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Russian Geology and Geophysics

2005 year, number 12

1.
EVOLUTION OF CONTINENTAL LITHOSPHERE, ORIGIN OF DIAMONDS AND THEIR DEPOSITS (to the 70th birthday of Academician NIKOLAI VLADIMIROVICH SOBOLEV)




2.
Main scientific achievements of Academician Nikolai V. Sobolev ( on the 70th anniversary of his birthday )


Pages: 1173-1175



3.
DIAMONDIFEROUS PERIDOTITES FROM OCEANIC PROTOLITHS: CRUSTAL SIGNATURES FROM YAKUTIAN KIMBERLITES

L.A. Taylor, Z.V. Spetsius*, R. Wiesli**, M. Spicuzza**, and J.W. Valley**
Planetary Geosciences Institute, Department of Earth & Planetary Sciences,
University of Tennessee, Knoxville, TN, 37996, USA
* ALROSA Co. Ltd., 6 ul. Lenina, Mirny, 678170, Russia
** Depaprtment of Geology & Geophysics, University of Wisconsin, Madison, WI, 53706, USA
Keywords: Peridotites, isotopes, protoliths, diamonds
Pages: 1176-1184

Abstract >>
It is now generally agreed that many of the eclogite xenoliths from kimberlites have protoliths that originated as ancient oceanic crust that was subducted beneath the major cratons worldwide. Similar types of evidence, based mainly upon oxygen isotopes, are used to question the paradigm that peridotite xenoliths in kimberlites are of mantle origin. The same general distribution of δ18O values about the mean mantle value is seen for the peridotites as for the eclogites, both with significant numbers of values distinctly outside of mantle range. It is concluded that at least some of these ultramafic xenoliths, including some diamondiferous ones, have their ultimate origin in the ancient oceanic crust. Indeed, some of the diamonds may have had their carbon derived from crustal sources as well.



4.
CHEMICAL, OPTICAL AND ISOTOPIC INVESTIGATION OF FIBROUS DIAMONDS FROM BRAZIL

A.A. Shiryaev1, 2, E.S. Izraeli3, E.H. Hauri4, O.D. Zakharchenko5, O. Navon3
1Institute of Crystallography of the RAS, 59 Leninsky prosp., Moscow 119333, Russia
2 Bayerisches Geoinstitut, 30 Universitatstr., Bayreuth 95440, Bayreuth, Germany
3 Institute of Earth Sciences, The Hebrew University, Jerusalem, 91904, Israel
4 Department of Terrestrial Magnetism, Carnegie Institution of Washington,
5241 Broad Branch Road, N.W., Washington, D.C. 20015, USA
5 TsNIGRI, 129B Varshavskoe shosse, Moscow, Russia
Keywords: Fibrous diamonds, carbonate and silicate inclusions, chemical and isotopic composition
Pages: 1185-1201

Abstract >>
One coated and two cubic fibrous diamonds from Brazil carry microinclusions that contain fluids with wide range of composition. Fluid chemistry is similar to that found in diamonds from Botswana and varies between a carbonatitic end-member rich in carbonate, CaO, FeO, MgO, and K2O and a silicic end-member rich in water, SiO2, Al2O3, and K2O. The main difference from the Botswanan set is the wider range of compositions sampled by individual diamonds. One diamond, BR-5, is unique and records growth from two contrasting compositions. The inner part grew from silicic fluid, and the outer part, from a carbonate-rich one. Carbon isotopic compositions vary between diamonds and radially within individual diamonds. Silicic fluids are associated with heavier isotopic compositions (most analyses >-5‰); carbonate-rich fluids with lighter values (most analyses <-5‰). Radial evolution in different diamonds is contrasting but is mostly towards the median value of -5‰. Nitrogen isotopes show more scatter but correlate positively with carbon isotopic composition. It is suggested that fluid chemistry and diamond isotopic composition are affected mainly by fractionation of carbonates and diamonds (and possibly silicates). Separation of CO2 and interaction of the fluid with host-rock carbon may also be important in controlling the isotopic composition.



5.
GENESIS OF LOW-CALCIUM KIMBERLITE MAGMAS

I.D. Ryabchikov and A.V. Girnis
Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry,
Russian Academy of Sciences, 35 Staromonetny per., Moscow, 119017, Russia
Keywords: Kimberlite, peridotite, experiment, melting of mantle, phase equilibria, solubility of CO2
Pages: 1202-1212

Abstract >>
Experimental data on melting of carbonatized mantle peridotites, phase equilibria during the high-pressure crystallization of kimberlite melts, and solubility of CO2 in kimberlite-like melts are analyzed. Melting of lherzolite at ~5 GPa in the presence of CO2 yields a wide spectrum of compositions depending on the content of the latter: from high-magnesium picritic magmas in CO2-free systems to dolomitic melts containing <5 wt.% CO2. Low-calcium melts of kimberlite composition from this series contain ~20 wt.% CO2. Experimental studies showed that the solubility of CO2 in kimberlite melts drastically increases at pressures of >4.5 GPa and reaches 20 wt.% at 5 GPa. At ~6 GPa, there is a stable garnet + orthopyroxene + magnesite association on the liquidus of CO2-saturated kimberlite melt. Experimental results indicate that the melting of magnesite-bearing harzburgite at ~6 GPa produces CO2-saturated kimberlite-like melts. Analysis of geochemical data showed that the ratios of CO2 to elements of a close degree of incompatibility (e.g., Th) in such hypothetic melts are nearly the same as in the primitive and depleted mantle. Thus, deep metasomatism of the mantle source is not necessary for the formation of kimberlite magmas, and high contents of incompatible elements might be the result of the extremely low degrees of rock melting. The proposed model for the formation of kimberlite magmas implies the interaction of melts from the asthenospheric mantle with garnet harzburgite in the lower continental lithosphere. This process can lead to CO2 saturation of the melts at a depth of ~200 km, which will favor a rapid magma ascent and transport of deep-seated minerals.



6.
THE ROLE OF GLOBAL FLUIDS IN THE GENESIS OF MANTLE HETEROGENEITIES AND ALKALINE MAGMATISM

L.N. Kogarko
Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences,
19 ul. Kosygina, Moscow, 117975, Russia
Keywords: Alkaline magmatism, carbonatites, mantle, metasomatism
Pages: 1213-1224

Abstract >>
Alkaline magmatism originated at 2.5-2.7 Ga and continuously developed throughout the Earth's history. The appearance of alkaline rocks coincided in time with change in geodynamic regime: The plume tectonics was supplemented by plate tectonics. Global plate-tectonic events at the Archean-Proterozoic boundary caused the subduction of the strongly oxidized volatile-enriched oceanic crust and gave rise to a large-scale mantle metasomatism, which then led to the formation of volatile-enriched reservoirs, the sources of alkaline-carbonatite magmatism. Ion microprobe studies of the metasomatized mantle material revealed impurities of primary carbonate melts highly enriched in trace elements. Based on the results obtained, a new model of the two-stage genesis of Ca-rich carbonatites is proposed: (1) metasomatic wehrlitization and carbonatization of the mantle material, (2) partial melting of the wehrlitized mantle, resulting in either carbonate-rich melts or immiscible liquids (in the excess of alkalies) - silicate, carbonatite, and sulfide (at the high activity of sulfur).
Metasomatic fluids were supplied, most likely, with plumes ascending from the core-mantle boundary.



7.
PARAGENESIS OF INCLUSIONS IN DIAMONDS FROM THE BOTUOBINSKAYA PIPE (Nakyn field, Yakutia)

S.I. Mityukhin and Z.V. Spetsius
ALROSA Co. Ltd., 6 ul. Lenina, Mirny, 678170, Russia
Keywords: Diamonds, inclusions, sulfides, kimberlites, Yakutia
Pages: 1225-1236

Abstract >>
This study was given to specific features and compositions of mineral inclusions in diamonds from the Botuobinskaya pipe. The initial collection of diamonds (91 specimens taken of the three hundred diamonds inspected) was represented by colorless and transparent crystals, chiefly, of octahedral shape, 3-4 mm in size. The imprisoned minerals were exposed to the day by polishing of diamonds and were instrumentally studied in situ.
The suite of revealed mineral inclusions combines a group of silicates and oxides, which were established in 28 crystals, and sulfides present in 65% of the crystals from the studied lot of diamonds. The silicate minerals are dominated by Cr-spinels with more than 61.0 wt.% Cr2O3, with subordinate amounts of garnet and olivine. Rutile, phlogopite, and sanidine are present as solitary inclusions. Sulfides are represented by pyrrhotite, pentlandite, and monosulfide solid solutions with scarce blocks and thin rims of chalcopyrite.
Analysis of compositions of mineral inclusions in diamonds of the Botuobinskaya pipe, with the sulfide phases taken into account, shows that the eclogite paragenesis makes up more than 50% of the bulk of inclusions in diamonds of this kimberlite body. The ultrabasic paragenesis makes up 45%, and about 3.5% of the crystals belong to the pyroxenite association. The high percentage of eclogite paragenesis among inclusions in diamonds is anomalous in the diamond populations in kimberlite pipes of the Yakutian Diamond Province. This suggests a specific composition of the medium of diamond formation and, correspondingly, upper mantle in the region of the new Nakyn kimberlite field.



8.
PALEOPROTEROZOIC COLLISIONAL SYSTEM AND DIAMONDIFEROUS LITHOSPHERIC KEEL OF THE YAKUTIAN KIMBERLITE PROVINCE

O.M. Rosen, A.V. Manakov*, and V.P. Serenko*
Geological Institute of the RAS, 7 Pyzhevsky per., Moscow, 109017, Russia
* Yakutian Research Geological Enterprise TsNIGRI, ALROSA Co. Ltd.,
7 Chernyshevskoe shosse, Mirny, 678174, Russia
Keywords: Collision, lithosphere keel, diamond, Archean, Proterozoic, Siberian craton, Yakutia
Pages: 1237-1251

Abstract >>
The Paleoproterozoic collisional system of the northeastern Siberian craton is compared with the underlying lithospheric mantle. This system appeared at about 1.9-1.8 Ga, by accreting microcontinents with an age of 3.1-2.5 Ga. Evidence comes from isotope dating of the formation of ancient terranes, their thermal transformation and melting of collisional granitoids. The crustal structure inferred from geological and geophysical data bear a relict signature of collisional systems, including deformations, up to 58 km thickened crust, and even slope of seismic surfaces along the predicted directions of collisional overriding.
The crustal structures are underlain by thick, up to 260-300 km, diamondiferous lithospheric mantle with higher seismic velocities, which thins out to ≤200 km toward the margins of the region. This local bulge may be identified as a lithospheric keel (root). The spatial relationship between this mantle keel and crustal collisional system of Proterozoic age is geometrically evident, and magmatic events are obviously coeval. But proportions of relevant processes are not clear. The simplest supposition is that the keel formed as a result of accretion of fragments of the Archean lithospheric mantle together with the crustal terranes attached on top. This supposition contradicts the existing ideas that only the crust participated in continental collision, whereas the underlying mantle slipped free and far away. It should be the subject of future studies.



9.
STRUCTURE AND EVOLUTION OF THE LOWER CRUST OF THE DALDYN-ALAKIT DISTRICT IN THE YAKUTIAN DIAMOND PROVINCE (from data on xenoliths)

V.S. Shatsky, L.V. Buzlukova, E. Jagoutz*, O.A. Koz'menko, and S.I. Mityukhin**
Institute of Mineralogy and Petrography, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
* Max Planck Institute for Chemistry, D-55122, Mainz, Germany
** ALROSA Co. Ltd., 6 ul. Lenina, Mirny, 678170, Russia
Keywords: Granulites, xenoliths, lower crust, geochronology
Pages: 1252-1270

Abstract >>
Study of xenoliths from the Udachnaya and Leningradskaya kimberlite pipes has shown that, among the rocks of the bottom of the Earth's crust in the Daldyn-Alakit district, the garnet granulites make up no less than 50%. Geochemical features of garnet granulites (high percentage of potassium and incompatible elements) show that they cannot be considered restites but are fragments of deep-seated intrusions crystallized in the lower crust. According to estimated pressure and temperature of equilibrium, the lower crust is dominated by garnet granulites (9-13 kbar), which upsection grade into two-pyroxene granulites (8.5-10 kbar).
The Sm-Nd dating of xenoliths in garnet granulites shows no isotope equilibrium. An inner isochron was obtained for none of the specimens. Model ages of the xenoliths evidence that most of the lower crust in the Yakutian Diamond Province was formed in the Archean (2.97-2.75 Ga). At the same time, the inner Pb-Pb isochron (1424±21 Ma) for a specimen of garnet granulites from the Leningradskaya kimberlite pipe as well as the model age (1.24 Ga) of granulite from the Udachnaya kimberlite pipe indicate the Neoproterozoic stage of thermal activity.
On the basis of data on velocities of travel of compressional waves in the lower crust (6.8-7.0 km/s) and obtained estimates of xenoliths of garnet granulites in kimberlite pipes, a conclusion is made that in the lower crust of the Daldyn-Alakit district garnet granulites form separate bodies in gneisses rather than a separate bed.



10.
EXPERIMENTAL MODELING OF MANTLE DIAMOND-FORMING PROCESSES

Yu.N. Pal'yanov, A.G. Sokol, and N.V. Sobolev
Institute of Mineralogy and Petrography, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Diamond, experiment, high pressure, model system, diamond genesis
Pages: 1271-1284

Abstract >>
Experimental data on diamond crystallization in model systems at 5.2-7.5 GPa and 1150-1800



11.
EXPERIMENTAL STUDY OF DIAMONDITE FORMATION IN CARBONATE-SILICATE MELTS: A MODEL APPROACH TO NATURAL PROCESSES

Yu.A. Litvin, G. Kurat*, and G. Dobosi**
Institute of Experimental Mineralogy, Russian Academy of Sciences,
Chernogolovka, Moscow region, 142432, Russia
* Nature History Museum, Vienna, Austria
** Laboratory of Geochemical Studies, Hungarian Academy of Sciences, Budapest, Hungary
Keywords: Syngenesis of diamondites and inclusions, carbonate parental melts, experiment
Pages: 1285-1299

Abstract >>
To simulate a natural carbonate-silicate medium parental for both diamondites and their syngenetic mineral inclusions, melanocratic carbonatites of the Chagatai complex, Uzbekistan, were used. The carbonatites are characterized by a high percentage of silicate components and high-pressure eclogitic silicate minerals. The experiments carried out at 7.0-8.5 GPa and 1800



12.
EQUILIBRIUM K-BEARING CLINOPYROXENE-MELT AS A MODEL FOR BAROMETRY OF MANTLE-DERIVED MINERAL ASSEMBLAGES

O.G. Safonov1,2, L.L. Perchuk1,2, and Yu.A. Litvin1
1Institute of Experimental Mineralogy, Russian Academy of Sciences,
4 ul. Institutskaya, Chernogolovka, Moscow Region, 142432, Russia
2Chair of Petrology, Geological Department, Lomonosov Moscow State University,
Vorob'evy Gory, Moscow, 119234, Russia
Keywords: K-bearing clinopyroxene, K-rich melts, mantle, inclusions in diamonds
Pages: 1300-1316

Abstract >>
Experimental data on synthesis of potassium-bearing clinopyroxene in systems of different compositions are reviewed. The partitioning of K between clinopyroxene and melt is governed mainly by such factors as pressure, concentrations of K, Si, and Al in coexisting aluminosilicate melt, and jadeite and Ca-Tschermak contents in clinopyroxene. Temperature has a minor effect on partition coefficient, especially for natural clinopyroxenes of complex composition. Based on these observations, a thermodynamic equation for the KAlSi2O6 (Cpx) = KAlSi2O6 (melt) equilibrium at 1-11 GPa and 1100-1900



13.
PHASE RELATIONS IN DIAMOND-FORMING CARBONATE-SILICATE-SULFIDE SYSTEMS ON MELTING

A.V. Shushkanova and Yu.A. Litvin*
Lomonosov Moscow State University, Vorob'evy Gory, Moscow, 119899, Russia
* Institute of Experimental Mineralogy, Russian Academy of Sciences,
4 ul. Institutskaya, Chernogolovka, Moscow Region, 142432, Russia
Keywords: Diamond genesis, carbonate-silicate-sulfide system, experiment
Pages: 1317-1326

Abstract >>
The goal of this work was to study phase relations on melting of the system CaCO3-Mg3Al2Si3O12 (pyrope)-FeS (pyrrhotite) modeling the compositions of mineral phases associated with natural diamonds. The experiments were carried out at 7 GPa and 1200-1600



14.
UHP-METAMORPHIC ROCKS FROM DORA MAIRA, WESTERN ALPS: CATHODOLUMINESCENCE OF SILICA AND TWINNING OF COESITE

H.-P. Schertl, O. Medenbach, and R.D. Neuser
Institut fьr Geologie, Mineralogie und Geophysik, Ruhr-Universitдt Bochum, 44780 Bochum, Germany
Keywords: Silica, coesite, cathodoluminescence, Western Alps
Pages: 1327-1332

Abstract >>
Hot cathode cathodoluminescence (CL) microscopy is introduced as a powerful tool to distinguish between different silica phases and between different generations of the same breakdown product of coesite. The rock type investigated is a fine grained pyrope-quartzite from Pangi, Dora Maira Massif, Italy. While coesite inclusions within pyrope display bright bluish-green luminescence colours, their palisade-like breakdown products of quartz are characterized by heterogeneously brownish-violet colours. Cracks within coesite are filled by quartz which shows brown luminescence colours indicating a different generation. Quartz of the matrix exhibits a homogeneously dark blue luminescence. Fine-grained, partly also fibrous chalcedony which is difficult to distinguish from quartz breakdown products of coesite under the polarizing microscope shows bright brownish-yellow luminescence colours. Consequently, even tiny crystals of coesite can easily be separated from different quartz generations or chalcedony due to their characteristic CL emission. Thus, CL-microscopy is an elegant and inexpensive method to discover chronological breakdown sequences of silica and to reveal specific features and to obtain information of minerals which may have become otherwise overlooked.
Twinning of metamorphic coesite resembles polysynthetic plagioclase exhibiting albite- and pericline-law individuals. Two respective types of lamellae were identified which intersect each other at an angle of ca. 90



15.
PROGRADE TRANSFORMATIONS OF GABBRONORITES DURING ECLOGITIZATION IN THE TEMPERATURE RANGE 600-700

S.P. Korikovsky
Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences,
35 Staromonetny per., Moscow, 109117, Russia
Keywords: Eclogitization, gabbronorites, corona structures, omphacite, garnet, plagioclase decomposition
Pages: 1333-1348

Abstract >>
Eclogitization of gabbronorites in the temperature range 600-700



16.
PILLOW BASALTS AND BLUESCHISTS ON BOL'SHOI LYAKHOVSKY ISLAND (THE NEW SIBERIAN ISLANDS) - FRAGMENTS OF THE SOUTH ANYUI OCEANIC LITHOSPHERE

A.B. Kuzmichev, E.V. Sklyarov*, and I.G. Barash*
Geological Institute of the RAS, 7 Pyzhevsky per., Moscow, 119017, Russia
* Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia
Keywords: Arctic tectonics, oceanic basalts, blueschists, South Anyui suture, New Siberian Islands, counterclockwise PT trend of metamorphism
Pages: 1349-1363

Abstract >>
Low-K medium-Ti tholeiite basic rocks represented by weakly altered pillow basalts as well as by their metamorphosed counterparts (amphibolites and blueschists) are exposed on tectonic slices in the southeast of Bol'shoi Lyakhovsky Island. The rocks are depleted in light rare-earth elements and were melted out of a depleted mantle source. A different mantle source enriched in incompatible high-field strength elements, such as Th, Nb, Zr, also contributed to the rock formation. The magma sources were not affected by subduction-related fluids or melts. The basalts represent the upper portion of oceanic crust of the Jurassic South Anyui ocean. The blueschists belong to the same oceanic crust subducted beneath the more southern Anyui-Svyatoi Nos arc to a depth of 25-30 km. Pressure and temperature of metamorphism indicate a