Felix V. Kaminsky

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.

Nyerereite and nahcolite inclusions in diamond: evidence for lower-mantle carbonatitic magmas

Abstract Nyerereite and nahcolite have been identified as micro- and nano-inclusions in diamond from the Juina area, Brazil. Alongside them are Sr- and Ba-bearing calcite minerals from the periclase-wüstite series, wollastonite II (high), Ca-rich garnet, spinels, olivine, phlogopite and apatite. Minerals of the periclase-wüstite series belong to two separate groups: wüstite and Mg-wüstite with Mg# = 1.9-15.3, and Fepericlase and periclase with Mg# = 84.9-92.1. Wollastonite-II (high, with Ca:Si = 0.992) has a triclinic structure. Two types of spinel were distinguished among mineral inclusions in diamond: zoned magnesioferrite (with Mg# varying from 13.5-90.8, core to rim) and Fe spinel (magnetite). Olivine (Mg# = 93.6), intergrown with nyerereite, forms an elongate, lath-shaped crystal and most likely represents a retrograde transformation of ringwoodite or wadsleyite. All inclusions are composed of poly-mineralic solid mineral phases. Together with previously found halides, sulphates and other mineral inclusions in diamond from Juina, they form a carbonatitic-type mineral paragenesis in diamond which may have originated in the lower mantle and/or transition zone. Wüstite inclusions with Mg# = 1.9-3.4, according to experimental data, may have formed in the lowermost mantle. The source for the observed carbonatitic-type mineral association in diamond is lower-mantle natrocarbonatitic magma. This magma may represent a juvenile mantle melt, or be the result of low-degree partial melting of deeply-subducted carbonated oceanic crust. This magma was rich in volatiles, such as Cl, F and H, which played an important role in the formation of diamond.

Thermal state and composition of the lithospheric mantle beneath the Daldyn kimberlite field, Yakutia

Abstract The proton microprobe has been used to study the distribution of trace elements in garnet and chromite concentrates from the Udachnaya kimberlite and three smaller, low-grade kimberlites from the Daldyn kimberlite field. Garnet thermobarometry and classical P-T estimates for megacrystalline peridotite xenoliths both suggest a Paleozoic geotherm beneath the Daldyn area that is close to a 35 mW/m 2 conductive model. Finer-grained xenoliths with T C scatter above this geotherm; high-temperature sheared xenoliths lie near or above a 40 mW/m 2 model geotherm at 1150–1400°C. The higher- T results are interpreted as the result of short-term heating, caused by magmatic intrusion and perturbation of a relatively cool conductive geotherm, especially near the base of the lithosphere. The stratigraphic distribution [inferred from nickel temperature ( T Ni )] of garnets with different major-element chemistry indicates that the lithosphere is strongly layered in terms of rock type; depleted lherzolites predominate to depths of ca. 150 km, harzburgites comprise up to 60% of the volume between 150 and 180 km, and these are underlain by a mixture of depleted and metasomatically enriched lherzolites. Zinc temperatures ( T Zn ) indicate that chromite-bearing peridotites are essentially absent at depths > 190 km. High- T lherzolite garnets carry a distinctive trace-element fingerprint showing enrichment in Zr, Ti, Y and Ga, interpreted as due to the infiltration of asthenosphere-derived melts. This melt-related metasomatic signature becomes the dominant one at ca. 220–230 km depth, and this is interpreted as the base of the lithosphere. This depth also corresponds approximately to the Lehman Discontinuity at the top of a pronounced low-velocity zone, observed in deep seismic sounding experiments across this part of the Siberian Platform. The techniques used here provide a means of mapping the lithosphere in terms of thermal structure, lithology and fluid-related processes; both lateral (3-D) and temporal (4-D) variations may be mapped using readily available garnet and chromite concentrates from the widespread kimberlite intrusions across the Siberian Platform.

Diamond potential of metamorphic rocks in the Kokchetav Massif, northern Kazakhstan

There are two known diamondiferous bodies in Kazakhstan, the Kumdy-Kol deposit and the Barchi-Kol occurrence. Both are located within the western part of a metamorphic belt that outcrops at the central part of the Kokchetav Massif. The metamorphic belt is interpreted as a mega-melange that comprises structural units underlain by HP and UHP rocks. Diamondiferous varieties of the UHP rocks within the Kumdy-Kol deposit form a narrow (~1 km thick), NE-trending band; they comprise less than one percent of the total UHP rock volume. The economically important diamondiferous zone consists of variegated rocks, from silicate to essentially carbonate varieties with a wide spectrum of mineral assemblages. Garnet-biotite gneiss makes about 80 % of the diamondiferous zone. Certain regularity is discernable in spatial distribution of diamonds: linear diamond-rich zones alternate with barren ones. The diamond grade ranges from several carats per ton (cpt) to several hundreds cpt. Different morphological varieties are present among the diamonds: octahedra, cubes, cube-octahedron combination forms, skeletal and spheroid crystals. A wide range of δ 13 C (−8.9‰ through −27 ‰) and δ 15 N (+5.3 ‰ through +25 ‰) values have been measured. Diamonds from different rock types differ in their carbon isotopic pattern: diamonds from gneiss have ‘lighter’ isotopic compositions relative to those of pyroxene-carbonate and garnet-pyroxene rocks. Graphite, coesite, clinopyroxene, rutile, titanite, kyanite, K-feldspar, biotite, phengite, quartz, phlogopite, albite, apatite, chlorite and carbonates are minerals forming intergrowths with diamonds. The most frequent combination is graphite +diamond. We conclude that the model of crustal fluid-metasomatic formation of diamonds in metamorphic rocks reflects the observed regularities best. The morphology of diamondiferous bodies favors the notion of their formation from a carbon-bearing fluid. The micrometer size of the diamonds, along with isotopic signatures, hint at their formation under metastable crustal conditions. This model presumes that the formative process occurred at relatively low temperature and pressure within an open disequilibrium catalytic system. It does not exclude an UHP episode in the geological history of the metamorphic rocks.

Oxygen potential of diamond formation in the lower mantle

Thermodynamic calculations have shown that when a metallic phase arising due to ferroan ion disproportionation is contained in lower-mantle rocks, carbon occurs as iron carbide and the oxygen fugacity corresponds to the equilibrium of ferropericlase with Fe-Ni alloy. The typical values of oxygen fugacity in zones of diamond formation in the lower mantle lie between the iron-wüstite buffer and six logarithmic units above this level. The processes that proceed in the lower mantle give rise to variation of

Possible primary sources of diamond in the North African diamondiferous province

f_{O_2 }

Nitrides and carbonitrides from the lowermost mantle and their importance in the search for Earth's “lost” nitrogen

within several orders of magnitude above the elevated