Vol 27, No 4 (2019)

ARTICLES
Early Cambrian syenite and monzonite magmatism in the southeast of the East European Platform: petrogenesis and tectonic setting
Nosova A.A., Voznyak A.A., Bogdanova S.V., Savko K.A., Lebedeva N.M., Travin A.V., Yudin D.S., Page L., Larionov A.N., Postnikov A.V.

Abstract

The paper reports new geochronological, petrological, and isotope-geochemical data on the syenites and alkali syenites of the Artyushki massif, and the monzonites of the Gusikha massif. These massifs are located along the southwestern and northeastern margins of the Pachelma aulacogen, in the southeastern part of the East European Platform (EEP). They have Early Cambrian ages of 524 ± 3 (Artyushki) and 514 ± 2 Ma (Gusikha) obtained by the U-Pb zircon method and similar ages of amphibole and K-feldspar by the 40Ar/39Ar method. This time period has previously been regarded as amagmatic in the EEP evolution.

The Artyushki massif is made up of Amp-Cpx syenite porphyries and Grt-Cpx alkali syenite porphyries and their fenitized varieties. As compared to the Amp-Cpx varieties the Grt-Cpx rocks are more peralkaline (A/NK > 0.9) and have higher LREE and HFSE, and fractionated HREE patterns. The metasomatized (fenitized) varieties are more potassic and bear geochemical evidence of fluid reworking (high Y/Ho ratios, significant Zn variations, and etc.). Bulk samples have weakly radiogenic Sr isotopic compositions: (87Sr/86Sr)520 are within 0.703066–0.703615. The values of εNd(520) vary from –0.69 to +1.64. The Grt-Cpx syenite porphyries have the positive εNd(520), while the Amp-Cpx and fenitized syenite porphyries feature negative εNd.

The Gusikha massif consists of biotite-amphibole and biotite monzonites. Similar to the Artyushki syenites in SiO2 contents, the Gusikha monzonites have higher Mg# (0.22–0.54 and 0.34–0.71 for the Artyushki and Gusikha massifs, respectively). They are also characterized by a negative Nb-Ta anomaly (Nb/Nb* = 0.5), high Ва/Sr ratio, and highly radiogenic (87Sr/86Sr)520 = 0.705204 and 0.705320. Their Nd-isotopic compositions correspond to εNd(520) = –6.7 and –7.0.

Two melts contributed to the formation of the Artyushki massif. One was a strongly contaminated melt (Amp-Cpx syenite porphyries, the other was weakly contaminated (Grt-Cpx syenite porphyries). The main contribution was phonolitic melt derived from the melting of a moderately metasomatized (carbonate- and amphibole-bearing) shallow lithospheric mantle. The earliest and deepest melt portions were carbonate-silicate in composition.

The geochemical, as well as the Sr and Nd isotopic compositions of the Gusikha monzonites indicate a predominant crustal contribution and pervasive reworking of the lithospheric mantle beneath southeastern Volgo-Uralia of the EEP in the Mesoproterozoic.

Both massifs feature the geochemistry of within-plate and supra-subduction derivatives, which suggests a postorogenic tectonic setting of the magmatism.

The presence of the Early Cambrian postorogenic magmatism within the East European Platform/Baltica is direct evidence for the involvement of Baltica in the collisional and/or accretionary events during the terminal Neoproterozoic – the beginning of the Paleozoic. This suggests reworking of the lithospheric mantle of Baltica during its collision with Timanian and East Avalonian/Cadomian terranes, including Scythia.

Петрология. 2019;27(4):357-400
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X-ray computed tomography as a method of reconstruction of 3d-characteristics of disseminated sulfides and spinel in plagiodunites from the Yoko-Dovyren intrusion
Korost D.V., Ariskin A.A., Pshenitsyn I.V., Khomyak A.N.

Abstract

The paper describes a methodology of applying X-ray computed tomography (CT) in studying textural–morphological characteristics of sulfide-bearing ultramafic rocks from the Yoko-Dovyren layered massif in the northern Baikal area, Buryatia, Russia. The dunites are used to illustrate the applicability of a reliable technique for distinguishing between grains of sulfides and spinel. The technique enables obtaining statistical characteristics of the 3D distribution and size of the mineral phases. The method of 3D reconstructions is demonstrated to be applicable at very low concentrations of sulfides: no than 0.1–0.2 vol %. Differences between 3D models are determined for sulfide segregations of different size, in some instances with features of their orientation suggesting the direction of percolation and accumulation of the sulfide liquids. These data are consistent with the morphology of the largest sulfide segregations, whose concave parts adjoin the surface of the cumulus olivine and simultaneously grow into grains of the poikilitic plagioclase. Detailed information of these features is useful to identify fingerprints of infiltration and concentration of protosulfide liquids in highly crystallized cumulate systems.

Петрология. 2019;27(4):401-419
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Oxygen and silicon β-factors of zircon estimated from first principles
Krylov D.P.

Abstract

Zircon β-factors have been calibrated against temperature for isotopic substitutions of 18O/16O and 30Si/28Si. Calculations were performed using the density functional theory (DFT) with the “frozen phonon” approach. The deduced geometric parameters of the zircon unit cell, and the phonon frequencies calculated, agree well with the experimental data. The results are expressed by the cubic polynomials on x = 106/T(K)2: 1000lnβzrn(18O/16O) = 9.83055x – 0.19499x2 + 0.00388x3;  1000lnβzrn(30Si/28Si) = 7.89907x – 0.17978x2 + 0.00377x3.

The expressions deduced can be utilized to construct geothermometers if combined with β-factors of coexisting phases. New calibrations of quartz-zircon are given. The new values of 1000lnβzrn and the estimated isotope fractionation factors between quartz and zircon (1000lnβqtz–1000lnβzrn) deviate considerably from previously used experimental, empirical, and semi-empirical calibration of the isotopic equilibrium.

Петрология. 2019;27(4):420-430
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Equation of state of the fluid system H2O-CO2-CaCl2 and properties of fluid phases at P-T parameters of the middle and lower crust
Ivanov M.V., Bushmin S.A.

Abstract

A numerical thermodynamic model is proposed for one of the most important geological fluid system – triple system H2O-CO2-CaCl2 at P-T conditions of the middle and lower crust, as well as for the crust-mantle boundary. The model is based on the equation for concentration dependence of the excess Gibbs free energy, proposed earlier, and for the first time obtained P-T dependencies of the coefficients of the equation of state (EOS) expressed via molar volumes of the components. The EOS allows predictions of the properties of the fluid, participating in the majority of the processes of depth petrogenesis: its phase state (homogeneous or multi-phase), densities of the fluid phases, concentrations of the components in the co-existing phases, and chemical activities of the components. The model precisely reproduces all available experimental data on the phase state of the ternary fluid system H2O-CO2-CaCl2 in the ranges of temperature 773.15–1073.15 K and pressures 0.1-0.9 GPa and allows, as well, correct application of the EOS beyond the experimentally studied domain of temperatures and pressures – namely up to P = 2 GPa and up to T = 1673.15 K. The possibility of the correct extrapolation of our EOS is ensured by using the parametrization of P-T dependencies via the molar volume of water. The latter remains in the experimental domain of values or very near to its boundaries, when increasing temperatures and pressures.

Петрология. 2019;27(4):431-445
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Behavior of the 238U, 235U, and 234U isotopes at weathering of volcanic rocks with u mineralization: a case study at the Tulukuevskoe deposit, Eastern Transbaikalia
Chernyshev I.V., Golubev V.N., Chugaev A.V., Mandzhieva G.V., Gareev B.I.

Abstract

The trend fractionation of the 238U and 235U isotopes and the extent of this process at the oxidative weathering of uranium minerals were evaluated by studying the variations in the U isotope composition of rocks, minerals, and fracture waters sampled in the quarry of the broadly known Tulukuevskoe uranium deposit in the Streltsovskoe ore field, eastern Transbaikalia. In the rock block in question, fine uranium minerals disseminated in the rocks were weathered under the effect of oxidizing fracture waters. Uranium isotope composition was measured in 22 water samples, eleven samples of the mineralized rocks, and eight uranium minerals. High-precision (±0.07‰, 2SD) measurements of the 238U/235U were carried out by MC-ICP-MS, using a 233U–236U double spike. The results involve the 238U/235U and 234U/238U ratios and the overall range of the δ238U variations determined in the rocks and waters (from –0.13 to –1.0‰ and from –0.22 to –0.59‰, respectively). Interaction between the waters and rocks induces U(IV) → U(VI) oxidation, U(VI) transfer into the aqueous phase, and 0.15–0.28‰ enrichment of U dissolved in the water in the 235U isotope. When the pitchblende is replaced by U(VI) minerals, the 238U and 235U isotopes also fractionate with ~0.3‰ enrichment of the younger U(VI) mineral phases in the light 235U isotope. The 238U/235U and 234U/238U ratios are proved to correlate, and hence, the fractionation of the 238U and 235U isotopes and the enrichment of the aqueous phase in the light 235U isotope proceed simultaneously with the well known shift in equilibrium the 238U–234U system with the accumulation of excess amounts of the 234U in the aqueous phase. Uranium leaching from uranium minerals, which is associated with the enrichment of the aqueous phase in excess amounts of the 234U isotope, can be viewed as a process that controls isotope fractionation in the 238U–235U system. This should be taken into account in describing the fractionation mechanism of the 238U and 235U isotopes at U(IV) → U(VI) oxidation. The fractionation of the 238U and 235U isotopes, which results in the isotopic "lightening" of U in the aqueous phase, largely controlled the complicated distribution pattern of the 238U/235U ratio in the quarry. In addition to isotope fractionation, this distribution was likely also affected by isotope exchange between uranium dissolved in the water and uranium in the finely dispersed minerals. The isotopically light uranium of the water could participate in forming U(VI) minerals at lower levels of the quarry.

Петрология. 2019;27(4):446-467
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