Sulfur K-edge XANES analysis of natural and synthetic basaltic glasses: Implications for S speciation and S content as function of oxygen fugacity

XANES analyses at the sulfur K-edge were used to determine the oxidation state of S species in natural and synthetic basaltic glasses and to constrain the fO₂ conditions for the transition from sulfide (S²⁻) to sulfate (S⁶⁺) in silicate melts. XANES spectra of basaltic samples from the Galapagos spreading center, the Juan de Fuca ridge and the Lau Basin showed a dominant broad peak at 2476.8 eV, similar to the spectra obtained from synthetic sulfide-saturated basalts and pyrrhotite. An additional sharp peak at 2469.8 eV, similar to that of crystalline sulfides, was present in synthetic glasses quenched from hydrous melts but absent in anhydrous glasses and may indicate differences in sulfide species with hydration or presence of minute sulfide inclusions exsolved during quenching. The XANES spectra of a basalt from the 1991 eruption of Mount Pinatubo, Philippines, and absarokitic basalts from the Cascades Range, Oregon, USA, showed a sharp peak at 2482.8 eV, characteristic of synthetic sulfate-saturated basaltic glasses and crystalline sulfate-bearing minerals such as hauyne. Basaltic samples from the Lamont Seamount, the early submarine phase of Kilauea volcano and the Loihi Seamount showed unequivocal evidence of the coexistence of S²⁻ and S⁶⁺ species, emphasizing the relevance of S⁶⁺ to these systems. XANES spectra of basaltic glasses synthesized in internally-heated pressure vessels and equilibrated at fO₂ ranging from FMQ − 1.4 to FMQ + 2.7 showed systematic changes in the features related to S²⁻ and S⁶⁺ with changes in fO₂. No significant features related to sulfite (S⁴⁺) species were observed. These results were used to construct a function that allows estimates of S⁶⁺/ΣS from XANES data. Comparison of S⁶⁺/ΣS data obtained by S Kα shifts measured with electron probe microanalysis (EPMA), S⁶⁺/ΣS obtained from XANES spectra, and theoretical considerations show that data obtained from EPMA measurements underestimate S⁶⁺/ΣS in samples that are sulfate-dominated (most likely because of photo-reduction effects during analysis) whereas S⁶⁺/ΣS from XANES provide a close match to the expected theoretical values. The XANES-derived relationship for S⁶⁺/ΣS as a function of fO₂ indicates that the transition from S²⁻ to S⁶⁻ with increasing fO₂ occurs over a narrower interval than what is predicted by the EPMA-derived relationship. The implications for natural systems is that small variation of fO₂ above FMQ + 1 will have a large effect on S behavior in basaltic systems, in particular regarding the amount of S that can be transported by basaltic melts before sulfide saturation can occur.     

Armalcolite stability as a function of pressure and oxygen fugacity

The stability of synthetic armalcolite of composition (Fe_0.5Mg_0.5Ti₂O₅ was studied as a function of total pressure up to 15 kbar and 1200°C and also as a function of oxygen fugacity (?O₂) at 1200°C and 1 atm total pressure. The high pressure experiments were carried out in a piston-cylinder apparatus using silver-palladium containers. At 1200°C, armalcolite is stable as a single phase at 10 kbar. With increasing pressure, it breaks down ($\text{dT}\text{dP}\text{= 20°C/}\text{kbar}$), to rutile, a more magnesian armalcolite, and ilmenite solid solution. At 14 kbar, this three-phase assemblage gives way ($\text{dT}\text{dP}\text{= 30°C/}\text{kbar}$) to a two-phase assemblage of rutile plus ilmenite solid solution.A zirconian-armalcolite was synthesized and analyzed; 4 wt % ZrO₂ appears to saturate armalcolite at 1200°C and 1 atm. The breakdown of Zr-armalcolite occurs at pressures of 1–2 kbar less than those required for the breakdown of Zr-free armalcolite. The zirconium partitions approximately equally between rutile and ilmenite phases.The stability of armalcolite as a function of ?O₂ was determined thermogravimetrically at 1200°C and 1 atm by weighing sintered pellets in a controlled atmosphere furnace. Armalcolite, (Fe_0.5Mg_0.5)-Ti₂O₅, is stable over a range ?O₂ from about 10^?9.5to 10^?10.5 atm. Below this range to at least 10^?12.8 atm, ilmenite plus a reduced armalcolite are formed. These products were observed optically and by Mössbauer spectroscopy, and no metallic iron was detected; therefore, some of the titanium must have been reduced to Ti³⁺. This reduction may provide yet another mechanism to explain the common association of ilmenite rims around lunar armalcolites.     

Nitrogen solubility in basaltic melt. Part I. Effect of oxygen fugacity

The role of the oxygen fugacity on the incorporation of nitrogen in basaltic magmas has been investigated using one atmosphere high temperature equilibration of tholeiitic-like compositions under controlled nitrogen and oxygen partial pressures in the [C-N-O] system. Nitrogen was extracted with a CO₂ laser under high vacuum and analyzed by static mass spectrometry. Over a redox range of 18 oxygen fugacity log units, this study shows that the incorporation of nitrogen in silicate melts follows two different behaviors. For log fO₂ values between −0.7 and −10.7 (the latter corresponding to IW − 1.3), nitrogen dissolves as a N₂ molecule into cavities of the silicate network (physical solubility). Nitrogen presents a constant solubility (Henry’s) coefficient of 2.21 ± 0.53 × 10⁻⁹ mol g⁻¹ atm⁻¹ at 1425°C, identical within uncertainties to the solubility of argon. Further decrease in the oxygen fugacity (log fO₂ between −10.7 and −18 corresponding to the range from IW − 1.3 to IW − 8.3) results in a drastic increase of the solubility of nitrogen by up to 5 orders of magnitude as nitrogen becomes chemically bounded with atoms of the silicate melt network (chemical solubility). The present results strongly suggest that under reducing conditions nitrogen dissolves in silicate melts as N³⁻ species rather than as CN⁻ cyanide radicals. The nitrogen content of a tholeiitic magma equilibrated with N₂ is computed from thermochemical processing of our data set as

     

Influence of oxygen fugacity and temperature on the redox state of iron in natural silicic aluminosilicate melts

The influence of oxygen fugacity (fO₂) and temperature on the valence and structural state of iron was experimentally studied in glasses quenched from natural aluminosilicate melts of granite and pantellerite compositions exposed to various T-fO₂ conditions (1100–1420°C and 10^?12–10^?0.68 bar) at a total pressure of 1 atm. The quenched glasses were investigated by Mössbauer spectroscopy. It was shown that the effect of oxygen fugacity on the redox state of iron at 1320–1420°C can be described by the equation log(Fe³⁺/Fe²⁺) = k log(fO₂) + q, where k and q are constants depending on melt composition and temperature. The Fe³⁺/Fe²⁺ ratio decreases with decreasing fO₂ (T = const) and increasing temperature (fO₂ = const). The structural state of Fe³⁺ depends on the degree of iron oxidation. With increasing Fe³⁺/Fe²⁺ ≥ 1, the dominant coordination of Fe³⁺ changes from octahedral to tetrahedral. Ferrous iron ions occur in octahedral (and/or five-coordinated) sites independent of Fe³⁺/Fe²⁺.     

Experimental investigation of large-ion-lithophile-element-, high-field-strength-element- and rare-earth-element-partitioning between calcic amphibole and basaltic melt: the effects of pressure and oxygen fugacity

The effects of pressure and oxygen fugacity (fO₂) on trace element partitioning between pargasitic amphibole and alkali-basaltic melts have been determined at pressures from 1.5 to 2.5 GPa and oxygen fugacities at 2 log units above and below the nickel–nickel oxide buffer. Amphibole crystallization experiments were performed in a piston cylinder apparatus and partition coefficients between amphibole and quenched melt of large-ion-lithophile elements (LILE: Rb, Sr, Ba), high-field-strength elements (HFSE: Zr, Nb, Ta, Hf, U, Th) and rare-earth elements (REE: La to Lu; +Y) were measured with a LASER ablation inductively coupled plasma – mass spectrometer. Increasing pressure from 1.5 to 2.5 GPa at similar temperatures and approximately constant fO₂ increases D _Rb but decreases D _Zr and D _Hf and D _REE (D _La, D _Ce, D _Pr). An empirical relationship was observed between D _Zr and (Ti/Al)_M2 in the amphibole, which can be described by:

Vanadium and niobium behavior in rutile as a function of oxygen fugacity: evidence from natural samples

Vanadium occurs in multiple valence states in nature, whereas Nb is exclusively pentavalent. Both are compatible in rutile, but the relationship of V–Nb partitioning and dependence on oxygen fugacity (expressed as fO₂) has not yet been systematically investigated. We acquired trace-element concentrations on rutile grains (n = 86) in nine eclogitic samples from the Dabie-Sulu orogenic belt by laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) and combined them with published results in order to assess the direct and indirect effects of oxygen fugacity on the partitioning of V and Nb into rutile. A well-defined negative correlation between Nb (7–1,200 ppm) and V concentrations (50–3,200 ppm) was found, documenting a competitive relationship in the rutile crystal that does not appear to be controlled by bulk rock or mineral compositions. Based on the published relationship of ^RtD_V and V valence with ?QFM, we suggest that the priority order of V incorporation into rutile is V⁴⁺ > V³⁺ > V⁵⁺. The inferred Nb–V competitive relationship in rutile from the Dabie-Sulu orogenic belt could be explained by decreasing fO₂ due to dehydration reactions involving loss of oxidizing fluids during continental subduction: The increased proportion of V³⁺ (expressed as V³⁺/∑V) and attendant decrease in ^RtD_V is suggested to lead to an increase in rutile lattice sites available for Nb⁵⁺. A similar effect may be observed under more oxidizing conditions. When V⁵⁺/∑V increases, ^RtD_V shows a dramatic decline and Nb concentration increases considerably. This is possibly documented by rutile in highly metasomatized and oxidized MARID-type (MARID: mica–amphibole–rutile–ilmenite–diopside) mantle xenoliths from the Kaapvaal craton, which also show a negative V–Nb covariation. In addition, their Nb/Ta covaries with V concentrations: For V concentrations <1,250 ppm, Nb/Ta ranges between 35 and 45, whereas for V > 1,250 ppm, Nb/Ta is considerably lower (5–15). This relationship is mainly controlled by a change in Nb concentrations, suggesting that the indirect dependence of ^RtD_Nb on fO₂, which is not mirrored in ^RtD_Ta, can exert considerable influence on rutile Nb–Ta fractionation.     

Influence of oxygen fugacity on mineral compositions in peralkaline melts: The Katzenbuckel volcano, Southwest Germany

The igneous rocks of the Katzenbuckel, Southwest Germany, represent a unique and unusual alkaline to peralkaline association within the European Volcanic Province. The magmatic activity can be subdivided into two main phases. Phase I comprises the main rock bodies of phonolite and nepheline syenite, which were later intruded by different peralkaline dyke rocks (tinguaites and alkali feldspar syenite dykes) of phase II. The dyke assemblage was accompanied by magnetite and apatite veins and was followed by a late-stage pneumatolytic activity causing autometasomatic alterations.

As is typical for alkaline to peralkaline igneous rocks, early mafic minerals of phase I rocks comprise olivine, augite and Fe–Ti oxides, which are substituted in the course of fractionation by Na-amphibole and Na-pyroxene. For the early magmatic stage, calculated temperatures range between 880 and 780 °C with low silica activities (0.4 to 0.6) but high relative oxygen fugacities between 0.5 and 1.9 log units above the FMQ buffer. Even higher oxygen fugacities (above the HM buffer) are indicated for the autometasomatic alteration, which occurred at temperatures between 585 and 780 °C and resulted in the formation of pseudobrookite and hematite.

The unusually high oxygen fugacities (even during the early magmatic stage) are recorded by the major element compositions of the mafic minerals (forsterite content in olivine between 68 and 78 mol%, up to 6.2 wt.% ZrO₂ and 8.5 wt.% TiO₂ in clinopyroxene), the unusual mineral assemblages (pseudobrookite, freudenbergite) and by the enrichment of Fe³⁺ in the felsic minerals (up to 2.8 wt.% Fe₂O₃ in alkali feldspar and up to 2.6 wt.% Fe₂O₃ in nepheline). These observations point to a metasomatically enriched and highly oxidized lithospheric mantle as a major source for the Katzenbuckel melts.     

The effects of temperature,oxygen fugacity and melt composition on the behaviour of chromium in basic and ultrabasic melts

Chromite was equilibrated with two natural basic liquids and one natural ultrabasic liquid at temperatures and oxygen fugacities appropriate to geological conditions. The experiments were designed to document changes in mineral and glass compositions between the iron-wüstite and nickel-nickel oxide buffers, with special emphasis on conditions along quartz-fayalite-magnetite. The Cr contents of the melts at chromite saturation increase strongly with increasing temperature and with decreasing oxygen fugacity.A relationship is described which accounts for the compositional dependence of the partitioning of Cr between spinels and silicate melts by considering the exchange of FeCr₂O₄ component between the crystalline and melt phases. Interpretation of the data in terms of this exchange suggests that Cr³⁺ in metaluminous melts occurs in octahedrally coordinated sites, and that it does not depend on charge-balancing by monovalent cations. In this model, Cr³⁺ is proposed to behave like network-modifying Al³⁺ and Fe³⁺, i.e., the excess aluminum and ferric iron which do not participate in tetrahedrally coordinated matrix or network-forming complexes.The results can also be applied to the problem of the formation of massive chromitites of great lateral extent in basic layered intrusions. The data are consistent with a model in which the crystallization of chromite is initiated through magma mixing, in combination with the rapid heat loss associated with periodic influxes of magma into a chamber. An alternative model, in which chromite crystallization is initiated by repeated fluctuations in oxygen fugacity, is possible only if the magma f_O2 is not controlled by an oxygen buffer such as QFM.     

Ferric-ferrous ratio and oxygen fugacity calculations for primitive mantle-derived melts: calibration of an empirical technique

Summary Several experimentally-based, empirical calibrations of the fO₂ of natural silicate melts at atmospheric pressure as a function of melt composition, melt Fe²⁺/Fe³⁺, and crystallization temperature have been developed (e.g.Sack et al., 1980;Kilinc et al., 1983;Kress andCarmichael, 1988;Borisov andShapkin, 1990). Cr-Al spinel is a liquidus phase of primitive mantle-derived melts, and is commonly found as inclusions in near-liquidus phenocrysts (mainly olivine). The established atmospheric pressure correlation between Fe²⁺/Fe³⁺ values in spinel and coexisting melts over a broad range of basaltic compositions (Maurel andMaurel, 1982) can be used to calculate the Fe²⁺/Fe³⁺ value of a melt if the composition of the equilibrium spinel is known. Compositions and crystallization temperatures of primitive melts can be determined by experimental studies of melt inclusions trapped by early-formed refractory phenocrysts. Thus, the association of spinel and melt inclusions in early liquidus phenocrysts can be used to estimate fO₂ conditions at the time of their crystallization.In this paper, we present a calibration of this method and discuss its applications. We conclude that combination of the equations ofMaurel andMaurel (1982) andBorisov andShapkin (1990) can be used to calculate fO₂ with an accuracy of ±0.71og units, when liquidus spinels have TiO₂ <2.5 wt% and Cr₂O₃ > 13 wt.%, and melt compositions are in the range from basaltic to picritic with H₂O contents <6 wt.%.Using this technique we find NNO fO₂ values of –0.8 to –1.4 for MORB dredged at the VEMA Fracture Zone in the Atlantic, and 0 to + 1 for Tongan high-Ca boninites.

---

Die Berechnung von Fe²⁺/Fe³⁺ und der Sauerstoff-Fugazitäten für primitive Mantelschmelzen: Kalibration einer empirischen Methode

Zusammenfassung Empirische, auf Experimenten basierende, Kalibrationen zur Berechnung von fO₂ natürlicher Silikatschmelzen bei atmosphärischem Druck in Abhängigkeit von der Schmelzzusammensetzung, des Fe²⁺/Fe³⁺ Verhältnisses und der Kristallisationstemperatur wurden z.B. vonSack et al. (1980),Kilinc et al. (1983),Kress undCarmichael (1988) undBorisov undShapkin (1980) entwickelt. Cr-Al-Spinell ist eine Liquidusphase primitiver Mantelschmelzen und kommt üblicherweise als Einschluß in near-liquidus Phänokristallen (hauptsächlich in Olivin) vor. Die Korrelation des Atmosphärendruckes zwischen Fe²⁺/Fe³⁺ in Spinell und koexistierender Schmelze kann dazu verwendet werden, das Verhältnis von Fe²⁺/Fe³⁺ der Schmelze für einen weiten Bereich basaltischer Zusammensetzungen zu berechnen, wenn die Zusammensetzung des im Gleichgewicht gebildeten Spinells bekannt ist (Maurel undMaurel, 1982). Die Zusammensetzungen und Kristallisationstemperaturen primitiver Schmelzen können durch experimentelle Studien von Schmelzeinschlüssen, die in früh gebildeten refraktären Phänokristallen eingeschlossen wurden, ermittelt werden. Daher lassen sich Spinelle und assoziierte Schmelzeinschlüsse in frühen Liquidus-Phänokristallen dazu benützen, die fO₂-Bedingungen während der Kristallisation abzuschätzen.In dieser Arbeit präsentieren wir eine neue Kalibration dieser Methode und diskutieren ihre Anwendungen. Wir schlußfolgern, daß unter Kombination der verwendeten Gleichungen vonMaurel undMaurel (1982) sowie vonBorisov undShapkin (1990) fO₂ mit einer Genauigkeit von ±0.7 log Einheiten berechnet werden kann, soferne die Liquidus-Spinelle < 2.5 Gew.% TiO₂ und > 13 Gew.% Cr₂O₃ haben und die Schmelzzusammensetzungen von basaltisch bis pikritisch, mit maximal 6 Gew.% H₂O, reichen.Unter Verwendung dieser Technik wurden die NNO fO₂ Werte für die von der VEMA Fracture Zone im Atlantik stammenden MORB Proben mit 0.8 bis - 1.4, die der der High-Ca Boninite von Tonga mit 0 bis + 1 bestimmt.

---

---

With 7 Figures     

Solubility and speciation of sulfur in silicate melts: The Conjugated Toop-Samis-Flood-Grjotheim (CTSFG) model

A Conjugated Toop-Samis-Flood-Grjotheim (CTSFG) model is developed by combining the framework of the Toop-Samis polymeric approach with the Flood-Grjotheim theoretical treatment of silicate melts and slags. Electrically equivalent ion fractions are computed over the appropriate matrixes (anionic and cationic) in a Temkin notation for fused salts, and are used to weigh the contribution of the various disproportionation reactions of type:

     

The oxidation state of europium as an indicator of oxygen fugacity

The distribution of Eu between plagioclase feldspar and magmatic liquid has been determined experimentally for basaltic and andesitic systems as a function of temperature and oxygen fugacity at one atmosphere total pressure. Using the approach of Philpotts the ratios $\text{Eu}{}^{\mathrm{2+}}\text{Eu}{}^{\mathrm{3+}}$ in plagioclase and coexisting magmatic liquid have been calculated. These ratios appear to be simply related to oxygen fugacity for the bulk compositions studied here. Using published trace element distribution data for natural rocks oxygen fugacities may be calculated from these experimental results. For terrestrial basalts calculated oxygen fugacities average 10^?7 with little dispersion from this value. Andesites average 10^?8.1 with considerable dispersion, while dacites and rhyodacites average 10^?9.1, also with considerable dispersion. Oxygen fugacities for lunar ferrobasalts cluster tightly around 10^?12.7. Data on achondritic meteorites are limited, but calculations indicate oxygen fugacities of two-to-five orders of magnitude lower than lunar ferrobasalts.     

The speciation of water in silicate melts

Previous models of water solubility in silicate melts generally assume essentially complete reaction of water molecules to hydroxyl groups. In this paper a new model is proposed that is based on the hypothesis that the observed concentrations of molecular water and hydroxyl groups in hydrous silicate glasses reflect those of the melts from which they were quenched. The new model relates the proportions of molecular water and hydroxyl groups in melts via the following reaction describing the homogeneous equilibrium between melt species: H₂O_molecular (melt) + oxygen (melt) = 2OH (melt). An equilibrium constant has been formulated for this reaction and species are assumed to mix ideally. Given an equilibrium constant for this reaction of 0.1–0.3, the proposed model can account for variations in the concentrations of molecular water and hydroxyl groups in melts as functions of the total dissolved water content that are similar to those observed in glasses. The solubility of molecular water in melt is described by the following reaction: H₂O (vapor) = H₂O_molecular (melt).These reactions describing the homogeneous and heterogeneous equilibria of hydrous silicate melts can account for the following observations: the linearity between f_H2O and the square of the mole fraction of dissolved water at low total water contents and deviations from linearity at high total water contents; the difference between the partial molar volume of water in melts at low total water contents and at high total water contents; the similarity between water contents of vapor-saturated melts of significantly different compositions at high pressures versus the dependence on melt composition of water solubility in silicate melts at low pressures; and the variations of viscosity, electrical conductivity, the diffusivity of “water,” the diffusivity of cesium, and phase relationships with the total dissolved water contents of melts.This model is thus consistent with available observations on hydrous melt systems and available data on the species concentrations of hydrous glasses and is easily tested, since measurements of the concentrations of molecular water and hydroxyl groups in silicate glasses quenched from melts equilibrated over a range of conditions and total dissolved water contents are readily obtainable.     

The effect of oxygen fugacity on ionic conductivity in olivine

The oxygen fugacity(f_O2) may affect the ionic conductivity of olivine under upper mantle conditions because Mg vacancies can be produced in the crystal structure by the oxidization of iron from Fe²⁺ to Fe3+. Here we investigated olivine ionic conductivity at 4 GPa, as a function of temperature, crystallographic orientation, and oxygen fugacity, corresponding to the topmost asthenospheric conditions. The results demonstrate that the ionic conductivity is insensitive to f_O2 under relatively reduced conditions(f_O2 below Re-ReO₂ buffer), whereas it has a clear f_O2-dependence under relatively oxidized conditions(f_O2 around the magnetite-hematite buffer). The ionic conduction in olivine may contribute significantly to the conductivity anomaly in the topmost asthenosphere especially at relatively oxidized conditions.     

Frequency dependent electrical properties of dunite as functions of temperature and oxygen fugacity

Using impedance spectroscopy, we have measured the electrical properties of two dunites and a single crystal olivine sample from 1000 to 1200° C as a function of oxygen fugacity (f _{o 2}). Two conduction mechanisms with resistances that add in series are observed for the dunites corresponding to grain interior and grain boundary conduction mechanisms. The conductivities for each mechanism were determined by analyzing the data using a complex nonlinear least squares fitting routine and the equivalent circuit approach. The grain interiors display a conductivity dependent on f _{o 2} to the 1/5.5–1/7 power, consistent with other determinations, and interpreted as indicating small polaron transport (Fe _Mg ^· ). The grain boundaries demonstrate a weaker f _{o 2} dependence that is dependent on temperature and material. Under certain conditions the f _{o 2} dependence of the grain boundary conductivity is negative. This result indicates that oxygen ion transport is probably not the dominant grain boundary charge transport mechanism; however, an unequivocal determination of the grain boundary mechanism has not been achieved. In some dunites the grain boundaries are more conductive than the grain interiors; in other dunites they are more resistive than grain interiors. The grain boundaries do not enhance the total conductivity of any of the materials of this study but are the controlling mechanism in some instances. Measurement of the complex electrical response at frequencies as low as 10^-4 Hz is required to determine the role of grain boundaries on the overall electrical properties of polycrystalline dunite.     

Associations of Mn-bearing minerals as indicators of oxygen fugacity during the metamorphism of metalliferous deposits

The paper summarizes experimental and calculation data on the effect of oxygen fugacity on the origin of mineral assemblages in Mn-bearing rocks and demonstrates the possibility of application of these data to the reconstruction of conditions under which metalliferous deposits were metamorphosed. A new variant of the T-log\(f_{O_2 } \) diagram is proposed for the Mn-Si-O system, which differs from previous ones by the location of the lines for the formation (decomposition) of braunite and tephroite. These two minerals are the most universal indicators of oxygen fugacity during the metamorphism of Mn-bearing deposits, because these minerals are widespread in nature and can be formed in diverse environments: braunite at high \(f_{O_2 } \) values in the pore solution, and tephroite at low \(f_{O_2 } \) values. The occurrence of Mn oxides and rhodonite (pyroxmangite) in a rock makes it possible to constrain the oxygen fugacity range. An original T-log\(f_{O_2 } \) diagram is constructed for the Ca-Mn-Si-O system. As follows from this diagram, a Ca admixture expands the stability field of rhodonite toward higher oxygen fugacity values. Johannsenite can be formed in these rocks at even higher \(f_{O_2 } \). The stability of both minerals is constrained in the region of low \(f_{CO_2 } \). The paper reports data on the Fe-Si-O and Mn-Fe-Si-O systems and discusses the possibility of applying the results of experiments in the Mn-Al-Si-O system to the estimation of conditions under which andalusite, spessartine, and galaxite can be formed in Mn-bearing rocks. Data on the mineralogy of numerous Mn deposits metamorphosed under various PTX parameters indicate that the origin of Mn-bearing mineral assemblages depends not so much on the temperature and pressure as on the oxygen fugacity, which is, in turn, controlled primarily by the composition of the pristine sediments (the presence or absence of organic matter in them) and host rocks and depends on the permeability of the rocks to oxygen, the P-T conditions, and the duration of the metamorphic processes.     

Time-dependent changes of the electrical conductivity of basaltic melts with redox state

The electrical conductivity of basaltic melts has been measured in real-time after fO₂ step-changes in order to investigate redox kinetics. Experimental investigations were performed at 1 atm in a vertical furnace between 1200 and 1400 °C using air, pure CO₂ or CO/CO₂ gas mixtures to buffer oxygen fugacity in the range 10⁻⁸ to 0.2 bars. Ferric/ferrous ratios were determined by wet chemical titrations. A small but detectable effect of fO₂ on the electrical conductivity is observed. The more reduced the melt, the higher the conductivity. A modified Arrhenian equation accounts for both T and fO₂ effects on the electrical conductivity. We show that time-dependent changes in electrical conductivity following fO₂ step-changes monitor the rate of Fe²⁺/Fe³⁺ changes. The conductivity change with time corresponds to a diffusion-limited process in the case of reduction in CO-CO₂ gas mixtures and oxidation in air. However, a reaction at the gas-melt interface probably rate limits oxidation of the melt under pure CO₂. Reduction and oxidation rates are similar and both increase with temperature. Those rates range from 10⁻⁹ to 10⁻⁸ m²/s for the temperature interval 1200-1400 °C and show activation energy of about 200 kJ/mol. The redox mechanism that best explains our results involves a cooperative motion of cations and oxygen, allowing such fast oxidation-reduction rates.     

Effect of oxygen fugacity on sulfur isotope compositions and magnetite concentrations in the Kirkpatrick Basalt,Mount Falla,Queen Alexandra Range,Antarctica

The δ³⁴S-values of total sulfur in the Jurassic tholeiite flows on Mt. Falla in Antarctica range from ?1.45 to +11.73‰. The concentrations of sulfur range from 80 to 480 ppm, which is typical of subaerial lava flows that lose varying proportions of sulfur by out-gassing of SO₂. The concentrations of magnetite range from less than 1% to more than 4% and appear to correlate inversely with the total Fe content of the flows. However, the five flows which are anomalously enriched in ³⁴S also have elevated magnetite concentrations. We suggest that the elevated magnetite concentrations and the ³⁴S enrichment were both caused by high oxygen fugacities (f_O2) in the melt. The magnetite concentrations are affected by the fugacity of oxygen through equilibrium in the FMQ buffer whereas the enrichment of the flows in ³⁴S resulted from outgassing of SO₂ at f_O2 greater than ～ 10^?8 atm. The dependence of δ³⁴S and the magnetite concentrations of the flows on f_O2 is supported by the stratigraphic variation of these parameters and by their direct linear correlation.     

Pressure dependence of the speciation of dissolved water in rhyolitic melts

Water speciation in rhyolitic melts with dissolved water ranging from 0.8 to 4 wt% under high pressure was investigated. Samples were heated in a piston-cylinder apparatus at 624-1027 K and 0.94-2.83 GPa for sufficient time to equilibrate hydrous species (molecular H₂O and hydroxyl group, H₂O_m + O ? 2OH) in the melts and then quenched roughly isobarically. The concentrations of both hydrous species in the quenched glasses were measured with Fourier transform infrared (FTIR) spectroscopy. For the samples with total water content less than 2.7 wt%, the equilibrium constant (K) is independent of total H₂O concentration. Incorporating samples with higher water contents, the equilibrium constant depends on total H₂O content, and a regular solution model is used to describe the dependence. K changes with pressure nonmonotonically for samples with a given water content at a given temperature. The equilibrium constant does not change much from ambient pressure to 1 GPa, but it increases significantly from 1 to 3 GPa. In other words, more molecular H₂O reacts to form hydroxyl groups as pressure increases from 1 GPa, which is consistent with breakage of tetrahedral aluminosilicate units due to compression of the melt induced by high pressure. The effect of 1.9 GPa (from 0.94 to 2.83 GPa) on the equilibrium constant at 873 K is equivalent to a temperature effect of 49 K (from 873 K to 922 K) at 0.94 GPa. The results can be used to evaluate the role of speciation in water diffusion, to estimate the apparent equilibrium temperature, and to infer viscosity of hydrous rhyolitic melts under high pressure.