Pyritization of fossil mollusk shells and some problems of supergene sulfide formation

The study of specific features of the pyritization of mollusk fossil shells has provided new evidence of the relationship between the generation of hydrosulfides during the bacterial reduction of sulfates and the composition of organic matter (OM) exploited by bacteria in processes of metabolism. The OM is represented by conchiolin of the ammonite shell frustule. Interaction between the bacterial H₂S and Fe²⁺ fosters the pseudomorphous replacement of conchiolin by the colloidal iron monosulfide that is subsequently transformed into pyrite. Hydrogen sulfide and/or monosulfide migrate into diagenetic cracks and cavities formed in the clayey—carbonate matrix that fills up the interior cavity of a shell. We believe that the data reported in this communication should be taken into consideration in the study of formation constraints of vein and disseminated sulfide mineralization in sedimentary rocks during the early diagenesis and related problems of ore formation.     

Ultramafic rocks from the Markov Deep in the rift valley of the Mid-Atlantic Ridge

The paper presents materials on the composition and texture of weakly serpentinized ultrabasic rocks from the western and eastern walls of the Markov Deep (5°30.6′–5°32.4′N) in the rift valley of the Mid-Atlantic Ridge (MAR). The predominant harzburgites with protogranular and porphyroclastic textures contain two major generations of minerals: the first generation composes the bulk of the rocks and consists of Ol _89.8–90.4 + En _90.2–90.8 + Di _91.8 + Chr (Cr#32.3–36.6, Mg#67.2–70.0), while the second generation composes very thin branching veinlets and consists of PlAn _32–47 + Ol _74.3–77.1 + Opx _55.7–71.9 + Cpx _67.5 + Amph _53.7–74.2 + Ilm. The syndeformational olivine neoblasts in recrystallization zones are highly magnesian. The concentrations and covariations of major elements in the harzburgites indicate that these rocks are depleted mantle residues (the high Mg# of minerals and whole-rock samples and the low CaO, Al₂O₃, and TiO₂ concentrations) that are significantly enriched in trace HFSE and REE (Zr, Hf, Y, LREE, and all REE). The mineralogy and geochemistry of the harzburgites were formed by the interaction of mantle residues with hydrous, strongly fractionated melts that impregnated them. The mineralogical composition of veinlets in the harzburgites and the mineralogical-geochemical characteristics of the related plagiogranites and gabbronorites suggest that these plagiogranites were produced by melts residual after the crystallization of gabbronorites. The modern characteristics of the harzburgites were shaped by the following processes: (i) the partial melting of mantle material simultaneously with its subsolidus deformations, (ii) brittle-plastic deformations associated with cataclastic flow and recrystallization, and (iii) melt percolation along zones of the maximum stress relief and the interaction of this melt with the magnesian mantle residue. Original Russian Text ? G.N. Savel’eva, N.S. Bortnikov, A.A. Peyve, S.G. Skolotnev, 2006, published in Geokhimiya, 2006, No. 11, pp. 1192–1208.     

Modeling climatic circulation of Onega Lake

The results of numerical modeling of Onega Lake climatic circulation are presented. The model used in this study was developed earlier and successfully applied to the reproduction of large-scale hydrodynamical processes in Ladoga Lake. The obtained results are of importance for the development of both models of nonreactive pollutant transport and ecosystem models of the water body. The developed model can be used to calculate currents and temperature fields for individual scenarios of external impacts on the water body, and the results of calculation of the climatic circulation can serve as initial approximations for scenario-based calculations.     

Dynamics of escape of high-energy electrons from a spherical plasma volume in the geomagnetic field

Using results of numerical modeling, the dynamics of escape of electrons, produced as a result beta-decay, to the external magnetic field from a spherical plasma volume with an expelled magnetic field is studied. The dependence of the fraction of escaped electrons on time and radius of the plasma volume has been obtained for two kinds of electron sources: a point isotropic source, located at the center of the sphere, and a volume isotropic source. It is shown that for a point source some part of electrons remains in the sphere, whereas for a volume source all electrons, at different values of the magnetic cavern radius, leave it at an identical relative escape rate.     

Dynamic processes in the system of interacting geospheres at the Earth’s crust-atmosphere boundary

Results of various instrumental observations of geodynamic processes in the Earth’s crust and geophysical fields at the crust-atmosphere boundary in the Oka area of the Nelidovo-Ryazan tectonic structure and adjacent fractures and in the zone of the Gornyi Altai earthquake of September 27, 2003, are presented. The correlations between the geophysical fields are determined from the results of processing and analysis of microseismic vibrations, the emanation field of natural radon, and variations in the electric field in ground and the magnetic field in the surface atmospheric layer. Tidal deformations of the crust were considered as external effects enhancing the interactions between the geospheres. It is noted that tectonic faults determining the block structure of the crust are recognizable as anomalous variations in the geophysical fields and their high cross-correlation.     

Seismicity and seismotectonic deformations of the crust in the Southern Baikal basin

This paper addresses the seismicity of the Southern Baikal basin, where the M _w = 6.0 earthquake of 1999 was the strongest over the period of instrumental observations in this region. Focal mechanisms of background earthquakes and aftershocks are analyzed in relation to faults mapped on flanks of and within the basin. Based on a supplemented catalog of focal mechanisms, the value and direction of seismotectonic strain are evaluated. The results show that the territory to the west of the transverse Angara fault (the Mishikhinskaya depression) experiences deformation of pure extension, while the E-W basin segment west of the fault is subjected to deformation of extension with shear (the transtension regime). The crustal deformation directions as determined from GPS measurements and seismological observations are found to agree well. The average seismotectonic strain rate of the crust amounts to 2.95 × 10^?9 yr^?1, which is about an order of magnitude smaller than the value obtained from geodetic observations.     

Stress-strain state of the lithosphere in the southern Baikal region and northern Mongolia from data on seismic moments of earthquakes

Investigation and understanding of the present-day geodynamic situation are of key importance for the elucidation of the laws and evolution of the seismic process in a seismically active region. In this work, seismic moments of nearly 26000 earthquakes with K _p ≥ 7 (M _LH ≥ 2) that occurred in the southern Baikal region and northern Mongolia (SBNM) (48°–54°N, 96°–108°E) from 1968 through 1994 are determined from amplitudes and periods of maximum displacements in transverse body waves. The resulting set of seismic moments is used for spatial-temporal analysis of the stress-strain state of the SBNM lithosphere. The stress fields of the Baikal rift and the India-Asia collision zone are supposed to interact in the region studied. Since the seismic moment of a tectonic earthquake depends on the type of motion in the source, seismic moments and focal mechanisms of earthquakes belonging to four long-term aftershock and swarm clusters of shocks in the Baikal region were used to “calibrate” average seismic moments in accordance with the source faulting type. The study showed that the stress-strain state of the SBNM lithosphere is spatially inhomogeneous and nonstationary. A space-time discrepancy is observed in the formation of faulting types in sources of weak (K _p = 7 and 8) and stronger (K _p ≥ 9) earthquakes. This discrepancy is interpreted in terms of rock fracture at various hierarchical levels of ruptures on differently oriented general, regional, and local faults. A gradual increase and an abrupt, nearly pulsed, decrease in the vertical component of the stress field S _v is a characteristic feature of time variations. The zones where the stress S _v prevails are localized at “singular points” of the lithosphere. Shocks of various energy classes in these zones are dominated by the normal-fault slip mechanism. For earthquakes with K _p = 9, the source faulting changes with depth from the strike-slip type to the normal-strike-slip and normal types, suggesting an increase in S _v . On the whole, the results of this study are well consistent with the synergism of open unstable dissipative systems and are usable for interpreting the main observable variations in the stress-strain state of the lithosphere in terms of spatiotemporal variations in the vertical component of the stress field S _v . This suggests the influence of rifting on the present-day geodynamic processes in the SBNM lithosphere.