Publishing House SB RAS:

Publishing House SB RAS:

Address of the Publishing House SB RAS:
Morskoy pr. 2, 630090 Novosibirsk, Russia



Advanced Search

Russian Geology and Geophysics

2004 year, number 5

1.
URANIUM-SERIES ISOTOPES AS PROXIES OF LATE PLEISTOCENE CLIMATE AND GEOCHRONOMETERS IN BOTTOM SEDIMENTS OF LAKE BAIKAL

E.P. Chebykin, D.N. Edgington*, E.L. Goldberg, M.A. Phedorin, N.S. Kulikova, T.O. Zheleznyakova, S.S. Vorob'yova, O.M. Khlystov, O.V. Levina, G.A. Ziborova, and M.A. Grachev
Limnological Institute, Siberian Branch of the RAS, 3 ul. Ulan-Batorskaya, Irkutsk, 664033, Russia
* Great Lakes WATER Institute, University of Wisconsin, Milwaukee, USA
Keywords: Climate proxy, uranium-series isotopes, activity ratios, ICP-MS, diatoms, Lake Baikal
Pages: 497-513

Abstract >>
238U, 234U, and 230Th concentrations in samples of Baikal bottom sediments from the submerged Akademichesky Ridge were measured by ICP-MS which is 100 times as fast as the traditional alpha spectrometry. These concentrations, and the respective activities A8, A4, and A0, show abrupt oscillations. The A4/A8 ratio varies more smoothly between 1.0 and 1.8 and correlates with the diatom record. A0 increases with depth in high-A4 intervals because 234U decays to 230Th ( = 245.3 kyr). According to our geochemical model, excess 234U (A4/A8 > 1) is supplied into Lake Baikal in the soluble form. This model, along with new data, was used to determine U-Th ages of six sediment intervals. In the suggested linear age-depth model for the core, the A4/A8 profile is similar to the SPECMAP record for the past 140 kyr. Fluxes of sediment components were estimated from sedimentation rates, water content, BiSi, and picnometric clay density. The clay flux during MIS 2, 3, 4 (controlled mostly by mountain glaciers) was four times as high as in the interglacials (MIS 5.5, 5.3, 5.1, and 1). The flux of authigenic uranium adsorbed by sediments from the Baikal water was almost zero during MIS 2 and 4, presumably because of strongly reduced Selenga discharge, the main source of dissolved uranium. The input of nutrients ceased for the same reason, and diatoms could not develop. Repeated cycles of aridization and the related discontinuity in river nutrient input may have been a major mechanism responsible for the formation of the unique assemblage of endemic organisms in Lake Baikal.
																								



2.
RECONSTRUCTION OF LATE PLEISTOCENE AND HOLOCENE ABIOTIC CONDITIONS IN LAKE BAIKAL

M.N. Shimaraev, and I.B. Mizandrontsev
Limnological Institute, Siberian Branch of the RAS, 3 ul. Ulan-Batorskaya, Irkutsk, 664033, Russia
Keywords: Climate, Holocene, Late Pleistocene, abiotic conditions, chemical weathering, Lake Baikal
Pages: 514-520

Abstract >>
Late Pleistocene (late Glacial) and Holocene abiotic conditions in Lake Baikal have been reconstructed for the past 16 kyr at a resolution of 200 yr on the basis of a detailed time scale of climate events in the Northern hemisphere and atmospheric CO2. Severe glacial conditions in the latest Pleistocene are reflected in an extremely low input of biogenic elements from the Baikal drainage basin which may be responsible for very low diatom abundances in the bottom sediments of the lake. Biogenic input increased significantly in a warm and humid climate between 11 and 9 kyr as a result of deglaciation, vegetation advance, and higher rates of chemical weathering supplying Si- and P-bearing minerals. The abrupt increase in diatom production about 8 kyr BP may have been caused by improved physical conditions (higher sub-ice insolation, thinner layer of convective mixing, etc.) and by rapidly increasing river input of nutrients.
																								



3.
CHROMIUM ASSEMBLAGE OF MINERALS IN MICACEOUS KIMBERLITES OF YAKUTIAN PROVINCE

S.I. Kostrovitsky and D. de Bruin*
Institute of Geochemistry, Siberian Branch of the RAS,
1a ul. Favorskogo, Irkutsk, 664033, Russia
* Geological Survey of South Africa,
private box X112, Pretoria, South Africa, 0001
Keywords: Micaceous kimberlite, high-Cr assemblage, zonal garnets, kosmochlor, phenocryst origin
Pages: 521-535

Abstract >>
Study was given to the composition of garnet and its inclusions from micaceous Cr-diopside-containing kimberlites making up a few pipes of the Daldyn field in the Yakutian kimberlite province. The garnets of almandine-pyrope composition containing up to 10% knorringite end-member are of lherzolite and, partly, wehrlite parageneses. The inclusions (kosmochlor + Cr-spinel + phlogopite) belong to high-Mg high-Cr assemblage and are characterized by considerable variations of composition reflecting nonequilibrium conditions of crystallization in a wide range of pressures and temperatures. Different types of garnet zoning suggest a complicate mechanism of its formation: garnet crystallization under decreased pressure with simultaneous recrystallization of the garnet matrix in dislocation microzones. A common explanation of zoning as a result of garnet recrystallization alone is also possible. Genesis of the assemblage is discussed. Alternative explanations are proposed which relate the origin of high-Cr assemblage of minerals to (1) crystallization from a kimberlite melt and (2) metasomatic influence of the kimberlite melt on garnet macrocrysts.
																								



4.
NOBLE-METAL MINERALIZATION IN RODINGITES OF THE SOUTHEASTERN EAST SAYANS

B.B. Damdinov, S.M. Zhmodik*, A.G. Mironov, and Yu.Ch. Ochirov
Geological Institute, Siberian Branch of the RAS, 6a ul. Sakh'yanovoi, Ulan Ude, 670047, Russia
* United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of the RAS,
3 prosp. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Rodingites, PGE, gold, subduction zone, East Sayans
Pages: 536-546

Abstract >>
Noble-metal mineralization in rodingites of the southeastern East Sayans was studied. Along with research into the lithology of the rocks, we carried out a complex analysis for their noble metals and major and trace elements. We elucidated some petrochemical features of the rocks and the distribution of PGE, Au, and Ag in rodingites from different areas. The rocks are shown to bear Pt-Pd mineralization. In addition, Pt and Au-Sn-Hg types of mineralization have been revealed in rodingites and pyrite-Pd-bearing metasomatites of the Khurai-Zhalga Massif. Platinoids accumulate mainly in the form of sulfides. Ore and noble-metal types of mineralization formed by transformation of rocks in the subduction zone and their enrichment with Pt, Pd, Au, Sn, and Hg. The ore components might have been supplied with fluids from the subduction zone as well as from the host rocks.
																								



5.
THE AGE OF CLINOPYROXENITES AND KOSWITES OF THE KONDER MASSIF (from their paleomagnetic characteristics)

A.S. Karetnikov
Institute of Tectonics and Geophysics, Far Eastern Branch of the RAS,
65 ul. Kim-Yu-Chena, Khabarovsk, 680063, Russia
Keywords: Paleomagnetic study, ultrabasites, petrographic analysis
Pages: 547-558

Abstract >>
Ultrabasites of the Konder Massif (57
																								



6.
NATURAL AND EXPERIMENTAL CONDITIONS OF FORMATION OF COARSE-GRAINED PRECIOUS OPAL (nucleation and number of centers of supramolecular crystallization)

D.V. Kalinin, V.V. Serdobintseva, and Ya.V. Kuznetsova
United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of the RAS,
3 prosp. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Precious opal, supramolecular crystal, granularity, crystal seed, number of nucleation points, coarse-grained structures
Pages: 559-564

Abstract >>
Natural coarse-grained precious opal structures result from the concentration of suspensions of monodisperse silica spheres and their subsequent supramolecular crystallization at low pH values. These processes occur in different media and at different times. Nucleation is determined by the diffusion of structural units at the interface, their large volume, and the insignificant incorporation of the system into the metastable zone. On synthesis of precious opal, the rate of supramolecular crystallization is determined by sedimentation of structural units, and the frequency of crystal seeding depends mainly on the viscosity of the dispersed medium.
																								



7.
REGULARITIES OF FORMATION OF LARGE PHOSPHORITE DEPOSITS

E.A. Eganov and E.L. Shkol'nik*
Institute of Petroleum Geology, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibisrk, 630090, Russia
* Far Eastern Geological Institute, Far Eastern Branch of the RAS,
159 prosp. Stoletiya Vladivostoka, Vladivostok, 690022, Russia
Keywords: Phosphorites, genesis of phosphorites, phosphorite deposit
Pages: 565-573

Abstract >>
Different hypotheses of phosphorite formation are based on the supposed existence of hypogene sources of phosphorus whose intensity is responsible for the rhythmicity of accumulation of large subsynchronous deposits of phosphorites. Comparison of volcanic and exhalation activities with epochs of phosphorite accumulation, considered to be of global importance, is not safe and is even false in some aspects. The formation of large deposits of rich phosphorites shows a poor correlation with the supposed intense supply of phosphorus from depth. It has been shown that a set of paleogeographical conditions rather than phosphorus concentration in oceanic waters by endogenetic processes is of critical importance. The intensity of an uncertain phosphorus source feeding the sea waters is not crucial for formation of large deposits of phosphorites. The critical factors are the way of mineralization of phosphates and mechanism of their accumulation to form a deposit.
																								



8.
LAKE ISSYK KUL' (TIEN SHAN): UNUSUALLY LOW HEAT FLOW IN AN ACTIVE INTERMONTANE BASIN

P. Vermeesch1, J. Poort2,1, A.D. Duchkov3, J. Klerkx2, and M. De Batist1
1 Renard Centre of Marine Geology, University of Ghent, Krijgslaan 281 S8, B-9000 Ghent, Belgium
2 Department of Geology and Mineralogy, Royal Museum for Central Africa,
Leuvensesteenweg 13, B-3080 Tervuren, Belgium
3 Institute of Geophysics, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Heat flow, geothermal gradients, geodynamical models, Lake Issyk Kul', Tien Shan, Kyrghyzstan
Pages: 574-584

Abstract >>
New geothermal data are presented for Lake Issyk Kul', a mountain lake in the northern Tien Shan, Central Asia. With 40 successful measurements done, the average heat flow is 48.3 mW/m2. This is 10 to 20 mW/m2 lower than estimates from previous surveys. Data from boreholes on the shores of the lake revealed an average heat flow of 54 mW/m2. All these values are remarkably low for an actively deforming area such as the Tien Shan. Two qualitative geodynamical models are suggested to explain these observations. If there is crustal thickening beneath Issyk Kul', the low heat flow can only be explained in a transient mode, with
																								



9.
CHARACTERISTIC TIMES OF IMPURITY EXTRACTION FROM SURFACTANT-BEARING MAGMA MELTS

V.N. Dorovsky
United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Magma melt, surfactant, extraction
Pages: 585-597

Abstract >>
Extraction of impurities from a magma melt containing dissolved surfactants and gas bubbles is analyzed for the simplest hydrodynamic conditions in which impurity particles move to the surface of a bubble. Their flux is proportional to D1/5 (D is the diffusion coefficient of surfactants), and the characteristic time of extraction in these conditions is very short. In the case of a rapidly expanding bubble, the particle flux is low. In the case of a contracting bubble, the flux is likewise proportional to D1/5 but the particles are concentrated in a halo around the bubble rather than on its surface.
																								



10.
SEISMOTECTONICS OF THE BAIKAL RIFT AND ITS MOUNTAINOUS SURROUNDINGS

V.V. Nikolaev† and R.M. Semenov
Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia
Keywords: Geomorphic structures, active faults, lithospheric block, earthquake source, seismic activity, earthquake mechanisms, earthquake magnitude
Pages: 598-604

Abstract >>
The Baikal rift is an active seismic region, as many continental rifts. Its high seismicity is associated with ongoing synrift tectonism with faulting of various geometries in rift basins and on rift ridges, under the effect of the ESE motion of the Transbaikalian block. In the suggested seismotectonic map of Siberia and the Russian Far East, the rift borders stand out against the almost aseismic Siberian craton, the moderately seismic Aldan-Okhotsk and Transbaikalian regions, and highly seismic North Mongolia and Baikal rift basins.