Range Of Oxygen Fugacities Research Articles

The effect of oxygen fugacity on the partitioning of Re between crystals and silicate melt during mantle melting

Interpretation of Re–Os isotopic systematics applied to mantle and mantle-derived rocks is currently hindered by the poorly understood behaviour of Re and Os during partial melting. Of particular interest is the incompatibility of Re and how it partitions between melt and the different mantle phases. Here, we study the partitioning behaviour of Re between the common upper mantle minerals (garnet, spinel, clinopyroxene, orthopyroxene, and olivine) and silicate melt under temperature (1275–1450 °C) and pressure (1.5–3.2 GPa) conditions relevant for basaltic magma genesis, over a range of oxygen fugacity (ƒO 2) large enough (QFM+5.6 to QFM−2.9) to demonstrate the effects of changing the oxidation state of Re from 4+ to 6+. Rhenium crystal/silicate-melt partition coefficients vary by 4–5 orders of magnitude, from moderately compatible to highly incompatible, for pyroxenes, garnet, and spinel as the oxidation state of Re changes from 4+ to 6+, but Re in either oxidation state is incompatible in olivine. Because the changeover from the one Re oxidation state to the other occurs over the range of ƒO 2s pertinent to partial melting in the Earth’s mantle, bulk Re crystal/silicate-melt partition coefficients during mantle melting are also expected to vary significantly according to the oxidation state of the system. For instance, assuming QFM−0.7 and QFM+1.6 as average ƒO 2 for mid-ocean ridge (MORBs) and island arc (IABs) basalts, respectively, a difference of at least one order of magnitude for bulk Re partition coefficients is expected (excluding any influence from a sulphide phase). Hence, Re is probably much more incompatible during the genesis of IABs compared to MORBs. Our results also demonstrate that Re 4+ has a partitioning behaviour similar to Ti 4+ rather than Yb, and is accordingly not a sensitive indicator of garnet in the source. The lower concentrations of Re observed in ocean island basalts (OIBs) compared to MORBs are therefore not a result of being generated deeper in the mantle where garnet is stable, leaving the hypothesis of late-stage loss of Re from OIB lavas by degassing as the most plausible explanation.

Valence of titanium and vanadium in pyroxene in refractory inclusion interiors and rims

The clinopyroxene in coarse-grained refractory inclusions contains significant amounts of Ti and V, two elements that are multivalent over the range of temperatures and oxygen fugacities under which the inclusions formed. The Ti 3+/Ti 4+ ratios and the valence of V of these pyroxenes are valuable recorders of nebular conditions. The former can be calculated stoichiometrically from electron probe analyses, but only for relatively Ti-rich grains (i.e., >∼4 wt% TiO 2 tot ). For Ti-poor pyroxene, and for measurement of V valence, another technique is needed. We have, for the first time, applied K-edge X-ray absorption near edge structure (XANES) spectroscopy to the measurement of Ti and V valence in meteoritic clinopyroxene in refractory inclusions. Use of MicroXANES, a microbeam technique with high (μm-scale) spatial resolution, allowed measurement of Ti and V valence along traverses across (a) Ti-, V-rich “spikes” in pyroxene in Type B1 inclusions; (b) typical grains in a Type B2 inclusion; and (c) the pyroxene layer of the Wark–Lovering rim sequence on the outsides of two inclusions. Measurements of Ti 3+/(Ti 3+ + Ti 4+), or Ti 3+/Ti tot, by XANES agree with values calculated from electron probe analyses to within ∼0.1, or ∼2 σ. The results show that Ti 3+/Ti tot increases sharply at the spikes, from 0.46 ± 0.03 to 0.86 ± 0.06, but the V valence, or V 2+/(V 2+ + V 3+), does not change, with V 2+ ≈ V 3+. We found that pyroxene in both Types B1 and B2 inclusions has Ti 3+/Ti tot and V 2+/V tot ratios between 0.4 and 0.7, except for the spikes. These values indicate, to first order, formation at similar, highly reducing oxygen fugacities that are consistent with a solar gas. The pyroxene in the rim on an Allende fluffy Type A coarse-grained refractory inclusion, TS24, has an average Ti 3+/Ti tot of 0.51 ± 0.08 and an average V 2+/V tot of 0.61 ± 0.06, determined by XANES. These values are within the range of those of pyroxene in the interiors of inclusions, indicating that the rims also formed under highly reducing conditions. Measurements of Ti 3+/Ti tot of pyroxene in the rim of a Leoville compact Type A inclusion, 144A, by both XANES and electron probe give a wide range of results. Of our 72 XANES analyses of this rim, 66% have Ti 3+/Ti tot of 0.40–0.71, and the remaining analyses range from 0 to 0.38. In data from Simon et al. [Simon J. I., Young E. D., Russell S. S., Tonui E. K., Dyl K. A., and Manning C. E. (2005) A short timescale for changing oxygen fugacity in the solar nebula revealed by high-resolution 26Al– 26Mg dating of CAI rims. Earth Planet. Sci. Lett. 238, 272–283.] for this sample, 7 electron probe analyses yield calculated Ti 3+/Ti tot values that are positive and 15 do not. In the probe analyses that have no calculated Ti 3+, Ca contents are anticorrelated and Al contents directly correlated with the total cations per 6 oxygens, and the data fall along trends calculated for addition of 1–7% spinel to pyroxene. It appears likely that electron probe analyses of pure pyroxene spots have Ti 3+/Ti tot values that are typical of refractory inclusions, in agreement with the majority of the XANES results. The average of the XANES data for 144A, 0.41 ± 0.14, is within error of that for TS24. The rim of 144A probably formed under reducing conditions like those expected for a solar gas, and was later heterogeneously altered, resulting in an uneven distribution of secondary, FeO-, Ti-bearing alteration products in the rim, and accounting for the measurements with low Ti 3+/Ti tot values.

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.

Externally-driven charge transfer in silicates at high pressure and temperature: A XANES study

Modest perturbations induced by an externally-applied electric field can generate significant variations in effective oxygen fugacity in high temperature silicates. This result has at least two-fold importance: first, it is a new petrologic technique to examine the behavior of a single sample under a large range of effective oxygen fugacity; and second, it is a mechanism for planetary electric fields to generate potentially significant chemical heterogeneities within planetary interiors. The redox state of Fe and V within a partially melted basaltic andesite was manipulated in situ in a piston-cylinder experiment with a DC power supply providing a source and sink of electrons to the sample. A 1 V electrical potential difference was applied across vanadium-doped synthetic basalt samples for 24 h. at 20 kbar and 1400 °C in a specially-designed piston cylinder sample assembly. Three experiments were performed: a control sample with no applied voltage, one with bottom ground and top anode (+ 1 V), and a third with top ground and bottom anode (+ 1 V). Synchrotron-based X-ray absorption near edge structure spectroscopy (XANES) was used to provide maps of iron and vanadium oxidation states with 5 μm × 5 μm spatial resolution throughout the recovered samples. Systematic increasing oxidation states of V and Fe were observed approaching the anode. Oxidation states were mapped to corresponding local oxygen fugacities by comparison with a series of samples synthesized under known oxygen fugacity conditions from previous studies. Both Fe and V markers indicate that the 1 V potential drop across the sample induces effective oxygen fugacity perturbations of 10 orders of magnitude. Therefore, it is possible that the presence of modest poloidal electric fields (∼ 10 − 6 V/m) within the Earth’s outer core may provide an electrochemical driving force for localized charge transfer reactions in certain overlying mantle areas, generating local order-of-magnitude differences in effective oxygen fugacity.

Experimental study of the effect of SiO2 on Ni solubility in silicate melts

The solubility of Ni in silicate melts with variable SiO2 content was studied at a total pressure of 1 atm within a wide range of temperature and oxygen fugacity. The maximum solubility of Ni (minimum activity coefficient of NiO) was observed in melts with ∼55–57 wt % SiO2, regardless of temperature and oxygen fugacity. Melts beyond this range showed significantly lower Ni solubility and, correspondingly, higher NiO activity coefficients. The analysis of our results and literature data led us to the conclusion that the NBO/T (number of nonbridging oxygen atoms per tetrahedrally coordinated atom) is inadequate to describe the effect of melt composition on Ni solubility.

Oxygen fugacity dependence of Os solubility in haplobasaltic melt

Os equilibrium solubilities were determined at 1350 °C over a wide range of oxygen fugacities (−12 < log fO 2 < −7) applying the mechanically assisted equilibration technique (MAE) at 10 5 Pa (= 1 bar). Os concentrations in the glass samples were analysed using ID-NTIMS. Additional LA-ICP-MS and SEM analyses were performed to detect, visualize and analyse the nature and chemistry of “nanonuggets.” Os solubilities determined range at a constant temperature of 1350 °C from 0.63 ± 0.04 to 37.4 ± 1.16 ppb depending on oxygen fugacity. At the highest oxygen fugacities, Os 3+ can be confirmed as the main oxidation state of Os. At low oxygen fugacities (below log fO 2 = −8), samples are contaminated by nanonuggets which, despite the MAE technique, were still not removed entirely from the melt. However, the present results indicate that applying MAE technology does reduce the amount of nanonuggets present significantly, resulting in the lowest Os solubility results reported to date under these experimental conditions, and extending the experimentally accessible range of fO 2 for these studies to lower values. Calculated metal/silicate melt partition coefficients are therefore higher compared to previous studies, making Os more siderophile. Neglecting the as yet unknown temperature dependence of the Os metal/silicate melt partition coefficient, extrapolation of the obtained Os solubilities to conditions for core-mantle equilibrium, results in a D Os Fe liq , ∞ / sil = 1.5 × 10 5 , while metallic alloy/silicate melt partition coefficients range from 1.4 × 10 6 to 8.6 × 10 7, in agreement with earlier findings. Therefore D Os met / sil remains too high by 2–4 orders of magnitude to explain the Os abundance in the Earth’s mantle as result of core-mantle equilibrium during core formation.

New calibration data for the Fe–Ti oxide thermo-oxybarometers from experiments in the Fe–Ti–O system at 1 bar, 1,000–1,300°C and a large range of oxygen fugacities

The current formulations of the Fe–Ti oxide thermobarometer (titanomagnetite–ilmenitess) fail to reproduce experimental results, in particular at the high temperatures that are relevant for basaltic assemblages. With the aim of improving the experimental basis of the calibration in the Fe–Ti–O system, we have synthesised assemblages of titanomagnetite–ilmenitess (Tmt–Ilmss), ilmenitess–pseudobrookitess (Ilmss–Psbss) and single-phase samples under a wide range of fO2 (fixed with CO/CO2 mixtures or by solid oxygen buffers) in sub-solidus conditions (1,000–1,300°C) at 1 bar. Runs lasted ≥24 h at 1,300°C and up to 240 h at 1,000°C and were terminated by quenching in water. All run products are polycrystalline, roughly equigranular aggregates, with grain sizes of 10–50 μm. They were examined and analysed with the SEM and EMP. Tmt compositions are broadly in accordance with the current models at moderate fO2, but significantly richer in Ti at low fO2 and high T, due to cationic vacancies. Ilmss compositions depart from the predicted values practically at all fO2 and T conditions, which is related to unsatisfactory thermodynamic models for the rhombohedral oxide. For Ilmss–Psbss assemblages the best agreement between our data and current calculations is at 1,000°C and moderately high fO2. Otherwise, experimental and calculated data strongly disagree. The experimental data set on the three Fe–Ti oxide solid solutions presented here is intended to support new versions of both the titanomagnetite–ilmenitess thermo-oxybarometer and the ilmenitess–pseudobrookitess oxybarometer.

Liquidus karrooite stability and composition at reducing conditions

We have determined the stability of rutile and karrooite on the liquidus of pseudobinary silicate melts of anorthite–diopside eutectic composition with, in addition, of up to 32 wt% TiO2 at one atm total pressure and at wide range of oxygen fugacities. At 1,300°C and at an fO2 below 10−11.2 atm rutile (TiO2) is replaced as liquidus phase by the pseudobrookite-type MgTi2O5–Ti3O5 solid solution with some Al in the crystal structure. The composition of karrooite was found to be strongly dependent on oxygen fugacity. Crystalline phases were identified by transmission electron microscopy (TEM). The results obtained in this study are relevant for understanding the chemistry of lunar armalcolites. Using “excess” rutile solubility at reducing conditions, we estimated the Ti4+ /Ti3+ ratio in silicate melts at 1,300°C as function of fO2.

The effect of TiO2on Pd, Ni, and Fe solubilities in silicate melts

We have determined Pd, Ni, and Fe solubilities in silicate melts of anorthite-diopside eutectic composition with variable TiO2 concentrations, from TiO2-free melts to melts with up to 26 wt% TiO2. The experiments were conducted with metal loops at 1300 °C, one atm total pressure, and at a wide range of oxygen fugacities. The glasses were analyzed with the electron microprobe and by laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS).

Experimental determination of siderite stability and application to Martian Meteorite ALH84001

The pressure-temperature equilibrium curve of the reaction siderite + hematite = magnetite + CO 2 was determined in the range 5-12 kbar and 480-650 °C by piston-cylinder experiments, with NaCl as a pressure medium. Silver oxalate was used as a CO 2 source and samples were buffered at hematitemagnetite oxygen fugacity. Reaction progress was monitored by extent of CO 2 gas loss and by X-ray diffraction (XRD) analysis. The data define a univariant curve, which is described by P = -14.599 + 0.025T + 0.000027 T 2 with P in kbar and T in °C. Calculations based on these data give ΔH 0 f (298K) siderite = -760.6 ± 0.9 kJ (kilojoules) from the oxides. The formation of siderite requires a specific range of ambient oxygen and carbon dioxide fugacities, dependent upon temperature and pressure. The stable assemblage of siderite and magnetite, at a given temperature and pressure, implies more restrictive ranges of oxygen and carbon dioxide fugacities, defined by reactions among siderite, magnetite, graphite, and hematite. Experimental and thermodynamic investigation of the Fe-C-O system indicates that the formation of magnetite along with Ca-Fe-Mg carbonate globules by inorganic processes is possible and may be relevant to Martian meteorite ALH84001. Decarbonation of the siderite component of the carbonate, either by a transient heating event or by a change in oxygen fugacity of a coexisting fluid, may have formed the observed grains, although this study does not address the size or morphology of magnetite grains formed by this mechanism.

XANES calibrations for the oxidation state of iron in a silicate glass

Fe K-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of anorthite-diopside eutectic glasses containing 1 wt% 57Fe2O3 quenched from melts equilibrated over a range of oxygen fugacities at 1409 °C. The Fe3+/ΣFe ratios were determined previously by 57Fe Mössbauer spectroscopy and vary between 0 (fully reduced) and 1 (fully oxidized). Using the Mössbauer results as a reference, various methods for extracting Fe3+/ΣFe ratios from XANES spectra were investigated. The energy of the 1s → 3d pre-edge transition centroid was found to correlate linearly with the oxidation state. Correlations also exist with the energy of the K absorption edge and the area of peaks in the derivative spectrum associated with the 1s → 4s and crest (1s → 4p) transitions. The Fe3+/ΣFe ratios determined from linear combinations of end-member spectra (Fe3+/ΣFe ~0 and ~1) were found to deviate significantly from the Mössbauer values. This may indicate the susceptibility of this method either to errors arising from the treatment of the background or to changes in Fe2+ or Fe3+ coordination with the Fe3+/ΣFe ratio. The general applicability of any XANES calibration for determining oxidation states is limited by variations in the Fe coordination environment, which affects both the intensity and energy of spectral features. Thus previous calibrations based on mineral spectra are not applicable to silicate glasses. Nevertheless, systematic trends in spectral features suggest that Fe3+/ΣFe values may be obtained from XANES spectra, with an accuracy comparable to Mössbauer spectroscopy, by reference to empirical calibration curves derived from compositionally similar standards.

The compositional evolution of differentiated liquids from the Skaergaard Layered Series as determined by geochemical thermometry

Based on the COMAGMAT-3.65 crystallization model a set of phase equilibria calculations (called geochemical thermometry) have been conducted at P=1 kbar and closed conditions with respect to oxygen for 65 rocks representing the principle units of the Layered Series of the Skaergaard intrusion. It allowed us to define the range of initial temperatures (1145 to 1085 C) and oxygen fugacities (1-1.5 log units above QFM to slightly below QFM) of the original crystal mush from which the rocks from LZa to UZa crystallized. In parallel, average major-element compositions of residual (interstitial) liquid were calculated demonstrating a trend of continual enrichment of FeO (up to 18 wt.%) and TiO2 (up to 5.5 wt.%) with only minor variations in the SiO2 contents (48 to 50 wt.%). Projection of the compositions onto the OLIV -CPX-QTZ diagram provides evidence that most of the Layered Series crystallized on the Ol-Pl-Cpx-oxide cotectic. Systematic dierences between the calculated residual liquid compositions for LZa/LZb and LZc to UZa (which are unlikely to reflect fractional crystallization) are within the accuracy of the COMAGMAT model, but may be also indicative of a late-stage process involving migration and re-equilibration of interstitial liquids. Estimated amounts of interstitial melts trapped in the Skaergaard cumulates range around 50 wt.%. Wager's compositions inferred from simple mass-balance were found to lie too far from the Ol- Pl-Cpx boundary to represent a realistic approximation of the low-pressure Skaergaard magma evolution. The main problem of genetic interpretations of the Skaergaard intrusion is a strong misbalance between the parental compositions followed from contact rocks and the results of geochemical thermometry and that of the whole dierentiated body. It is most apparent for TiO2 and P2O5 which are almost twice as high in the average intrusion composition compared to the proposed parents. Moreover, the intrusion composition has of 2-4 wt.% less SiO2 and much more iron. One possible explanation is to assume the Skaergaard magma came to the chamber with an amount of crystals (Ol +Pl) equilibrated with the calculated parental liquid. However, even if some amount of hidden troctolitic material exists, it is unlikely that crystallization in a closed system could produce large volumes of rocks rich in Fe-Ti oxides without complementary more felsic dierentiates.

Experimental constraints on the chemical evolution of large icy satellites

We derive experimental constraints on the interior structure and mineralogy of large icy satellites by reacting material of chondritic chemistry with water at a pressure of 1.5 GPa, temperatures between 300°C and 800°C and a range of oxygen fugacities. Our results document the existence of three chemical processes that probably occur in large icy satellites as a result of high pressure hydrothermal processing: (1) the formation of low-density hydrated silicates, (2) the alloying of iron and sulfur to form FeS-dominated cores, and (3) the instability of organic material relative to carbonates. We construct new internal models of the thermal and structural state of Ganymede, and infer that the magnetic field of this body arises from convection within a mostly iron sulfide core. Simple thermochemical calculations are conducted to further explore the likely effects of composition and oxygen fugacity on the high pressure chemistry undergone by organic material within icy satellites. Both experimental and calculated results show that primordial organics are likely to have been largely oxidized to carbonates through hydrothermal processing early in Ganymede’s history, potentially sterilizing Ganymede’s H2O layer.

Oxygen fugacity and geochemical variations in the martian basalts: implications for martian basalt petrogenesis and the oxidation state of the upper mantle of Mars

The oxygen fugacity of the Dar al Gani 476 martian basalt is determined to be quartz-fayalite-magnetite (QFM) −2.3 ± 0.4 through analysis of olivine, low-Ca pyroxene, and Cr-spinel and is in good agreement with revised results from Fe-Ti oxides that yield QFM −2.5 ± 0.7. This estimate falls within the range of oxygen fugacity for the other martian basalts, QFM −3 to QFM −1. Oxygen fugacity in martian basalts correlates with 87Sr/86Sr, 143Nd/144Nd, and La/Yb ratios, indicating that the mantle source of the basalts is reduced and that assimilation of crust-like material controls the oxygen fugacity. This allows constraints to be placed on the oxidation state of the martian mantle and on the nature of assimilated crustal material. The assimilated material may be the product of early and extensive hydrothermal alteration of the martian crust, or it may be amphibole- or phlogopite-bearing basaltic rock within the crust. In either case, water may play a significant role in the oxidation of basaltic magmas on Mars, although it may be secondary to assimilation of ferric iron-rich material.

THE VALENCE AND SPECIATION OF SULFUR IN GLASSES BY X-RAY ABSORPTION SPECTROSCOPY

The geochemical behavior of sulfur in magmas depends strongly on the oxidation state of sulfur, but this is not easily determined by standard analytical methods. We have measured XANES absorption spectra at the sulfur K-edge and have found that such measurements are useful to characterize the oxidation state and speciation of sulfur in silicate glasses of geological rel evance. Measured spectra of a set of reference minerals show the effects of different oxidation states and coordination numbers of sulfur; there is a large shift in energy (~10‐12 eV) of the sulfur K-edge between S 2‐ and S 6+ . This large and easily detectable difference makes possible the measurement of the valence of sulfur in unknown samples by measuring the shift in energy of the absorption edge. This approach is applicable to both crystalline and glassy materials, and useful results have been obtained on samples with as little as 450 ppm S. We have used XANES measurements to characterize oxidation state and speciation of sulfur in a set of natural and synthetic sulfur-bearing glasses. The samples cover a range of composition from basaltic to almost rhyo litic, and some were synthesized over a range of pressure, temperature and oxygen fugacity; glass S content varies between 450 and 3000 ppm. XANES analyses, carried out in fluorescence mode at LURE, allowed determination of the sulfur oxidation state in all of the samples and clearly show that some samples contain a mixture of S 2‐ and S 6+ ; no other sulfur species were observed. Quantitative determination of the abundance of sulfide and sulfate shows good agreement with independent measurements based on electron-microprobe determination of the wavelength shift of sulfur K X-rays.

Formation of orange hibonite, as inferred from some Allende inclusions

Abstract— We studied three fluffy Type A refractory inclusions from Allende that contain orange hibonite. The melilite in the present samples is very Al‐rich, averaging Åk6, Åk14, and Åk12 in the three samples studied. Hibonite in two inclusions, unlike that in Murchison, has low rare earth element abundances of &lt;10 × CI; in the other inclusion, the hibonite, melilite and perovskite have Group II‐like patterns. The hibonite and melilite in all three inclusions studied have excess 26Mg consistent with (26Al/27Al)I = 5 × 10−5. Much of the hibonite and some of the spinel in these inclusions is corroded. These phases are found enclosed in melilite, but based on bulk compositions and phase equilibria, hibonite should not be an early‐crystallizing phase in these inclusions. We conclude that the hibonite and probably some of the spinel is relic. Reversely zoned melilite, rounded spinel and isotopically heavy Mg in the inclusions probably reflect reheating events that involved melting and evaporation. Alteration of the gehlenitic melilite gave rise to some rare phases, including corundum and nearly pure CaTs pyroxene.Studies have shown that blue hibonite contains Ti3+ while orange hibonite does not (Ihinger and Stolper, 1986; Beckett et al., 1988). Orange hibonite formed either under oxidizing conditions (such as at oxygen fugacities at least seven orders of magnitude greater than that of a solar gas at 1700 K), or under conditions reducing enough (e.g., solar) that it contained Ti3+, which was later oxidized in situ. Although V and Ce oxides are volatile at the temperature and range of oxygen fugacities at which orange hibonite is known to be stable, we find that (a) the hibonite is V‐rich (∼1 wt% V2O3) and (b) there are no negative Ce anomalies in Allende hibonite. This indicates that the hibonite did not form by condensation under oxidizing conditions. In addition, there are slight excesses of Ti + Si cations relative to Mg + Fe cations (up to 0.1 of 0.8 cations per 19 oxygen anions), probably reflecting the original presence of Ti3+. The results of this study strongly support the suggestion (Ihinger and Stolper, 1986) that Allende hibonite originally formed under reducing conditions and was later oxidized. Oxygen fugacities within ∼2–3 orders of magnitude of that of a solar gas are implied; otherwise, strong Ce and V depletions would be observed.

Titaniferous magnetite–ilmenite thermometry and titaniferous magnetite–ilmenite–orthopyroxene–quartz oxygen barometry in granulite facies gneisses, Bamble Sector, SE Norway: implications for the role of high-grade CO2-rich fluids during granulite genesis

Oxygen fugacities have been estimated for a wide distribution of samples from the granulite facies terrane (region C) of the Bamble Sector, SE Norway using both the titaniferous magnetite–ilmenite and orthopyroxene–titaniferous magnetite–quartz oxygen barometers. These oxygen fugacities are estimated using temperatures calculated from the titaniferous magnetite–ilmenite thermometer of Ghiorso and Sack (1991) and are both internally consistent with each other as well with the thermometer. In samples for which the estimated temperature is high, the two oxygen barometers show good agreement whereas agreement is poor for low temperature samples. In these low temperature samples, oxygen fugacities estimated from titaniferous magnetite–ilmenite are considerably less than those estimated from orthopyroxene–titaniferous magnetite–quartz. An increase in this discrepancy with decrease in temperature appears to reflect preferential resetting of the hematite component in the ilmenite grains without significant alteration of the more numerous titaniferous magnetite grains. This is due, in part, to greater re-equilibration of the ilmenite grains during retrograde interoxide resetting between the ilmenite grains and the titaniferous magnetite grains. The mean temperature for the non-reset samples, 791 ± 17 °C (1σ), is in good agreement with temperatures obtained from garnet–orthopyroxene KD exchange thermometry in the same region, 785–795 °C (1σ) (Harlov 1992, 2000a). Most non-reset oxygen fugacities range from log10 f O2=−14 to −11.8 or approximately 0.5–1.5 log units above quartz–fayalite–magnetite at 7.5 kbar. Both these temperatures and the range of oxygen fugacities are in good agreement with those estimated using the titaniferous magnetite–ilmenite thermometer/oxygen barometer of Andersen et al. (1991). The QUIlP equilibrium (quartz–ulvospinel–ilmenite–pyroxene) is used to project self-consistent equilibrium temperatures and oxygen fugacities for samples reset due to hematite loss from the ilmenite grains. These projected temperatures and oxygen fugacities agree reasonably well with the non-reset samples. The mean projected QUIlP temperature is 823 ± 6 °C (1σ). This result supports the conclusion that low titaniferous magnetite–ilmenite temperatures (down to 489 °C) and accompanying low oxygen fugacities are the result of hematite loss from the ilmenite grains. Non-reset oxygen fugacities lie approximately 1.5 log10 units above the upper graphite stability curve indicating that the stable C–O–H fluid phase interacting with these gneisses, whether regionally or locally, was CO2. This is borne out by the presence of numerous CO2-rich fluid inclusions in these rocks.

Magnetic characterization of reduction in Mount Fuji basaltic tree‐mold

A Mount Fuji basaltic tree‐mold presents the mineralogical record associated with a large oxygen fugacity gradient during the cooling of the basalt. Magnetic exchange anisotropy is postulated to be characteristic of cation excess in some spinel titanomagnetites. The exchange anisotropy is characterized by loop shifting along the field and magnetization axes when cooled in a magnetic field, large coercivity increase, and magnetic viscosity which has discrete activation fields and logarithmic to linear decay characteristics. The cryogenic loop characteristics attributable to exchange anisotropy are not found in the tree‐mold where iron and ilmenite are the main phases. The coupling has been created in the laboratory with ordinary basalt artificially reduced in Hydrogen gas. The coupling is not found in spinel oxidation non‐stoichiometry. These results imply that we are able to identify the presence of spinels that have equilibrated in the low oxygen fugacity range of the spinel stability field where cation excess is characteristic.

Geochronology and geochemistry of late Cenozoic basalts from the Leiqiong area, southern China

The Leiqiong area, which includes the Leizhou Peninsula and the northern part of the Hainan Island, is the largest province of exposed basalts in southern China. Ar–Ar and K–Ar dating indicates that incipient volcanism in the Leiqiong area may have taken place in late Oligocene time and gradually increased in tempo toward the Miocene and Pliocene Epoch. Volcanic activities were most extensive during Pleistocene, and declined and ended in Holocene. Based on radiometric age dating and geographic distribution, Pliocene and Quaternary volcanism in Hainan Island can be grouped into two stages and six eruptive regions. The early volcanism is dominated by flood type fissure eruption of quartz tholeiites and olivine tholeiites whereas the later phase is dominated by central type eruption of alkali olivine basalts and olivine tholeiites. The systematic decrease of MgO, ΣFeO and TiO 2 with increasing SiO 2 content for basalts from Hainan Island indicates that fractional crystallization of olivine, clinopyroxene and Ti-bearing opaques may have occurred during magmatic evolution. From coexisting Fe–Ti oxide minerals, it is estimated that the equilibrium temperatures range from 895–986°C and oxygen fugacities range from 10 −13.4 to 10 −10.7 atmospheres in the basaltic magmas. The incompatible element ratios and the chondrite-normalized REE patterns of basalts from the Leiqiong area are generally similar to OIB. The Nb/U ratios (less than 37) in most of the tholeiitic rocks and the negative Nb anomaly observed in the spidergram of some basalts indicated that the influence of a paleo-subduction zone derived component can not be excluded in considering the genesis of the basalts from the Leiqiong area. The tholeiites in the Leiqiong area may have mixed with a more enriched lithospheric mantle component as well as undergone relatively larger percentages of partial melting than the alkali basalts.

Nitrogen Solubility in Silicate Melts as a Function of Oxygen Fugacity and Melt Composition

Nitrogen is the most abundant element of the atmosphere. Nevertheless, the cycle of this element between mantle and atmosphere is poorly understood. First, nitrogen might have been incorporated into the Earth during a primitive magma ocean stage. Presently, nitrogen is outgassed from the solid earth through partial melting and degassing of mantle derived magmas. So, nitrogen solubility in silicate melt and gas phase is a critical parameter. Moreover, at the present time, oxygen fugacity during magma generation is oxidizing, but the possible occurrence of a hydrogen-rich atmosphere during the accretion of the Earth might have imposed much more reducing conditions. Therefore, it is important to determine the nitrogen solubility in silicate melt over a large range of oxygen fugacity. We have developed new experimental and analytical methods based on: (i) high temperature equilibrium of silicate melt under controlled nitrogen and oxygen fugacities, and (ii) laser extraction in high vacuum line, purification of extracted volatiles and static vacuum mass spectrometric measurement of nitrogen. In this paper, we detailed the influence of oxygen fugacity and melt compositions on this nitrogen solubility.