Vol 27, No 6 (2019)

Experimental study of the system peridotite–basalt–fluid: phase relations at supra- and sepercritical P-T parameters
Gorbachev N.S., Kostyuk A.V., Nekrasov A.N., Gorbachev P.N., Soultanov D.M.

To obtain new data on the phase relationships in the fluid-containing upper mantle at P up to 4 GPa, T up to 1400°C, partial melting of H2O-containing peridotite, basalt, as well as peridotite-basalt association with an alkaline-carbonate fluid was experimentally studied as a model of the mantle reservoir with protoliths of the subdued oceanic crust. At partial melting of H2O-containing peridotite at P = 3.7–3.9 GPa, T = 1000–1300°C, critical ratios were observed in the whole studied interval P and T. At partial melting of H2O-containing basalt critical relationships between the silicate melt and the aqueous fluid were observed at T = 1000°C, P = 3.7 GPa. At T = 1100°C, Na-alkaline silicate melt coexisted with garnetite, at T = 1150 and 1300°C – with clinopyroxenite. Signs of critical relationships between the carbonated silicate melt and the fluid were observed in peridotite-basalt-alkaline-water-carbonate fluid system at Р = 4 GPa, T = 1400°C. The reaction ratios among the minerals of peritotite restite with the substitutions of OlOpx ← Ca-Cpx ← K-Amf indicated a high chemical activity of the supercritical fluid melt. The results of the experiments suggest that in the fluid-containing upper mantle with supercritical Р-Т there are areas of partial melting (asthenosphere lenses), containing near-solidus supercritical fluid-melts enriched with incompatible elements, with high reactivity. Mantle reservoirs with supercritical fluid-melts, similar in geochemical terms to the “enriched” mantle, can serve as a source of magma enriched with incompatible elements. The modal and latent metasomatism of the upper mantle under the influence of supercritical fluid-melts leads to the peridotite refertilization due to the enrichment of restite minerals with incompatible elements and its eclogitization.

Петрология. 2019;27(6):606-616
Physical and chemical parameters of processes producing rare-metal deposits in granitoid systems with fluorine: experimental data
Shapovalov Y.B., Chevychelov V.Y., Korzhinskaya V.S., Kotova N.P., Redkin A.F., Konyshev A.A.

The problem of the origin of rare-metal deposits in granites is considered, taking into account the results of experimental studies, which make it possible to introduce restrictions on the interpretation of geological materials and the construction of genetic models. The role of both magmatic and hydrothermal-metasomatic factors in the formation of various types of rare-metal deposits is discussed. The saturation concentrations of Ta and Nb in granite melt significantly depend on the melt composition, varying from ~ 2–5 to ~ 0.1 wt.%. They depend on temperature and pressure, to a lesser extent. The partitioning of Ta and Nb between the fluorine-containing fluid and the granite melt is sharply biased in favor of the melt. The principal difference in the partitioning of W and Ta, Nb, Sn in melt granite-salt systems is shown. Fluoride water-salt phase is a very effective extractant of tungsten, while Ta, Nb and Sn completely remain in the aluminosilicate melt. The model magmatic fluid, that is in equilibrium with Li-F granite melt, is multiphase and contains significant amounts of SiO2 and fluorides of Na, Al, Li and K. The solubility of ore minerals in this fluid is insignificant, but the concentration of Nb substantially exceeds that of Ta. The concentrations of HF in high-temperature magmatic fluids were estimated that could reach ~ 0.5–1 M HF. The experimentally determined solidus temperatures of Li-F granites are ~ 570–630°C at a pressure of 100–200 MPa. At T = 300–550°С and P = 50–100 MPa the actual hydrothermal transfer of Ta and Nb is possible only by sufficiently highly concentrated solutions (fluids) of HF and, possibly, KF. At alkaline sodium solutions, hydrothermal transport of Nb is quite probable, and for Ta it is difficult, at that the solubility of pyrochlore is higher than the solubility of columbite.

Петрология. 2019;27(6):617-637
Partitioning of volatile components (Cl, F, CO2) in water-saturated fluid-magmatic systems of various composition
Chevychelov V.Y.

The results of experimental studies of the behavior of volatile components (Cl, F, CO2 and H2O) in fluid-magmatic systems are presented. The maximum chlorine content in magmatic melts mainly depends on the composition of the melt and, to a lesser extent, on pressure (10–300 MPa) and temperature (800–1000°C). The Cl content in the melt increases from 0.2–0.3 to 3–5 wt.% with increasing in the Ca content during the transition from polymerized granitoid to depolymerized basaltic melts. The pressure dependence of solubility has a maximum at a pressure of about 100–200 MPa. A tendency of increasing in the Cl content and decreasing in the F content in the melt during the transition from acidic and alkaline to basaltic melts has been established. The maximum Cl content in the melt significantly increases from rhyolitic (up to 0.25 wt.%) to phonolitic (up to 0.85 wt.%) and dacitic (up to 1.2 wt.%) at temperatures of 1000–1200°C and pressure of 200 MPa. The addition of CO2 to the system causes an increase in the Cl content in the melt by 20–25 relative %, which is apparently associated with an increase in the Cl activity in the fluid. In this case, the H2O content in the melt decreases by ~ 0.5–1.0 wt.%. A strong effect of hydrolysis was shown in the interaction of an alumina-rich granitic melt with ~ 0.5–1N chloride fluid. This effect shows that at hypabyssal magmatic conditions (P = 100 MPa, T = 750°C), the fluid is acidic (the pH after the experiment is ~ 1–1.5) and it is characterized by high dissolving power. It was established experimentally that as a result of the interaction of aqueous Na-K-Ca-chloride fluid of variable composition with granodioritic and granitic melts in the pressure range of ~ 100–200 MPa and temperatures of 820–1000°C and with increasing in the total salt content, the Na and K replace Ca in the silicate melt, displacing the latter into the fluid, that is enriched in CaCl2 and is depleted in NaCl. Experimental results on the joint partitioning of Cl and F provide a quantitative basis for understanding the degassing processes in the course of the evolution of alkaline and basaltic magmas. They are important for assessing the extent of the removal of Cl and F into the earth’s atmosphere during volcanic activity and the effects of this removal on climate change.

Петрология. 2019;27(6):638-657
Petrological-geochemical characteristics of lavas, sources and evolution of magmatic melts of the Kazbek neovolcanic center (Greater Caucasus)
Parfenov A.V., Lebedev V.A., Chernyshev I.V., Vashakidze G.T., Yakushev A.I., Goltsman Y.V., Chugaev A.V., Oleynikova T.I., Kanunnikova E.M., Gabarashvili Q.A.

The results of petrological-geochemical and isotope-geochemical studies of the Late Pleistocene-Holocene lavas of the Kazbek Neovolcanic Center, one of the largest centers of youngest magmatism in the Greater Caucasus, are presented.

It has been established that the volcanic rocks of the Kazbek center arise a continuous compositional series basaltic (trachy-)andesites–(trachy-)andesites–dacites with a predominance of calc-alkaline intermediate and moderately-acid lavas. The obtained results indicate that the processes of fractional crystallization and mixing of melts had a leading role in the petrogenesis of the rocks. The crustal assimilation was of limited importance; its influence is noticeable only in the rocks of the earliest and late pulses of magmatic activity within the Kazbek center. The common crustal lithologies participated in the assimilation were presented by metamorphosed Jurassic sediments (mainly shales and sandstones), forming the foot of the Kazbek center, and Mesozoic mafic metamorphosed volcanites very rarely. The specific features of AFC processes during the development of the studied magmatic system (including the presence of noticeable amount of water in the melt, the leading role of Amp in the cumulus and the absence of Pl fractionation) led to the appearance of dacitic lavas with geochemical signs of adakites as an evolutional end-member.

The volcanic rocks of the Kazbek center are derived from trachybasalt magmas, the source of which was presented by the mantle reservoir of OIB-type. Recent and previously published results of studies of the Neogene-Quaternary magmatism manifested within the Greater Caucasus show that the main petrological and geochemical characteristics of this regional mantle reservoir remained constant from the end of the Miocene to the present time.

Петрология. 2019;27(6):658-689
Genesis and evolution of the mantle melts of the devonian mafic-ultramafic rocks from the Eastern Azov region (Dnieper-Donetsk rift, Ukraine) based on the clinopyroxene geochemistry study
Sazonova L.V., Nosova A.A., Yutkina E.V., Kondrashov I.A., Shumlyanskyy L.V.

The Devonian magmatic association of the Eastern Azov region, which is part of the Pripyat-Dnieper-Donetsk rift zone, was studied. The association includes gabbroids, peridotites, pyroxenites, and lamprophyre dikes of the Pokrovo-Kireevsky massif (PKM) and picrites, picrobasalts and basalts of the Anton-Taramskaya suite (ATS). The clinopyroxenes of different generations from the micaceous PCM gabbro and the alkaline ATS picrite were studied. It was obtained the information on the mantle source composition and the evolution of melts, who determined the close spatial-temporal location of kimberlites, basites, ultramafic rocks, including alkaline ones, in the Eastern Azov region.

Clinopyroxenes from micaceous gabbro are composed of Cpx1 (Mg# = 0.87–0.88) or Сpx2 (Mg# = 0.80–0.81) cores and Cpx3 external zones (Mg# = 0.70–0.76). Clinopyroxenes in alkaline picrite are composed of Сpx2 cores (Mg # = 0.80–0.84) and external Cpx3 zones (Mg# = 0.71–0.78). The multielement spectra of clinopyroxenes are generally dome-shaped in nature, with enrichment with LREE, depletion of Ba, Nb, TREE, Zr-Hf negative anomaly, a negative Sr-anomaly appears in Cpx2 and Cpx3 also. The resulting compositions of the model melt for Сpx2 from the micaceous gabbro are very close to the composition of this gabbro, and the compositions of the model melt for Cpx2 from the alkaline picrite coincide with those of this picrite. The high Mg# value and concentrations of Cr in Cpx1 cores indicate that the earliest weakly differentiated composition close to the primary could serve as the equilibrium melt. The presence in the Cpx1 geochemical spectra of a negative Zr-Hf anomaly at ZrPM <HfРМ may be evidence of the origin of melts that once contained these clinopyroxenes, due to the melting of metasomatized, possibly carbonated garnet-contained peridotites. Probably, the Cpx1 cores are relics of phenocrysts crystallized from the earliest melt during the formation of the PCM and the ATS.

An important feature of the Eastern Azov rocks is a very high content of Ti (up to 7.3 wt.% TiO2) in the high-Mg (Mg# = 0.48–0.65) and deep (CaO/Al2O3> 0.8) melts, which formed picrobasalts and lamprophyres. The geochemical features of the early Cpx1 cores compared with the geochemistry of clinopyroxenes from ilmenite-containing mantle metasomatites are consistent with the assumption that carbonated ilmenite-containing peridotites, possibly also phlogopite-containing (PIC), are the source of ultrahigh-Ti primary melts for the Eastern Azov lamprophyres.

Петрология. 2019;27(6):690-714
Geodynamic nature of magmatic sources of North-West Pacific: an interpretation data on isotope composition of Sr and Nd in rocks dredged at Stalemate ridge, Ingenstrem depression, and Shirshov Rise
Silantyev S.A., Kostitsyn Y.A., Shabykova V.V., Krasnova E.A., Ermakov Y.Y., Dogadkin D.N., Zhilkina A.V.

First data on isotope composition of Sr and Nd in rocks dredged at different area’s belong to lithosphere of the NW Pacific are present. All samples examined were obtained from NW termination of Stalemate Ridge (NW Pacific) and Central part of Shirshov Rise (Western Bering Sea). Results of conducted study allow sure enough to judge on geodynamic affinity of the central segment of Shirshov Rise. Mafic-ultramafic rocks dredged here originated due evolution of magmatic melt formed by partial melting of source parental for MORB belongs to mantle wedge perhaps. Thus, this interpretation means that Shirshov Rise is remnant Back-Arc Spreading Center. Data on petrology and isotope chemistry of rocks from Stalemate magmatic assemblage demonstrate geochemical heterogeneity of their possible magmatic sources. The presented data allow to assume participation in magmatism of this region of NW Pacific source that responsible for formation of most older volcanic seamounts from NW Termination of Hawaiian-Emperor volcanic chain. There is petrographic similarity between rock assemblage recovered at NW Stalemate and plutonic rocks composed of xenoliths from volcanic effusions of Aleutian Island Arc exists. Considering the scarcity of existing information about the structure of the lithosphere in the NW Pacific it is possible to assume with caution the participation in the construction of the oceanic slope of the Aleutian Trench and the adjacent segment of the Stelmate Ridge fragments of Aleutian Arc basement.

Петрология. 2019;27(6):715-736

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