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Abstract: The paper presents the first experimental results on the chemical interdiffusion of major components (SiO2, Al2O3, Na2O, CaO, MgO, and FeO) and the \(\text{CO}_{3}^{2-}\) anion at interaction between basalt and kimberlite melts under moderate pressures. The research was carried out using a high gas pressure apparatus of original design at Ar or CO2 pressures of 100 MPa and a temperature of 1300°C, with the use of the method of diffusion pairs. It is established that the rate of the oncoming chemical diffusion of all major components of melts (SiO2, Al2O3, Na2O, CaO, and MgO) and \({\text{CO}}_{3}^{{2 - }}\) anion is almost identical at the interaction of model basalt and kimberlite carbonate-containing melts and is approximately one order of magnitude higher than the diffusion rate of these components at the interaction of melts in the more polymerized andesite–basalt model system. The latter is explained by the significantly lower viscosity of the boundary melt (Montana boundary), which is formed during the interaction of model basalt and kimberlite melts. The equal diffusion rates of CaO and the \({\text{CO}}_{3}^{{2 - }}\) anion indicate that the CaCO3 carbonate diffuses from kimberlite to basalt (both model and natural) melts by means of the diffusion of the end members. The pattern of the diffusion processes significantly changes when melt of natural magnesian basalt interacts with model kimberlite. Thereby calcite diffuses into magnesian basalt also by means of diffusion of the end members. The diffusion rates of all other components of the melts (SiO2, MgO, and FeO) significantly increase. A weak exponential concentration dependence of all diffusing components is determined, with this dependence close to D(i) = constant. PubDate: 2022-06-01
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Abstract: Numerical models are presented for metasomatic processes caused by the coupled heat and solute transfer from a granite source to the host metapelite of given composition in the mode of pervasive and channelized vertical fluid flow. The temperature change is calculated by the equations of conductive and advective heat transport in a permeable medium, and the results of fluid–rock interaction are calculated by means of the HCh software package. The mineral composition of rocks, the extent of their transformation, and the degree of fluid–protolith disequilibrium depend on the fluid flux, time and distance from the source. At a flux of 10–10 m/s, only multiphase mineral assemblages characteristic of contact metamorphic zoning are formed. In permeable channels, at a flux of 10–8–10–6 m/s, a vertical metasomatic zoning is formed. It changes with time as the channel is heated and the fluid/rock ratio increases. The velocity of replacement fronts in this zoning differs by several orders of magnitude. At a given composition and temperature of the fluid source and the composition of the protolith, the formation of the specific metasomatic assemblages is mainly determined by two factors: the volume fluid/rock ratio and the temperature difference between the source and the fluid. The increase in the fluid acidity with temperature decrease is most pronounced in narrow single channels, where conductive heat transfer prevails, and the temperature gradient persists for a long time. In wide closely spaced channels, the temperature is determined by advective heat transfer by fluid, its gradient disappears, and the influence of the fluid source composition becomes predominant. PubDate: 2022-06-01
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Abstract: The Velitkenay monzonite-granite-migmatite massif is a granite-core gneiss dome on the Arctic coast of Chukotka and a key outcrop for understanding mid-Cretaceous magmatism and metamorphism of the Arctic Alaska–Chukotka terrane. The petrology and geologic history of the complex were investigated using whole rock major and trace element and isotopic analyses (Sr, Nd and Pb), as well as electron microprobe thermobarometry, zircon U-Pb geochronology, trace element geochemistry, and O and Lu-Hf isotopic methods. The massif consists of two distinctive Albian plutonic suites. Deformed 106–103 Ma monzonitoids represent the early phase of plutonism; they were melted from a relatively mature crustal source with the bulk composition εNd(i) from –5.5 to –7.9; TNd (DM-2st) = 1.4–1.6 Ga, and zircon composition εHf(i) from –11 to –7, δ18O from 10 to 8.4; and they evolved via assimilation and fractional crystallization processes. Late phase 102–101 Ma leucogranites (εNd(i) from –3.8 to –6.7, TNd (DM-2st) = 1.2–1.4 Ga) formed from melting of Neoproterozoic orthogneisses with mantlelike zircon (εHf(i) from +11 to +13, δ18O ~ 5.8) and differ from early phase monzonitoids by the systematic presence of inherited Neoproterozoic (660–600 Ma) zircon xenocrysts. Migmatized Neoproterozoic orthogneisses are exposed in the central part of the dome, whereas the country rock on the flanks of the dome are paragenisses and schists with Devonian protolith ages. The intrusion of monzonitoid magmas was syntectonic with early stages of exhumation of the Velitkenay massif, whereas the leucogranite phase of magma intruded after peak metamorphism and does not exhibit ductile deformation. Based on subhorizontal mineral stretching lineations along the flanks of the dome and overall sigmoidal plan view of the massif, structural doming appears related to localized transtension in a more regional dextral strike-slip geodynamic environment. U-Pb isotope-geochronological data make it possible to distinguish seven episodes of granitoid magmatism, of which three major, in terms of volume (Aptian Bilibino, Albian Chaun, and Turonian-Coniacian Okhotsk-Chukotka granitoid magmatism subprovince), reflect the maximum rates of growth and modification of the Chukotka crust. Subordinate, in terms of volume, magmatic events of the Neoproterozoic, Devonian, Permian-Triassic, Late Jurassic, and Valanginian-Hauterivian have also been reconstructed. PubDate: 2022-06-01
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Abstract: Various ore deposits, mostly those of gold, tin, copper and mercury, are spatially and temporally associated with Mesozoic magmatic belts in the surroundings of the Kolyma–Omolon terrane in eastern Yakutia. The paper discusses the metallogenic significance of redox conditions in ore–magmatic processes with regard to the proportions of ferrous and ferric oxides in the magmatic rocks. It has been established that igneous rocks of the Main and Northern batholith belts, transverse rows (western continuation of the Kolyma loop), and small zones in the southwestern surroundings of the Main batholith belt crystallized under reducing conditions (ilmenite series). All of the tin and tungsten ore deposits and occurrences are concentrated above the intrusive zone. The ilmenite-series rocks are framed by magnetite-series rocks, which were formed under oxidizing conditions. The accumulations of chalcophile elements are associated with the magnetite-series rocks. The position of the boundary between the Mesozoic ilmenite- and magnetite-series rocks does not depend on the age of the rocks, their crystallization depths, and petrochemical composition. Zoning in the redox conditions had occurred before the formation of the igneous rocks and continued throughout the Cretaceous. PubDate: 2022-06-01
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Abstract: One of the mechanisms explaining relationships between CO2-rich fluids and granitoid magmas at high-temperature crustal metamorphism is the melting of protoliths that originally contained carbonate minerals. In order to study the coupled processes of dehydration/decarbonation and melting, experiments were conducted with carbonate–biotite gneiss from an Archean greenstone belt at pressures of 6, 10, and 15 kbar in the temperature range of 800–950°C, and phase relations in this rock were modeled using the pseudo-section method. The experiments and modeling revealed a subvertical positive dP/dT slope of the solidus of the rock. In comparison to the calculated solidus temperatures, the experiments showed higher melting temperatures (~800°C at 6 kbar and ~850°C at 10 and 15 kbar). The products of the experiments at pressures of 6 and 10 kbar and temperatures >850°C were found out to contain assemblages of clinopyroxene, orthopyroxene, and ilmenite. The products of the experiments at 15 kbar did not contain either orthopyroxene or ilmenite, but calcium garnet and rutile were stable. The first portions of the near-solidus melts at 6 and 10 kbar were poor in SiO2 (44–50 wt %) and were formed because carbonate phases were involved in the melting reactions. With a temperature increase, the melts acquired a granite composition that was close to the composition of melts formed during the melting of the carbonate-free plagioclase + biotite + quartz assemblage. An aqueous–carbonic fluid containing Ca–Mg–Fe carbonate components coexisted with the melts. The phase assemblages and compositions of the granite melts obtained in the experiments are consistent with the modeling results. Comparison of the experimental results with published data on the partial melting of the carbonate-free plagioclase + biotite + quartz assemblages led us to the preliminary conclusion that Ca–Mg–Fe carbonates are able to decrease the melting temperature. The experiments have demonstrated that granite magmas can be derived together with aqueous–carbonic fluids from a carbonate-bearing protolith during high-grade metamorphism in the middle and lower crust. The occurrence of clinopyroxene or two-pyroxene assemblages in granitoids can be considered as a mineralogical indicator of this process. PubDate: 2022-06-01
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Abstract: Abstract Overthrusting of the allochthonous Svecofennian block onto the autochthonous block of the Karelian Craton margin caused compression and decompression in the Meyeri thrust zone of the Northern Ladoga area, which are recorded by the thermobarometry of mineral paragenesis in para- and orthogneisses. The highest calcium garnets in paragenesis with medium-calcium plagioclase, as well as biotite and/or muscovite yielded the mineral formation pressures up to 8–9 kbar, which is 2–3 kbar higher than metamorphic pressures typical of rocks surrounding the thrust zone. This is related to the additional pressure caused by the tectonic load on the rocks in the thrust zone. The subsequent evolution of the PT parameters of gneiss metamorphism indicates a simultaneous decrease in temperature and pressure until reaching the minimum values of Т = 500–550°С and Р = 1.6–3 kbar. The water activity in a metamorphic fluid was determined from mineral reactions with hydrous minerals within the range of ~0.20–0.44. Despite the narrow range, aH2O shows some variations at the present-day erosion level of the thrust zone, with the lowest value found in pre-muscovite garnet–biotite parageneses. An increase of water fraction in the metamorphic fluid and the appearance of muscovite parageneses did not lead to an increase in aH2O due to a simultaneous increase in the content of salt components in the fluid. The salt composition of the metamorphic fluid is revealed from the replacement of early minerals by late ones, the formation of which requires the presence of Na+ and K+ in the fluid. PubDate: 2022-04-01
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Abstract: Abstract The structure of the Central Asian Orogenic Belt (CAOB) is determined by a combination of Neoproterozoic and Paleozoic paleoceanic and island-arc complexes and blocks with Precambrian basement. The Dzabkhan terrane, one of the largest blocks in the central segment of the CAOB, was initially considered as a microcontinent with an Early Precambrian basement. It included the Baidarik, Tarbagatai, Songino and proper Dzabkhan terranes, which are attributed to the Early Precambrian “cratonic” terranes. At present, the Early Precambrian complexes were established only in the northwestern Baidarik terrane and the Ider complex of the Tarbagatai terrane. Their folded structure was formed at ~ 1855 Ma, which is typical of the basement of ancient Northern Eurasia craton. The Early Proterozoic charnockitoids of the Baidarik terrane with an age of 1854 ± 5 Ma contain granulite-facies xenoliths. Previously, these xenoliths were considered as belonging to the Bumbuger Complex, which hosts the charnockitoid massif. However, zircons from two-pyroxene granulite xenoliths yielded an age of 1850 ± 4 Ma, which almost coincided with the age of 1854 ± 5 Ma determined for host charnockites. These data showed that charnockitoids generated at the lower crustal depth brought up the fragments of the Early Proterozoic lower crust to the amphibolite-facies metamorphic zone. The obtained new age data on zircon (2413 ± 12 Ma) extracted from two-pyroxene quartz diorite (enderbite) xenolith in these charnockitoids provided new insight into Early Proterozoic granulite facies metamorphism, which reflects the stages of the lower crustal growth in the geological evolution of the Early Precambrian complexes of the Baidarik terrane. PubDate: 2022-04-01
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Abstract: Early Precambrian retrogressed eclogites are abundant in the Belomorian Province of the Fennoscandian Shield. This study reports unique features of retrogressed eclogites in the Samylino study area. The eclogites preserve a massive texture and the assemblage of Omp + Grt + Rt + Qz. The authors studied zoned crystals of garnet and omphacite with inclusions of diopside–plagioclase symplectites. The garnet crystals consist of three growth zones (central, intermediate, and marginal) and bear microinclusions of individual mineral grains and symplectites. These zones host three mineral assemblages of metamorphic minerals successively formed at different P–T conditions. The early assemblage includes Di–Pl symplectites, hornblende, and quartz, which were formed at T = 700–760°C and P = 12–14.5 kbar, and belongs to the first retrograde stage of the early eclogites. The second assemblage occurs in the intermediate zone of the zoned garnet crystals. This assemblage consists of Omp (Jd 32–36%) inclusions and Grt, makes up the intermediate zone, and was in equilibrium with Omp. These Omp and Grt crystallized at T = 710–820 °C and P = 14–16 kbar during later eclogite metamorphism. The third assemblage comprises garnet of the marginal zone with inclusions of Cpx, Pl, and Hbl. These minerals were formed at T = 650–730°C and P = 8.5–12 kbar, which are consistent with parameters of the high-pressure amphibolite facies of the latest retrograde metamorphism. The authors have distinguished four different age groups of zircons from the earlier and later eclogites based on U–Pb (SHRIMP II) dating: about 2.8, 2.7, 2.46, and 1.93 Ga. The early zircons form the cores of zircon grains, and many of them are interpreted as igneous, which crystallized in the Mesoarchean mafic protolith. All other groups of the zircons are of metamorphic origin, indicating polymetamorphic transformations of the early eclogites. The Neoarchean zircons contain microinclusions of omphacite and zoisite. However, we cannot reliably confirm the Neoarchean eclogite-facies metamorphism. Zircons dated at 1.93 Ga contain inclusions of omphacite, garnet, and zoisite. These grains were formed during the later eclogite-facies metamorphism. Thus, the Early Precambrian Belomorian eclogites are polymetamorphic rocks that bear two eclogite-facies mineral assemblages (according to petrological data). Structural and geochronological data also point to a polymetamorphic origin of the retrogression of the eclogites. Our geochronological data make it possible to date the second high-pressure (eclogite-facies) event as Paleoproterozoic (1.93 Ga) and the first one as presumably Neoarchean (ca 2.7 Ga). PubDate: 2022-04-01
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Abstract: The composition of spinel lherzolite xenolith KLB-1 from the Kilbourne Hole volcanic crater, United States, which is close to the composition of the Earth’s primitive mantle, was used for thermodynamic modeling of phase relations in the Na2O–CaO–FeO–MgO–Al2O3–SiO2 system (NCFMAS) using the Perple_X software package in the temperature range of 900–2000 °C and pressures of 0.0001–30 GPa. The calculated phase diagram is in good agreement with published thermodynamic data on KLB-1 composition and reveals the peculiarities of mantle mineral assemblages at P–T parameters on which experimental data are insufficient or absent. The results showed that the mineral assemblage of garnet wehrlite (garnet + olivine + clinopyroxene), the least common type of mantle peridotite on the Earth’s surface, prevails in the upper mantle since the Archean. Mineral assemblage of garnet lherzolite (garnet + olivine + clinopyroxene + orthopyroxene), which is a variety of mantle peridotites most widely found on the Earth’s surface, is formed in the lithospheric mantle because its temperatures are lower than those of the convecting mantle. Thermodynamic modeling reveals a ringwoodite-free field in the P–T diagram (located at the bottom of the mantle transition zone), which is crosscut by Archean adiabats and by the geotherms of Archean and the hottest Phanerozoic plumes. This area causes a change, from negative to positive, in the slope of the boundary between the lower mantle and the mantle transition zone. A positive slope of the boundary in the Archean should have stimulated the ascend of lower mantle plumes through the transition zone. Conversely, this boundary has a negative slope for most Phanerozoic plumes, rising from the lower mantle, and as a result, the plumes either slow down or stop. PubDate: 2022-04-01
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Abstract: Abstract Phase equilibria were studied experimentally in three rock samples of the Ary-Bulak massif at T = 800–700°C, P = 1 kbar, water content 10 wt %, and oxygen fugacity corresponding to the Ni–NiO and Mt–Hem buffers. Liquidus phases in melts obtained by melting of Ca-rich ongonites are fluorite, topaz, quartz, and plagioclase (andesine, bytownite). The liquid immiscibility of silicate and F–Ca salt melts described in the Ary-Bulak ongonites was not found in the experimental samples. The liquidus temperature of Ca-rich samples is ≥800°C, which is not typical for highly evolved differentiated granite melts. Based on the obtained factual material, it was concluded that simple experiments on melting–crystallization of rock samples of the Ary-Bulak massif do not fully reproduce its formation. It is possible that an important role in nature was played by irreversible processes that are not taken into account in this experimental series: interaction with host rocks, a sharp significant change in the fluid regime or P-T parameters, etc. The relicts of immiscible silicate and salt F–Ca melts described in natural Ca–F-rich samples also could be caused by one of these processes. PubDate: 2022-04-01
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Abstract: Abstract Plate-tectonic paleogeodynamic setting of volcanic massive sulfide-bearing complexes of the Magnitogorsk island-arc megazone was reonstructed based on petrological-geochemical study and available geophysical data. The correlation between La/Yb ratio, Th, Yb, TiO2, and Zr concentrations in the basalts of ore regions, and ore volumes (Cu + Zn, in kt) in volcanic massive sulfide deposits was established. This indicates that the ore compositions and volumes in the ore-magmatic systems depend on the melting degree of suprasubduction mantle wedge. It was concluded that supraduction asthenospheric diapirs participated in the genesis of massive sulfide-bearing complexes. The obtained information can be used to predict new areas promising for massive sulfide mineralization. PubDate: 2022-02-01 DOI: 10.1134/S0869591122010052
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Abstract: The Urumieh-Dokhtar Magmatic Arc (UDMA) appears as a nearly linear suite of magmatic rocks that extends from NW to SE Iran parallel with the orogenic suture of the Zagros Fold-Thrust belt. The Qazan granitoids formed along the central part of UDMA and span a wide range of felsic rocks, including granodiorite, quartz diorite, diorite, and monzogranite. Their common rock-forming minerals are mainly quartz, feldspar, amphibole, biotite and clinopyroxene. These granitoids contain abundant mafic microgranular enclaves (MME). Bulk-rock major and trace element compositions returned a relatively low SiO2 content (ca. 51–55 wt %) and high Mg# (ca. 40–50) for MME samples, potentially reflecting a mantle-derived origin. The granitoid host rocks are metaluminous (A/CNK = ca. 0.7–1) I-type rocks with arc-related calc-alkaline affinity. They yield higher SiO2 contents with a comparatively larger variation (ca. 57–66 wt %) and lower Mg# (ca. 35–48), consistent with derivation from partial melting of lower continental crust. Mixing of two contrasting mafic (sourced from an enriched subcontinental lithospheric mantle wedge) and felsic (derived from lower continental crust) melts in the Neo-Tethyan subduction-related oxidizing system has resulted in generation of Qazan granitoid melts. Thermometry of Qazan intrusive rocks indicates that the ascending melt was crystallized at 1200–1100°C in lithospheric mantle to lower continental crust. PubDate: 2022-02-01 DOI: 10.1134/S0869591122010064
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Abstract: bstract The mineral zoning of corundum-bearing rocks of the Belomorian mobile belt, whose genesis remains controversial, has been studied at one of the typical occurrences: Khitoostrov. Based on the estimates of the P–T parameters of the formation of corundum-bearing rocks by the multiequilibrium thermobarometry method, pseudosections were constructed using the Perple_X 6.9.0 software package in P–T, T–µ(SiO2), µ(SiO2)–µ(Na2O), and µ(SiO2)–µ(K2O) space, with CO2–H2O fluid, to model the metasomatic transformation of migmatized kyanite–garnet–biotite gneisses of the Chupa sequence. It has been established that the mineral zoning of the corundum-bearing rocks reflects the successive transition of SiO2, Na2O and K2O to a mobile state. Desilication in the outer zones led to the formation of` quartz-free associations, alkaline sodic metasomatism concurrent with ongoing desilication led to the formation of associations with staurolite and plagioclase, and K2O removal led to the replacement of biotite with calcium amphibole. Further desilication and sodic metasomatism resulted in mineral assemblages with corundum. This process is reflected in an increase in the sodium content in the calcic amphibole and variations in the Ca content of the garnet at a practically constant composition of the plagioclase. PubDate: 2022-02-01 DOI: 10.1134/S0869591122010027
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Abstract: The plutonic rocks studied in the Ashadze Complex contain four groups of mineral assemblages: (I) coarse-grained gabbronorite, which is dominant; (IIa) micrograined differentiated gabbro (oxide microgabbro) enriched in ilmenite and apatite and sporadically containing zircon grains smaller than 70 µm; (IIb) local oxide microgabbro aggregates with zircon grains as large as 200 µm and quartz; and (III) biotite- and orthopyroxene-bearing plagiogranite veinlets. As an interpretation, a model of sequential differentiation of MORB-type melt is proposed. The compositional variations of the plagioclase and orthopyroxene are consistent with the general sequence of crystal fractionation and show partial overlap of assemblages (II) and (III). In the sequence of zircon-bearing assemblages (IIa), (IIb), and (III), zircon shows a systematic enrichment in Hf. Zircon of assemblage (IIb) hosts melt inclusions. The inclusions were completely remelted at 910°C and quenched into homogeneous glass. The analyzed granitoid (75–76 wt % SiO2) water-bearing (H2O ~ 3 wt %) composition of the inclusions was interpreted as a residual melt after crystallization of the evolved vein gabbro rich in ilmenite. The results offer an example of an evolved granitoid product of MORB differentiation and are consistent with high-degree magmatic differentiation in the oceanic crust, a model according to which the crystallization of evolved oxide gabbro results in a residual silicic melt. PubDate: 2022-02-01 DOI: 10.1134/S0869591122010076
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Abstract: > The coastal northwestern part of the Sea of Okhotsk hosts local volcanic fields of olivine–two pyroxene andesites and basaltic andesites of the Kytyima volcanic complex, which were dated at about 48 ± 2 Ma (U–Pb, 40Ar/39Ar, and K-Ar methods). The age of the eruptions correlates with that of bottom sediments in the Sea of Okhotsk and may reflect local stages of extension on the continental margin of northeast Asia. The geochemistry of the lavas makes it possible to classify them as highly magnesian, calc-alkaline, moderately potassic volcanic series with clearly seen negative Nb, Ta, and Ti anomalies and with positive Sr and Pb ones. The isotopic composition of the rocks [εNd(T) = 2.4–5.5, 87Sr/86Sr(0) = 0.703415–0.704175] and the relatively “young” Cambrian Nd model ages of the Paleogene volcanic rocks indicate that the melts were derived from a mantle source depleted in radiogenic isotopes (presumably, fragments of the Cretaceous slab under the complexes of the Uda–Murgal ensimatic island arc), which later experienced fluid-assisted metasomatism and enrichment in trace incompatible elements. Variations in isotope composition and in concentrations of major and trace elements in the lavas were controlled by decompressional fractional crystallization without indications of any significant crustal contamination. PubDate: 2022-02-01 DOI: 10.1134/S0869591122010039
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Abstract: Abstract This paper synthesizes available and original U-Pb geochronological and hafnium and oxygen isotope data on zircon from gabbro and peridotites in the oceanic core complexes (OCC) of the Mid-Atlantic Ridge (MAR) extending for 2000 miles along its crest zone. We attempted to reproduce the evolution of MAR magmatism and to determine the geochemical and geodynamic nature of zircon protolith in OCC. We show that the relicts of old continental lithosphere have been preserved locally beneath the axial ridge zone and were involved in the partial melting of a shallow mantle during the entire magmatic evolution of the MAR rift valley. Age variations of zircon from plutonic rocks of the oceanic basement of fracture zones at some distance from the rift valley suggest young magmatism that differs in age from established magnetic anomalies. During the geological history of the Atlantic Ocean, the evolution of the melt originated in the rift valley of MAR, with zircon crystallization at the final stages, was influenced by aqueous (or aqueous–saline) fluid. Obtained conclusions confirm the fundamental significance of the interaction between hydrothermal and magmatic systems in the slow-spreading mid-ocean ridges. PubDate: 2022-02-01 DOI: 10.1134/S0869591122010040
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Abstract: Carbon contents and isotopic compositions were compared in the basalt groundmass of the oceanic crust of different age in the zone of the East Pacific Rise. In samples the basalt groundmass of the ancient oceanic crust (~270 Ma, ODP Site 801C) in which a carbonate phase was found, the isotopic composition of the oxidized carbon (δ13C = ±1.5‰) indicates that this carbon was formed by the precipitation of seawater dissolved inorganic carbon (DIC). In the samples in which no carbonate phase was identified, the low concentration (<0.1 wt % CO2) of oxidized dispersed carbon and its isotopic composition (δ13C < –7‰) are in the range of values typical of carbon dissolved in basalt glasses without crystallinity. This makes it possible to relate the oxidized dispersed carbon to residual carbon dissolved in the magmatic melt after CO2 degassing. The precipitation of DIC results in a positive correlation between the concentration of total carbon and its δ13C values, along with the formation of a carbonate phase. The application of this criterion to basalt groundmass samples of the young crust (~15 Ma, ODP Site 1256D) showed that oxidized dispersed carbon in the young oceanic crust groundmass was not formed by the precipitation of DIC, contradicting the generally accepted paradigm. Constant concentration and δ13C values of the reduced dispersed carbon in the basalt groundmass of the young and ancient oceanic crusts, including lithological zones where microbial activity has not been recorded, indicate that the most probable model is high-temperature abiogenic generation of reduced dispersed carbon near the ridge axis. The Fischer–Tropsch synthesis and/or Bell–Boudouard reaction provide a possible basis for the abiogenic model. The Bell–Boudouard reaction 2CO = C + CO2 leads to the formation of an adsorbed layer of elemental carbon on the fresh surfaces of minerals during background alteration of the oceanic basalt crust. The CO2–CO gas-phase equilibrium maintains the necessary depletion of the newly formed elemental carbon in the 13C isotope to δ13C < –20‰. Abiogenic models for the origin of the isotopically light reduced dispersed carbon in the basalt groundmass do not assume the presence of carbon depleted in the heavy 13C isotope in the magmatic melt. PubDate: 2021-11-01 DOI: 10.1134/S0869591121060060
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Abstract: High-precision analysis of sulfur isotope composition was carried out for sulfide fractions from ten samples of olivine gabbronorite that composes a thick (approximately 300 m) swell of a ore-bearing apophysis that is parallel to the basal part of the Yoko-Dovyren massif in northern Baikal area, Russia. The δ34S values were found out to widely vary from +11‰ to –1.9‰. The maximum enrichment in isotopically heavy sulfur was identified within the basal horizon, which is 10 m thick, whereas the minimum values of δ34S were observed near the upper contact of the intrusive body. Sulfide droplets in chilled picrodolerite from the lower contact zone (Pshenitsyn et al., 2020) show a narrow range of δ34S (+8.65 ± 0.34‰, n = 5). Lower values of δ34S ranging from +2.09 to +2.53‰ are characteristic of the sulfide-rich net-textured ores, the mineralized olivine gabbronorite, and a cutting leucogabbro dike. The sulfur isotope compositions of two samples of pyrite-bearing rocks from the host carbonate–terrigenous rocks display discrete values of δ34S = +2.20‰ and δ34S = +9.40 ± 0.14‰ at a whole-rock sulfur concentration up to 3.5 wt %. Simple scenarios of the additive mixing of isotope-contrasting reservoirs corresponding to a juvenile magmatic source (δ34S = 0 and +2‰) and a provisionally chosen contaminant (δ34S = +9.4‰) are demonstrated to require a high degree of assimilation of host rocks (as much as 60–80%) and complete isotope equilibration of the hybrid system. In the contact picrodolerite with rare globular sulfides, the mixing mechanism is inconsistent with the estimated sulfur solubility in its parental magma: approximately 0.08 wt % (Ariskin et al., 2016). The high δ34S values in rocks from the basal part of the apophysis may be explained, under the assumption that contact-metamorphic H2S-bearing fluid was introduced into the magmatic system, by the thermal decomposition of pyrite coupled with dehydration of the host rocks. The proposed mechanism does not require a volume assimilation of crustal materials and is consistent with petrological and geochemical characteristics of the Dovyren magmas and derivative cumulates. PubDate: 2021-11-01 DOI: 10.1134/S0869591121060023
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Abstract: New experimental data on interaction of synthetic iron carbide Fe3C with pure hydrogen were acquired in a unique in-house built IHPV, which allows long-lasting high-T runs under hydrogen pressure, in the temperature range T = 1273–1423 K and pressure P = 30–100 MPa. The run products consist of metal with a very low carbon content (0–2 ± 0.3 at %) and a fluid phase enriched in methane (CН4/(CH4 + H2) up to 0.37). Raman spectroscopy revealed peaks of hydrogen and disordered carbon in the metal product of one selected run, which indicates the possibility for both elements to enter the metal structure. Thermodynamic calculations at high P–T have shown that even in equilibrium with a methane-rich binary CН4–Н2 fluid, the metal should dissolve but a minor amount of C, while the H2 solubility can be rather high. Iron carbides can be stable only under conditions close to carbon saturation, when temperature does not exceed that of methane pyrolysis at a given P, and correspondingly, the hydrogen mole fraction in the fluid is very low. PubDate: 2021-11-01 DOI: 10.1134/S0869591121060072
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Abstract: Here we present summarizing of isotopic compositions and element ratios of noble gases, nitrogen, carbon and hydrogen in carbonatites of different generations of the Guli massif (West Siberia, Russia) obtained by stepwise crushing. The data point to the subcontinental lithospheric mantle (SCLM) as a primary source of the fluid phase in Guli carbonatites. However, the estimated 40Ar/36Ar ratio in the Guli mantle source of about 5400 is similar to the Kola plume value of 5000 ± 1000 (Marty et al., 1998). One explanation of such a low estimated 40Ar/36Ar ratio in the mantle end-member with SCLM type helium (4Не/3Не ~ 120000) and neon (21Nе/22Nеmantle ~ 0.7) is an admixture of atmospheric argon to the local mantle source. This assumption is supported by the Ar-Ne systematics as well as by the data for hydrogen isotopic composition. Early carbonatite differs significantly from the later ones by the concentration of highly volatile components, as well as by the isotopic compositions of carbon (CO2), argon, and hydrogen (H2O). The mantle component dominated in fluids at the early formation stages of the Guli massif rocks, whereas the late stages of carbonatite formation were characterized by an additional fluid source, which introduced atmospheric argon and neon, and most likely a high portion of CO2 with isotopically heavy carbon. The argon-neon-hydrogen isotope systematics suggest that the most plausible source of these late stage fluids are high temperature paleometeoric waters. The absence of a plume signature could be explained in terms that Guli carbonatites have been formed at the waning stage of plume magmatic activity with an essential input of SCLM components. PubDate: 2021-11-01 DOI: 10.1134/S0869591121060035
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