REE distribution in the resin-asphaltene fractions as a geochemical criterion for identification of sources of trace elements in oil

This paper reports REE data on resin-asphaltene components of oil from six oil-gas-bearing provinces and on bitumoids from inferred oil-source rocks (domanikites and bazhenites). It was shown that, regardless of geological-tectonic structure of the regions, oil composition, depth of reservoirs, and host lithologies, oil exhibits significant REE fractionation, and, unlike bitumoids, positive Eu anomaly. The (Eu/Sm)_n ratio increases from asphaltenes to resins and further to oils. Based on REE distribution in oil, source rocks, and bitumoids, it was concluded that deep-seated fluids were one of the possible sources that defined the trace element composition of oil.     

U-Th-Pb systematics of precambrian carbonate rocks: Dating of the formation and transformation of carbonate sediments

The current state in the Pb-Pb dating of the formation and epigenesis of Precambrian carbonate sediments has been reviewed. The geochemistry of admixture elements (U, Th, Pb, Rb, Sr, Mn, and Fe) constituting carbonate minerals and the redistribution of the elements in the course of early diagenesis and epigenesis have been considered. The advantages of choosing samples for Pb-Pb dating on the basis of geochemical criteria similar to those applied for Sr-isotope stratigraphy and the potentialities of applying different methods of decontamination of specimens, analyzed from surface contamination and possible epigenetic carbonate phases, were illustrated. By the example of a series of carbonate formations, distinctions in susceptibility of U-Pb and Rb-Sr systems of carbonate minerals to secondary transformations have been demonstrated. The prospects for using the method of step solution allowing noncogenetic carbonate generations to be separated and thus the accuracy and reliability of Pb-Pb dates to be upgraded were considered.     

Dynamics of suspended matter concentration in Chupa Inlet (Karelian Coast,the White Sea) in summer and autumn

Studies on the water and suspended matter concentration dynamics were performed for the summer and autumn periods in Kruglaya Bay, Chupa Inlet (Karelian Coast, the White Sea). The tidal cycle appeared as the major factor defining the water salinity, temperature, and suspended matter concentration. The water temperature and suspended matter concentration increased during low tide, and the opposite pattern was observed for water salinity; all the processes were true for both surface and bottom water layers. Therefore, we conclude that major bulk of the allochtonous suspended matter is carried into the sea by the ebb stream.     

U-Pb isotopic dating of zircons (SHRIMP-II) from granulites of the Ol’khon region of Western Baikal area

Doklady Earth Sciences -     

Geochemical evidence for the ophiolitic nature of the gabbros,gabbroanorthosites, and ultramafic rocks of the Gerbikan fragment of the Baladek Massif of the southeastern framing of the North Asian craton

The Baladek Massif represents a tectonic block located at the boundary between the SE framing of the North Asian craton and the fold structures of the Galam terrane of the Mongol-Okhotsk fold belt. According to the existing concepts, the rocks of this massif are regarded as Precambrian complexes of the aforementioned craton. This paper reports the results of studying the fragment of the Baladek Massif in the Gerbikan River basin. New geochemical data obtained on the gabbros, gabbroanorthosites, and ultramafic rocks testify to their similarity with typical ophiolite complexes. An underlying complex of metamorphic ultrabasic rocks, a layered complex of pyroxenites and “cumulate” gabbros, gabbroanorthosites, and a complex of isotropic gabbros were distinguished within this fragment. It is highly possible that the basic volcanic rocks and jaspers of the Silurian-Lower Devonian sequences of the Galam segment of the Mongol-Okhotsk fold belt also belong to the inferred ophiolite complex.     

Noble-metal mineralization in the Semenov-2 hydrothermal field (13°31′N), mid-atlantic ridge

The porous fine-grained to microcrystalline copper-zinc ore of the Semenov-2 hydrothermal field, a site in the Semenov hydrothermal cluster discovered in 2007 (13°31′N, MAR), is anomalously enriched in Au (22–188 ppm) and Ag (127–1787 ppm). Chalcopyrite, isocubanite, würtzite, and opal are major minerals; sphalerite, marcasite, pyrite, and covellite are auxiliary; and galena, pyrrhotite, native gold, silver telluride, barite, and aragonite are sporadic. Gold containing 0.31 to 23.07 wt % Ag occurs as up to 9-μm-sized subhedral, dendritelike, and elongated grains mostly hosted in opal and less frequently in sphalerite and in pores within isocubanite-chalcopyrite aggregate. An elongated grain (2 × 4 μm in size) of the Ag-Te phase was found in a pore. So far only basalts have been dredged from the Semenov-2 field, but anomalous gold and silver concentrations suggest the influence of ultramafic rocks; the latter were found 1.5 km westward, in the Semenov-1 hydrothermal field. Mineral assemblage and morphology of gold particles indicate its primary origin in contrast to the hydrothermal fields hosted in basalts, where gold is a product of remobilization. Zonal gold grains, found on oceanic floor for the first time, are characterized by low Ag content in the cores and high Ag content in the outer rims, reflecting variation in formation conditions.