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

2020 year, number 5-6

1.
GEODYNAMICS, PETROLOGY, AND MINERALOGY: GLOBAL PROBLEMS, EXPERIMENTS, AND KEY CASES

N.L. Dobretsov1,2, A.V. Sobolev3,4, N.V. Sobolev2,5, S.V. Sobolev6, J.L.R. Touret7
1Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 1, Novosibirsk, 630090, Russia
3Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 119991, Russia
4Universite Grenoble Alpes-CNRS, 1381 rue de la Piscine, BP 53, 38041 Grenoble Cedex 9, France
5V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
6Helmholtz Centre Potsdam, Potsdam, Germany
7Institut de Mineralogie et de Physique des Milieux, Sorbonne Universites, 4 Place Jussieu, F-75005 Paris, France

DOI: 10.15372/RGG2020143

																				  																		



2.
PLATE TECTONICS VS. PLUME TECTONICS INTERPLAY: POSSIBLE MODELS AND TYPICAL CASES

N.L. Dobretsov1,2
1Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 1, Novosibirsk, 630090, Russia
Keywords: Plate tectonics, plume tectonics, seismic tomography, gravity field, lower mantle, core, models of lower mantle plumes, plume magmatism, compression, extension

Abstract >>
The interplay of plume and plate tectonics is discussed with reference to well-documented late Paleozoic and Mesozoic-Cenozoic structures. The considered issues include models of lower mantle plumes; the typical case of the Hawaii plume; plate motions in the Pacific basin under the effect of plume activity and subduction processes; the role of plumes in the breakup of continents and rifting for the cases of the East African rifts and Gondwana breakup; large continental igneous provinces of Siberia and Tarim, with meimechites and kimberlites; and the formation of large granitic batholiths and related metallogeny. The study contains several points of novelty: integrated use of lower mantle seismic tomography and satellite altimetry data on gravity patterns; correlation of plume heat with plate velocities assuming that some plumes may dissolve in the asthenosphere; and correlation of rifting with compression and extension zones.

DOI: 10.15372/RGG2020102
																				  																		



3.
LARGE-SCALE FLUID TRANSFER BETWEEN MANTLE AND CRUST DURING SUPERCONTINENT AMALGAMATION AND DISRUPTION

J.L.R. Touret1, J.M. Huizenga2,3
1Institut de Mineralogie et de Physique des Milieux, Sorbonne Universites, 4 Place Jussieu, F-75005 Paris, France
2James Cook University, Townsville, Queensland, QLD 4811, Australia
3University of Johannesburg, Auckland Park, 2006, Johannesburg, South Africa
Keywords: CO, Supercontinents, ultrahigh-temperature granulite, fluid inclusions, brines

Abstract >>
Supercontinents are a unique feature of the planet Earth. A brief review of supercontinents formed since the Archean shows that before the Eocambrian, supercontinents, notably Gondwana and Rodinia, amalgamated through high-temperature mobile belts, all of them containing ultrahigh-temperature granulite occurrences. During the final stage of the amalgamation, the lower continental crust was brought to magmatic temperature (from ~900 to more than 1000 C) during a variable time span, from less than 10 Ma in the recent short-lived orogens to more than 150 Ma in the Eocambrian (Gondwana) or Neoproterozoic (Rodinia) long-lived orogens. Ultrahigh-temperature granulites worldwide contain the same types of fluid inclusions, namely, dense CO2 and highly saline aqueous brines. The fluid amount in the peak metamorphic conditions is indicated by the amount of preserved fluid inclusions (especially CO2) and by the secondary effects caused by the fluids when they left the lower crust, including regional feldspathization, albitization or scapolitization, and formation of megashear zones, either oxidized (quartz-carbonate) or reduced (graphite veins). While some fluids may be locally derived either from mineral reactions or from inherited sediment waters, carbon isotope signature and petrographical arguments suggest that most fluids, both CO2 and high-salinity brines, are derived from carbonatite melts resulting from partial melting of metasomatized mantle. Ultrahigh-temperature metamorphism is critical for supercontinent amalgamation, but the associated fluid causes instability and disruption shortly after amalgamation.

DOI: 10.15372/RGG2019128
																				  																		



4.
FOUR STAGES OF THE THERMAL EVOLUTION OF ECLOGITES FROM THE MAKSYUTOV COMPLEX (South Urals)

V.V. Fedkin
D.S. Korzhinskii Institute of Experimental Mineralogy, Russian Academy of Sciences, ul. Akademika Osipyana 4, Chernogolovka, Moscow Region, 142432, Russia
Keywords: Maksyutov Complex, eclogite, HP/UHP metamorphism, garnet-clinopyroxene equilibrium, geothermobarometry

Abstract >>
Based on a detailed electron probe microanalysis of the composition and zoning of coexisting minerals (garnet, clinopyroxene, and plagioclase), we have studied the P-T conditions of formation of high-pressure (HP/UHP) rocks of the Maksyutov eclogite-blueschist complex in the South Urals. We have established its periodic evolution and have determined the specific thermodynamic parameters of mineral formation at each stage of its geodynamic history, from the conditions of crystallization of the protolith to the final stages of retrograde greenschist metamorphism. The new analytical data on the composition of coexisting phases confirm the high-pressure formation of eclogites present as numerous lenses, boudins, and interlayers among blueschist and feldspar-mica schists in the lower part of the complex. Thermobarometric calculations of metamorphism parameters were performed for the Grt + Cpx Pl + Qz paragenesis using a Grt-Cpx geothermometer and a Pl-Cpx-Qz geobarometer. Garnet in eclogites of the Maksyutov Complex, being conservative to changes in the - conditions, has a direct, reverse, or inverse chemical zoning. Under equilibrium of garnet with omphacitic clinopyroxene, this zoning recrods conjugate progressive and regressive - paths reflecting the thermodynamic conditions at the certain stages of the terrane evolution. Based on the data obtained, we have recognized at least four P-T stages of progressive metamorphic transformations of the Maksyutov Complex: (1) >800-910 C, ~2.5-3.5 GPa; (2) 540-790 C, 2.0-3.5 GPa; (3) 410-690 C, 1.1-2.5 GPa; and (4) 310-520 C, 1.0-1.2 GPa. The estimated P-T parameters of the conjugate regressive stages of metamorphism are as follows: (1) 870-625 C, 3.5-2.5 GPa; (2) 745-615 C, 3.5-2.0 GPa; (3) 690-550 C, 1.5-1.0 GPa; and (4) 590-460 C, 1.2-0.6 GPa, respectively. The age data for the certain stages, along with the parameters of metamorphism, form a single P-T-t path of the complex, which determines the position of the gradient of the metamorphic field during the complex exhumation.

DOI: 10.15372/RGG2019182
																				  																		



5.
OCEANIC ZIRCON AS A PETROGENETIC INDICATOR

L.Ya. Aranovich, N.S. Bortnikov, A.A. Borisov
Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Staromonetnyi per. 35, Moscow, 119017, Russia
Keywords: Zircon, mid-ocean ridge, gabbro, plagiogranites, geothermometry

Abstract >>
We present results of a study of the morphology, internal structure, and chemical composition of oceanic zircon, which show that zircon is a sensitive indicator of tectonic and physicochemical processes occurring in the lower oceanic crust. Crystallization of magmatic zircon in gabbroids is not an instantaneous process; it proceeds in the course of differentiation of parental melts in the tectonically active mid-ocean ridge (MOR) setting. The main geochemical indicator of crystallization differentiation during magma cooling is an increase in Hf content toward the zircon grain edge. Zoning is also observed in magmatic zircons from oceanic plagiogranites (OPG), but it is weaker, apparently because of the narrower temperature range of zircon crystallization in these rocks. The OPG zircons are depleted in REE as compared with the least altered magmatic zircons of gabbro, which is probably due to the formation of OPG during the partial melting of gabbro with the participation of concentrated water-salt fluid, a derivate of seawater, and due to the co-crystallization of zircon and apatite. High-temperature hydrothermal processes within slow-spreading MORs lead to a partial or complete recrystallization of zircon as a result of dissolution/redeposition. A significantly reduced cerium anomaly and the presence of microinclusions of xenotime, uranium and thorium oxides or silicates, and, sometimes, baddeleyite in zircon alteration zones indicate a reducing type and high alkalinity of the hydrothermal fluid. The fluid, a derivate of seawater, acquires these features during circulation near the axial zone of ridges as a result of phase separation in the system H2O-NaCl and interaction of the fluid with abyssal peridotites of oceanic core complexes. The estimated solubility of zircon in basic melts indicates that even near-solidus crystallization of zircon is highly unlikely in anhydrous basaltic melts but is possible in differentiates of deep-seated hydrous basic magmas. The Ti-in-Zrn geothermometer must be used with caution, because variations in the Ti content in zircon might be controlled not only by temperature but also by other factors, especially when mineral inclusions in zircon testify to a drastic change in its growth (dissolution) conditions. A geothermometer based on the distribution of Zr and Hf between zircon and the host rock has several advantages over indicators of the crystallization temperature of magmatic zircon that are based on the zircon saturation index and the content of Ti in zircon. It does not depend on the composition of melt and on the correct estimates of the SiO2 and TiO2 activity. In addition, reconstruction of the Zr and Hf fractionation trends during crystallization of zircon from granitoid melts makes it possible to evaluate the temperature of separation of more differentiated melt fractions.

DOI: 10.15372/RGG2019187
																				  																		



6.
MELTING AND PARAGENESES OF GLOBAL SUBDUCTING WATER-ENRICHED SEDIMENT IN CLOSED AND OPEN SYSTEMS: EXPERIMENT AND THERMODYNAMIC MODELING

A.L. Perchuk1,2, A.A. Serdyuk2, N.G. Zinovieva1, M.Yu. Shur1
1Lomonosov Moscow State University, Leninskie Gory 1, Moscow, 119234, Russia
2D.S. Korzhinskii Institute of Experimental Mineralogy, Russian Academy of Sciences, Chernogolovka, Moscow Region, 142432, Russia
Keywords: Subducting sediment, experiment, subduction, melting, thermodynamic modeling, open and closed systems

Abstract >>
The phase relations and melting of subducting sediment were studied in two series of high-pressure runs at 750-900 ºC and 2.9 GPa and by thermodynamic modeling. In the runs we used a chemical mixture corresponding in composition to global subducting sediment (GLOSS) but enriched in water (2 = 15.52 wt.% as compared with 7.29 wt.% in GLOSS). The first series of runs was carried out in open capsules, and the second series was performed by the same procedure and at the same - - t parameters but in closed (welded) capsules. The products of the runs of both series showed agreement on the P - T conditions of melting and the presence of garnet, carbonate, kyanite, SiO2 phase, and phengite in the parageneses. However, the products of all runs in open capsules contain omphacite, including magmatic one, whereas the products of the runs in welded capsules lack it, except for those obtained in the subsolidus run (at 750 ºC). The results of thermodynamic modeling for the composition of the experimental mixture (H2O-GLOSS) in the closed system are in agreement with the experimental data on the hydrous solidus and the stability of most minerals, showing a decrease in the content of omphacite as melt appears. This specific effect caused by the increased pressure of aqueous fluid in the closed system is observed in the welded capsules. Thermodynamic modeling for the H2O-GLOSS composition also shows that a complete decomposition of slab carbonates under hot subduction conditions is possible, but this result is not confirmed by our experimental data. Since the melting and mineral growth processes in subduction zones are controlled by migrating fluids and melts, it seems correct to rely on the results of runs in open capsules.

DOI: 10.15372/GiG2019177
																				  																		



7.
DISTRIBUTION OF FELSIC-MAFIC INTRUSIVE AND EXTRUSIVE ROCKS IN THE EARTHS CRUST: CORRELATION WITH MAGMA VISCOSITY REGULARITIES

E.S. Persikov
D.S. Korzhinskii Institute of Experimental Mineralogy, Russian Academy of Sciences, ul. Akademika Osipyana 4, Chernogolovka, Moscow Region, 142432, Russia
Keywords: Viscosity, hydrous magmas, granite, andesite, basalt, Earths crust, regularities, structural and chemical model, rhyolite, diorite, gabbro, temperature, pressure

Abstract >>
New regularities of the viscosity of near-liquid felsic, medium, and mafic hydrous magmas have been established in a wide range of thermodynamic parameters and the Earths crust depths (1-30 km). The study was carried out using our new structural and chemical model of reliable predictions and by calculation of the viscosity of magmas of almost any composition. It is shown that these regularities are actually a quantitative physicochemical basis explaining the cause of the relative distribution of intrusive and effusive felsic and mafic rocks in the Earths crust. This confirms V.S. Sobolevs idea of the relationship between the relative distribution of intrusive and effusive felsic, medium, and mafic rocks in the Earths crust and the laws of changes in the viscosity of hydrous magmas in a variable P-T field.

DOI: 10.15372/RGG2019102
																				  																		



8.
ESTIMATION OF CO2 CONTENT IN THE GAS PHASE OF MELT INCLUSIONS USING RAMAN SPECTROSCOPY: CASE STUDY OF INCLUSIONS IN OLIVINE FROM THE KARYMSKY VOLCANO (Kamchatka)

N.L. Mironov1, D.P. Tobelko1, S.Z. Smirnov2,3, M.V. Portnyagin1,4, S.P. Krasheninnikov1
1Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 119991, Russia
2V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
3Novosibirsk State University, ul. Pirogova 1, Novosibirsk, 630090, Russia
4GEOMAR Helmholtz Centre for Ocean Research, Kiel, Wischhofstr. 1-3, 24148 Kiel, Germany
Keywords: Olivine-hosted melt inclusions, gas bubble, Raman spectroscopy, CO2, parental magmas, subduction zones

Abstract >>
Carbon dioxide (CO2) is one of the main volatile components of natural magmas, but estimation of its initial contents remains a challenge. Study of melt inclusions in minerals permits a direct estimation of the content of CO2 in the melts. For the precise determination of its content in melt inclusions, it is necessary to analyze the contents of CO2 both in glass and in the fluid daughter phase of the inclusions. In this work we constructed a calibration dependence of the density of CO2 in the range 0.01-0.22 g/cm3 on the distance between its characteristic peaks in Raman spectra (Fermi diads). The accuracy of density determination is 0.03 g/cm3. The calibration was used to estimate the density of CO2 in the gas phase of melt inclusions in magnesian olivine (Fo84.8-88.5) from basalts of the Karymsky Volcano, eastern Kamchatka. The estimated density was 0.03-0.21 g/cm3. Using these values, we evaluated the minimum initial content of CO2 in the parental magmas of the Karymsky Volcano, 0.45 wt.%. These data, along with the known initial content of water (~4.5 wt.%), indicate that the parental magmas began to crystallize at a pressure of at least 7 kbar (depth of >25 km). To increase the reliability of the above method of estimation of the CO2 content in olivine-hosted melt inclusions, we propose to carry out preliminary experimental reheating of inclusions for complete homogenization of the fluid phase and determination of the 3D size of melt inclusions. The performed study provides a reliable evaluation of the content of CO2 in parental magmas, the depth of crystallization, and the degree of magma degassing and permits a comparison of the compositions of magmatic fluids and high-temperature volcanic gases.

DOI: 10.15372/GiG2019169
																				  																		



9.
PETROLOGY, GEOCHEMISTRY, AND ORIGIN OF SULFIDE-BEARING AND PGE-MINERALIZED TROCTOLITES FROM THE KONNIKOV ZONE IN THE YOKO-DOVYREN LAYERED INTRUSION

A.A. Ariskin1,2, L.V. Danyushevsky3, M.L. Fiorentini4, G.S. Nikolaev2, E.V. Kislov5, I.V. Pshenitsyn1,2, V.O. Yapaskurt1, S.N. Sobolev1
1Lomonosov Moscow State University, Leninskie Gory 1, Moscow, 119234, Russia
2Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academ of Sciences, ul. Kosygina 19, Moscow, 119991, Russia
3University of Tasmania, Private Bag 79, Hobart, TAS 7001, Australia
4The University of Western Australia, 35 Stirling Highway, 6009 Crawley, Perth, Western Australia, Australia
5Geological Institute, Siberian Branch of the Russian Academy of Sciences, ul. Sakhyanovoi 6, Ulan-Ude, 670047, Russia
Keywords: Sulfide-bearing troctolite, mss, iss, PGE mineralization, precious metals, fractionation of Se and Te, sulfide separation, infiltration, accumulation, Dovyren

Abstract >>
The exposures, structure, mineralogy, and composition of unusual sulfide-bearing troctolites from the Yoko-Dovyren layered intrusion in the northern Baikal area (Russia) are described in detail for the first time. The troctolite succession (referred to as the Konnikov Zone) is characterized by the presence of pegmatoid poikilite sulfides and sulfide dissemination with diverse PGE mineralization. The former are dominated by pyrrhotite-troilite products of exsolution of monosulfide solid solution (mss), and the latter is composed mostly of cubanite-chalcopyrite assemblages produced from an intermediate Ni-Cu-Fe solid solution (iss). The positive covariations between the contents of sulfur and chalcogens (Se, Te) along with the sublinear dependence of the Pd, Pt, Au, and Cu contents on the Te contents indicate a sulfide control of the distribution of these elements in troctolite cumulates. According to the sulfide-normalized contents of these elements in rocks, the average «100% sulfides in the samples are subdivided into two groups: (1) strongly depleted in PGE, Au, Cu, and Te and (2) with 10-50-fold enrichment in them. This division is consistent with the morphological and mineralogical differences between the groups. Of genetic significance is the fact that the mss assemblages are somewhat poorer in PGE and Te than the primitive sulfides from the Dovyren basal zone, whereas the assemblages with predominant copper sulfides are significantly richer in these elements. This fact is confirmed by LA-ICP-MS data on the trace-element composition of the sulfide phases. The established specific features indicate a limited scale of fractionation of immiscible sulfides during the solidification of the troctolite cumulates. The formation of PGE- and Te-rich assemblages can be related to the course of crystallization of a sulfide precursor similar to the most primitive sulfide liquid. This is consistent with the known laws of crystallization of sulfide systems and explains the abnormally high S/Te ratios in pegmatoid troctolites enriched in mss products. Thus, sulfide melts act as an agent that transports precious metals and chalcogens in the troctolite cumulate area. This conclusion requires specification of the physical mechanisms and parameters (rheology, permeability, wettability by sulfides of different phases, etc.) of the cumulus medium favoring the spatial separation of a monosulfide solution and Cu-containing PGE-rich fractions with their subsequent infiltration and deposition at the boundaries of critical low permeability.

DOI: 10.15372/RGG2019185
																				  																		



10.
CRYSTALLOGRAPHIC ORIENTATION AND GEOCHEMICAL SPECIFICS OF MINERAL INCLUSIONS IN DIAMONDS

N.V. Sobolev1,2, Yu.V. Seryotkin1,2, A.M. Logvinova1,2, A.D. Pavlushin3, S.S. Ugapeva3
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 1, Novosibirsk, 630090, Russia
3Diamond and Precious Metal Geology Institute, Siberian Branch of the Russian Academy of Sciences, pr. Lenina 39, Yakutsk, 677980, Russia
Keywords: High and ultrahigh pressures, eclogite, peridotite, olivine, garnet, chromite, diamond, coesite, mutual orientation of diamond and inclusions, paragenesis, mineral equilibria, geothermobarometry, morphology of inclusions, high-density fluid inclusions

Abstract >>
The orientation of 76 mineral inclusions - olivine (25), pyrope (13), and magnesiochromite (38) - was measured in 16 diamond samples from major primary diamond deposits in Yakutia, such as the Mir, Udachnaya, Internatsionalnaya, Aikhal, and Yubileinaya kimberlite pipes. The novelty of the performed research is a special approach to a choice of samples containing not only olivine inclusions, plenty of which have been studied in recent years after the publication of the book «Carbon of the Earth in 2013, but others as well. Our collection comprises 25% of the studied worlds diamond samples, including the most typical diamonds of prevailing peridotite paragenesis, which were found in almost all known kimberlites. Neither our experiments nor similar studies carried out by the worlds researchers in 2014-2019 revealed any inclusions with orientation meeting the epitaxial criterion in the diamonds. Only a few magnesiochromite inclusions in three diamonds have a near-regular orientation. The carbon isotope composition shows a significant correlation with the composition of mineral inclusions in the diamonds of peridotite and eclogite parageneses and no correlation with other properties of the inclusions, which can be considered the geochemical specifics of the latter. At the same time, there are numerous literature and our data on the intricate growth history of diamonds and on the wide variations in the composition of mineral inclusions in different zones of some diamonds. Taking into account this fact and the different morphology of diamonds, we assume the possible coexistence of syngenetic and protogenetic inclusions in the same diamond. This hypothesis is confirmed by finding of peridotite and eclogite xenoliths with garnet- or olivine-enclosed diamonds in kimberlites. All the samples contain heavy hydrocarbons, from pentane (5H12) to hexadecane (C16H34), prevailing in fluid inclusions in kimberlite and placer diamonds and in pyrope and olivine of diamond-bearing peridotite xenoliths.

DOI: 10.15372/RGG2020144
																				  																		



11.
EXPERIMENTAL MODELING OF DECARBONATION REACTIONS RESULTING IN Mg, Fe-GARNETS AND CO2 FLUID AT THE MANTLE P-T PARAMETERS

Yu.V. Bataleva1, I.D. Novoselov1,2, A.N. Kruk1, O.V. Furman1,2, V.N. Reutsky1, Yu.N. Palyanov1,2
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 1, Novosibirsk, 630090, Russia
Keywords: Decarbonation, CO fluid, mantle carbonates, garnet, high-pressure experiment, experimental modeling

Abstract >>
Experimental modeling of decarbonation reactions with the formation of Mg,Fe-garnets and CO2 fluid during mantle-crust interactions was carried out in a wide range of the upper-mantle pressures and temperatures. Experimental studies were performed in the MgCO3-Al2O3-SiO2 and (Mg,Fe)CO3-Al2O3-SiO2 systems in the pressure range 3.0-7.5 GPa and temperature range 950-1450 C ( t = 10-60 h), using a multianvil high-pressure apparatus of the split-sphere type (BARS). Experiments were carried out with a specially designed high-pressure buffered cell with a hematite container that prevents the diffusion of hydrogen into a Pt-capsule with a sample. It has been experimentally established that in the MgCO3-Al2O3-SiO2 system decarbonation occurs by the schematic reaction MgCO3 + SiO2 + Al2O3 → Mg3Al2Si3O12 + CO2 at 1100 20 (3.0 GPa), 1150 20 (6.3 GPa), and 1400 20 (7.5 GPa) and in the (Mg,Fe)CO3-Al2O3-SiO2 system, by the reaction (Mg,Fe)CO3 + SiO2 + Al2O3 → (Mg,Fe)3Al2Si3O12 + CO2 at 1000 20 (3.0 GPa), 1150 20 (6.3 GPa), and 1400 20 (7.5 GPa). Based on Raman spectroscopic characterization of the synthesized garnets, the position of the main modes R , υ 2, and υ 1 in the pyrope has been determined to be 364, 562, and 924-925 cm-1, respectively, and that in pyrope-almandine, 350-351, 556-558, and 918-919 cm-1. The effectiveness of the hematite container was demonstrated by means of mass spectrometry analysis. It has been found that the fluid composition corresponded to pure CO2 in all experiments. The P-T positions of decarbonation curves leading to the formation of a CO2 fluid in assemblage with pyrope and pyrope-almandine have been experimentally reconstructed and compared with the previous calculation and experimental data. It has been established that the experimentally reproduced reaction lines with the formation of pyrope + CO2 or pyrope-almandine + CO2 assemblages are shifted to lower temperatures by 50-150 relative to the calculated ones. When considering the obtained results with regard to the stability of natural carbonates of various compositions in subduction settings, it has been found that at depths of ~90-190 km Mg,Fe-carbonates react with oxides in the temperature range 1000-1250 C, and at depths of ~225 km, at 1400 C.

DOI: 10.15372/RGG2020115
																				  																		



12.
COMPOSITION OF REDUCED MANTLE FLUIDS: EVIDENCE FROM MODELING EXPERIMENTS AND FLUID INCLUSIONS IN NATURAL DIAMOND

A.G. Sokol1,2, A.A. Tomilenko1, T.A. Bulbak1, I.A. Sokol1, P.A. Zaikin3, N.V. Sobolev1,2
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 1, Novosibirsk, 630090, Russia
3N.N. Vorozhtsov Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Lavrentieva 9, Novosibirsk, 630090, Russia
Keywords: Mantle, fluid, inclusions in diamond, hydrocarbons, deep cycle of carbon and nitrogen

Abstract >>
Experimental modeling in the C-O-H, C-O-H-N, and peridotite-C-O-H-N systems, combined with analyses of fluid inclusions in natural diamonds, is used to reconstruct the compositions of fluids that can be stable in the reduced mantle. Hydrocarbons (HCs) in the upper mantle can form either by reactions of carbonates with iron/wüstite and water or by direct hydrogenation of carbon phases (graphite, diamond, and amorphous carbon) interacting with reduced fluids. Carbon required for the formation of HCs can come from diamond, graphite, or carbonates. Mainly light alkanes are stable at the mantle pressures and temperatures in the C-O-H and C-O-H-N systems as well as in the peridotite-fluid system under ultrareduced to moderately reduced redox conditions at the oxygen fugacity from -2 to +2.5 lg units relative to the IW (Fe-FeO) buffer. Some oxygenated HCs can be stable in fluids equilibrated with carbonate-bearing peridotite. Ammonia and, to a lesser degree, methanimine (CH3N) are predominant nitrogen species in reduced fluids in the conditions of the subcratonic lithosphere or the Fe0-bearing mantle. The presence of HCs as common constituents of reduced mantle fluids is supported by data on inclusions from natural diamonds hosted by kimberlites of the Yakutian province and from placer diamonds of the northeastern Siberian craton and the Urals. Fluid inclusions have minor amounts of H2O, methane, and other light alkanes but relatively high concentrations of oxygenated hydrocarbons, while the H/(H + O) ratio varies from 0.74 to 0.93. Hydrocarbon-bearing fluids in some eclogitic diamonds have high CO2 concentrations. Also, the fluid inclusions have significant percentages of N2 and N-containing species, Cl-containing HCs, and S-containing compounds. Both the experimental results and the analyses of fluid inclusions in natural diamonds indicate that HCs are stable in the upper mantle conditions. The set of hydrocarbons, mainly light alkanes, might have formed in the mantle from inorganic substances. Further research should focus on the causes of the difference between experimental and natural fluids in the contents of methane, light alkanes, oxygenated hydrocarbons, and water and on the stability of N-, S-, and Cl-containing fluid components.

DOI: 10.15372/RGG2020103
																				  																		



13.
OXIDIZING-TYPE FUMAROLES OF THE TOLBACHIK VOLCANO, A MINERALOGICAL AND GEOCHEMICAL UNIQUE

I.V. Pekov1,2, A.A. Agakhanov3, N.V. Zubkova1, N.N. Koshlyakova1, N.V. Shchipalkina1, F.D. Sandalov1, V.O. Yapaskurt1, A.G. Turchkova1, E.G. Sidorov4
1Lomonosov Moscow State University, Leninskie Gory 1, Moscow, 119991, Russia
2Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 119991, Russia
3Fersman Mineralogical Museum, Leninskii pr. 18/2, Moscow, 119071, Russia
4Institute of Volcanology and Seismology, Far Eastern Branch of the Russian Academy of Sciences, bul. Piipa 9, Petropavlovsk-Kamchatsky, 683006, Russia
Keywords: Fumarole, volcanic sublimate, exhalative mineral formation, highly oxidizing conditions, new mineral, Tolbachik Volcano, Kamchatka

Abstract >>
We overview recent data on the mineralogy of oxidizing-type fumaroles of the Tolbachik Volcano (Kamchatka, Russia), with the main focus on the chemical specifics of the minerals. The active fumarole fields of Tolbachik are the most prominent mineral-forming exhalative system of this type in the world. About 350 mineral species, including 123 minerals first discovered there, are reliably identified in the Tolbachik fumaroles. The species diversity and specifics of this mineralization are due to the unique combination of the physicochemical conditions and mechanisms of its formation: high temperatures, atmospheric pressure, superhigh oxygen fugacity, gas transport of most of chemical elements, and direct deposition of many high-temperature minerals from volcanic gases with a specific geochemical composition, including strong enrichment in alkaline metals and chalcophile («ore) elements. Sublimate silicates and minerals of As, Cu, Zn, Mn, Ti, Sn, Sb, Se, Au, Ag, Cs, Tl, and F are briefly described in terms of mineral geochemistry.

DOI: 10.15372/RGG2019167