N. V. Sobolev

Diamond formation from mantle carbonate fluids

Analysis of inclusions has shown that natural diamond forms at pressures of 5-6 GPa and temperatures in the range 900-1,400 °C. In non-metallic systems,, diamond has been synthesized only at pressures greater than 7 GPa and temperatures of more than 1,600 °C. We find that diamond can crystallize in alkaline carbonate-fluid melts at pressures and temperatures that correspond to those of natural diamond formation.

Mineral inclusions in diamonds from the Sputnik kimberlite pipe, Yakutia

Abstract The Sputnik kimberlite pipe is a small “satellite” of the larger Mir pipe in central Yakutia (Sakha), Russia. Study of 38 large diamonds (0.7-4.9 carats) showed that nine contain inclusions of the eclogitic paragenesis, while the remainder contain inclusions of the peridotitic paragenesis, or of uncertain paragenesis. The peridotitic inclusion suite comprises olivine, enstatite, Cr-diopside, chromite, Cr-pyrope garnet (both lherzolitic and harzburgitic), ilmenite, Ni-rich sulfide and a Ti-Cr-Fe-Mg-Sr-K phase of the lindsleyite-mathiasite (LIMA) series. The eclogitic inclusion suite comprises omphacite, garnet, Ni-poor sulfide, phlogopite and rutile. Peridotitic ilmenite inclusions have high Mg, Cr and Ni contents and high Nb Zr ratios; they may be related to metasomatic ilmenites known from peridotite xenoliths in kimberlite. Eclogitic phlogopite is intergrown with omphacite, coexists with garnet, and has an unusually high TiO 2 content. Comparison with inclusions in diamonds from Mir shows general similarities, but differences in details of trace-element patterns. Large compositional variations among inclusions of one phase (olivine, garnet, chromite) within single diamonds indicate that the chemical environment of diamond crystallisation changed rapidly relative to diamond growth rates in many cases. P - T conditions of formation were calculated from multiphase inclusions and from trace element geothermobarometry of single inclusions. The geotherm at the time of diamond formation was near a 35 mW/m 2 conductive model; that is indistinguishable from the Paleozoic geotherm derived by studies of xenoliths and concentrate minerals from Mir. A range of Ni temperatures between garnet inclusions in single diamonds from both Mir and Sputnik suggests that many of the diamonds grew during thermal events affecting a relatively narrow depth range of the lithosphere, within the diamond stability field. The minor differences between inclusions in Mir and Sputnik may reflect lateral heterogeneity in the upper mantle.

Evidence of fluid inclusions in metamorphic microdiamonds from the Kokchetav massif, northern Kazakhstan

Abstract 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.

The role of mantle ultrapotassic fluids in diamond formation.

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/K2CO3/H2O/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.

Diamond formation through carbonate-silicate interaction

Abstract Crystallization of diamond and graphite from the carbon component of magnesite, upon its decarbonation in reactions with coesite and enstatite at pressures of 6-7 GPa and temperatures of 1350-1800°C has been accomplished experimentally. In a series of experiments, diamond was obtained in association with enstatite, coesite, and magnesite, as well as with forsterite, enstatite, and magnesite. Octahedral diamond crystals with sizes up to 450 mm were studied by FTIR spectroscopy and were found to contain nitrogen and hydrogen, which are known as the most abundant impurities in natural type Ia diamonds. We found that growth of diamond on the cubic faces of seed crystals proceeds with formation of a cellular surface structure, which is similar to natural fibrous diamonds. The isotopic composition of synthesized diamonds (δ13C = -1.27‰) was determined to be close to that of the initial magnesite (δ13C = -0.2‰)

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

Abstract 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.

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.

Diamondiferous Eclogites from the Udachnaya Kimberlite Pipe, Yakutia

Xenoliths from the upper mantle have undergone a wide variety of processes at varied temperatures and pressures, as recorded by mineral compositions and textures. Eclogite xenoliths in kimberlites are a unique source from which to obtain information about such processes. Furthermore, eclogites that contain accessory diamond yield important compositional constraints on the deeper upper mantle. Whether eclogites have mantle or crustal origins is still a subject of controversy. Mineralogy and petrography of 29 eclogite xenoliths from the Udachnaya kimberlite in Yakutia, Siberia are presented and combined with previous studies of these eclogites by our group (Jerde et al., 1993, 1994; Snyder et al., 1993a; Sobolev et al., 1994). Five different petrographic groups are defined, based on texture, mineral color, and degree of alteration. Chemical compositions of eclogitic minerals span a complete range from high to low-jadeite content in clinopyroxenes and from pyrope-almandine to grossular in garnets. Eclogites ...

Extreme Chemical Diversity in the Mantle during Eclogitic Diamond Formation: Evidence from 35 Garnet and 5 Pyroxene Inclusions in a Single Diamond

We report major- and trace-element variations in 35 garnet inclusions extracted from a single eclogitic diamond from the Mir kimberlite pipe, Yakutia. The range in CaO and Mg# in these garnets is large-from 3.84 to 9.66 wt% and 45 to 56, respectively. These ranges cover nearly half of the total range in eclogitic garnet compositions from diamonds worldwide. An extremely wide range in trace elements such as Y, Zr, and Sr also covers nearly the total range recorded for garnets included in diamonds, as well as that known for rock-forming minerals of diamondiferous eclogite xenoliths-15.1 to 48.9 ppm Y; 2.2 to 40.8 ppm Zr; and 0.5 to 9.1 ppm Sr. The widest ranges in REE between different grains (by an order of magnitude) are noted for LREE and MREE. Such compositional ranges are the first recorded for garnets available as inclusions in a single eclogitic diamond and cover nearly the complete range of compositions known for all Yakutian eclogites. Five clino-pyroxene inclusions exhibit more moderate ranges in ...

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.