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Russian Geology and Geophysics

2003 year, number 9

1.
Ni-RICH SULFIDE INCLUSIONS IN EARLY LAMPROITE MINERALS

V.V. Sharygin, L.N. Pospelova, S.Z. Smirnov, and N.V. Vladykin*
United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
* Institute of Geochemistry, Siberian Branch of the RAS, 1a ul. Favorskogo, Irkutsk, 664033, Russia
Keywords: Lamproite, liquid immiscibility, sulfide globule, monosulfide solid solution, chalcopyrite, pentlandite, heazlewoodite, godlevskite, violarite
Pages: 817-828

Abstract >>
Magmatic sulfide inclusions were found in early lamproite minerals from three localities: in olivine-1 of olivine hyalolamproite from Smoky Butte (Montana, USA), in olivine-1 of olivine orendite from Leucite Hills (Wyoming, USA), in orthopyroxene and phlogopite of fortunite from SE Spain. Sulfide blebs usually coexist with fluid and silicate melt inclusions in the host mineral. Occasionally they are found in silicate melt inclusions. Sulfide inclusions contain the following assemblages: pentlandite + chalcopyrite, monosulfide solid solution (Mss) + chalcopyrite, violarite - in the Smoky Butte lamproites; Mss + pentlandite and Mss + chalcopyrite in the Leucite Hills orendites; pentlandite + heazlewoodite, pentlandite + pyrrhotite and pentlandite + godlevskite - in the SE Spain fortunites. Pentlandite broadly varies in Ni/(Ni+Fe) ratio from 0.44 to 0.71. The Smoky Butte Mss has the highest concentrations of Ni (up to 58.2 wt.%) and is close to ideal (Ni,Fe)S (Me/S - 0.98-1.00), while the Leucite Hills Mss contains up to 27.5 wt.% Ni and has a composition intermediate between hexagonal (Fe,Ni)9S10 (Me/S - 0.9) and monoclinic (Fe,Ni)7S8 (Me/S - 0.875) compositions. Chalcopyrite is similar to ideal composition of the CuFeS2-type. Pyrrhotite compositions vary from FeS to (Fe,Ni)9S10. Ni-rich minerals were found in enstatite of fortunites. They are identified chemically as heazlewoodite solid solution (Ni - 51.1-62.4, Fe - 8.5-17.6, S - 28.7-30.9 wt.%) and godlevskite and/or godlevskite solid solution (Ni - 57.3-62.9, Fe - 4.4-10.3, S - 32.1-32.6 wt.%). Probable composition for initial sulfide melt (ISM) calculated for globules vary for different lamproite localities. In the Smoky Butte lamproites it is undersaturated in sulfur and close in composition to pentlandite (Me/S - 1.10) with approximately equal Ni/(Ni+Fe) ratio (0.51-0.53) and contains up to 33.3 wt.% Ni and up to 5.3 wt.% Cu. Initial sulfide melt in fortunites is also depleted in sulfur (Me/S - 1.11-1.21), but it shows broad variations in Ni content, from 27 to 51 wt.%. Initial sulfide melt in the Leucite Hills orendites is saturated in sulfur (Me/S - 0.91) and contains up to 23.4 wt.% Ni and up to 5.1 wt.% Cu.
The presence of sulfide globules indicates that a small volume of sulfide melt was separated from the silicate liquid at the early stage of evolution of a primitive lamproite magma under T >> 1000
																								



2.
SPECTROSCOPIC STUDY OF THE SURFACE OF STOICHIOMETRIC PYRRHOTITE CRYSTALS OBTAINED IN THE PRESENCE OF CADMIUM UNDER HYDROTHERMAL CONDITIONS

V.L. Tauson
Institute of Geochemistry, Siberian Branch of the RAS, 1a ul. Favorskogo, Irkutsk, 664033, Russia
Keywords: X-ray photoelectron spectroscopy, Auger spectroscopy, trace elements, incompatible elements, uptake mechanism, sorption, endocrypty, pyrrhotite, cadmium
Pages: 829-833

Abstract >>
Auger and X-ray photoelectron spectroscopies were used to study crystals of stoichiometric pyrrhotite produced in the system Fe-S-CdS-NH4Cl-H2O under hydrothermal conditions at 450
																								



3.
IMPURITIES AND CARBON ISOTOPE COMPOSITIONS OF MICRODIAMONDS WITH EXTRA FACES FROM THE UDACHNAYA KIMBERLITE PIPE

D.A. Zedgenizov, V.N. Reutsky, V.S. Shatsky, and E.N. Fedorova
Institute of Mineralogy and Petrography, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Diamond, nitrogen, carbon isotope composition
Pages: 834-841

Abstract >>
Microdiamonds with extra faces from the Udachnaya kimberlite pipe have no morphologic analogs among the known microdiamonds. The cause of the appearance of such faces on octahedral microdiamonds is still obscure. The results obtained in this study shed light upon the defect-impurity and carbon isotope compositions of the Udachnaya microdiamonds with additional faces. Predominant nitrogen defects in the diamond structure are A and B1. Most of the microcrystals contain hydrogen impurities as well. A specific feature of these microdiamonds is low concentration and significant aggregation of nitrogen defects. The obtained values of 13C vary from -2.76 to -7.16
																								



4.
U-Pb, Ar-Ar, and Sm-Nd ISOTOPE-GEOCHRONOLOGICAL STUDY OF PORPHYRITIC SUBALKALIC GRANITES OF THE TARAKA PLUTON (Yenisei Range)

A.D. Nozhkin, E.V. Bibikova*, O.M. Turkina, and V.A. Ponomarchuk
United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
* Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences,
19 ul. Kosygina, Moscow, 117975, Russia
Keywords: Granites; U-Pb, Ar-Ar, and Sm-Nd dating; Precambrian, Yenisei Range
Pages: 842-852

Abstract >>
We present first results of complex U-Pb, Ar-Ar, and Sm-Nd isotope study of porphyritic subalkalic granites of the Taraka pluton (Yenisei Range), one of the largest Proterozoic Na-K plutons on the southwestern and southern periphery of the Siberian Platform. The obtained U-Pb dates for zircon indicate that the subalkalic granites formed 1837 3 myr BP. The trace-element composition and Nd isotope characteristics of the granites evidence that they were produced from a highly differentiated mature sialic source no older than 2566 Ma. With Ar-Ar dating of the granite minerals, we made an attempt to elucidate the time of subsequent tectonothermal events, each of which opened a new epoch in the development of the Ar-Ar systems.
																								



5.
GEOCHEMISTRY AND CONDITIONS OF FORMATION OF THE SOLGON GRANITOID BATHOLITH, KUZNETSK ALATAU

V.I. Grebenshchikova and Yu.V. Maksimchuk
Institute of Geochemistry, Siberian Branch of the RAS, 1a ul. Favorskogo, Irkutsk, 664033, Russia
Keywords: Geochemistry, granitoids, batholith, modeling
Pages: 853-869

Abstract >>
Under consideration is the geology and geochemistry of the granitoid rocks (from gabbro to leucogranite) making up a large Early Paleozoic batholith of the Kuznetsk Alatau. The early (progressive) stage of batholith formation was due to magmatic substitution and partial melting of the metabasite crust under the effect of a subalkaline mantle-derived magma. The rocks formed at this stage are exocontact hornfelses and endocontact hybrid diorites and monzonite of hybrid habit. The transitions between these rocks are illustratively displayed in the pattern of distribution of rare-earth elements. The next (regressive) stage was due to the differentiation of a monzonitoid melt and its crystallization, which proceeded against the background of a considerable decrease in alkalinity of the residual melt and led to the formation of the main facies rock variety - granodiorite-tonalite. It is characterized by higher contents of Ba, Sr, B, and IGE, lower contents of K, Li, Rb, and Cs, and by very low concentrations of Be, Sn, W, Nb, and Ta as compared with palingenic granitoids of other regions.
The granitic magma was generated in a separate chamber and was not produced by differentiation of a granodiorite melt. In composition, the hornblende-biotite granites are close to trondhjemites (Na/K > 1). A small volume of the residual melt formed during the fractionation of the granitic magma appeared in the stocks and dikes of subalkalic leucogranites.
The presence of postbatholith dikes of basic and intermediate compositions and their closeness to the composition of monzonitoid rocks of batholith suggest that a deep-seated source of magma (and heat) had existed for a long time. The postbatholith dikes of quartz porphyries (ongonites) are characterized by higher, as compared with the other rocks of the batholith, contents of rare ore elements (Rb, Be, B, Sn, Nb, Ta, F) and higher temperatures of formation, which reflects the metamagmatic (fluid-magmatic) stage and is typical of the granites of the same kind in regions of
																								



6.
PORPHYRITIC POTASSIUM-RICH ALKALINE-ULTRABASIC ROCKS OF THE CENTRAL TOMTOR MASSIF (Arctic Siberia): CARBONATIZED LAMPROITES

S.M. Kravchenko
Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences,
35 Staromonetny per., Moscow, 109017, Russia
Keywords: Lower ore horizon, lamproite, volcanic rock, carbonatization
Pages: 870-883

Abstract >>
We have discovered almost unaltered porphyritic potassium-rich alkaline-ultrabasic rocks in the central Tomtor Massif and have first carried out an X-ray microanalysis of the hosted minerals (phlogopite, leucite, clinopyroxene, amphibole, K-feldspar, etc.). Using modern analytical methods (neutron activation and X-ray fluorometry), we determined the contents of 43 major, trace, and radioactive elements and CO2 in 16 samples of differently carbonatized porphyritic rocks and estimated the content of CaO in the unaltered rocks. A correlation analysis showed that during carbonatization, the rocks received Ca, Nb, REE, Sr, and Ba and lost Si, Ti, Mg, and Na, thus transforming into rocks compositionally similar to kamafugites. The obtained structural data on concentric faults, horizontal separations, phenocryst orientation, etc. suggest that the studied potassium-rich alkaline-ultrabasic rocks are volcanics that occur in a subsidence caldera 6-10 km across. Based on the classification diagrams of Holmes, Foley et al., and La Roche, we have established that the porphyritic rocks are dominated by lamproites. The Tomtor Massif includes a unique
																								



7.
THE STRUCTURE AND STRESS-STRAIN FIELD OF THE BAIKAL-PATOM FOLDBELT: IMPLICATIONS FOR ORIGIN

A.V. Sintsov
Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia
Keywords: Orogen, fold, fault, strain, stress
Pages: 884-892

Abstract >>
The structure and stress-strain field of the Baikal-Patom orogen (fold arc) show its origin under uniaxial N-S compression. Testing of two alternative models implying pressure from the south or from the north suggests that the stress field and the related strain formed during southward (in the present frame of reference) motion of the Siberian craton.
																								



8.
THERMAL STATE OF LITHOSPHERE IN MONGOLIA

S.V. Lysak and R.P. Dorofeeva
Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia
Keywords: Temperature, geothermal gradient, thermal conductivity, heat production, heat flow, hot springs, tectonic activity, thermal evolution
Pages: 893-903

Abstract >>
The thermal state of lithosphere in Mongolia has been modeled on the basis of measured regional heat flows and predicted temperatures at depths of 0 to 50 km. The highest temperatures of 1100-1200
																								



9.
FAST TWO-DIMENSIONAL SIMULATION OF HIGH-FREQUENCY ELECTROMAGNETIC FIELD IN INDUCTION LOGGING

M.I. Epov and V.N. Glinskikh
Institute of Geophysics, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Forward problem, approximation approaches, electromagnetic field, background medium model, HFIL soundings
Pages: 904-915

Abstract >>
New approximate algorithms applied to axisymmetrical induction logging are based on generalized Neumann representation of high-frequency electromagnetic field. Different approximations to forward problems in HFIL soundings use uniform or layered models of the background medium. The accuracy of the linear approximations depends on the background conditions, sizes of perturbations, and electrical conductivity contrasts. Accuracy can be improved by appropriate selection and correction for the background field whereas the perturbation component is represented as a linear anomaly. It is proposed to model the background medium with regard to attenuation of electromagnetic field in complex media.
																								



10.
DIFFERENCES BETWEEN STATIC AND DYNAMIC ELASTIC MODULI OF ROCKS: PHYSICAL CAUSES

E.I. Mashinsky
Institute of Geophysics, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Nonlinear seismics, inelastic behavior, hysteresis, strain amplitude dependence of wave velocities and attenuation, static and dynamic elastic moduli
Pages: 916-938

Abstract >>
Elastic moduli are controlled mainly by the viscoelastic and microplastic behavior of rocks if stress and strain remain below the proportionality limit. Differences between measured static and dynamic elastic moduli are caused by different inelastic contributions to stress-strain relationships which behave as a function of strain amplitude and frequency (energy and strain rate). Static and dynamic elastic moduli can be appropriately compared at equal strain amplitudes and frequencies and at identical physical properties of solids.