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Russian Geology and Geophysics

2008 year, number 8

Analysis of experimental data on the diffusion coefficients of Fe, Mn, Mg, and Ca in garnets

V.N. Korolyuk, G.G. Lepezin*
Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Diffusion coefficients; compensation law; garnets; metamorphism; duration
Pages: 557-569

Abstract >>
Experimental data on the diffusion coefficients of Fe, Mn, Mg, and Ca in garnets are generalized. Frequency factors (cm2/s), activation energies (cal/mole), and activation volumes (cm3/mole) are optimized. Based on the compensation law, the dependences of diffusion coefficients on pressure (kbar) and temperature (K) have been derived:
D Fe = 1.0 · 10-3 exp [ - (61450 + 23.9 · 8 · P ) / (RT) ] ,
D Mn = 2.3 · 10-6 exp [ - (44600 + 23.9 · 6 · P) / (RT) ] ,
D Mg = 4.9 · 10-3 exp [ - (65900 + 23.9 · 9.2 · P ) / (RT) ] ,
D Ca = 4.1 · 10-5 exp [ - (62450 + 23.9 · 11.2 · P) / (RT)] .

Petrology of the Early Mesozoic ultramafic-mafic Luchina massif ( southeastern periphery of the Siberian craton )

I.V. Buchko a , A.A. Sorokin a , A.E. Izokh b , A.M. Larin c , A.B. Kotov c , E.B. Sal'nikova c , S.D. Velikoslavinskii c , A.P. Sorokin a , S.Z. Yakovleva c , Yu.V. Plotkina c
a Institute of Geology and Nature Use, Far East Branch of the RAS, 2 ul. Khmel'nitskogo, Blagoveshchensk, 675000, Russia
b Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Koptyuga, Novosibirsk, 630090, Russia
c Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, 2 nab. Makarova, St.Petersburg, 199034, Russia
Keywords: Layered massifs; primary melt; geochronology; U-Pb method; subduction; plume; Dzhugdzhur-Stanovoy superterrane
Pages: 570-581

Abstract >>
Three zones of layered series - lower, middle, and upper - composed of dunites and plagiodunites, troctolites and olivine gabbros, gabbros and gabbronorites, respectively, have been recognized in the Luchina massif. The melt that produced the massif rocks was of picrite-basaltic composition (15-16% MgO), and its crystallization took place at 1300-1000

Tectonic control of mafic intrusions in the Tastau volcanoplutonic complex (eastern Kazakhstan)

K.A. Dokukina a , V.G. Vladimirov b
a Geological Institute, Russian Academy of Sciences,7 Pyzhevsky per., Moscow, 119017, Russia
b Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Melt injection; melt fragmentation; synkinematic magmatism; shear zone; peperite
Pages: 582-593

Abstract >>
We discovered unusual very small (2 to 70 cm) mafic bodies intruded into metamorphosed Early Carboniferous sandstone and shale in the Late Paleozoic Tastau volcanoplutonic complex (Char shear zone, eastern Kazakhstan). The small intrusions possibly emplaced during shearing when sediments experienced cataclasis and ensuing viscosity decrease. The shear rate was comparable to that in historic crustal earthquakes. Rapid shear motion concurrent with magmatism caused fragmentation of the injecting low-viscosity melt and its dispersal in globular and other bodies along linear breccia zones. The origin of the intrusions is explained in a tectonic model.

Mechanisms and species of gold incorporation into crystals of cadmium, lead, and iron sulfides

V.L. Tauson, N.V. Smagunov, V.V. Akimov, V.A. Datkov
Institute of Geochemistry, Siberian Branch of the RAS, 1a ul. Favorskogo, Irkutsk, 664033, Russia
Keywords: Gold; species; real crystals; galena; greenockite; pyrrhotite; pyrite; endocrypty; defects
Pages: 594-601

Abstract >>
The mechanisms and species of gold incorporation into crystals of simple sulfides are described. The concept of endocrypty in its modern treatment and the corresponding technique of experimental study were used. The technique is based on the principle of phase composition correlation and variation of parameters specifying the concentrations of active crystal defects. Gold species are determined with the method of statistical samples of analytical data for single crystals (SSADSC). The highest gold incorporation limits have been established for PbS and CdS under high sulfur fugacity (2.4 · 10-2 and 7 · 10-3 wt.%, respectively) at 500

The Late Cenozoic of northern Transbaikalia and paleoclimates of southern East Siberia

F.I. Enikeev
Chita Geological Survey, 91/15 ul. Amurskaya, Chita 672090, Russia
Keywords: Late Cenozoic; Pleistocene; paleoclimate; palynology; Chara basin; Transbaikalia
Pages: 602-610

Abstract >>
Comprehensive studies have been applied to a core from well 126 in the Chara basin (eastern Baikal-Stanovoy upland). The 1180 m thick drilled section comprises Upper Pleistocene sediments (0-173.1 m) with two intervals of varved silt coeval with the Wurm Glacial, two Middle Pleistocene glacial intervals at 180-263.5 m and 337-424.7 m, earliest Pleistocene (424.7-550 m), Pliocene (550-1036 m), and Miocene (1036-1180 m) strata. The core records the history of Late Cenozoic climate and vegetation for the past 20 Myr (with two large gaps), which has been reconstructed using the pollen analysis. Interpolation of long- and medium-period climate oscillations tied to the time scale allows predicting a trend of possible climate change for the future 2-2.5 kyr.

Distribution of local earthquakes in the crust beneath central Lake Baikal

V.D. Suvorova, Ts.A. Tubanovb
a Institute of Petroleum Geology and Geophysics, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
b Geological Institute, Siberian Branch of the RAS, 6a ul. Sakhyanovoi, Ulan-Ude, 670047, Russia
Keywords: hypocenter location; crust; Baikal rift
Pages: 611-620

Abstract >>
We have located earthquake hypocenters in the area of central Lake Baikal using the Hypoellipse inversion of direct P traveltimes collected by a local seismological network of seven stations in 2001 through 2005. The location accuracy depends on the agreement between the reference velocity model and the real subsurface. Traveltime residuals have no bearing on the accuracy of depth estimates due to origin time errors.
Hypocenter location with reference to a DSS-derived layered velocity model predicts that the crust is seismogenic between 10 and 22 km. The hypocenters cluster densely in belts trending in NE and SE directions along and across Lake Baikal. Earthquakes become deeper from 10 to 22 km toward the Selenga basin.

Numerical solution of Maxwell's equations for anisotropic media using the Laguerre transform

A.F. Mastryukov, B.G. Mikhailenko
Institute of Computing Mathematics and Mathematical Geophysics, Siberian Branch of the RAS, 6 prosp. Akad. Lavrentieva, Novosibirsk, 630090, Russia
Keywords: Maxwell's equations; electromagnetic waves; anisotropic media; relaxation time; conductivity; dielectric permittivity; Laguerre transform; finite-difference time-domain method; linear equation system
Pages: 621-627

Abstract >>
We apply the spectral Laguerre transform in the time domain to solve 2D Maxwell's equations for propagation of electromagnetic waves in lossy anisotropic media. The new algorithm is simple and efficient as Maxwell's equations are reduced to a harmonic series of linear algebraic equations where the matrix is independent of the harmonic order and is the same for all harmonics.
The efficiency of the algorithm is improved by fitting a specially introduced free parameter of the Laguerre transform. If it is large, the solution spectrum shifts toward higher harmonics which is formally equivalent to the case of ray approximation.
The Laguerre solution is comparable with high-order accurate finite-difference time-domain (FDTD) solutions. The method is stable both in the region of the wavefield, where conductivity approaches zero and the spectral Fourier method is unstable, and in the high-conductivity region, where the explicit FDTD code requires a too small time step.

A two-loop frequency-domain electromagnetic induction system for detection of anomalous objects

A.I. Chelovechkov, B.M. Chistoserdov, S.V. Baidikov
Institute of Geophysics, Uralian Branch of the RAS, 100 ul. Amundsena, Ekaterinburg, 620016, Russia
Keywords: Two-loop measurement system; field focussing; frequency-domain electromagnetic induction sounding
Pages: 628-633

Abstract >>
We discuss a two-loop frequency-domain electromagnetic induction sounding system which allows vertical focussing of the primary magnetic field thus reducing the disturbance from shallow subsurface. The frequency dependence of apparent resistivity estimated from the real part of the measured magnetic field turns out to be more sensitive than that found from the absolute total field. The advantage of the suggested FD EMI system has been validated by experiments.