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.     

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.     

New data on the material composition of mud volcano products on Kerch Peninsula

The results of recent studies of the isotope and chemical composition of the waters and breccias that erupted from mud volcanoes of Kerch Peninsula are presented. It was found that the waters of the volcanoes considered were characterized by low salinity, a chloride–hydrocarbonate–sodium composition, and an increased ¹⁸O content. The breccias were enriched in organic matter and some microelements (Li, Se, Hg, etc.). As for petrogenic elements, depletion of Ca in the breccias is noted.     

New data on the trace metal composition of the planktonic foraminifera microfossils of the Atlantic Ocean

This paper reports new data on the trace metal composition of planktonic foraminifer shells from surface sediments and cores (fraction >0.1 mm) in the central part of the Atlantic Ocean. This investigation has made it possible to identify a considerable accumulation of trace elements from water due to calcite entering into the crystal lattice under biomineralization and adsorption on the shell surface and pores, despite the fact that the shells are depleted in trace elements relative to pelagic clays. The trace element content in planktonic foraminifer microfossils is characterized by temporal variability, which is the most pronounced in long cores (Holocene–Upper Pleistocene) and reflects the sedimentation paleoenvironment in the ocean.     

New data on the composition and age of basement rocks of the Tagil paleo-island arc system

The Tagil paleo-island arc terrane is composed of Late Ordovician-Devonian intrusive, volcanic, and volcano-sedimentary complexes. The western margin of the terrane is comprised of dunite-clinopyroxenite-gabbro massifs of the Ural platinum-bearing belt, which are fringed by rock strata of widely different metamorphic grades. Work on isotope systematics of olivine gabbros enabled us to infer a Vendian age (550–540 Ma) of homogenization of the Sm-Nd isotopic system of the Kytlym and Knyaspa massifs within the Ural platinum-bearing belt. The Sm-Nd ages for metamorphic rocks of the Belaya Gora complex surrounding the studied massifs also agree with a Vendian age (573–574 Ma). Our results suggest that metamorphites of the Belaya Gora complex (amphibolites, plagiogneisses, two-mica and biotite gneisses, schists containing garnet, cordierite, staurolite, gedrite, and sillimanite) and dunite-clinopyroxenite-gabbro intrusions of the proto-platinum-bearing belt may have been formed in a convergence setting above a mantle plume, most likely in a back-arc (?) extension region. Reactivation of this plume during the late Precambrian resulted in the opening of the Ural paleo-ocean. The Late Ordovician-Early Devonian times were marked by metamorphic reworking and tectonic transport of pre-Paleozoic complexes into an accretionary prism setting of the Tagil paleo-arc that was accompanied by generation of gabbroid and granitoid magmas. Based on the obtained results, the Tagil terrane can be now considered as part of the Paleozoic paleo-island arc system developed on a heterogeneous Proterozoic basement.     

New data on the structure and composition of stromatolite buildups (Northern Anabar Region)

Lamination, a characteristic feature of stromatolites, is related to thin alternation of mineral and organogenic layers. The present paper is devoted to structures of Lower Riphean stromatolites in the Fomich River area (northern Anabar Uplift) that were described for the first time and studied with an electron microscope. Two Colonnella forms with different microstructures were examined. It has been established that a banded texture is observed if an organogenic layer is mainly composed of tightclusters of filiform (theread like) particles. However, a clotted texture is recorded if the layer is represented by isolated coccomorphic (rounded) particles. The chemical composition of these tiny structures, which can accumulate several elements, is discussed and analyzed. In the past, the organogenic layer represented a biofilm that comprised a complex-function cyanobacterial community.     

New data on the composition of stable isotopes in glendonites of the White Sea and their genesis

Several samples including host sediments, recent molluscan shells, glendonites, and associated carbonate concretions were subjected to the thorough investigation. The obtained analytical data characterize the unique locality of Holocene and Recent carbonate structures in the White Sea basin. The microscopic examination of glendonites revealed several microtextural types of calcite and the succession of their formation. It is established that they are composed of carbonate material, which was discretely formed due to biochemical destruction of terrigenous sedimentary material. The revealed differences in the carbon isotope composition in different glendonites are likely explained by local geochemical properties, which existed within host sediments, or the asynchronous formation of carbonate structures due to changes in the chemical composition of waters that saturated sediments.     

An ordinary chondrite impactor for the Popigai crater, Siberia

With a diameter of ∼100 km, Popigai in Northern Siberia is the largest crater known in the Cenozoic. The concentrations in platinum group elements (PGE) were analyzed in twenty samples of homogeneous impact melt collected in the northwestern flank of the crater to identify the composition of the projectile. The method selected was preconcentration by NiS fire assay followed by inductively coupled plasma-mass spectrometry (ICP-MS). This technique measures all the PGE (except Os) and by using aliquots >10g, the results are highly reproducible. The major and trace element composition of the impact melt resembles that of gneissic lithologies of the Anabar shield, which are representative of the target rock. The PGE are enriched in the melt by factors of 3 to 14 compared to the main target lithology, but the meteoritic contamination is only around 0.2 wt.%. Using plots of elemental ratios such as Ru/Rh vs. Pt/Pd or Ru/Rh vs. Pd/Ir, the Popigai impactor is clearly identified as an ordinary chondrite and most likely l-chondrite. This study indicates that PGE elemental ratios allow discrimination of the type of impactor, even in the case of low meteoritic contamination. This study confirms that a significant fraction of the crater-forming projectiles presently documented could have an ordinary chondrite composition. Their probable source, the S-type asteroids, appears to form the majority of the bodies in the main asteroid belt and among Near Earth Objects (NEOs). The ordinary chondrite origin of the Popigai projectile supports an asteroidal origin for the late Eocene impacts as a plausible alternative to the comet shower scenario proposed by Farley et al. (1998).     

Comparative data on the volatile component composition of magma

Ferrihydrite (2.5 Fe₂O₂-4.5 H₂O) is an unstable colloidal mineral. It dissolves in highly alkaline solutions and is precipitated from them in the form of goethite. Jarosite is stable at very low pH but is decomposed at higher values of pH with separation of iron oxides. Experiments show that in rapid decomposition of jarosite a protohematite substance, ferrihydrite, is formed. This transformation occurs at moderate pH values when solutions percolate through the aggregates of jarosite. Ferrihydrite, an unstable colloidal hydrated oxide of ferric iron, changes spontaneously to stable hematite with time. Very slow decomposition of jarosite results in its replacement by iron hydroxide, goethite. Under laboratory conditions in alkaline solutions lepidocrocite may be obtained from jarosite. The synthesis of this iron hydroxide passes through a stage of intermediate products: ferrihydrite and hydrated ferric oxide - ferriprotolepidocrocite, formed by solution of ferrihydrite in strongly alkaline solutions. The transformation of ferriprotolepidocrocite into lepidocrocite may be regarded as a topotactic reaction. —Authors.