The Archean Varvarinsky block was distinguished in the southern Vorontsovka terrane of Eastern Sarmatia in the late 1960s. It was proved to consist of Paleoproterozoic I-type sodic diorites and granodiorites dated at 2.09 Ga. Isotopic and geochemical characteristics of its rocks (highly radiogenic Nd composition, enrichment in Cr and Ni, depletion in LILE, HFSE, and LREE, as well as low Sr/Y values) suggest their island-arc nature. The sources were aqueous melts from the mantle wedge, metasomatized by fluids during the melting of a subducted oceanic slab, followed by fractionation in intermediate chambers. The finds of the most ancient precollisional granitoid intrusions provide direct evidence that island arc structures occurred in the Volga−Don Ocean and were composed of calc-alkaline volcanics of basalt−andesite−dacite−rhyolite (BADR) composition. They are the intrusive roots of the destroyed volcanic island arcs produced during the subduction of oceanic slabs in the Volga−Don Ocean.

Experimental data on the solubility of niobium and tantalum oxides and oxyfluorides in fluoride solutions were reviewed. Experimental solubility data for pyrochlore (CaNa)Nb2O6F and microlite (CaNa)Ta2O6F were used to calculate their thermodynamic properties at 300–800°C. The thermodynamic properties of albite, andalusite, muscovite, paragonite, and pyrophyllite were refined in the temperature range 300–550°C. The influence of solution composition and aluminosilicate mineral association on the solubility of pyrochlore and microlite in the supercritical region of physicochemical parameters was modeled by thermodynamic calculations. The calculations showed that the solubility of the minerals is very low, and niobium and tantalum cannot be removed by metamorphosed solutions. Mechanisms were proposed for HF accumulation, which could play an important role in niobium and tantalum dissolution, as well as recrystallization and replacement of ore mineral phases over limited distances.

Abstract The solubility of phenakite (Be2SiO4) in granite melts was experimentally studied at temperatures of 1000 and 1100°C and pressures of 1 and 4 kbar in dry conditions and in the presence of 10 wt % H2O. The starting materials were granite glasses with agpaitic coefficient of 1–2.5 and natural phenakite. It was found that the solubility of phenakite increases with increasing agpaitic index (Na + K)/Al of the melt, and the solubility of BeO in hydrous melts is higher than in dry ones. The solubility of phenakite also increases with pressure. The obtained experimental data were generalized with the previous data in the form of an equation describing the solubility of BeO in alkaline-granite melts coexisting with crystalline Be phases depending on the agpaitic index, temperature, and pressure. The results of the experiments and their generalizations support the model of Be concentration in alkaline hydrous melts—the products of differentiation of granite magmas.

In this paper, we examine the compositions of basaltic melts from ocean islands using our regularly updated database of analyses of melt inclusions in minerals and glasses of igneous rocks. Mean concentrations of major, trace, and volatile components in melts were calculated for the global dataset (22 550 analyses from 33 island systems) and for the most extensively studied complexes (Iceland, Hawaii, Canaries, Galapagos, and Reunion). It was found that the mean contents of most elements fall between the mean compositions of magmas from mid-ocean ridges (most depleted) and intraplate continental complexes (most enriched). Variations of element ratios in particular complexes were considered in detail, and it was found that they can be described by mixing magmas from a depleted source and variably enriched materials. The abundances of trace elements in the supposed mantle sources were estimated. The depleted source is most clearly manifested in Iceland and is almost identical to the depleted mantle the melting of which produces mid-ocean ridge basalts.

The paper presents experimental data on melting model compositions of basaltic komatiite (BK) and enstatite chondrite (ECH) at a temperature of T = 1300°C and hydrogen pressure $${{P}_{{{{{\text{H}}}_{{\text{2}}}}}}}$$ = 100 MPa. The experiments modeled interaction between the magma ocean and the early Earth’s hydrogen atmosphere. The experimental products consist of silicate glass (quenched melts), which is notably depleted in FeO but enriched in lithophile oxides and H2O, and iron with minor Si and O admixtures. The equilibrium oxygen fugacity in the experimental runs was approximately two logarithmic units below the Fe−FeO buffer. Calculation of the fractional crystallization of the melts indicates that the complete crystallization products are granodiorite, which consists of two feldspars, clinopyroxene, and quartz with a minor amount of black mica (for the starting composition obtained in the run with BK), or quartz−two feldspars granite with minor amounts of biotite and muscovite (for the starting composition obtained in the run with ECH). Crystallization of zircon from the ECH melt might occur at T = 730−750°C. Our proposed model is the first that explains generation of melts enriched in SiO2 and H2O by internal processes of planetary evolution and does not invoke pre-hydrated upper crust for generating the Earth’s first felsic material.

Carbonate minerals in oceanic crust are formed through CO2 interaction with silicate minerals of ultramafic and mafic rocks. Carbonation leads to the generation of numerous carbonate veins, filling interstices in the rock matrix and producing partially and/or completely carbonated rocks that compose the protolith of slow-spreading mid-ocean ridges and are present in ophiolite complexes. Silantyev et al. (2023) proposed a conceptual model of the main stages in the formation of carbonated serpentinites in different segments of the Mid-Atlantic Ridge. In this study, we examined isotope variations (δ18O, δ13С, Rb, Sr, Sm, Nd) in the previously studied carbonated ultramafic rocks from oceanic core complexes of the slow-spreading mid-oceanic ridge. The carbon and oxygen isotope compositions obtained in our study are well consistent with results of our previous studies and make it possible to quantify the duration and spatial position of sea fluid interaction with serpentinites of different segments of the Mid-Atlatnic Ridge in the oceanic crust sequence. Peridotite groups previously distinguished based on the mineral and petrographic features are well consistent with relations of obtained parameters or with water/rock ratio calculated using Sr-Nd isotope systematics, and reflect the sequence of carbonation stages in the ultramafic protolith of oceanic crust and duration of its residence on the seafloor. Our results indicate that the oceanic core complexes containing the studied rocks were exhumed to the seafloor surface during different time periods.

Based on the geochemical and isotopic (δ18О, δD) data, the thermal and fluid conditions during the formation of the Eldjurta granite massif were reconstructed. Analysis of rocks collected from the core of the Tyrnyauz Superdeep Well (TSW) within the depth range of 1427–3923 m revealed their homogeneous isotopic parameters: the δ18O values of bulk samples, quartz, feldspars, and biotite in 12 samples of biotite granites are 8.50 ± 0.33, 9.55 ± 0.22, 8.40 ± 0.33 and 5.45 ± 0.40‰, respectively. The δD values in the biotite vary from −103.3 to −95.6‰. The closure temperatures of the oxygen isotope system of quartz are 440–980°C. The rock cooling history was reconstructed using a new approach based on the analysis of single quartz grains. This approach can be used for detailed reconstructions of thermal history during formation of intrusive bodies. The definite samples were used to demonstrate that Dodson’s equation is valid for description of the δ18O values of quartz in a granite system. The data obtained suggest that the studied part of the massif was formed in at least two almost simultaneous stages. The lower part of the massif was crystallized first, and the second injection of granite melt arrived immediately after the first portion has been crystallized, but had no yet had time to cool significantly. The Tc values in the lower part of the massif indicate the re-opening of the oxygen isotope system of quartz, with subsequent long-term isotope re-equilibration between minerals. This leads to decrease of the observed Tc values and the calculated cooling rates, which is related to increasing volume of the intrusive body and cooling within already heated rocks. Estimates of the isotopic parameters of the water component indicate the absence of exotic water fluid (meteoric or buried waters) during cooling of the massif. The variations of the δ18O values in the minerals of the Eldjurta biotite granites can be described in terms of a simple retrograde exchange at the cooling stage.

Crystal size distributions (CSD) of olivine were obtained for 17 samples of plagiodunite and Pl‑bearing dunite from the central part of the Yoko-Dovyren massif, northern Baikal region, Russia. Three types of CSD were identified: loglinear, bimodal, and lognormal. Combining these data with the results of petrological reconstructions, which earlier revealed two main types of the Dovyren magmas (using the method of geochemical thermometry), we proposed a basic scenario of interaction between magmatic suspensions of different temperature to explain the diversity of the CSD. The intratelluric olivine transported by magmas of different temperature, which had not subjected to abrupt cooling or heating in the chamber, retained an original loglinear CSD. For some portions of the hottest magma (∼1290°C), it is assumed that the original olivine evolved into a bimodal CSD due to accelerated crystallization at faster cooling of the high-temperature injections contacting relatively cold crystal mush (∼1190°C). An interpretation of the lognormal CSD suggests that part of the olivine crystals composing the protocumulate systems efficiently interacted with the pore melt infiltrating upward during the compaction of the underlying crystal mush. This led to cycles of partial dissolution and regrowth of the olivine grains resulting in a final lognormal CSD. The infiltrating hot melt, which was undersaturated with immiscible sulfide liquid, could dissolve sulfides preexisting in the low-temperature mush. This produced dunites with lognormal CSD relatively depleted in sulfur and chalcophile elements. The lognormal CSD is considered to be a marker of crystal mush regions through which the focused infiltration of the pore melt proceeded.

Mineral reactions were studied in metamorphic rocks from the Meyeri tectonic zone, and the P–T path of the development of this structure was calculated. According to the P–T path, the Proterozoic granulite complex of the Svecofennian Belt was thrust onto low-temperature rocks of the Archean Karelian Craton. Relict staurolite and other minerals preserved as inclusions in the garnet porphyroblasts made it possible to identify the pre-peak stage of metamorphism with P–T parameters no higher than the low-temperature amphibolite facies of moderate and low pressure. The peak metamorphic conditions of the tectonic zone are estimated at T > 700°C and P ~ 7 kbar using the composition of relict minerals, while the temperature on the prograde trend of metamorphism was 500–600°C at a pressure of about 5 kbar. The post-peak stage began with a distinct decompressional P–T path at the aforementioned high temperatures, with a change from granulite hypersthene-containing assemblages to lower-temperature hydrous ones. The subsequent metamorphic retrogression was characterized by the development of numerous hydrous minerals as a result of the activation of fluids in the shear zone. The P–T path of the tectonic zone is clockwise and reflects the exhumation of the Svecofennian granulite complex during the orogenic events.

The paper presents the results of detailed petrographic and petrogeochemical studies of metagabbro-dolerites from the central part of the Kara astrobleme. The data obtained made it possible to narrow the petrographic diversity of the rocks to two varieties: quartz-free metagabbro-dolerites and quartz-bearing metagabbro-dolerites, which were formed through differentiation of a single magmatic melt. The geochemical features of the rocks indicate the heterogeneity of the magma-generating source. The rock-forming minerals of the quartz-free and quartz-bearing metagabbro-dolerites are characterized by the presence of planar elements of the low and middle stages of impact metamorphism.