Home – Home – Jornals – Russian Geology and Geophysics 2012 number 11

Carbon, though being abundant in the Solar system, barely exceeds 0.01 wt.% in the silicate mantle, whereas it is ~3.6 wt.% in primitive chondritic meteorites that most likely formed our planet. This deficit may be due to redistribution of carbon in the liquid metal phase and then in the core at the stage of magma ocean fractionation, because carbon is much more soluble in Fe-Ni ± S melt than in silicate melts. The terrestrial heat and mass transfer are controlled mainly by layered convection and periodic peaks of plume activity as fast mantle jets that rise from the core. Plumes carry significant amounts of CO₂, H₂O, and K₂O (most probably in the form of carbonatite or hydrous carbonatite melts) released by the degassing core on its interaction with oxidized silicate material. There are two mechanisms that may maintain fast plume ascent: (1) local melting at the plume front as a result of doping with volatiles (H₂O, CO₂) as in a gas burner (rise rate 60-110 cm/yr) or (2) flow controlled by diffusion transport of silicate components in carbonatite melt (rise rate ~100 cm/yr).

The Garevka metamorphic complex (GMC), located at the junction of the Central Angara and Isakovka terranes (western part of the Transangarian Yenisei Ridge), was studied in terms of its tectonometamorphic evolution and geodynamic processes in the Neoproterozoic history of the region. Geological, structural, geochronological, and petrological data permitted the recognition of two stages in the GMC evolution, which differ in thermodynamic regimes and metamorphic field gradients. These stages were related to crustal contraction and extension within the Yenisei regional shear zone, a large lineament structure in the region. Stage 1 was marked by the formation of metamorphic complexes in the middle to upper amphibolite facies moderate-pressure regional metamorphic settings at ~960 Ma, P = 7.7-8.6 kbar, and T = 582-631?C. This suggests subsidence of the area to the middle continental crust with dT/dH = 20-25?C/km. During stage 2, the rocks experienced Late Riphean (~880 Ma, SHRIMP II U-Pb and ⁴⁰Ar/³⁹Ar dating) dynamic metamorphism under epidote-amphibolite facies conditions ( P = 3.9-4.9 kbar; T = 461-547?C), indicating a metamorphic field gradient of dT / dH no greater than 10?C/km, with the formation of blastomylonites in narrow zones of ductile and brittle deformations. In these zones, high-grade GMC blocks were exhumed to the upper continental crust and underwent low-temperature metamorphism. Comparison of the structural, geologic, and other evolutionary features (nearly identical age constraints in view of exhumation rate, similar PT -paths, and different types of metamorphism associated with different geodynamic settings, etc.) of the Garevka and Teya complexes suggests that they constitute a single polymetamorphic complex.

Sesimic data on the southern (Laptev Sea) extremity of the Lomonosov Ridge were used to develop a new structural model for the sedimentary cover. It permitted a correlation between the seismic cross-sections of the ridge crest and two deep-sea basins: the Podvodnikov Basin and the Amundsen Plain. It is the first time that a seismic model has taken into account both regional seismic-reflection profiles obtained from NP drifting ice stations and recent high-resolution CDP data. Our seismic model agrees both with geological data on the Laptev Sea continental margin and the data obtained from deep-sea drilling into the Lomonosov Ridge under the IODP-302 project. The sedimentary cover of the southern Lomonosov Ridge and adjacent parts of the Amundsen Plain and Podvodnikov Basin was dated at the Aptian-Cenozoic. The sedimentary section is divided by two main unconformities, of Campanian-Paleocene and Oligocene-Early Miocene ages. The cover contains a structurally complicated graben system, which is an extension of the New Siberian system of horsts and grabens, recognized in the shelf. Sedimentation began in the grabens in the Aptian-Albian and ended with their complete compensation in the Paleocene.

We present new data on the material composition of terrigenous deposits in the pre-Jurassic basement of the West Siberian Plate, which formed on the margin of the Kazakhstan paleocontinent. Studies of the sections of several wells drilled in the east of the Kurgan Region showed that the Middle-Upper Devonian and lower Lower Carboniferous continental and littoral deposits are graywackes rich in rock and plagioclase clastics. Quartz grains amount to no more than 3-5%. The rock clastics are mainly volcanics of basic and normal compositions, which were supplied, most likely, from acting volcanoes of the marginal belt. By genesis, these are predominantly tephroids. Along with allothigenic components of sandstones, we consider authigenic minerals, which permit treating the postsedimentational transformation of the Middle Devonian and Early Carboniferous rocks as early catagenesis. By the vitrinite reflectance, the Lower Visean sandstones correspond to gradations MC₂ and MC₃ (medium catagenesis).

The formation and evolution conditions for alkaline magmatism and associated igneous rocks in the western framing of the Siberian craton are shown by the example of alkaline and subalkaline intrusive bodies of the Yenisei Ridge. We present petrographic, mineralogical, geochemical, and geochronological data for the rocks of the Srednetatarka and Yagodka plutons, located within the Tatarka-Ishimba suture zone. Ferroan and metaluminous varieties enriched with rare elements (Nb, Ta, Zr, Hf, REE) are making up most of the studied rocks. They formed at the stages of fractional crystallization of alkaline magma in a setting of active continental margin in the west of the Siberian craton in the Late Neoproterozoic (710-690 Ma). As differentiates of mantle magmas, these rocks associate with Nb-enriched rocks - A -type leucogranites and carbonatites. Sm/Nd and Rb/Sr isotopic data imply a predominance of the mantle component in the magmatic sources of the mafic and intermediate rocks as well as contamination of various volumes of continental crustal material by this magma.

We present the first results of studying the major- and trace-element composition of microinclusions in the coats of variety IV diamonds from the Sytykanskaya pipe. These microinclusions are of silicate-carbonate composition. Similar compositions are reported for the microinclusions in diamonds from the placers of the northeastern Siberian craton and cuboids from the Internatsional'naya pipe. The microinclusions studied are close to kimberlites and carbonatites in trace-element composition but depleted in HFSE, Mg, and transition metals and enriched in K and LILE. The distribution of incompatible elements in the microinclusions studied is similar to the "table" pattern, which was observed for high-density hydrous-silicic fluids.

High-Ba (~11 wt.% BaO) phlogopite was found for the first time in olivinites of the Guli intrusion in the Maimecha-Kotui province of ultrabasic alkaline rocks and carbonatites. The high-Ba mica occurs in assemblage with a paragenesis of olivinite minerals - clinopyroxene, titanomagnetite, apatite, and ilmenite.
High-Ba mica is an early phlogopite generation. Its magmatic crystallization led to a decrease in Ba content. Low-Ba mica is a late phlogopite generation. The high Ba/K ratios at the early stages of evolution of a mantle magmatic system are necessary for the formation of high-Ba minerals and point to magma formation at great depths and the contribution of mantle metasomatism to the geochemical characteristics of parental magmas.

The total hydrocarbon composition and average structural-group characteristics of typical Paleozoic crude oils of the Timan-Pechora petroliferous basin are described. The hydrocarbon (HC) types of crudes are compared, which were conventionally determined from the composition of their gasoline fractions, by mass-spectrometric analysis of the HC composition of crudes, and by structural-group analysis of crudes, based on radiospectrometric data. The analyses have shown the presence of 30 structural types of HCs with up to 43 carbon atoms. The whole series of members up to C₄₃ are specific only for HCs with no more than three rings in the molecule.
It has been established that the carbonate strata in the basin under investigation, independently of the depth of their occurrence and the age of the host deposits, generate and accumulate heavy high-resin high-sulfur oils rich in alicyclic structures and assigned by HC composition to the naphthene-methane or even naphthene type.
Terrigeneous reservoirs here abound in crudes of methane-naphthene type. The contents of sulfur and resinous substances and the fraction of carbon atoms in alicyclic HC structures decrease as the depth of occurrence of the host deposits grows, thus reflecting the known gradual process of "methanization" of petroleum composition.

We present new paleomagnetic data for continental flood basalts (Siberian traps) obtained from cores of two boreholes in the northwestern Noril'sk area, within the Kharaelakh and Vologochan basins. Paleomagnetic measurements of lava and tuff samples from KhS-59 and SSV-19 boreholes allowed reconstructing and correlating the polarity patterns. Thus, multiple paleomagnetic anomalies (PA) have been discovered as short-term polarity changes in narrow intervals of the magnetostratigraphic section above the principal reversal at the boundary between the Ivakin and Syverma Formations.
The most prominent anomalies are observed at the bases of the Morongo and Mokulai Formations. The samples from the anomalous intervals differ from those of other intervals neither in magnetic properties nor in mineralogy and magnetic grain sizes. Therefore, the revealed PMA record excursions of the geomagnetic field. Comparison of the results with the Meishan Global Stratotype Section and Point (GSSP) of the Permian-Triassic boundary implies a revision to the P-T position in the trap basalt succession of the Noril'sk area. Judging by the EMF behavior, basalts in this part of the trap province erupted for at least 500 Kyr during the interval of stable normal polarity.

Testing TEM systems has to include field experiments with physical models commensurate to the real transmitter-receiver configurations and to the target subsurface features. A large horizontal loop closed across a known resistance is a convenient model in this respect. It is convenient to lay in the field, it has manageable parameters, and its natural response is easy to calculate.
A field-size experiment and numerical modeling were applied to investigate the model wire loop response to eddy current in a uniform conductive ground, both at early and late times. The higher the resistivity of the ground, the larger the time range in which the measured response matches the predicted one, other things being equal.
The experiments show that (i) closed loops laid near a transmitter-receiver system are applicable to test the quality of the latter as a tool for TEM or other similar resistivity surveys; (ii) current induced in the model loop can be used to infer the resistivity of the ground; (iii) a closed loop slows down the growth and reduces the amplitude of voltage induced at early times in a receiver loop or in a multiturn coil.