Buffer Equilibrium Research Articles

Oxygen fugacity buffering in high-pressure solid media assemblies from IW-6.5 to IW+4.5 and application to the VK-edge oxybarometer

AbstractControl of oxygen fugacity during high-temperature phase equilibrium experiments is required to simulate the conditions that exist in natural systems. At high pressures, oxygen fugacity may be imposed using solid buffer equilibria via the classic “double capsule” technique. This design becomes untenable, however, at temperatures above the melting points of commonly used noble metal capsule materials and/or where buffer assemblages may alloy with the capsule or contaminate the sample. Here we introduce and test a modified double capsule approach that includes a solid metal-oxide buffer in close proximity to but separate from the sample of interest. Buffers used include (in order of most oxidized to reduced) Ni-NiO, Co-CoO, W-WO3, Fe-FeO, Mo-MoO2, Cr-Cr2O3, V-V2O3, Ta-Ta2O5, and Nb-NbO. At a fixed temperature, these buffers span a wide range—up to 10 log fO2 units. To demonstrate the buffering capacity of this double capsule approach, secondary redox equilibria and V-doped CaO-MgO-Al2O3-SiO2 system glasses were studied in experiments using the double capsule geometry. The secondary equilibria provide an independent verification of the oxygen fugacity established in the double capsule environment. The glasses proved difficult to interpret, and our results provide guidance to future efforts to utilize the glass oxybarometer at reducing conditions. Application of this modified double capsule technique to studies of V valence in MgAl2O4 spinels led to the recognition of several factors that will affect V valence in this system: temperature of equilibration, duration of experiment, and spinel bulk composition. We have synthesized V-bearing MgAl2O4 spinel at the reduced conditions of the Cr-Cr2O3, (IW-3.51), Ta-Ta2O5, (IW-5.37), and Nb-NbO buffers (IW-5.44). This spinel exhibits a very small V3+ pre-edge peak consistent with its reduced nature. The absence of evidence for V2+ suggests that MgAl2O4 spinel excludes V2+ due to the preference of V for octahedral sites. This finding is supported by DFT calculations for spinels of variable composition, and in agreement with some other indirect evidence for preference for V3+ in aluminous spinels (Bosi et al. 2016; Paque et al. 2013).

Crystallisation and zircon saturation of calc-alkaline tonalite from the Adamello Batholith at upper crustal conditions: an experimental study

The understanding of the geochemical and petrophysical evolution of magmas forming intermediate calc-alkaline batholiths at shallow crustal levels critically depends on knowledge of the phase equilibria relations along the liquid line of descent. Here, we present experimental results for a tonalitic system at a pressure of 200 MPa and under water-saturated conditions. Melting experiments were performed at temperatures between 700 and 1000 °C in externally heated HCM pressure vessels, with oxygen fugacity controlled close to the Ni–NiO buffer equilibrium (NNO) employing an argon–methane mixture as pressure medium and Co–Pd redox sensors to verify fO2 conditions. Natural rock powder of a medium-K tonalite from the Adamello Batholith in Northern Italy served as experimental starting material. Based on compositional data of stable phases in the run products and images of entire run charges, mass balance calculations as well as image processing were performed to investigate the evolution of the crystal/melt ratio with respect to temperature. Furthermore, compositional trends of minerals as well as the liquid line of descent of residual melts were obtained. Orthopyroxene, clinopyroxene and plagioclase were identified as near-liquidus phases (below 990 °C). At 900 °C, amphibole joins the solid-phase assemblage at the expense of clinopyroxene, indicating the existence of a peritectic relationship. After an initial near-linear decrease with temperature, residual melt fractions exhibit a plateau of 45–55 vol. % between 750 and 850 °C, followed by a rapid decrease coinciding with quartz saturation at 725 °C. Compositions of residual liquids evolve along a typical calc-alkaline differentiation trend with decreasing temperature (increasing SiO2 and decreasing TiO2, Al2O3, CaO, MgO and FeO contents) and become peraluminous below 900 °C. Intermediate to acidic rocks from the Adamello follow the experimental liquid line of descent indicating that the observed compositional spread of the natural intermediate composition rock record can be explained by low-pressure magma differentiation and liquid extraction. Experimentally determined zircon saturation levels are at low temperatures distinctly lower compared to existing and often used Zr-saturation models, but fully consistent with observed and modelled Zr-evolution trends from the natural rock record of the Southern Adamello Batholith inferring that zircon saturation in these intermediate to felsic plutonic rocks occurred at 800–830 °C corresponding to a melt fraction of about 50 vol. %.

Impliciations of Transport and pH Effects on Electrocatalytic CO2 Reduction

Electrocatalytic CO2 reduction has become a highly promising route to sustainable and economical carbon-neutral fuels. So far, however, a basic understanding of the underlying chemical processes is still lacking, mainly due to the highly complex, large reaction network involving a variety of C1 and C2 products. Experimental investigations are additionally aggravated by apparent transport limitations of CO2 to the surface which are also determined by buffer equilibria in solution. Additionally, pH gradients at the solid-liquid interface can be important to consider, since local pH effects have shown to critically influence product selectivities. Such effects overlay (electro-)chemical kinetics which can be modeled e.g. by Density Functional Theory (DFT) in combination with micro-kinetic modeling approaches. However, in order to utilize these methods for the transport limited regime, these need to be extended by equations governing diffusion and migration effects as well as chemical reactions in the electrolyte. Using a coupled kinetic-transport model based on Poisson-Nernst-Planck theory and ab initio calculations of reaction and activation energies, we show transport limitations to be crucial to determining C2 to C1 product selectivity. We present strategies for catalyst structural design to optimize the selectivity towards higher value C2 products.

Redox conditions during crystallization of ultramafic and gabbroic rocks of the Yoko–Dovyren massif (Based on the results of measurements of intrinsic oxygen fugacity of olivine)

We present the results of electrochemical measurements of the intrinsic oxygen fugacity of olivine separates representing seven rock types from the central part and southwestern termination of the Yoko–Dovyren mafic—ultramafic massif. The $${f_{{O_2}}}$$ values were determined using a high-temperature solid-electrolyte double-cell assembly developed at the Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences. A total of 59 experiments were performed (from 7 to 16 for each sample) at the atmospheric pressure and within the temperature range of 800–1050°C at the 30–50°C increment. The results were interpreted using the calculated log $${f_{{O_2}}}$$ – 1/T(K) and log $${f_{{O_2}}}$$ – T(°C) dependences. It was shown that the subsolidus temperature range of the rocks (below ~1050°C) is characterized by lowest intrinsic oxygen fugacity of olivine, which is 1–4 log units below the QFM buffer. For the solidus temperatures of ~1100°C, the more oxidized conditions ranging approximately from QFM to ~QFM-3 were measured. Extrapolating the log $${f_{{O_2}}}$$ – T°C dependences to the temperatures of the original magmas (~1200–1300°C) produces the maximum scatter in oxygen fugacities from ~QFM+2.5 to QFM-1. The estimated range of redox conditions for the Dovyren magma crystallization lies between the QFM and ~QFM-2.5 buffer equilibria. This is consistent with the complete absence of primary magmatic titanomagnetite and the presence of ilmenite in the Dovyren rocks.

Application of IR and Raman spectroscopy for the determination of the role of oxygen fugacity in the formation of N–С–О–Н molecules and complexes in the iron-bearing silicate melts at high pressures

Large-scale melting of the Earth’s early mantle under the effect of global impact processes was accompanied by the generation of volatiles, which concentration was mainly controlled by the interaction of main N, C, O, and H gas-forming elements with silicate and metallic melts at low oxygen fugacity (fO2), which predominated during metallic segregation and self-oxidation of magma ocean. The paper considers the application of Raman and IR (infrared) Fourier spectroscopy for revealing the mechanisms of simultaneous dissolution and relative contents of N, C, O, and H in glasses, which represent the quench products of reduced model FeO–Na2O–Al2O3–SiO2 melts after experiments at 4 GPa, 1550°C, and fO2 1.5–3 orders of magnitude below the oxygen fugacity of the iron—wustite buffer equilibrium (fO2(IW)). Such fO2 values correspond to those inferred for the origin and evolution of magma ocean. It was established that the silicate melt contains complexes with N–H bonds (NH3, NH2+, NH2-), N2, H2, and CH4 molecules, as well as oxidized hydrogen species (OH– hydroxyl and molecular water H2O). Spectral characteristics of the glasses indicate significant influence of fO2 on the N–C–O–H proportion in the melt. They are expressed in a sharp decrease of NH2+, NH2-(O–NH2), OH–, H2O, and CH4 and simultaneous increase of NH2-(≡Si–NH2) and NH3 with decreasing fO2. As a result, NH3 molecules become the dominant nitrogen compounds among N–C–H components in the melt at fO2 two orders of magnitude below fO2(IW), whereas molecular СН4 prevails at higher fO2. The noteworthy feature of the redox reactions in the melt is stability of the ОН– groups and molecular water, in spite of the sufficiently low fO2. Our study shows that the composition of reduced magmatic gases transferred to the planet surface has been significantly modified under conditions of self-oxidation of mantle and magma ocean.

Integrated Modeling of Transport Processes, Buffer Equilibria, and Biochemical Reactions in Chromatography Columns Using CADET

Integrated Modeling of Transport Processes, Buffer Equilibria, and Biochemical Reactions in Chromatography Columns Using CADET

Integrated modeling of transport processes, buffer equilibria and biochemical reactions in chromatography columns using CADET

Integrated modeling of transport processes, buffer equilibria and biochemical reactions in chromatography columns using CADET

Experimental study of the influence of water on the buffer equilibrium of magnetite–wüstite and wüstite–metallic iron

In experimental studies, water decomposition under reducing conditions results in the high fugacity of hydrogen, which is able to form interstitial solid solutions with wusite. New experiments demonstrate that the hydrogen fugacities attained in the presence of high water pressure and at the oxygen fugacities imposed by the buffer equilibria magnetite–wustite (MW) and wustite-iron (WI) are sufficiently high for the occurrence of hydrogen–oxygen interactions and for the formation of H2-saturated compound. Instead of buffer equilibria, hydrogen fugacity was directly controlled in the experiments at 700–1200°C and 200 MPa using the improved Shaw membrane technique in the form of a specially designed cell with Ar–H2 mixtures; H2 mole fraction in the mixtures ranged from 0.0 to 0.8. According to the phase rule for systems with perfectly mobile independent components, buffer reactions in the presence of water-bearing phases change into divariant equilibria. The X-ray study of phase composition along Mag–Wus divariant field shows an increase of wustite content relative to magnetite with increase of hydrogen mole fraction in a fluid. Mass-spectrometric study showed that the relative bulk solubility of hydrogen in buffer phases at 950°C decreases with an increase of iron content in wustite owing to the change in wustite stoichiometry under more reducing conditions. Experimental data indicate that the hydrogen solubility in wustite results in a shift of the magnetite stability field toward reducing region for \(0.65\log f_{O_2 } \) at 700°C and for \(1.2\log f_{O_2 } \) at 1200°C, whereas the stability of wustite with metallic iron practically coincides with anhydrous equilibrium, which is related to the close values of hydrogen solubility in wustite and in the metallic iron at temperatures below 950°C. However, the hydrogen solubility in metallic iron significantly increases with increasing temperature and its divariant field is shifted to a reducing region.

NH3 Permeability versus CO2 Permeability: Insights from Mathematical Modeling

Exposing a cell to a solution containing NH3/NH4+ leads to a rapid NH3 influx, which causes a rise in intracellular pH (pHi) and a transient decrease in surface pH (pHS). Conversely, exposing a cell to a solution containing CO2/HCO3− leads to a rapid CO2 influx, which causes a fall in pHi and a transient increase in pHS.In our laboratory we use the maximal change in the height of the pHS transient (ΔpHS) as a semiquantitative index of membrane permeability to NH3 (PM,NH3) or to CO2 (PM,CO2). As a first step in extracting absolute membrane permeability from pHS measurements, in 2012 we developed a reaction‐diffusion mathematical model of CO2 influx to investigate the dependence of ΔpHS predicted by the model on PM,CO2 values (Somersalo et al, J Theor Biol, 2012). To account for physiological data on gas channels, the model predicts that,contrary to widespread assumptions, the membrane must offer considerable resistance to the movement of CO2.Here, we investigate the dependence of ΔpHS versus PM,NH3 with a reaction‐diffusion mathematical model of NH3 influx into a spherical cell. The model is an extension of the model of Somersalo et al, in which now we include the NH3/NH4+ buffer equilibrium and passive transmembrane flux of NH3. After having checked that the model correctly predicts the transient changes in pHi and pHS that one would expect as NH3 enters a cell, we performed simulations in which we systematically decreased PM,NH3 from that corresponding to a thin film of water. By comparing the dependence of ΔpHS versus a range of PM,NH3 (present study) with the dependence of ΔpHS versus PM,CO2 (Somersalo et al), we conclude that, contrary to the Overton's prediction (which considers only solubility in the membrane), the background PM,NH3 (in absence of gas channels) is far higher than the background PM,CO2.

Effect of the sources and evolution of solutions on the composition of metasomatites

The effect of the composition of the sources and the thermal and baric evolution of metasomatic solutions on the character of metasomatic products was studied by numerically simulating solution-rock interaction. The rock sources of the solutions assumed in these simulations were shale, granite, and granodiorite. The relative acidity of the rocks and solutions interacting with them was suggested to estimate using the parameter \(A_R = \log \left( {a_{R^ + } /a_{H^ + } } \right) + \frac{1} {2}\log a_{H_2^O } \) (where R = K, Na). The values of this parameter in rocks differently vary with temperature and pressure and are controlled by buffer equilibria of minerals composing these rocks. The acidity of the solution relative to the source increases at cooling because of the more rapid dissociation of acids than salts and bases. However, the solution can be either more acidic or more alkaline relative to protoliths of other composition. Solutions derived from various sources but cooling in similar regime produce metasomatites of different composition. As the solution cools, feldspar metasomatites can grade into quartz-muscovite ones, which corresponds to the vertical temperature zoning of metasomatic aureoles. The alkalinity of solutions relative to their sources increases at their decompression, but the solutions may be less alkaline relative to protoliths of other composition. In contrast to cooling regime, solutions derived from all of the examined types of sources at decompression produce one of the feldspars and Ca, Mg, and Fe minerals, such as biotite, chlorite, amphibole, and magnetite. Quartz-free varieties of feldspathic metasomatites can be produced at either cooling or decompression but only by alkaline solutions. The potassic or sodic character of feldspathization at decompression depends on the composition of the source and the P-t conditions of solution generation.

Solution behavior of C-O-H volatiles in FeO-Na2O-Al2O3-SiO2 melts in equilibrium with liquid iron alloy and graphite at 4 GPa and 1550°C

In order to elucidate the solution behavior of carbon and hydrogen in iron-bearing magmatic melts in equilibrium with a metallic iron phase and graphite at oxygen fugacity (fO2) values 2–5 orders of magnitude below the iron-wustite buffer equilibrium, fO2 (IW), experiments were carried out at 4 GPa and 1550°C with melts of FeO-Na2O-SiO2-Al2O3 compositions. Melt reduction in response to an fO2 decrease was accompanied by a decrease in FeO content. The values of fO2 in the experiments were determined on the basis of equilibrium between Fe-C-Si alloy and silicate liquid. Infrared and Raman spectroscopy showed that carbon compounds are formed in FeO-Na2O-SiO2-Al2O3 melts: CH4 molecules, CH3 complexes (Si-O-CH3), and complexes with double C=O bonds. The content of CO2 molecules and carbonate ions (CO 3 2− ) is very low. In addition to carbon-bearing compounds, dissolved hydrogen occurs in melt as H2 and H2O molecules and OH− groups. The spectral characteristics of FeO-Na2O-SiO2-Al2O3 glasses indicate the occurrence of redox reactions in the melt, which are accompanied at decreasing fO2 by a significant decrease in H2O and OH−, a slight decrease in H2, and a significant concomitant increase in CH4 content. The content of species with the double C=O bond increases considerably at decreasing fO2 and reaches a maximum at ΔlogfO2(IW) = −3. According to the obtained IR spectra, the total water content (OH− + H2O) in the glasses is 1.2–5.8 wt % and decreases with decreasing fO2. The high H2O contents are due largely to oxygen release related to FeO reduction in the melt. The total carbon content at high H2O (4.9–5.8 wt %) is approximately 0.4 wt %. The carbon content in liquid iron alloys depends on silicon content and, probably, oxygen solubility and ranges from 0.3 to 3.65 wt %. Low carbon contents were observed at a significant increase in Si content in liquid iron alloy, which may be as high as ∼13 wt % at fO2 values 4–5 orders of magnitude below fO2(IW).

Electrostatic Effects of the Ion Atmosphere on Nucleosome Core Particle Interactions

Nucleosome-nucleosome attraction is known to be a vital aspect of chromatin compaction. The attraction is primarily electrostatic and is affected by the addition of divalent (or higher valence) ions. The specific mechanisms of this compaction, however, have yet to be determined. In particular there has been recent debate about the role of the nucleosome tails in nucleosome-nucleosome attraction. using buffer equilibrium atomic emission spectroscopy, we have determined the ion composition of the nucleosome in both wildtype and tail-modified mutants under a variety of ionic conditions. These data, when compared to atomic-scale Poisson-Boltzmann equation predictions, will shed light on the electrostatic and tail effects that drive inter-nucleosome interactions in solution.

Computer simulation in hydrothermal systems with allowance for nonideality of sphalerite and pyrrhotite

To enhance the computer simulation of hydrothermal processes using the HCh program package, an external ZnS_FeS module has been created on the basis of a nonideal asymmetric model of sphalerite solid solution. FeS and ZnS activity coefficients computed in line with this model within a temperature range 200–350°C lead to the decrease in FeS mole fraction (XFeS) in sphalerite by 3.0–1.5 times as compared with the ideal model. The calculated data on composition of sphalerite at the pyrite-pyrrhotite buffer with allowance for pyrrhotite nonideality are consistent with experimental results within the limits of 2% XFeS of its value (0.215). A nonlinear relationship logXFeS versus \(\left( {\log f_{S_2 } } \right)\). has been established, involving additional calculated data on equilibria of sphalerite with pyrite and magnetite, as well as pyrite and barite. With transition from pyrrhotite to magnetite and barite, a FeS mole fraction in sphalerite decreases to 0.1 and 0.006, respectively, because of increase in sulfur fugacity. The feasibility of using the calculation results based on the nonideal model of sphalerite for interpretation of natural data is exemplified in the Rainbow ore occurrence at the Mid-Atlantic Ridge (MAR). The computed pyrite-pyrrhotite and pyrite-cubanite-chalcopyrite buffer equilibria (XFeS = 0.215 and 0.10–0.12, respectively) are consistent with compositions of sphalerite in the pyrrhotite-cubanite-sphalerite and sphalerite ores (XFeS = 0.20–0.33 and 0.05–0.14, respectively).

Mineral reaction buffering of Venus’ atmosphere: A thermochemical constraint and implications for Venus-like planets

The equilibrium suggested as a buffer for CO 2 in the Venus atmosphere, CaCO 3 + SiO 2 = CaSiO 3 + CO 2, cannot act as a buffer at the Venus surface/troposphere – the pressure–temperature slope of the equilibrium and that of the atmosphere (dry adiabat with significant greenhouse heating) do not provide buffering capacity (if indeed CaCO 3 were present). Instead, perturbations to T or P(CO 2) can produce catastrophic expansion or collapse of the atmosphere. This instability can be generalized to all devolatilization reactions that produce a radiatively active gas in a planetary atmosphere dominated by such gases, and gives a simple thermochemical criterion for whether a reaction could buffer such an atmosphere. Simple decarbonation reactions fail this criterion, suggesting that the abundance of CO 2 in a CO 2-dominated atmosphere cannot be buffered by chemical reactions with the surface; a similar conclusion holds for the abundance of H 2O in an H 2O-dominated (steam) atmosphere. Buffering of minor gases is more likely; a mineral buffer equilibrium for SO 2 proposed for Venus, FeS 2 + CO 2 = Fe 3O 4 + SO 2 + CO, passes the thermochemical criterion, as does a reaction involving Ca sulfate. These inferences can be generalized to atmospheres in ‘moist’ adiabatic equilibria, and to extrasolar Venus-like planets, and will help in interpreting the compositions of their atmospheres.

Studying the forms of dissolution of hydrogen, carbon, nitrogen and oxygen volatiles in magmatic melts of the early earth's mantle by the methods of IR and Raman spectroscopy

Citation: Kadik, A. A., V. V. Koltashev, E. B. Kryukova, V. G. Plotnichenko (2011), Studying the forms of dissolution of hydrogen, carbon, nitrogen and oxygen volatiles in magmatic melts of the early earth's mantle by the methods of IR and Raman spectroscopy, Vestn. Otd. nauk Zemle, 3, NZ6031, doi:10.2205/2011NZ000161. Experimental studies of the features of dissolution of Н, C, N, O volatiles in the melting products of the early Earth’s mantle are continued. As a model system we have chosen the aluminosilicate melt (albite NaAlSi3O8 80 wt. %) + metallic Fe phase (FeO 20 wt. %) + H+C+N with nitrogen addition (Si3N4 1, 3, 5, 7 wt. %) which at quenching the products of experiments provides НС-N-O-containing silicate glasses. The quenching was carried out at a high pressure 4 GPa, high temperature 1550 о С, and low values of the chemical oxygen potential (fugacity fO2) from -2 to -4 below the iron-wustite buffer equilibrium lgfO2 (IW). The investigation techniques were IR (infrared) micro spectroscopy and micro Raman. The IR absorption spectra (Fig. 1) were measured on a vacuum Fourier-transform FTIR spectrometer “Bruker IFS-113v” with an optical microscope “IR Microscope A590”, focusing the radiation passing through the investigated samples into a spot from 15 to 400 m in diameter. The Raman spectra measurements (Fig. 2) were provided on a Triple Raman spectrograph Т-64000 (Jobin Yvon) in which the exciting radiation from Ar + -laser was focused on a sample surface into a 2 m spot. The application of these techniques has allowed to establish the influence oxygen fugacity (fO2) on the ratio between the oxidized (H2O, OH - CO2, C=O, CO3 2- ) and reduced (H2, CH4, SiC, C-C) Н- and C-species in the melts, as well as O2, N2, N-O, C-N and others N-H complexes (NH3, NH4 + , NH2 - (≡Si-NH2), NH2 + (≡Si-O-NH2)) in a rather wide region of the redox state

A new method to estimate the oxidation state of basaltic series from microprobe analyses

The oxygen fugacity and therefore the iron redox state of a melt is known to have a strong influence on the liquid line of descent of magmas and thus on the composition of the coexisting melts and crystals. We present a new method to estimate this critical parameter from electron probe microanalyses of two of the most common minerals of basaltic series, plagioclase and clinopyroxene. This method is not based on stoichiometric calculations, but on the different partitioning behaviour of Fe 3+ and Fe 2+ between both minerals and a melt phase: plagioclase can incorporate more Fe 3+ than Fe 2+, while clinopyroxene can incorporate more Fe 2+ than Fe 3+. For example, the effect of oxidizing a partly molten basaltic system (Fe 3+ is stabilized with respect to Fe 2+) results in an increase of FeO total in plagioclase, but a decrease in the associated clinopyroxene. We propose an equation, based on published partition coefficients, that allows estimating the redox state of a melt from these considerations. An application to a set of experimental and natural data attests the validity of the proposed model. The associated error can be calculated and is on average < 1 log unit of the prevailing oxygen fugacity. In order to reduce the number of different variables influencing the Fe 2+/Fe 3+ mineral/melt equilibrium, our model is restricted to basaltic series with SiO 2 < 60% that have crystallized at intermediate to low pressure (< 0.5 GPa) and under relatively oxidizing conditions (∆FMQ > 0; where FMQ is the fayalite–magnetite–quartz oxygen buffer equilibrium), but it may be parameterized for other conditions. A spreadsheet is provided to assist the use of equations, and to perform the error propagation analysis.

Comparison of Cretaceous granitoids of the Chaun tectonic zone to those of the Taigonos Peninsula, NE Asia: rock chemistry, composition of rock forming minerals, and conditions of formation

Abstract. The Cretaceous granitoid complexes of the Eastern Taigonos and the Prybrezhny Taigonos belts (southern part of the Taigonos Peninsula), Tanyurer pluton of the Okhotsk-Chukotka volcanic belt, and the Peekiney, Moltykan, and Telekay plutons of the Chaun tectonic zone are discussed in relation to their structural position, petrography, rock and mineral chemistry and physicochemical conditions of melt crystallization. These granitoid plutons were generated through melting of a compositionally heterogeneous crustal source, with direct contribution from mafic melts produced in the mantle wedge above active or extinct Benioff zones. Variations of the trace-element composition of granitoids are controlled to a greater extent by local compositional peculiarities of the source regions than by the geodynamic regime as such. The final crystallization of these plutons occurred at comparatively shallow depths, between 1–2 and 6–7 km, in a temperature interval of 700–770°C. The depth of emplacement of the bodies decreases with increasing distance from the areas with oceanic and transitional type crust, as does the degree of incompatible element enrichment of the mantle and crustal sources of melts. Variations in fo2 values at the late stages of crystallization of the plutons reach 3–4 orders of magnitude, exceeding the limits of the quartz-fayalite-magnetite (QFM) and nickel-nickel oxide (NNO) buffer equilibria, which likely results from local variations of the source composition.

Magnetite-ilmenite equilibria in Archean enderbites from the Sutam Complex (Aldan shield)

The composition of disintegrated titanomagnetites and ilmenites from Archean enderbites of the Sutam Complex of the Aldan shield has been reconstructed. The values of oxygen fugacity and temperature of titanomagnetite‐ilmenite mineral equilibrium at 7.5 kbar general pressure are calculated. In the process of granulite metamorphism oxygen volatility varied within = ‐14.823 ± ‐16.868 at temperatures T = 723 ° C‐910 ° C, corresponding to a line parallel to the quartz‐ferrosillite‐magnetite buffer equilibrium, but about 1.5 orders of above it. The probable values of oxygen volatility for protoliths of Sutam enderbites were not lower than calculated ones. Detailed study of iron‐titanium oxide paragenesis occurring in metamorphic rocks has been given little attention so far. In the meanwhile, a purposeful approach with reconstruction of the primary composition of iron‐titanium oxides may supply valuable data on the temperature and oxidation potential at the time of metamorphism. Study of these minerals is especially important when they occur in ancient magmatic rocks. In this case, hypothesizing a minor change in the oxygen potential during metamorphism makes it possible to obtain the value of the oxidation potential of the magmatic rock protolith. In this way we can assess the oxygen regime of the Earth’s crust during the earliest stages of our planet’s evolution. In our paper this task is being solved on the basis of petrological study of magnetite‐ilmenite equilibria occurring in enderbites of the Sutam metamorphic complex. The Sutam Complex forms a tectonic block of the same name to the south of Aldan shield in the Sutam River basin (Fig. 1). It is distinguished by ultrahigh-pressure conditions of regional metamorphism at depth. It was described first by A.A. Marakushev [1]. According f O2 log f O2 log to [2, 3], the Sutam Complex consists of two strata: the low one involving mainly hypersthene plagiogneisses (enderbites) with minor amounts of garnet, garnet‐ biotite plagiogneisses, and garnet‐sillimanite, cordierite, hypersthene‐sillimanite gneisses, and quartzites. Reconstruction of the primary nature of high-grade rocks of the low and upper strata of the complex [2, 3] revealed that volcanic rocks of calc‐alkaline series, andesites, dacites, and rhyodacites (andesite‐dacite association) dominate in the low strata. Volcanic rocks of the komatiite‐tholeiite series presented by peridotite komatiites and komatiitic and tholeiitic basalts (komatiite‐basalt association) make up an insignificant part of the succession (not more than 10%). The initial composition of the upper strata is reconstructed as intercalation of volcanic rocks of the andesite‐dacite association with greywackes, pelites, volcanoclastics, as well as chemogenic‐sedimentary, sil

Solubility of platinum and palladium in silicate melts under high water pressure as a function of redox conditions

The solubility of platinum and palladium in a silicate melt of the composition Di55An35Ab10 was determined at 1200°C and 2 kbar pressure in the presence of H2O-H2 fluid at an oxygen fugacity ranging from the HM to WI buffer equilibria. The influence of sulfur on the solubility of platinum in fluid-bearing silicate melt was investigated at a sulfur fugacity controlled by the Pt-PtS equilibrium at 1200°C and a pressure defined in such a way that the \( f_{H_2 O} \) and \( f_{O_2 } \) values were identical to those of the experiments without sulfur. The experiments were conducted in a high pressure gas vessel with controlled hydrogen content in the fluid. Oxygen fugacity values above the NNO buffer were controlled by solid-phase buffer mixtures using the two-capsule technique. Under more reducing conditions, the contents of H2O and H2 were directly controlled by the argon to hydrogen ratio in a special chamber. The hydrogen fugacity varied from 5.2 × 10−2 bar (HM buffer) to 1230 bar (\( X_{H_2 } \) = 0.5). Pt and Pd contents were measured in quenched glass samples by neutron activation analysis. The results of these investigations showed that the solubility of Pt and Pd increases significantly in the presence of water compared with experiments in dry systems. The content of Pd within the whole range of redox conditions and that of Pt at an oxygen fugacity between the HM to MW buffer reactions are weakly dependent on \( f_{O_2 } \) and controlled mainly by water fugacity. This suggests that, in addition to oxide Pt and Pd species soluble at the ppb level in haplobasaltic melts, much more soluble (ppm level) hydroxide complexes of these metals are formed under fluid-excess conditions. Despite a decrease in water fugacity under reducing conditions, Pt solubility increases sharply near the MW buffer. It was shown by electron paramagnetic resonance spectrometry that, in contrast to dry melts, fluid-saturated silicate melts do not contain a pure metal phase (micronuggets). Therefore, the increase in Pt solubility under reducing conditions can be explained by the formation of Pt hydride complexes or Pt-fluid-silicate clusters. At a sulfur fugacity controlled by the Pt-PtS equilibrium, the solubility of Pt in iron-free silicate melts as a function of redox conditions is almost identical to that obtained in the experiments without sulfur at the same water and oxygen fugacity values. These observations also support Pt dissolution in iron-free silicate melts as hydroxide species.

Rehydration of Calcined Mg-Al Hydrotalcite in Acidified Chloride-Containing Aqueous Solution

The rehydration of periclase-like Mg-Al mixed oxide obtained by calcination of hydrotalcite- like precursor with a Mg/Al molar ratio of 2 was carried out in KCl-HCl solutions at various pH and constant concentration of Cl-. A buffer equilibrium accompanied by leaching out of Mg2+ cations from the solid was observed during rehydration, when reconstruction of the layered hydrotalcite structure takes place. With increasing HCl concentration in the rehydration solution, the Mg/Al molar ratio in the obtained solid gradually decreased from about 1.5 to 0.3. An anomaly was found, when the increasing concentration of acid resulted in increasing pH of final solution. The XRD measurements showed that the initially formed hydroxide form of hydrotalcite is transformed into randomly interstratified forms of the Mg-Al hydroxide hydrate/Mg-Al chloride hydroxide hydrate, which are accompanied by the release of OH- anions into solution. The crystalline structure of rehydrated product decreased as Mg2+ cations were released into solution, which was accompanied by incorporation of Cl- into the solid.