Effect Of Oxygen Fugacity Research Articles

The effect of oxygen fugacity on the evaporation of boron from aluminoborosilicate melt

Abstract. We present the results of B2O3 evaporation experiments from Ca- and Mg-bearing aluminoborosilicate melts. Our experiments were conducted at 1245 to 1249 ∘C and 1350 to 1361 ∘C for different run times (60–1020 min), and at oxygen fugacities (logfO2) relative to the fayalite–magnetite–quartz (FMQ) buffer of FMQ−6 to FMQ+1.5, and in air. Our results show that with increasing fO2, evaporation of B from the melt increases by a factor of 5 compared to reducing conditions. Using Gibbs free energy minimization calculations, we suggest two possible evaporation reactions for B2O3 which constrain its speciation in the gas phase to be either 3+ or 4+ (B2O3(g) and BO2(g)). The measured B2O3 contents of the B evaporated residual glasses were used to calculate evaporation rate constants (ki) for B2O3 in oxidizing conditions (air, ki=2.09×10-4 cm min−1 at 1350 ∘C) and reducing conditions (FMQ−4, ki=4.46×10-5 cm min−1 at 1350 ∘C). The absence of diffusion profiles in the experimental glasses suggests that the evaporation rates are slower than B2O3 diffusion rates and therefore the rate-limiting process. Overall, the rate of B evaporation in air is approximately a factor of 5 higher compared to reducing conditions at FMQ−4.

Effect of oxygen fugacity on the storage of water in wadsleyite and olivine in H and H–C fluids and implications for melting atop the transition zone

Abstract. This study aims to experimentally constrain the water storage capacities of olivine and wadsleyite at a depth near 410 km (12–14 GPa) under water-saturated conditions, as a function of temperature, oxygen fugacity, and the presence of carbon (molar H / C of 2). Experiments have been conducted in the multi-anvil press, with sealed double capsules to preserve fluids, at 1200 to 1400 ∘C and three different oxygen fugacities fixed at the rhenium–rhenium oxide buffer (RRO), nickel–nickel oxide buffer (NNO), and iron-wüstite (IW) for oxidizing, intermediate, and reducing conditions, respectively. The water contents of minerals were measured by Raman spectroscopy that allows a very small beam size to be used and were cross-checked on a few samples with NanoSIMS analyses. We observe an effect, although slight, of fO2 on the water storage capacity of both wadsleyite and olivine and also on their solidus temperatures. At 1200 ∘C, the storage capacity of the nominally anhydrous minerals (NAMS) increases with increasing oxygen fugacity (from the IW to the RRO buffer) from 1 wt % to 1.5 wt % H2O in wadsleyite and from 0.1 wt % to 0.2 wt % in olivine, owing to the increase in H2O / H2 speciation in the fluid, whereas at 1400 ∘C the storage capacity decreases from 1 wt % to 0.75 wt % H2O in wadsleyite and down to 0.03 wt % for olivine. At high temperature, the water storage capacity is lowered due to melting, and the more oxidized the conditions are the more the solidus is depressed. Still, at 1400 ∘C and IW, wadsleyite can store substantial amounts of water: 0.8 wt % to 1 wt % H2O. The effect of carbon is to decrease water storage capacity in both wadsleyite and olivine by an average factor 2 at 1300–1400 ∘C. The trends in water storage as a function of fO2 and C presence are confirmed by NanoSIMS measurements. The solidus at IW without C is located between 1300 and 1400 ∘C in the wadsleyite stability field and drops to temperatures below 1300 ∘C in the olivine stability field. With the addition of C, the solidus is found between 1200 and 1300 ∘C in both olivine and wadsleyite stability fields.

The effects of oxygen fugacity and sulfur on the pressure of vapor-saturation of magma

Abstract Geobarometers are commonly used to determine the pressure (and hence depth) of magmatic bodies. For instance, at equilibrium, the concentration of dissolved volatiles in a vapor-saturated melt can be used as a barometer: this is the pressure of vapor-saturation (Psatv). Most determinations of Psatv assume that melt and vapor contain only oxidized C-O-H species. However, sulfur is the third most abundant volatile element in magmas, and oxygen fugacity (fO2) exerts a strong influence on the speciation of the melt and vapor. To explore how S and fO2 affect calculations of Psatv, we model a Hawaiian tholeiite that contains both reduced and oxidized C-O-H-S species in the melt and vapor. We find that excluding reduced C-O-H species in the system can result in significant underestimations of Psatv under reducing conditions (ΔFMQ < 0). The effect of S on Psatv is small except in the vicinity of the “sulfur solubility minimum” (SSmin; 0 < ΔFMQ < +2), where excluding S-bearing species can result in underestimates of Psatv. The implications of these results depend on the volatile concentration of the system being investigated, its fO2, and the melt composition and temperature. Our results suggest there will be little impact on Psatv calculated for mid-ocean ridge basalts because their fO2 is above where reduced C-O-H species become important in the melt and vapor and yet below the SSmin. However, the fO2 of ocean island and arc basalts are close enough to the SSmin and their S concentrations high enough to influence Psatv. However, high-CO2 and high-H2O concentrations are predicted to reduce the effect of the SSmin. Hence, Psatv calculated for shallowly trapped melt inclusions and matrix glass are more affected by the SSmin than deeply trapped melt inclusions. Lunar and martian magmas are typically more reduced than terrestrial magmas, and therefore accurate Psatv calculations for them require the inclusion of reduced C-O-H species.

Effects of Oxygen Fugacity on Hydroxyl Incorporation in Garnet at 1–3 GPa and 800–1000°C and Implications for Water Storage in the Mantle

AbstractGarnet, an important mineral component in Earth's interior, usually contains minor water as structural OH, but experimental data on OH incorporation in the mineral as a function of chemical and thermodynamic parameters remain rare. In particular, the role of oxygen fugacity (fO2) that is variable in the Earth has not been well assessed. We examine the effects of fO2 on the incorporation of OH at 1–3 GPa and 800–1000°C, by H‐annealing gem‐quality spessartine‐rich, grossular‐rich, and pyrope‐rich garnets. For each type of annealed garnet, the OH peak positions and spectral shapes are the same, suggesting the same OH incorporation mechanism(s). Under identical conditions, spessartine‐rich and grossular‐rich garnets are rich in OH, with solubility orders of magnitudes greater than for pyrope‐rich garnets. Over the studied conditions, the solubility of OH increases with pressure and temperature but decreases with fO2. The negative correlation of fO2 and OH solubility is probably due to the Fe3+ to Fe2+ reduction at more reduced conditions and the coupled OH incorporation in the structure. The effects of fO2 on OH solubility are more significant in Fe‐rich garnets. The ascertained small OH solubility in peridotite garnets, together with comparisons with relevant studies, suggest that mantle garnet is unlikely to be a major water carrier. In combination with literature‐reported positive correlation of fO2 and OH solubility in olivine, the partition coefficient of OH between garnet and olivine in the upper mantle is expected to be redox‐dependent.

Effects of oxygen fugacity on hydroxyl incorporation in garnet at 1–3 GPa and 800–1000 °C and implications for water storage in the mantle

Effects of oxygen fugacity on hydroxyl incorporation in garnet at 1–3 GPa and 800–1000 °C and implications for water storage in the mantle

The effect of oxygen fugacity on ionic conductivity in olivine

The oxygen fugacity (fO2) 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 fO2 under relatively reduced conditions (fO2 below Re-ReO2 buffer), whereas it has a clear fO2-dependence under relatively oxidized conditions (fO2 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.

The effect of oxygen fugacity on diamond resorption in ascending kimberlite melt

When transported by magmas to the Earth's surface, diamond crystals underwent resorption, the intensity of which significantly differed in various kimberlite pipes. We experimentally simulated diamond resorption at different oxygen fugacities (fO2) in ascending kimberlite magma enriched in CO2 and H2O. The experiments were carried out using specially prepared unaltered Group I kimberlite from the Udachnaya East pipe (Yakutia) and model carbonatite at 3.0 GPa, 1200–1400 °C, and fO2 in a range of NNO–2 to NNO + 3.2 log units (where NNO is Ni-NiO buffer). Over the investigated range of conditions, resorption of octahedral diamond crystals is found to occur according to a single scenario. Negative trigons and shield-shaped laminae develop on the {111} faces and crystal edges are truncated by the surfaces of tetrahexahedroids. The rate of diamond resorption increases in all studied systems as fO2 and temperature are raised. In this case, water-enriched melts are the most aggressive media in the investigated T–fO2 interval. Among the most oxidized high-temperature melts, it is carbonatite melts depleted in SiO2 that provide the maximum rate of diamond resorption. Furthermore, the rates of diamond resorption we obtained are an order of magnitude higher than those previously measured in silicate melts containing CO2 and H2O, at fO2 values from the NNO buffer to NNO-2. Therefore, high oxygen fugacity, a temperature of ~1400 °C, and essentially carbonate composition of water-containing magma could provide a high intensity of diamond resorption at the mantle stage of magma ascent to the surface. Apparently, this process primarily influenced the formation of the appearance and preservation of natural diamond crystals in kimberlite pipes.

Towards the investigation of ternary compound in the Ti-Al-Zr-O system: Effect of oxygen fugacity on phase formation

The Ti-Al-Zr-O system has received significant attention as it hosts a variety of phases of industrial interest. In this paper, new experiments on the Ti-Al-Zr-O system aimed at clarifying phase formation depending on the redox conditions with special attention to the formation of ternary compounds and their results have been reported. For the first time a monophase AlTi2O5 sample with a ratio of titanium oxides TiO2:Ti2O3 = 2:1 was obtained and characterized. Single-crystal X-ray diffraction analysis of β-(ZrxTi1-x)2O4 crystals was carried out. The synthesis of the ternary Ti-Al-Zr compound was performed in an evacuated quartz ampoule and accompanied by the extraction of silicon from the ampoule. The compound has pyrochlore-type cubic structure A24+B23+O7 (Fd-3m) and the composition close to (Ti4+1,47(2)Zr0.41(2)Si0.12(1))2(Al1.15(4)Ti3+0.85(4))2O7. We believe that our study is a starting point for future research as it provides first experimental data on the ternary compound in the Ti-Al-Zr-O system.

Cation distribution and valence in synthetic Al–Mn–O and Fe–Mn–O spinels under varying conditions

ABSTRACTThe spinel-group minerals, found in a range of igneous rocks, are resistant to weathering and can incorporate several multivalent elements, meaning they have the potential to provide insight into the redox conditions of parental magmas. Naturally occurring spinel can contain varying quantities of Mn, an element which occurs terrestrially and extra-terrestrially as Mn2+, Mn3+, Mn4+ and Mn5+. However, a lack of information on the effects of oxygen fugacity ($f_{{\rm O}_{\rm 2}}$) on: (1) Mn valence state and cation distribution; and (2) on spinel-melt partitioning means that the potential for a Mn-in-spinel oxy-barometer remains largely untested. Here, we use electron probe microanalysis, micro-focus X-ray Absorption Near Edge Structure (XANES) spectroscopy and single-crystal X-ray diffraction (SC-XRD) to investigate cation distribution and valence state in spinels in the Al–Mn–O and Fe–Mn–O systems synthesized at ambient pressure under varying $\hskip 2pt f_{{\rm O}_{\rm 2}}$ conditions. In contrast to previous studies, we find that the spectral resolution of the Mn K-edge XANES spectra is insufficient to provide quantitative data on Mn valence state and site occupancy, although it does verify that Mn is incorporated as both Mn2+ and Mn3+, distributed over tetrahedral and octahedral sites. Combination of data from XANES and SC-XRD refinements can, however, be used to model Mn, Al and Fe valence and site occupancy. It would be expected that Mn–Fe spinels have the potential to record $f_{{\rm O}_{\rm 2}}$ conditions in parental melts due to changes to the octahedral site under conditions that were more reducing. However, decoupling the effects of temperature and oxygen fugacity on the TFe3+–TMn2+ exchange in the Mn–Fe spinels remains challenging. In contrast, little variation is noted in Mn–Al spinels as a function of $\hskip 2pt f_{{\rm O}_{\rm 2}}$, implying that crystal chemistry and cation site geometry may significantly influence cation distribution, and by inference, crystal-melt partitioning, in spinel-group minerals.

The solubility of Pd and Au in hydrous intermediate silicate melts: The effect of oxygen fugacity and the addition of Cl and S

The solubilities of Pd and Au in a hydrous trachyandesitic melt were experimentally determined at 1000 °C and 200 MPa at oxygen fugacity (ƒO2) from 0.45 log units below to 6.55 log units above the Ni-NiO buffer (NNO). The effect of adding metal-binding ligands (i.e. Cl and S) to the silicate melt was also studied. The solubility of Au increases from 0.15 ± 0.1 to 3.85 ± 1.48 ppm in Cl- and S-free melts with ƒO2 increasing from NNO−0.45 to NNO+6.55 with a slope that suggests that it is present in 1+ oxidation state over the entire studied ƒO2 range. On the other hand, Pd solubility, shows a more moderate increase with ƒO2, especially in the lower half of the studied range, increasing from 2.66 ± 0.25 ppm at NNO−0.45 to only 3.62 ± 0.38 ppm at NNO+1.72 in Cl- and S-free melts. Overall, the variation in Pd solubility as a function of ƒO2 indicates Pd being dissolved in the silicate melt in both zero and 1+ oxidation state, with the former being dominant below NNO+4.5. At NNO−0.45 to +3.48, the addition of 3170–4060 ppm Cl to the silicate melt increased the solubility of Au by an average factor of 1.5, in comparison to Cl-free melts. However, at NNO+6.55, Au solubility increased by a factor of 2.5. The addition of Cl had a negligible effect on the solubility of Pd except for a large increase (factor of 2.4) at NNO+6.55. At reducing conditions (NNO−0.45), the addition of 170 ppm S to the silicate melt increased the solubility of Au by a factor of ∼4 but did not change the solubility of Pd in comparison to S-free melts. The observation that Pd is dominantly present as Pd0 at NNO < +4.5 may explain the lack of complexation with potential reduced sulfur-bearing and chloride ligands. As one may expect similar behavior in fluids degassing from magmas at depth, the lack of oxidized Pd species could be an important factor behind the scarcity of economically viable Pd-rich magmatic-hydrothermal deposits observed in nature.

The effect of oxygen fugacity and Na/(Na+K) ratio on iron speciation in pantelleritic glasses

Reduction experiments were conducted to study the structural role (redox state and coordination) of iron in peralkaline rhyolitic glasses as a function of oxygen fugacity and alkali atomic ratio ([Na/(Na+K)]=0.0, 0.69 and 1.0). The glasses investigated, analogues of natural pantellerites of the Kenya Rift Valley, have been synthesized at ambient pressure, 1250°C and equilibrated under controlled redox conditions. Oxygen fugacities, ranging from NNO+7 to NNO- 6.8 (relative to Ni/NiO assemblage buffer), were adjusted by different gases and/or gas mixtures (air, O2, CO2, Ar/H2/H2O, Ar/H2) in a horizontal gas-mixing furnace.Iron oxidation state and coordination geometry have been inferred by Fe K-edge X-ray Absorption Near-Edge Structure (XANES) spectroscopy, which suggested the presence of Fe3+ in four-fold coordination and Fe2+ having an average coordination number intermediate between four- and five-fold in all the glasses.A colorimetric method has been employed to determine the Fe2+/Fetot ratios for all the samples. We verified in the samples a continuous increase of divalent iron for more reducing conditions (e.g. Fe2+/Fetot ranging from 0.09 in air to 0.93 in Ar/H2 atmosphere) but a non-linear relation between [Na/(Na+K)] molar ratio and iron redox was observed.The weak dependence of log (Fe3+/Fe2+) on log (fO2/bar) indicates a strong deviation from ideal mixing of iron species in pantelleritic compositions which suggests a different behaviour of alkali dominated peralkaline melts compared to other aluminosilicate melts in terms of fO2 and alkalis effects.

Effect of oxygen fugacity on the glass transition, viscosity and structure of silica- and iron-rich magmatic melts

Iron oxidation state affects the physical properties and structure of natural melts and, ultimately, magmatic and volcanic processes such as magma transport and eruptive style. Here, we study the glass transition temperature and melt viscosity of an iron-rich peralkaline rhyolite as a function of the iron oxidation state. Glass transition temperature is determined using a differential scanning calorimetry up to 680°C. High-temperature viscosity is measured at 1385°C using the concentric cylinder technique. Both the glass transition temperature and viscosity increase with oxygen fugacity conditions. Although significant changes in structure with temperature are expected, we observe a direct relationship between the effect of oxygen fugacity on bulk properties at high and low temperature and polymerization of glass structure as inferred by Raman spectroscopy. Our results show that viscosity models should consider the effect of oxygen fugacity on the melt viscosity at eruptive temperature together with the effect of water and crystal content.

Experimental determination of Pb partitioning between sulfide melt and basalt melt as a function of P, T and X

We have measured the partition coefficient of Pb (KdPb) between FeS melt and basalt melt at temperatures of 1250–1523°C, pressures of 1.0–3.5GPa and oxygen fugacities at iron–wustite and wustite–magnetite. The total observed range of KdPb is 4.0–66.6, with a strong negative dependence on pressure and a strong negative dependence on FeO of the silicate melt (Fe+2 only). The FeO control was constrained over a wide range of FeO (4.2–39.5%). We found that the effect of oxygen fugacity can be subsumed under the FeO control parameter. Prior work has established the lack of a significant effect of temperature (Kiseeva and Wood, 2015; Li and Audétat, 2015). Our data are parameterized as: KdPb=4.8+(512−119*P in GPa)*(1/FeO−0.021).We also measured a single value of KdPb between clinopyroxene and basalt melt at 2.0GPa of 0.020±0.001. This experimental data supports the “natural” partitioning of Pb measured on sulfide globules in MORB (Patten et al., 2013), but not the low KdPb of ∼3 inferred from sulfides in abyssal peridotites by Warren and Shirey (2012). It also quantitatively affirms the modeling of Hart and Gaetani (2006) with respect to using sulfide to buffer the canonical Nd/Pb ratio for MORB and OIB (Hofmann, 2003). For the low FeO and pressure of segregation typical of MORB, KdPb∼45, and the Nd/Pb ratio of erupted basalts will be the same as the Nd/Pb ratio of the mantle source. The remaining puzzle is why MORB and OIB have the same Nd/Pb when they clearly have different FeO and pressure of melt segregation.

Effect of oxygen fugacity on OH dissolution in olivine under peridotite-saturated conditions: An experimental study at 1.5–7 GPa and 1100–1300 °C

The dissolution of OH in olivine by experimental studies at simulated conditions has attracted increasing interest over the past three decades, and the influence of pressure, temperature and composition has been relatively well constrained. Oxygen fugacity is highly heterogeneous in the upper mantle, on both temporal and spatial scales, and is an important parameter in characterizing many chemical and physical processes in the mantle. However, less attention has been devoted to the effect of oxygen fugacity on OH dissolution in olivine, and the only few available reports on this topic have led to significant inconsistency and debate. In this study, the correlation between oxygen fugacity and OH solubility in Fe-bearing olivine has been systematically investigated by conducting experiments at 1.5–7GPa and 1100–1300°C and under peridotite- and fluid-saturated conditions, with natural gem-quality olivine single crystals and fresh peridotite xenoliths as starting materials and with oxygen fugacity controlled by the Fe–FeO, Ni–NiO and Fe2O3–Fe3O4 oxygen buffer pairs. The water concentrations were determined by polarized analyses using a Fourier-transform infrared spectroscopy. The results show that, at all the experimental conditions, the OH bands at both high frequency (∼3650–3450cm−1) and low frequency (∼3450–3100cm−1) are prominent. The intensity of OH bands at ∼3355 and 3325cm−1 increases positively with oxygen fugacity, suggesting a dominant role of Fe3+ in their incorporation. Under otherwise identical conditions, the water content is gradually enhanced with increasing pressure, temperature or oxygen fugacity. The effect of oxygen fugacity on the enhancement of OH solubility appears not sensitive to temperature (1100–1300°C) at a given pressure, but becomes progressively stronger with increasing pressure from 1.5 to 7GPa given the temperature. Relative to oxygen fugacity buffers, the OH solubility is on average increased by ∼50% between Fe–FeO and Ni–NiO, independent of pressure and temperature. The enhanced OH solubility at relatively oxidizing conditions could be explained by the role of oxygen fugacity in changing the point defects of olivine. Experimental work concerning the storage capacity of water in olivine and in the upper mantle must consider the effect of oxygen fugacity. The OH solubility of olivine in the reducing deep upper mantle obtained from experiments under oxidizing conditions would be overestimated. The initial OH information of many natural olivines may have been modified by some recent secondary events, e.g., during their shallow residence and/or transport to the surface, and in principle, olivine equilibrated in the upper mantle may be D-depleted relative to its coexisting minerals. Some preliminary constraints on the water storage capacity in the upper mantle of the Earth and the Mars are also provided.

Recommended mineral-melt partition coefficients for FRTEs (Cu), Ga, and Ge during mantle melting

First-row transition element (FRTE) concentrations in primitive mantle-derived melts have been used as direct indicators of mantle source mineralogy (e.g., Ti, Mn, Fe, Co, Ni, Zn) and as proxies to trace the oxidation state of the mantle (e.g., Sc, V, Cu, Zn). Ga and Ge, which share chemical similarities with FRTEs, may also have the ability to trace mineralogical heterogeneities in the source of mantle-derived melts. Although the partitioning behaviors of most FRTEs are well constrained during mantle melting, partition coefficients of Cu, Ga, and Ge between mantle minerals and melt are still uncertain. Here we report new measurements that constrain partition coefficients of Cu, Ga, and Ge between olivine (Ol), orthopyroxene (Opx), clinopyroxene (Cpx), and basaltic melt from graphite capsule experiments carried out at 1.5–2 GPa and 1290–1500 °C. We suggest that discrepancies between recent experimental studies on Cu partitioning reflect one or more of the following causes: compositional control on partitioning, the effect of oxygen fugacity, Cu loss, Fe loss, non-Henrian behavior, and/or lack of complete chemical equilibrium. The partitioning values obtained from this study are 0.13 (±0.06), 0.12 (±3), and 0.09 for D CuOl/melt, D CuOpx/melt, and D CuCpx/melt, respectively. Using values from this study and from the literature, we show that melting of a sulfide-bearing peridotite source with an initial D Cu peridotite/melt ranging from 0.49 to 0.60 can explain the Cu content of primitive MORBs. Here, we also support the hypothesis that Ga partitioning between pyroxenes and melt strongly depends on the Al2O3 content of pyroxenes. Using pyroxene compositions from experiments, and previous partition data from literature, we recommend D GaPx/melt values for low- P (1.5 GPa) spinel peridotite melting ( D GaOpx/melt = 0.23 and D GaCpx/melt = 0.28), intermediate- P (2.8 GPa) spinel peridotite melting ( D GaOpx/melt = 0.42 and D GaCpx/melt = 0.40), high- P (3 GPa) garnet peridotite melting ( D GaOpx/melt = 0.38 and D GaCpx/melt = 0.37), high- P (4 GPa) garnet peridotite melting ( D GaOpx/melt = 0.26 and D GaCpx/melt = 0.30), and MORB-like eclogite melting at 2–3 GPa ( D GaCpx/melt = 0.78). Consistent with previous studies, we find that Ga is incompatible in olivine during low- P peridotite melting ( D GaOl/melt = 0.08). Using values from this study and from the literature, we support the hypothesis that the Ga, Ga/Sc, and Ti contents of most mantle-derived melts require garnet in their source, but that additional lithologies (e.g., metasomatic veins) may be necessary to explain the chemical variability of those melts. Here we also obtain Ge partition coefficients applicable to low- P peridotite melting of 0.67, 1.04, and 1.12 for D GeOl/melt, D GeOpx/melt, and D GeCpx/melt, respectively. Last, to provide a comprehensive picture of FRTE, Ga, and Ge partitioning during mantle melting, we provide a complete set of recommended partitioning values, based on results from this study and from the literature, for all FRTEs, Ga, and Ge, relevant for partial melting of spinel and garnet peridotite, as well as for MORB-like eclogite.

Disequilibrium iron isotopic fractionation during the high-temperature magmatic differentiation of the Baima Fe–Ti oxide-bearing mafic intrusion, SW China

Iron isotopic compositions of olivine, clinopyroxene and titanomagnetite of the Baima Fe–Ti oxide-bearing layered mafic intrusion, SW China, are used to investigate Fe isotopic fractionation during the formation of the Fe–Ti oxide ore bodies and to constrain the origin of silicate minerals and Fe–Ti oxides. The Baima intrusion comprises the Lower Zone of interlayered troctolite, clinopyroxenite and oxide ores and the Upper Zone of gabbros. Significant differences of Fe isotope values between olivine (δFe56=−0.01 to +0.11‰), clinopyroxene (δFe56=+0.11 to +0.22‰), and titanomagnetite (δFe56=+0.20 to +0.31‰) were observed in rocks and ores of the Lower Zone. Iron isotopic fractionation between olivine and clinopyroxene, olivine and titanomagnetite and clinopyroxene and titanomagnetite exhibits large variations of 0.06‰ to 0.22‰, 0.12‰ to 0.27‰, and 0.00‰ to 0.20‰, respectively, suggesting disequilibrium fractionation. This disequilibrium cannot be explained by thermal or chemical gradient-induced kinetic fractionation or by possible subsolidus exsolution of granular ilmenite from titanomagnetite. Instead, it could be attributed to crystallization of silicates and titanomagnetite from two immiscible Si-rich and Fe-rich liquids. Continuous segregation of the Fe-rich liquid changed the Fe isotopic composition of the Si-rich liquid and thus the Fe isotopic compositions of olivine and clinopyroxene crystallized from it, resulting in disequilibrium Fe isotopic fractionation between them. The effect of oxygen fugacity on the crystallization order of titanomagnetite and ilmenite from the Fe-rich melts, on the other hand, gave rise to the disequilibrium Fe isotope fractionation between titanomagnetite and olivine/clinopyroxene.

The effect of oxygen fugacity, melt composition, temperature and pressure on the oxidation state of cerium in silicate melts

Ce LIII-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of synthetic glasses prepared over a range of oxygen fugacities (fO2s, from −10 to +11 in logarithmic units relative to the quartz–fayalite–magnetite, QFM, buffer), temperatures (1300–1500°C), and pressures (1atm and 1GPa). The oxidation state ratio of Ce, Ce4+/∑Ce (where ∑Ce=Ce3++Ce4+), was determined from the spectra allowing the relationships between Ce4+/∑Ce and fO2, temperature, pressure and melt composition to be determined. Ce4+/∑Ce varied systematically with fO2 from 0 to ~0.8 over the range of conditions studied. Ce4+ is stabilised relative to Ce3+ by less polymerised compositions and lower temperatures, while pressure appears to have almost no effect (possibly stabilising Ce4+). Ce4+ in an Fe2+-bearing melt is not preserved on cooling to a glass due to the reaction Ce4++Fe2+=Ce3++Fe3+. Ce4+/∑Ce in natural melts is exceedingly small but may be recorded in the mineral zircon as an increased abundance of Ce relative to the other rare earth elements, which occur exclusively in the trivalent state. The magnitude of this Ce anomaly has considerable potential as an oxy-barometer.

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 Mossbauer 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+.

Effect of oxygen fugacity on the H 2O storage capacity of forsterite in the carbon-saturated systems

High pressure experiments have been performed in the systems Mg 2SiO 4–C–O–H and Mg 2SiO 4–K 2CO 3–C at 6.3 GPa and 1200 to 1600 °C using a split-sphere multi-anvil apparatus. In the Mg 2SiO 4–C–O–H system the composition of fluid was modeled by adding different amounts of water and stearic acid. The fO 2 was controlled by the Mo–MoO 2 or Fe–FeO oxygen buffers. Several experiments in the Mg 2SiO 4–C–O–H system and all experiments in the Mg 2SiO 4–K 2CO 3–C system have been conducted without buffering the fO 2. Forsterite in the system Mg 2SiO 4–K 2CO 3–C does not reveal OH absorption bands in the IR spectra, while forsterite coexisting with carbon-bearing fluid and silicate melt at log fO 2 from FMQ-2 to FMQ-5 (from 2 to 5 log units below fayalite-magnetite-quartz oxygen buffer) contains 800–1850 wt. ppm H 2O. The maximum concentrations were detected at 1400 °C and FMQ-3.5. We observed an increase in the solidus temperature in the system Mg 2SiO 4–C–O–H from 1200 to above 1600 °C with log fO 2 decreasing from FMQ-2 to FMQ-5. The increase of the solidus temperature and the broadening of the stability field of the H 2O–H 2–CH 4 subsolidus fluid phase at 1400–1600 °C explain the high H 2O storage capacity of forsterite relative to that crystallized from carbon-free, oxidized, hydrous, silicic melt. At temperatures above 1400 °C liquidus forsterite precipitated along with diamond from oxidized (FMQ-1) carbonate–silicate melt and from silicate melt dissolving the moderately reduced C–O–H fluid (from FMQ-2 to FMQ-3.5). Formation of diamond was not detected under ultra-reduced conditions (FMQ-5) at 1200–1600 °C. Olivine co-precipitating with diamond from dry carbonate–silicate or hydrous–silicic fluid/melt can provide information on the H 2O contents and speciation of the diamond-forming media in the mantle. The conditions for minimum post-crystallization alteration of olivine and its hydrogen content are discussed.

The effect of oxygen fugacity on the olivine to wadsleyite transformation: Implications for remote sensing of mantle redox state at the 410 km seismic discontinuity

High-pressure and -temperature multianvil experiments were performed to test the effect of varying oxygen fugacity on the olivine to wadsleyite transformation. Two capsules, containing samples of (FeMg) 2 SiO 4 , were placed in each experiment; the first buffered the oxygen fugacity with an assemblage of Re and ReO 2 . whereas the second ensured the lowest possible ferric iron concentra- tion through the presence of excess Fe metal. Measurements of coexisting olivine, wadsleyite. and ringwoodite compositions from the Fe metal saturated experiments were used to accurately determine the pressure in each experiment using established phase relations. Under the more oxidizing conditions of the Re-ReO 2 buffer, the stability field of wadsleyite was found to expand with respect to both the olivine and ringwoodite stability fields. Mössbauer spectroscopy measurements reveal Fe 3+ /∑Fe ratios for wadsleyite buffered by Re-ReO: of 0.1-0.25. while olivine appears to be FeJ"-free. A thermodynamic model that employs the wadsleyite end-members (Fe 3+ 5/3 □ 1/3 )Fe 3+ O 4 -Fe 2 SiO 4 -Mg 2 SiO 4 is used to examine the effect of varying bulk mantle Fe 3+ /∑Fe ratio on the depth and depth interval of the 410 km seismic discontinuity. Fe 3+ /SFe ratios in the range 0.02-0.12 would cause the depth interval or thickness of the 410 km discontinuity to increase from -8 to 15 km but would haw very little effect on the seismically observable absolute depth. Very large bulk mantle Fe 3+ /∑Fe ratios (&gt;0.2), unrepresented in recovered mantle samples, would be required to explain recent seismic observations that the depth interval of the 410 km may be &gt;20 km beneath certain regions. Such observations are more likely to be explained by moderate local enrichments in both ferric iron and H;0 in the mantle, most likely as a result of slab interaction.