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Russian Geology and Geophysics

2004 year, number 9

1.
THERMOCHEMICAL PLUMES

A.A. Kirdyashkin, N.L. Dobretsov, and A.G. Kirdyashkin
United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Thermochemical plume, free convection, outer core, lower mantle, eutectics, heat and mass transfer of plume, melting point, concentration
Pages: 1005-1024

Abstract >>
The thermal conditions under which thermochemical plumes formed and ascended were analyzed. A thermochemical plume forms at the boundary of two layers, with heat flow coming from the lower layer. Locally, a chemical dope is supplied, which lowers the melting point near the base of the upper layer. As soon as the melting point becomes lower than the temperature at the boundary, the upper layer begins to melt and the plume ascends.
The physicochemical conditions at the core-mantle boundary at which a thermochemical plume forms were explored. Plausible reactions at core-mantle boundary involving H2 and CH4 are presented, which produce compounds reducing the melting point of the lower mantle. Probable composition and eutectic temperatures resulting from the reactions at the core-mantle boundary have been assessed.
General criteria and related equations have been deduced for heat and mass transfer of a thermochemical plume. Heat and mass transfer was analyzed near the base and roof of the plume. Formulas have been derived for the plume source heating power, source diameter, and mass flux of a chemical dope. The velocity and time of plume ascent as well as ultimate height of the plume have been determined. Basic equations have been derived to calculate the main plume parameters. Using these equations, we have determined patterns of temperature distribution and dope concentrations in the plume channel. We have also estimated plausible concentrations of a chemical dope at the base of the plume and a decrease in melting point owing to the dope. The minimum diameter, mean velocity and time of the plume ascent have been calculated, with physical properties of the lower mantle taken into account.
																								



2.
PETROLOGY OF ORDOVICIAN COLLISION-RELATED LAYERED PERIDOTITE-GABBRO INTRUSIONS (exemplified by the Mazhalyk intrusion, Southeastern Tuva)

E.V. Borodina, V.V. Egorova, and A.E. Izokh
Institute of Geology, Siberian Branch of RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090
Keywords: Layered intrusions, peridotite-gabbro complexes, fractional crystallization, parental magma, komatiites, rare-earth elements, mantle source
Pages: 1025-1042

Abstract >>
The Mazhalyk intrusion, a reference of the Mazhalyk peridotite-gabbro complex of Southeastern Tuva, has a rhythmically layered internal structure. Its age (484.22.3 Ma) corresponds to the Ordovician accretion-collisional step of evolution of the Earth's crust in Central Asia. The intrusion contains rocks of marginal facies and layered series, including ultramafic, subultramafic, mafic, and anorthosite groups of rocks. The upsection succession of cumulate associations is as follows: Ol+Sp=>Ol+Sp+Cpx=>Ol+Sp+Cpx+Pl=>Ol+Sp+Cpx+Pl+Opx=>Ol+Sp+Cpx+Pl+Opx+Amf=>Cpx+Pl+Opx=>Cpx+Pl+Opx+Amf. Variations in mineral compositions are: Fo83.1 to Fo71.5 for olivine; An90.14 to An81.72 for plagioclase; En53.19-43.04 Fs16.1-6.71 Wo48.05-32.16 for clinopyroxene, with mg# = 100 Mg/(Mg+Fe) ranging from 88.6 to 76.3; En82.17-71.68 Fs27.85-16.03 Wo2.01-0.18 for orthopyroxene, with mg# ranging from 84.0 to 73.1, and amphibole with mg# = 81.63 to 62.99. Petrochemistry of the Mazhalyk rocks is consistent with fractional crystallization of a picritic parental magma. The low content of alkalies (under 1-2 wt.%) in rocks and accumulation of total iron in later differentiates suggest that the parental magma of the Mazhalyk intrusion is similar to a tholeiite magma. All the Mazhalyk rocks, irrespective of their position in section, are characterized by a weakly differentiated trend of REE distribution. The content of REE in them is less than 10 chondrite units and exceeds the REE content in the primitive mantle and in the N-MORB source by a factor of 2-4. The geochemistry of the Mazhalyk intrusion is distinguished by a Eu maximum expressed in all rocks and depletion in HREE, which suggests that garnet is present in the mantle source.Mineralogical, petrographic, and geochemical features of the Ordovician collisional peridotite-gabbro intrusions (Mazhalyk intrusion) are similar to those of island-arc intrusions of Vendian-Cambrian age (Central intrusion). This similarity suggests that the parental magmas of both collisional and island-arc intrusions were formed by partial melting of depleted subduction-related mantle source, corresponding in composition to garnet lherzolite. The Mazhalyk intrusion is distinguished from island-arc intrusions by higher mg# of parental magma, which is due to higher degree of its melting under the effect of the mantle plume.
																								



3.
EARLY PALEOZOIC SNEZHNAYA GABBRO-SYENITE INTRUSION IN SOUTHWESTERN BAIKAL REGION (trace-element signature of crust/mantle mixing)

M.I. Grudinin, S.V. Rasskazov*, S.N. Kovalenko*, and A.M. Il'yasova*
Irkutsk State University, 3 ul. Lenina, Irkutsk, 664003, Russia
* Institute of the Earth's Crust, Siberian Branch of RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia
Keywords: Early Paleozoic, collision, alkaline rocks, trace elements, Baikal region
Pages: 1043-1052

Abstract >>
The Snezhnaya gabbro-syenite intrusion is composed of medium-alkaline and transitional rocks, namely gabbroics, syenites, and granodiorites. It intruded after the formation of the metamorphic complex of the southwestern Baikal region associated with the Early Ordovician collision. Gabbroics crystallized at the early stage of intrusion and show mantle and subduction signature. Granodiorites are crosscut by thin veins of S-type gabbroics. The final intrusion stage produced syenite veins with I-type features. Besides magmatism, the Snezhnaya rocks bear traces of metasomatism which accompanied the intrusion of syenites. Many samples show better or worse pronounced Eu peaks or dips indicating fractionation of plagioclase. Ore gabbro has no Eu anomalies.
																								



4.
TECTONIC SETTING AND COMPOSITION OF HIGH-POTASSIUM ULTRAMAFIC-MAFIC IGNEOUS COMPLEXES OF FOLDED AREAS (exemplified by the Altai-Sayan and other regions)

V.N. Dovgal', A.E. Izokh, G.V. Polyakov, and A.E. Teleshev
Institute of Geology, Siberian Branch of RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Magmatism, high-potassium, ultramafic-mafic, geodynamic conditions, Altai-Sayan Folded Area
Pages: 1053-1064

Abstract >>
Study was given to the character of location and geotectonic setting of ultramafic-mafic associations containing high- and ultrapotassium rocks of the Altai-Sayan Folded Area. It has been shown that the high-potassium ultramafic-mafic associations of high alkalinity are concentrated in long-lived sutures separating large diachronous blocks of the Earth's crust: at the junction of the Siberian craton with folded orogens of the protero-Sayans in the east, and in the zone of the Kuznetsk-Altai fault, in the west. The regular character of this distribution and intimate temporal relationship of high-potassium ultramafic-mafic complexes with postcollisional processes not only in folded but also in within-craton regions are illustrated by numerous examples from the rest of the world.
																								



5.
EXOTIC AMPHIBOLITES OF THE ILMENY MOUNTAINS (South Urals): COMPOSITION AND GEOLOGIC SETTING

V.G. Korinevsky and E.V. Korinevsky
Institute of Mineralogy, Uralian Branch of the RAS, Miass, Chelyabinsk Region, 456301, Russia
Keywords: Metamorphic strata, olistoliths of anorthite amphibolites, serpentinite melange, rodingites, protoliths, ancient weathering crust
Pages: 1065-1079

Abstract >>
In this paper, we report the chemical composition and contents of REE, Cr, and Ni of amphibolites making up olistoliths in aposedimentary matrix and exotic blocks in metaultrabasic rocks. The latter are large fragments of ancient metamorphosed serpentinite melange, which form chaotic accumulations (olistostrome) in the Riphean-Vendian quartzite-schist strata of the Ilmeny complex in the Urals. The amphibolites are extremely depleted in SiO2 (<40-45%), rich in Al2O3 (>17-22%) and CaO (9-20%), and have noticeable amounts of alkalies (Na2O = 0.6-2%, K2O = 0.1-0.7%) and extremely high contents of SrO in the hosted plagioclases. In these features the amphibolites have no chemical analogs among the known igneous rocks. Moreover, they bear finest zircon grains and have abnormally high contents of REE (REE = 40-1246 ppm) and elevated contents of Ni and Cr (30-225 and 35-655 ppm, respectively). Such amphibolites are lacking in the other strata of the Riphean-Vendian Ilmeny metamorphic complex. The specific composition of the studied amphibolites might be explained by the fact that their protolith was the disintegration products of ancient weathering crust developed over basic and ultrabasic rocks.
																								



6.
PLACER PGE MINERALS FROM THE FILIPPA CLINOPYROXENITE-DUNITE MASSIF (Kamchatka)

E.G. Sidorov, N.D. Tolstykh*, M.Yu. Podlipsky*, and I.O. Pakhomov**
Institute of Volcanology, Far Eastern Branch of the RAS, 9 ul. Piipa, Petropavlovsk-Kamchatsky, 683000, Russia
* Institute of Geology, Siberian Branch of the RAS, 3 prosp. Akad Koptyuga, Novosibirsk, 630090, Russia
** Gertsen Russian State Pedagogical University, 48 nab. Moiki, St. Petersburg, 191186, Russia
Keywords: PGE minerals, placer, Ural-Alaskan type, ore-forming system
Pages: 1080-1097

Abstract >>
Study is given to association of PGE minerals from the Maior Brook placer in the Filippa zoned massif of the Ural-Alaskan type. The massif rocks feeding placers (dunites, wehrlites, and clinopyroxenites) are seriously crushed and metasomatized. The PGE minerals are dominated by Pt-Fe alloys with Ir-rich impurities. The PGE mineral association involves native iridium grains (20%). The isoferroplatinum bears inclusions of PGE sulfides, arsenides, sulfoarsenides, and antimonides. There are also all varieties of PGE-thiospinels, including Pb- and Co-bearing ones, hollingworthite RhAsS with up to 15.7 wt.% Sb, and malanite-carrolite CuPt2S4-CuCo2S4 (up to 47 mol.%), kashinite-bowieite Ir2S3-Rh2S3, and, more seldom, laurite-erlichmanite RuS2-OsS2 isomorphous series. Their inclusions bear PGE-containing mss and iss solid solutions as well as rare minerals such as rhodarsenide (Rh,Pd,Pt)2As, polkanovite Rh12As7, genkinite (Pt,Pd)4Sb3, stumpflite PtSb, mertieite II (Pd,Pt)8Sb3, and unnamed phases IrAs(Sb,S), Pt(As,Sb,S), (Rh,Pt,Os,Fe)2(S,As)3, and (Fe,Cu)(Pd,Pt)3 (S,Sb,As)3. The conclusion is drawn that the ore-forming system of the source of the studied mineral association, namely, the Filippa massif, had mainly an iridium composition at the early magmatic stage and showed high activities of S and As during the postmagmatic transformations of Pt-Fe alloys. At the late evolution stage, the system became enriched in Pb and Co.
																								



7.
AMPLITUDE-OFFSET ANALYSIS OF WAVEFIELD CONTINUATION

S.V. Gol'din and E.V. German*
Institute of Geophysics, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
* Novosibirsk State University, 2 ul. Pirogova, Novosibirsk, 630090, Russia
Keywords: Ray method, wavefield continuation, geometric spreading, wave amplitude, reflector
Pages: 1098-1106

Abstract >>
Offset wavefield continuation can reduce seismograms to a zero-offset case which simplifies data processing, e.g., migration or AVO analysis. Then it is important to constrain amplitude moveout associated with DMO correction. We suggest to estimate amplitude moveout in DMO as a boundary-value solution for Fomel's partial differential equation, using the standard ray method. DMO correction causes no amplitude moveout for plane reflectors, and moveout arising in reflection from curved interfaces is a complex function of curvature, dip, and offset. The obtained equations can be applied to correct DMO data to be further used in AVO analysis.
																								



8.
ELECTROMAGNETIC FIELD IN A NONUNIFORM SPHERE (3D case)

V.V. Plotkin
Institute of Geophysics, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Electric and magnetic modes, deep electromagnetic soundings, induction and galvanic sources, impedance, inhomogeneous medium, electrical conductivity
Pages: 1107-1119

Abstract >>
The paper presents a new approach to the problem of global electromagnetic field simulated as a nonuniform sphere excited by external induction or galvanic source currents. The forward problem with arbitrary nonuniform sources is solved taking into account the dependence of electrical conductivity on both radial and angular coordinates. The angular and radial dependences and amplitude variations of electric and magnetic fields are computed assuming that deviations from spherical symmetry are caused by mid-mantle or surface conductivity perturbations associated with induction and galvanic excitation. The inverse problem is solved using a mode approach.