [2]. Diamonds of this joint variety are easily distinguished among other diamonds from placers of the northeastern part of the Siberian platform for the following reasons: (1) these diamonds are represented by crystals of octahedral habitus (rarely) or their inter growths; (2) they occur in the forms of dissolution of dodecahedroids and intermediate forms between octahedrons and dodecahedroids (often); (3) crystals are overfilled with black flaky inclusions (graphite along the walls of flattened vacuoles with fluids; (4) the surface of crystals is covered by scars and fissures of leaching along the vacuoles brought out into the open surface during dissolution (Fig. 1a); (5) these dia monds are characterized by increased size (crystals less than 1 mm are practically absent); (6) lightened isotope composition of carbon, δ13С varies from –19 to –25‰, which corresponds to the characteristics of crustal carbon of organic origin established for a sig nificant part of diamonds from eclogite paragenesis [10]; (7) extremely high structural admixture of nitro gen mostly in the A form (up to 1200 ppm); (8) abundance of fluid inclusions (mostly such compo nents as СО2, Н2О, N); (9) reduced density (3.500– 3.508 g/cm3, diamonds of the I variety show density values about 3.51543 g/cm3). Eclogite paragenesis of these diamonds is proved by occurrences of coesite inclusions [7]. Such diamonds are absent in the known kimberlites and lamproites of different ages in the Siberian platform and worldwide.

Synthesis of heavy hydrocarbons under P-T conditions of the Earth’s upper mantle

32 The Earth’s mantle, especially the upper mantle, is highly stratified according to the oxidation–reduction conditions [1]. The main trend of variations in physi cochemical conditions recorded in the mantle is a decrease in oxygen fugacity with depth. According to recent concepts, at a depth of 150 km the zone of sta bility of the oxidized form of carbon (carbonates) is followed by a stability zone of elemental carbon forms (graphite/diamond) [2]. At a depth of approximately 250 km, the oxida tion–reduction conditions of the upper mantle corre spond to the stability of metallic iron [3, 4]. Given these data, in the framework of the conceptual model of the global carbon cycle [5], the probability of decomposition of carbonates entering into the mantle in the subduction zones with formation of solid carbon phases (graphite/diamond) and volatile hydrocarbon compounds is of interest. The formation of the latter is especially acute for scientists considering the possibility of their abiotic origin. During experimental studies [6–9], attempts were made to synthesize hydrocarbons from СаСО3 in aqueous media under the P–T conditions of the Earth’s upper mantle. As reducing agents, FeO or metallic Fe were used. Experiments were carried out using different types of high pressure apparatuses. In all cases, СаСО3 was decomposed. In the gas phase, СН4 was recorded. In addition, a mixture of hydrocarbons corresponding to the hydrocarbon portion of natural gas in chemical composition was produced [7, 9]. However, heavy hydrocarbons (HHCs), components of crude oil, were not synthesized at the P–T conditions of the Earth’s upper mantle. This work provides the first results of HHC synthe sis from the carbonate material at high P–T parame ters. A series of three experiments was carried out using the multi anvil high pressure split sphere appa ratus (BARS) in a working cell made of ZrO2 and MgO with a tubular graphite heater under the following mode: pressure buildup, sample heating, and cooling and pressure release after holding under the given P–T conditions. The research procedure is given in works [9, 10]. Experiments were carried out in Pt ampoules sealed by welding with an electric arc in the open air. The first experiment (no. 4 6 13) was carried out for 17 hours in the following system: СаCO3 (140.3 mg)–Ca(OH)2 (10.1 mg)–Femetal (10.5 mg) at P = 4.5 GPa and T = 1600°C. The sample cooling was performed by interruption of the power supply to the high pressure apparatus. The second experiment (no. 4 8 13) was carried out for 5 hours in the system MgCO3 (15.4 mg)– Ca(OH)2 (14.3 mg)–Femetal (51.9 mg)–SiO2 (20.8 mg) at P = 4.0 GPa and T = 1400°C. The sample was cooled to room temperature under high pressure for 14 hours. In this experiment, the Pt ampoule was coated with 0.05 mm W foil (132.6 mg). The third experiment (no. 4 10 13) was carried out for 24 hours in the sys tem MgCO3 (7.7 mg)–Ca(OH)2 (7.0 mg)–Femetal (26.0 mg)–SiO2 (11.1 mg) at P = 3 GPa and T = 1400°C with subsequent cooling by quenching. For the third experiment, crushed and mixed com ponents were put into an ampoule (height of 6 mm, a diameter of 5.0/3.3 mm, weight of 328.4 mg), which, in turn, was inserted into the Pt ampoule. Since the experiment was conducted in the closed system (to preserve volatile products) titanium was used as an oxygen scavenger, forming as a result of reduction of СО2 and Н2О. After the experiments, volatile components in the samples were analyzed by combined gas chromatogra phy mass spectrometry. Pt ampoules were opened with a punch in a special device put into the gas chro matograph circuit ahead of the analytical column, which was heated in the carrier gas (He, purity 99.999%) flow at temperature of 140°С for 90 min utes. The gas mixture was analyzed using a Thermo Synthesis of Heavy Hydrocarbons under P–T Conditions of the Earth’s Upper Mantle

Eitelite in sheared peridotite xenoliths from Udachnaya-East kimberlite pipe (Russia) – a new locality and host rock type

For the first time eitelite Na 2 Mg(CO 3 ) 2 was observed as a daughter phase in the melt inclusions in olivine from one of the deepest known mantle rocks sampled by kimberlite magma – sheared peridotite xenoliths (190 – 230 km), taken from the Devonian (~370 Ma) Udachnaya-East kimberlite pipe (Siberian craton, Russia). Eitelite was identified by confocal Raman spectroscopy and confirmed by energy-dispersive X-ray spectroscopy. Raman spectra of eitelite in the melt inclusions are characterized by a very strong band at 1105 cm −1 attributed to CO 3 2− symmetric stretching, and weaker bands at 207–208 and 260–263 cm −1 due to lattice vibration. Our findings of eitelite in the melt inclusions entrapped by olivine of mantle xenoliths indicate that this rare carbonate can crystallise from primitive mantle-derived alkaline carbonatite melt.

Experimental constraints on the role of chloride in the origin and evolution of kimberlitic magma

graphite tube isolated from a capsule with the sample by the MgO insulator was applied as a heater. The sample powder was loaded in a Au-Pd or Pt capsule and after accurate drying welded by arc welding. The inner walls of Ptcapsules were shielded by Re foil for minimization of iron loss. Each cell contained two capsules: one with the composition of model kimberlite (this study) and another with the composition of natural kimberlite. The results of experiments with the natural composi� tion will be presented in the next paper. The tempera� ture in each run was controlled by a W97Re3-W75Re25 thermocouple located in the center of the heater and isolated by the Al2O3 insulator from it. The tempera� ture gradient in the sample measured using a two� pyroxene thermometer did not exceed 50°С at 1500°С and 5 GPa. The pressure in the sample was maintained by the calibration performed at room tem� perature by change in the electric resistance for the transitions Bi I-II (2.55 GPa), Ba I-II (5.5 GPa), and Bi III-IV (7.7 GPa) and corrected for high tempera� tures by phase transitions graphite-diamond, quartz- coesite, and garnet-perovskite in the CaGeO 3 system. The error in pressure measurement was estimated on the level of 0.2 GPa. The composition of phases was determined on a JEOL Superprobe JXA�8800 micro� probe analyzer at Tohoku University.

Geochemical evolution of rocks at the base of the lithospheric mantle: Evidence from study of xenoliths of deformed peridotites from kimberlite of the Udachnaya pipe

This paper presents the results of study of the geochemical and mineralogical compositions of deformed peridotite xenoliths from the Eastern Udachnaya kimberlite pipe. The performed investiga� tions were mainly aimed at obtaining information on the composition and evolution of the lower layers of the lithospheric mantle in the central part of the Sibe� rian Craton. Xenoliths of deformed peridotites are present in kimberlite pipes of all old cratons and are the deepest of all mantle nodules as is evident from the PTcondi� tions of equilibrium. These rocks provide the main information on the composition and evolution of the lower layers of the lithospheric mantle; because of this, their study is of significant interest (1-7). Because of the small sizes and serpentinization (partial or com� plete) of the deformed peridotites, most of the previ� ously performed investigations concerned only the composition of individual rockforming minerals of xenoliths. There is a lack of data on the bulk composi� tion of these rocks at present, and they were obtained for partially serpentinized xenoliths (7-9). Representative collection of xenoliths of deformed peridotites from the eastern body of the Late Devo� nian Udachnaya pipe related to the Daldyn kimberlite field of the Yakutian kimberlite province, which is the largest diamond deposit in Russia, was selected for geochemical study. This body consists of two com� bined kimberlite bodies: Western Udachnaya and Eastern Udachnaya; the eastern body is unique in rela� tion to the amount deepseated rock xenoliths, their diversity, and preservation. Xenoliths are presented by a large set of facial groups and varieties, among which deformed peridotites occupy a significant place (2, 3). Samples of deepseated rocks selected for the study are uniquely fresh. These xenoliths contain no signs of secondary alterations, and the L.O.I. value in the stud� ied rocks is close to zero. The weight of the samples varies from 500 g to several kilograms. Only the central parts of xenoliths, without surfaces in the contact with kimberlite, were taken for analysis of the bulk compo� sition. Most of the xenoliths from this collection are typi� cal lherzolites composed of olivine, orthopyroxene, clinopyroxene, and garnet in various proportions. According to the content of clinopyroxene (2-4 wt %), five samples may be formally attributed to garnet harzburgites. In all samples, large (up to 1 cm) grains of rockforming minerals and the matrix composed of finegranular olivine form porphyroclastic or mosaic� porphyroclastic textures. Olivine in the studied xeno� liths is the prevailing mineral; its content varies from 60 to 85%. The concentration of the forsterite mole� cule in olivines from nodules ranges from 86.4 to 91.3 mol %. Orthopyroxene, as well as olivine, is pre� sented by relatively large grains (porphyroclasts) with a size from 2 to 10 mm and small grains (neoblasts) with a size of <0.5 mm. The amount of orthopyroxene in the studied samples varies from 5 to 18%. The study of the orthopyroxene composition demonstrates that it relates to highmagnesium varieties (En = 88.2- 92.9 mol %), however being more ironrich in compar� ison with orthopyroxenes from granular garnet peridot� ites. Orthopyroxenes contain admixtures of Al 2O3 (0.42- 0.68 wt %), Cr2O3 (0.11-0.52 wt %), and TiO2 (0.21- 0.6 wt %). The clinopyroxene content varies from 2 to 14% of rock volume. This mineral is characterized by a low concentration of the diopside component (Ca/(Ca + Mg) = 35.9-43.3 mol %) that results from high temperatures of the equilibrium of characterized associations. The concentration of garnet in rocks var� ies from 4 to 15%. Garnet grains are practically not deformed, have an isometric shape, and often form linearly elongated chains. The chemical composition of garnets corresponds to the lherzolite paragenesis