New data on the composition of tagamites of the Popigai astrobleme

New data on the composition of tagamites (impact melt rocks) of the Popigai atsrobleme (Siberia) are presented. The preliminary results support the following major conclusions: (1) the matrix of the studied tagamites is crystallized and is composed of identified minerals; (2) the degree of the matrix crystallization and the amount of inclusions in tagamites can vary significantly; (3) tagamites are identical to the intracrater gneisses of the Popigai astrobleme by the REE contents and are distinct from them by significantly lower SiO₂ and higher Al₂O₃, FeO, MgO, and CaO contents; (4) the Popigai tagamites and intracrater gneisses are enriched in Ni (on average) in comparison with gneisses of the Khapchan Group; and (5) the various (often sufficient) degrees of crystallization of tagamites indicate the duration of the cooling of the melt, when diamonds could possibly have been dissolved.     

Impact diamonds of the Popigai astrobleme: Main properties and practical use

In the 1960s, the so-called “schistose” placer diamonds were found in sandy sediments of Ukraine. Their primary source remained unknown and their origin was supposed to be cosmic, by analogy with the diamonds detected in some meteorites. In the early 1970s, similar diamonds were found in terrestrial rocks of the Popigai impact crater (astrobleme) in northern Siberia. It was shown that these diamonds are products of the transformation of graphite contained in gneisses (Masaitis et al., 1972). Later, such diamonds were found in other astroblemes in Russia, Ukraine, Germany, Finland, and Canada. Impact diamonds markedly differ from diamonds hosted in kimberlites and lamproites in their appearance and in other features. In the 1970s–1990s, diamonds from the Popigai astrobleme were studied in many research institutions of Russia, Ukraine, and outside the USSR. The results of these investigations have been mostly published, and my brief review is based on these data.     

Fluid+melt intrusions in lechatelierite from the Popigai suevites: A product of dynamic interaction melts and fluids during the shock melting of the target gneiss

The paper presents data on lechatelierite form suevites of the Daldyn Formation in the Popigai astrobleme. Some of the lechatelierite samples show a complicated structure and contain block of diaplectic quartz glass and dynamic “intrusions” of glasses of types I, II, and III. The glasses of types I and II abound in fluid inclusions and display evidence of partial homogenization with lechatelierite. The glasses of type III are clearly separated from all other glasses but show evidence of dynamic interaction with them in the molten state. Fluid inclusions in the glasses of types I and II are syngenetic but have notably different densities from those of completely liquid or gaseous inclusions at 20°C. As is indicated by cryometric data, the liquid phase of the inclusions is aqueous solution of low salinity (5–8 wt % NaCl_equiv). The bulk petrochemistry of the glasses of type I characterizes them as highly silicic (96.04 wt % SiO₂ on average), with elevated K and Na concentrations (Na₂O + K₂O = 0.72 wt % on average), with 0.73 wt % Al₂O₃ (on average) and analytical totals 1.97 wt % less than 100%. The glasses of type II are also rich in SiO₂ (91.51 wt % SiO₂ on average) but contain a broader spectrum of concentrations of major oxides (totaling 5.53 wt % on average) and deficient analytical totals (by 2.96 wt % on average). The glasses of type III are completely equal to impactites produced by melting gneisses of the Popigai astrobleme. The glasses of type I are interpreted to be the intrusion products of the “early” highly mobile and H₂O-rich fluid+melt mixtures, whose protolithic material was K-Na feldspars of the target rocks. The derivation of these melts was associated with the capturing of much silica and water at a highly mobile behavior of K and Na and an inert behavior of Al. The glasses of type II were produced by the extensive mixing of silica and water at the limited involvement of apogneiss melts, and these glasses are sometimes deficient in Al. The glasses of type III are usual mixed apogneiss melts. Excess silica in the glasses of types I and II and their richness in water and deficiency in Al suggest impact anatexis and the selective separation of components during their derivation; the parental fluid-melt mixtures of these glasses were derived from such “hydrous” varieties of the target gneisses as diaphthorized and fractured rocks. The evolution and partial vitrification of lechatelierite and the glasses of types I and II proceeded under residual shock pressures, as follows from data on the dense (from ∼0.5 to 1 g/cm³) aqueous inclusions in these glasses, which suggest that the inclusions were captured in the glasses under pressures from ∼0.8 to 3.3 GPa. It follows that our lechatelierite samples have a complex multistage genesis, and their quenching facilitated the preservation of “intrusions” of various stages of shock melting, including the products of the “early” impact anatexis of the gneisses with the selective separation of components at the active participation of water.     

Discovery of a double impact crater in Libya: the astrobleme of Arkenu

Using orbital imaging radar, we detected a double circular structure, located in the southeastern part of the Libyan Desert, which is partially hidden under sandy sediments. Fieldwork confirmed it to be an unknown double impact crater, each crater having a diameter of about 10 km, younger than 140 Ma. Sampling on the site enabled the observation of quantities of shatter cone structures and impact breccias containing planar fractures. To cite this article: P. Paillou et al., C. R. Geoscience 335 (2003).     

A new natural,super-hard,transparent polymorph of carbon from the Popigai impact crater,Russia

A natural shockwave event led to the formation of a new crystalline polymorph of carbon in gneisses from the Popigai crater, Russia. The new species occupies the interior of a multiphase assemblage and is entirely enveloped by lonsdaleite and graphite. Polishing hardness of this new phase is greater than that of lonsdaleite. Micro-beam synchrotron X-ray diffraction, imaging and fluorescence studies revealed a pure transparent carbon phase. The diffraction pattern is indexed in terms of a cubic cell (a=14.697 Å, space group Pm3m.). This species was neither encountered in static or dynamic high-pressure experiment nor predicted by theoretical calculations. To cite this article: A. El Goresy et al., C. R. Geoscience 335 (2003).     

Moissanites of the Popigai Astrobleme

Moissanites were found in tagamites of the Popigai meteorite crater along with impact diamonds. We have studied 55 samples including 49 individual polytypes and six intergrowths. The numbers of 6H, 15R, 4H, 6H/15R, and 6H/4H polytypes are 82, 7, 5, 4, and 2%, respectively. By the assemblage of polytypes, the moissanites of the Popigai astrobleme are distinct from kimberlite moissanites, as well as from synthetic SiC, which is characterized by the absence of the 4H polytype and the presence of more diverse inclusions (including Fe-bearing). The Popigai astrobleme is one of few objects with reliable natural moissanite. Technogenic contamination is excluded, since any researcher can find this mineral in tagamites.     

HRTEM study of Popigai impact diamond: heterogeneous diamond nanostructures in native amorphous carbon matrix

High-resolution transmission electron microscopy was applied for the detailed nanostructural investigation of Popigai impact diamonds with the aim of revealing the nature of the amorphous carbon of the matrix. The successful application of two complementary specimen preparation methods, focused ion beam (FIB) milling and mechanical cleavage, allowed direct imaging of nanotwinned nanodiamond crystals embedded in a native amorphous carbon matrix for the first time. Based on its stability under the electron beam, native amorphous carbon can be easily distinguished from the amorphous carbon layer produced by FIB milling during specimen preparation. Electron energy loss spectroscopy of the native amorphous carbon revealed the dominance of sp ²-bonded carbon and the presence of a small amount of oxygen. The heterogeneous size distribution and twin density of the nanodiamond crystals and the structural properties of the native amorphous carbon are presumably related to non-graphitic (organic) carbon precursor material.     

Water and other volatile systematics of olivine-hosted melt inclusions from the Yellowstone hotspot track

Major oxide, trace element and volatile (H₂O, CO₂, S, F, and Cl) compositions have been analyzed for olivine-hosted melt inclusions in eight basalt samples from Yellowstone National Park and the Snake River Plain (SRP) to identify the least differentiated melt compositions and assess the volatile budget of the Yellowstone hotspot. Melt-inclusion chemistry was evaluated to understand potential overprinting effects in the shallow mantle and crust of magmas derived from deeper levels. Maximum water concentrations of 3.3 wt% and CO₂ up to 1,677 ppm have been observed in olivine-hosted melt inclusions from the Gerritt Basalts at Mesa Falls, Idaho (SRP region), which is significantly higher than the maximum concentrations measured in lavas from other hotspots such as Hawaii (~0.8–0.9 wt%). Maximum water concentrations were generally observed in the least differentiated melt inclusions in terms of incompatible major oxide concentrations, indicating that high water concentrations are characteristic of the mantle or perhaps lower crust rather than resulting from differentiation enhancement within the shallow crust, even taking into account the fact that water behaves as an incompatible element during crystal fractionation. Enrichment in Ba coupled with depletion in Th in many of the melt inclusions and their host rocks is a characteristic of many arc lavas and may indicate that volatiles in Yellowstone-Snake River Plain basalts could have a subduction zone origin.     

Homogeneous impact melts produced by a heterogeneous target?: Sr-Nd isotopic evidence from the Popigai crater, Russia

The 35.7 ± 0.2 Ma old Popigai crater, Siberia, with a diameter of about 100 km is one of the best preserved large terrestrial impact structures. The heterogeneous target at the impact site consists of Archean to Lower Proterozoic metamorphic rocks of the crystalline basement, Upper Proterozoic quartzites and other clastic deposits, as well as Cambrian to Cretaceous clastic sediments and sedimentary rocks, including carbonate rocks. Moreover, Proterozoic and Permo-Triassic dolerite dykes are found in the target area. We report major element, Sr and Nd isotope data for 13 of these target rocks and for various types of impactites. The 15 analysed impactite samples include tagamites (impact melt rocks), suevites and impact glass from small veins. Furthermore, two impact breccias and two impact glass-coated gneiss bombs were analysed. We discuss the relation of these impactites to the target lithologies, and evaluate on the basis of literature data the relation of microkrystites (and associated microtektites) in Upper Eocene sediments to the Popigai event.The impactites have SiO₂ abundances ranging from 59 to 66 wt.% and show significant variations in the content of Fe, Ca, and Ti. They have present day ⁸⁷Sr/⁸⁶Sr ratios between 0.7191 and 0.7369. Their Sr model ages T^Sr_UR range from 1.9 to 2.3 Ga. The ¹⁴³Nd/¹⁴⁴Nd ratios for the impactite samples cluster between 0.5113 and 0.5115. The Nd model ages T^Nd_CHUR range from 1.9 to 2.1 Ga.In an ε_CHUR(Nd)-ε_UR(Sr) diagram, the impactites and Upper Eocene microkrystites (and associated microtektites) plot in a field delimited by Popigai target lithologies. The impactites are restricted to the field of crystalline basement rocks and Upper Proterozoic quartzites, but they show different isotopic signatures in different crater sectors. Impactites and Upper Eocene microkrystites plot in different, only partly overlapping clusters. The leucocratic microkrystites and microtektites have a higher affinity to the post-Proterozoic rocks in the target area than the impactites. Seemingly, the melanocratic microkrystites originated mostly from crystalline basement. This data alignment supports the assumption that Popigai is the source crater for all three types of ejecta. For the first time, clear relations are established of the geochemically variable Upper Eocene microkrystites and associated microtektites to specific target lithologies at Popigai crater. Finally, the observed range in Sr and Nd isotope parameters determined for impact melt lithologies that originated during the Popigai event show a much higher variability than known from other craters. This result indicates that mixing of impact melt which later formed tagamite sheets and glass particles in different impact breccias, was incomplete at the time of ejecta dispersal.     

Morphology of Crystals Growing in Impact Diamonds of the Popigai Astrobleme (Experimental Data)

Doklady Earth Sciences - Experimental results on the growth of cubic diamond on lonsdaleite-bearing diamonds from the Popigai astrobleme at 5.5 GPa and 1450°C in the Fe–Ni–C system...     

川西甲基卡花岗伟晶岩型锂矿床中熔体、流体包裹体固相物质研究

川西甲基卡二云母花岗岩和伟晶岩内发育大量原生熔体包裹体和富晶体流体包裹体。为了查明甲基卡成矿熔体、流体性质与演化特征,运用激光拉曼光谱和扫描电镜鉴定了甲基卡花岗伟晶岩型锂矿床中二云母花岗岩及伟晶岩脉不同结构带内的原生熔体、流体包裹体的固相物质。分析结果表明,甲基卡二云母花岗岩石英内熔体包裹体的矿物组合为磷灰石+白云母、白云母+钠长石、白云母+石墨;伟晶岩绿柱石内富晶体流体包裹体的矿物组合主要为刚玉、富铝铁硅酸盐+刚玉+锂辉石、锂辉石+石英+锂绿泥石;伟晶岩锂辉石内富晶体流体包裹体的矿物组合主要为磷灰石、锡石、磁铁矿、石英+钠长石+锂绿泥石、萤石、富钙镁硅酸盐+富铁铝硅酸盐+富铁硅酸盐+石英;花岗岩浆熔体与伟晶岩浆熔体(流体)具有一定的差异,成矿熔体、流体成分总体呈现出碱质元素(Na、Si、Al)、挥发分(F、P、CO_2)含量增高及基性元素(Fe、Mg、Ca)降低的特征;包裹体中子矿物与主矿物的化学成分具有一定的差别,揭示出伟晶岩熔体(流体)存在局部岩浆分异作用,具不混溶性及非均匀性。因此认为,伟晶岩熔浆(流体)为岩浆分异与岩浆不混溶共同作用的产物,挥发分含量的增高(F、P、CO_2)使伟晶岩能够与稀有金属组成各类络合物或化合物,这对于稀有金属成矿起到了至关重要的作用。     

Ultramafic inclusions in basaltic rocks from Hawaii

Ultramafic inclusions and the enclosing basaltic rocks were collected from a number of localities in the Hawaiian Islands; these and other specimens were studied by standard pétrographic techniques and with an electron microprobe. Emphasis was on determination of mineral assemblages, mineral compositions, and variations in composition. Sixty-eight inclusions and thirteen basaltic rocks are described, with partial chemical analyses (Ti, Al, Cr, Fe, Mn, Ni, Mg, Ca, Na, K) of olivines, orthopyroxenes, clinopyroxenes, and some feldspars and other minerals. Inclusions range from dunite to anorthosite, and basaltic hosts range from olivine nephelinite to olivine tholeiite. The inclusions are separable into three categories, which correlate with three groups basaltic hosts: Lherzolite inclusions are relatively poor in Fe, and the component minerals have limited ranges of composition. In Hawaii, lherzolite inclusions occur preferentially in extremely undersaturated hosts (olivine nephelinite, nepheline basanite, and ankaratrite). Other varieties of inclusions (dunite, wehrlite, feldspathic peridotite, pyroxenite) are relatively rich in Fe, and the component minerals have wider ranges of composition. These inclusions, together with gabbro, occur preferentially in hosts which are but moderately undersaturated (alkaline olivine basalt, hawaiite, and ankaramite). The sparse inclusions in nearly-saturated basalt (olivine tholeiite) are petrographically distinct from those in the other two categories. These correlations suggest that the inclusions and the enclosing basaltic rocks are genetically related. As the three suites of inclusions differ chemically, mineralogically, physically, and texturally, more than one origin is probable.     

Polyphase hydrocarbon inclusions in garnet from the Mir diamondiferous pipe

The garnet nodules from the enrichment concentrate of the Mir kimberlite pipe, which have different saturation levels of regularly oriented mineral and primary polyphase fluid inclusions, are investigated. The garnets that contain inclusions are classified in the grossular-almandine-pyrope series and are usually characterized by an increased content of Si, Mg, Al, and the absence of Fe³⁺. The polyphase fluid inclusions are studied using IR-Fourier- and Raman spectroscopy. The presence of polyaromatic hydrocarbons, H₂O and CO₂ is established in their structure, which indicates the hydrocarbon specific character of fluids that participate in the processes of deep mineral formation.     

The analysis of the volatiles from fluid inclusions in hydrothermal fluorite

Volatiles from primary fluid inclusions in hydrothermal fluorite were studied. The gases released were analysed with a mass spectrometer using an internal standard; water-vapour pressure was measured manometrically.To extract the volatiles, both heating and grinding in vacuum were used. In the thermal treatment, volatiles from other sources besides the inclusions were also found: H₂ and hydrocarbons, as well as additional amounts of H₂O and CO₂. The vacuum grinding, on the other hand, leads to volatile deficiency, especially with respect to H₂O and CO₂, due to retention of these components on the ground material.The study of the dependence of amount of volatiles released upon heating, on the grain size of the mineral fractions, was used as additional information for evaluating the amounts of volatiles coming from a source other than inclusions.The thermal studies were supplemented by decrepitophonic measurements.It is concluded that the volatiles from inclusions are represented practically only by CO₂ and H₂O in a 1:100 molar ratio. Conclusions about the conditions of mineral formation are drawn.     

Trace elements in sulfide inclusions from Yakutian diamonds

 Sulfide inclusions in diamonds may provide the only pristine samples of mantle sulfides, and they carry important information on the distribution and abundances of chalcophile elements in the deep lithosphere. Trace-element abundances were measured by proton microprobe in >50 sulfide inclusions (SDI) from Yakutian diamonds; about half of these were measured in situ in polished plates of diamonds, providing information on the spatial distribution of compositional variations. Many of the diamonds were identified as peridotitic or eclogitic from the nature of coexisting silicate or oxide inclusions. Known peridotitic diamonds contain SDIs with Ni contents of 22–36%, consistent with equilibration between olivine, monosulfide solid solution (MSS) and sulfide melt, whereas SDIs in eclogitic diamonds contain 0–12% Ni. A group of diamonds without silicate or oxide inclusions has SDIs with 11–18% Ni, and may be derived from pyroxenitic parageneses. Eclogitic SDIs have lower Ni, Cu and Te than peridotitic SDIs; the ranges of the two parageneses overlap for Se, As and Mo. The Mo and Se contents range up to 700 and 300 ppm, respectively; the highest levels are found in peridotitic diamonds. Among the in-situ SDIs, significant Zn and Pb levels are found in those connected by cracks to diamond surfaces, and these elements reflect interaction with kimberlitic melt. Significant levels of Ru (30–1300 ppm) and Rh (10–170 ppm) are found in many peridotitic SDIs; SDIs in one diamond with wustite and olivine inclusions and complex internal structures have high levels of other platinum-group elements (PGEs) as well, and high chondrite-normalized Ir/Pd. Comparison with experimental data on element partitioning between crystals of monosulfide solid solution (MSS) and sulfide melts suggests that most of the inclusions in both parageneses were trapped as MSS, while some high-Cu SDIs with high Pd±Rh may represent fractionated sulfide melts. Spatial variations of SDI composition within single diamonds are consistent with growth histories shown by cathodoluminescence images, in which several stages of growth and resorption have occurred within magmatic environments that evolved during diamond formation. Received: 5 July 1995 / Accepted: 21 February 1996