Effect Of Oxygen Fugacity Research Articles

Diamond oxidation at atmospheric pressure: development of surface features and the effect of oxygen fugacity

The oxidation of cubic diamond plates was experimentally investigated at 100 kPa, at 1000, 1050 and 1100 °C and at variable oxygen fugacity ( f O 2 ) to evaluate the effect of f O 2 on diamond preservation and to examine the development of secondary diamond morphology. The diamond oxidation rate was determined from weight and size changes of the diamonds and shows a similar f O 2 -dependence at three temperatures ( T ): Ln V = 0.57 log f O 2 + T term , where V is diamond oxidation rate and T term is the term dependent on temperature. The derived relationship can be used to evaluate the significance of the earlier established variations in the redox state in several EKATI mine property kimberlites, N.W.T., Canada. The surfaces of etched diamonds from the experiments were studied using high-resolution field emission scanning electron microscopy. At 100 kPa the shape, orientation and size of the etch pits have no consistent correlation with temperature, f O 2 and the reaction rates. Increase in the CO 2 -content of the reacting gas results in an evolution from irregular cavities to pits formed by the development of (100) faces and further to more regular forms with (111) faces. These results agree with high pressure data and suggest that the features developed on oxidized diamonds are determined by variations in the reaction rates in different directions of the diamond lattice, which are dictated by the molecular composition of the reacting fluid (or gas). A high CO 2 content favours fast oxidation rates in the [111] direction.

Effect of chemical environment on the hydrogen-related defect chemistry in wadsleyite

The effect of chemical environment on the hydrogen-related defect chemistry in wadsleyite was investigated using Fourier-transform infrared (FTIR) spectroscopy. Samples were annealed at P = 14-16 GPa and T = 1230-1973 K using Kawai-type multi-anvil apparatus. The effect of oxygen fugacity (fO₂) was investigated using three metal-oxide buffers (Mo-MoO2, Ni-NiO, and Re-ReO2). The effect of water fugacity (fH₂₂O) was studied using two different capsule assemblies (“nominally dry” and “dry” assemblies). A range of total OH concentration (COH,Total) of studied wadslyeites varies between <50 H/106Si (<3 wt ppm H2O) and 23 000 H/106Si (1400 wt ppm H2O). The observed FTIR spectra were classified into four different classes, i.e., peaks at 3620 (“3620”), 3480 (“3480”), and 3205 cm-1 (“3205”) and the others (Group O), where the Group O includes peaks at 3270, 3330, and 3580 cm−1. The variation in OH concentration corresponding to each peak was analyzed separately. The OH concentrations correspond to “3620,” “3480,” and “3205” were found to be highly dependent on both fH₂₂O and fO₂₂. Assuming COH,Group O = 2[(2H)xM] (COH,Group O is OH concentration of Group O), present data were analyzed by using thermodynamic model for concentration of hydrogen-related defects. Based on analytical results, OH concentration of “3620” and “3480” was found to be reasonably explained by q = 1/2 and r = 1/12 (q and r are fH₂O and fO₂ exponents, respectively), whereas that of “3205” was consistent with q = 1/2 and r = -1/12. These results suggest that “3620” and “3480” correspond to HM′ whereas “3205” corresponds to H·, respectively, under the charge neutrality condition of [FeM′] = 2[VM″ ].

Rheological Consequences of Redox State

Within the upper mantle of Earth, there is a gradient from a relatively more oxidizing near-surface region, with oxygen fugacities near those of the fayalite-magnetite-quartz buffer (FMQ), to more reducing conditions at depth near the iron-wustite buffer (IW). Oxygen fugacity appears to vary laterally, as well as vertically, by as much as a factor of 10 4 . As flow within the interior of the Earth and other terrestrial planets occurs due to the (mostly) solid-state deformation of rocks, an understanding of the effect of oxygen fugacity on creep is critical in modeling planetary interior dynamical behavior. This is especially important for the asthenosphere, that anomalously weak region in the uppermost mantle that accommodates isostasy and largely decouples mantle convection from plate motions. Experimental studies of the rheological behavior of iron-bearing minerals have demonstrated that oxygen fugacity can play an important role in deformation. We have shown that olivine rich rocks deformed near FMQ deform in the dislocation creep regime about a factor of 6 faster when buffered near FMQ than at IW. Experiments on olivine single crystals and aggregates indicate that this difference in behavior results from an increase in the concentration of silicon vacancies under more oxidizing conditions, as dislocation creep is rate-limited by the climb of dislocations, which is controlled by diffusion of silicon defects. Although fewer data are available for the effects of oxygen fugacity on pyroxene deformation, clinopyroxene appears to be stronger under more oxidizing conditions, while the data on orthopyroxene deformation show no dependence on oxygen fugacity. These results indicate that vertical and lateral variations in oxygen fugacity may result in, at most, an order of magnitude difference in viscosity, while other factors, such as water fugacity and lithology may be more significant.

Dynamic crystallization of shock melts in Allan Hills 77005: Implications for melt pocket formation in Martian meteorites

A series of crystallization experiments have been performed on synthetic glasses matching the composition of a melt pocket found in Allan Hills (ALH) 77005 in order to evaluate the heterogeneous nucleation potential of the melt and the effect of oxygen fugacity on crystallization. The starting temperature of the experiments varied from superliquidus, liquidus and subliquidus temperatures. Each run was then cooled at rates of 10, 500 and 1000 °C/h at FMQ. The results of this study constrain the heating and cooling regime for a microporphyritic melt pocket. Within the melt pocket, strong thermal gradients existed at the onset of crystallization, giving rise to a heterogeneous distribution of nucleation sites resulting in gradational textures of olivine and chromite. Skeletal olivine in the melt pocket center crystallized from a melt containing few nuclei cooled at a fast rate. Nearer to the melt pocket margin elongate skeletal shapes progress to hopper and equant euhedral, reflecting an increase in nuclei in the melt at the initiation of crystallization and growth at low degrees of supercooling. Cooling from post-shock temperatures took place on the order of minutes. An additional series of experiments were conducted for a melt temperature of 1510 °C and a cooling rate of 500 °C/h at the FMQ buffer, as well as 1 and 2 log units above and below FMQ. Variation in chromite stability in these experiments is reflected in crystal shapes and composition, and place constraints on the oxygen fugacity of crystallization of the melt pocket. We conclude that the oxygen fugacity of the melt pocket was set by the Fe 3+/Fe 2+ ratio imparted by melting of the host rock, rather than external factors such as incorporation of CO 2-rich Martian atmosphere, or melting and injection of oxidized surface (e.g., regolith) material. Comparison with previous crystallization experiments on melt pockets in Martian basalts indicate that the predominance of dendritic crystal shapes reflects the likelihood that those melt pockets with lower liquidus temperatures will be more completely melted, destroying most or all nuclei in the melt. In this case, crystal growth takes place at high degrees of supercooling, yielding dendritic shapes. It appears as though the melting process is just as important as cooling rate in determining the final texture of the melt pocket, as this process controls elimination or preservation of nuclei at the onset of cooling and crystallization.

The effect of oxygen fugacity on hydroxyl concentrations and speciation in olivine: Implications for water solubility in the upper mantle

The effect of oxygen fugacity on hydroxyl speciation and solubility in San Carlos olivine was investigated in a series of piston cylinder experiments at 2.0 GPa and 1100 and 1300 °C. The presence of enstatite was intended to fix silica activity and experimental fO 2 in most runs was regulated by lining the inner walls of the sample-containing Pt capsules with either Fe, Ni or Re foil. Polarised infrared absorbance spectra show that olivine annealed in capsules lined with Fe foil only contain hydroxyl defects with vibrational energies between 3650 and 3400 cm − 1 (relatively high frequencies). Spectra of crystals annealed in capsules lined with Ni or Re foil show that these samples contain hydroxyl defects with vibrational energies between 3650 and 3400 cm − 1 (high frequency), and at 3355 and 3325 cm − 1 (low-frequency). The concentrations of hydroxyl defects responsible for absorptions at 3355 and 3325 cm − 1 change as a function of oxygen fugacity and are, therefore, interpreted as Fe 3+-related OH centres. Hydroxyl solubility in our experiments is substantially less than current solubility laws might suggest. These differences may be attributed to variations in experimental procedure, but they also demonstrate that the incorporation of hydroxyl in olivine must be better understood before an accurate model to describe water solubility in the deep Earth can be constructed. Our data show that, when experiments conducted at identical P– T conditions are compared, olivine annealed at low fO 2 (below Fe–FeO) contains less than half as much water as crystals from relatively more oxidised experiments. Because natural mantle conditions approximate to those of our low fO 2 experiments, current solubility laws – which are based on data from relatively high fO 2 experiments – overestimate the amount of water that can be hosted in olivine in the upper mantle. From a consideration of defect equilibria we propose a mechanism for the incorporation of OH in olivine in a way that is appropriate for the actual range of conditions expected in the mantle.

Metal–silicate partitioning of cesium: Implications for core formation

Here we present the first set of metal–silicate partitioning data for Cs, which we use to examine whether the primitive mantle depletion of Cs can be attributed to core segregation. Our experiments independently varied pressure from 5 to 15 GPa, temperature from 1900 to 2400 °C, metallic sulfur content from pure Fe to pure FeS, silicate melt polymerization, expressed as a ratio of non-bridging oxygens to tetrahedrally coordinated cations (nbo/ t) from 1.26 to 3.1, and fO 2 from two to four log units below the iron–wüstite buffer. The most important controls on the partitioning behavior of alkalis were the metallic sulfur content, expressed as X S, and the nbo/ t of the silicate liquid. Normalization of X S to 0.5 yielded the following expressions for D-values as a function of nbo/ t: log D Na = −2.0 + 0.44 × (nbo/ t), log D K = −2.4 + 0.67 × ( nbo/ t), and log D Cs = −3.2 + 1.17 × (nbo/ t). Normalization of nbo/ t to 2.7 resulted in the following equations for D-values as a function of S content: log D Na = −4.1 + 6.4 × X S, log D K = −7.7 + 13.9 × X S, and log D Cs = −12.1 + 23.3 × X S. There appears to be a negative pressure effect up to 15 GPa, but it should be noted that this trend was not present before normalization, and is based on only two measurements. There is a positive trend in cesium’s metal–silicate partition coefficient with increasing temperature. D Cs exhibits the largest change and increased by a factor of three over 500 °C. The effect of oxygen fugacity has not been precisely determined but in general, lowering fO 2 by two log units resulted in a rise in all D-values of approximately an order of magnitude. In general, the sensitivity of partition coefficients to changing parameters increased with atomic number. The highest D-value for Cs observed in this study is 0.345, which was obtained at nbo/ t of 2.7 and a metal phase of pure FeS. This metallic composition has far more S than has been suggested for any credible core-forming metal. We therefore conclude that the depletion of Cs in Earth’s mantle is either caused by radically different behavior of Cs at pressures higher than 15 GPa or is not related to core formation. Even so, we have shown that a planet with a sufficient S inventory may incorporate significant amounts of alkali elements into its core.

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.

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.

Effect of oxygen fugacity on the etching rate of diamond crystals in silicate melt

Experimental data on the etching of diamond crystals in basaltic melt at 1130°C with variable oxygen fugacity in the environment are considered. The oxygen fugacity was set with the HM and NNO buffers. The study was carried out on a 0.6–0.8 mm fraction (powder) of natural diamond crystals. It has been established that, at the same temperature, the rate of diamond etching (oxidation) in silicate melt depends on the oxygen fugacity in the environment. The etching rate decreases with decline in the oxygen fugacity from the case where the melt comes into contact with atmospheric air to the conditions controlled by the HM and NNO buffers. Under the conditions of the HM and NNO buffers, oxidation was accompanied by surface graphitization of diamond crystals.

The solubility of copper in hydrous rhyolitic melts: The effects of oxygen, chlorine and sulfur

Porphyry-Cu deposits are unquestionably linked to subduction zone (arc) magmas. Yet the source magmas for these deposits remain unclear, despite recognition that arc-magmas are water-bearing and relatively oxidised. This study focuses on understanding some of the chemical parameters that may affect the solubility of Cu in hydrous melts using experimental petrology. In particular, the effect of oxygen fugacity and Cl and S content of the melt was investigated.The experiments were conducted in an internally heated pressure vessel where the oxygen fugacity is controlled by the addition of H2 to the Ar pressure medium. The sample charges consisted of powdered synthetic glass of rhyolitic composition containing water (7 wt%) and various amounts of NaCl or a S-bearing phase (FeS or CaSO4 depending on fO2). The samples were sealed in welded AuCu capsules and run at 4 kbars and 800 °C and 900 °C for up to 10 days. Twelve samples can be run simultaneously in the same vessel enabling the P, T and fO2 conditions to be identical for each sample. The fO2 was checked using solid Ni–Pd and Co–Pd sensors. The fO2 ranged from NNO−1 to approx. NNO+3; the Cl contents varied from O to 9000 ppm at 900 °C, and O to 6500 ppm at 800 °C; the S content in the melt was very low for all runs, typically <80 ppm. Results show that although there is a very slight positive correlation between Cl and Cu, Cl does not appear to play a dominant role in the solubility of Cu. The main control on Cu solubility is oxygen fugacity, which correlate linearly, Cu increasing with fO2. The oxidation state of Cu in these experiments can be determined using the reaction Log aCuOglassn = n/2 x log ∞O2 + C, where 2n is the valence of dissolved Cu. A slope of 0.25–0.30 suggests dissolution as a Cu2O-like compound where Cu is present as Cu+. This explains the strong correlation between oxidised magmas and porphyry-Cu deposits. The solubility of S was too low to be varied significantly within detection limits and excess sulfur was found to form a discrete sulfide phase. At lower oxygen fugacities (approx. NNO−1) it consists of a Cu–Fe–S phase, whereas at higher oxygen fugacities (up to NNO+3) a Cu–S phase precipitates. This is contrary to the expectation that at high oxygen fugactities sulfide is no longer stable and suggests that Cu greatly expands the stability field of sulphide liquids.

Experimental quantification of the fractionation of Fe isotopes during metal segregation from a silicate melt

Fractionation of Fe isotopes between a silicate melt and metallic alloys has been quantified experimentally at 1500 °C. The effects of oxygen fugacity and run duration have been investigated to distinguish between kinetic and equilibrium fractionations. A new experimental setup is presented, in which metallic Fe is produced by reduction of oxidized iron from a silicate melt due to a change of redox conditions. This metallic Fe is physically removed from the silicate and sequestered as a (Pt,Fe) alloy. Bulk analyses of the silicate and metallic fractions using multi-collector ICP-MS methods are coupled with in situ analyses using an ion microprobe. Experimental results indicate that significant isotopic fractionation of Fe occurs during metal segregation. During the early stages of the experiments, we find evidence for kinetic fractionation caused by faster diffusion of 54Fe along concentration gradients in the alloy. This process leads to the formation of a metallic phase which is isotopically light compared to the residual silicate (metal–silicate fractionation up to − 2.35‰/a.m.u.). The data are used to quantify the difference in diffusion coefficient of each isotope of iron, providing a means to predict Fe isotope fractionation during Fe diffusion in metallic alloys. On the other hand, this state of affairs is transient in nature, and at superliquidus temperature, the systems rapidly reach a state of isotopic equilibrium in which the metal is isotopically heavier than the silicate melt by about + 0.2 ± 0.15‰/a.m.u. These results are consistent with the range of variation observed in natural samples. These kinetic and equilibrium data provide an experimental framework to understand the observed variability among igneous and meteoritic materials formed at high temperature, particularly for iron and pallasite meteorites.

The effect of halogens on Zr diffusion and zircon dissolution in hydrous metaluminous granitic melts

Diffusion of Zr and zircon solubility in hydrous, containing approximately 4.5 wt% H2O, metaluminous granitic melts with halogens, either 0.35 wt% Cl (LCl) or 1.2 wt% F (MRF), and in a halogen-free melt (LCO) were measured at 1.0 GPa and temperatures between 1,050 and 1,400 °C in a piston-cylinder apparatus using the zircon dissolution technique. Arrhenius equations for Zr diffusion in each hydrous melt composition are, for LCO with 4.4±0.4 wt% H2O: $$D = 2.88 \pm 0.03x10^{ - 8} \exp \left( {{{ - 140.1 \pm 33.9} \over {RT}}} \right)$$ , for LCl with 4.5±0.5 wt% H2O: $$D = 2.33 \pm 0.05x10^{ - 4} \exp \left( {{{ - 254.8 \pm 64.1} \over {RT}}} \right)$$ and for MRF with 4.9±0.3 wt% H2O: $$D = 2.54 \pm 0.03x10^{ - 5} \exp \left( {{{ - 223.8 \pm 15.5} \over {RT}}} \right)$$ . Solubilities determined by the dissolution technique were reversed for LCO +4.5±0.5 wt% H2O by crystallization of a Zr-enriched glass of LCO composition at 1,200 and 1,050 °C at 1.0 GPa. The solubility data were used to calculate partition coefficients of Zr between zircon and hydrous melt, which are given by the following expressions: for LCO $$\ln D_{Zr}^{zircon/melt} = 1.63\left( {{{10000} \over T}} \right) - 5.87$$ , for LCl $$\ln D_{Zr}^{zircon/melt} = 1.47\left( {{{10000} \over T}} \right) - 4.75$$ and, for MRF by $$\ln D_{Zr}^{zircon/melt} = 1.47\left( {{{10000} \over T}} \right) - 4.91$$ . Experiments on the same compositions, but with water contents down to 0.5 wt%, demonstrated reductions in both the diffusion coefficient of Zr and zircon solubility in the melt. The addition of halogens at the concentration levels studied to metaluminous melts has a small effect on either the diffusion of Zr in the melt, or the solubility of zircon at all water concentrations and temperatures investigated. At 800 °C, the calculated diffusion coefficient of Zr is lowest in LCl, 9×10–17 m2 s–1, and is highest in LCO, 4×10–15 m2 s–1. Extrapolation of the halogen-free solubility data to a magmatic temperature of 800 °C yields solubilities of approximately one-third of those directly measured in similar compositions, predicted by earlier studies of zircon dissolution and based upon analyses of natural rocks. This discrepancy is attributed to the higher oxygen fugacity of the experiments of this study compared with previous studies and nature, and the effect of oxygen fugacity on the structural role of iron in the melt, which, in turn, affects zircon solubility, but does not significantly affect Zr diffusion.

The effect of pressure, temperature, oxygen fugacity and composition on partitioning of nickel and cobalt between liquid Fe-Ni-S alloy and liquid silicate: implications for the earth’s core formation

Experiments have been conducted with a multi-anvil apparatus on two natural chondrites in four different sample containers. Partition coefficients for Ni and Co between liquid Fe-Ni-S-alloy and liquid silicate ( D Ni LA/LS and D Co LA/LS) were determined over a pressure range between 2 and 25 GPa and a temperature range between 2100 and 2623 K. Multi-variable linear regression analysis was performed on the measured D Ni LA/LS and D Co LA/LS, using a formula that is partially based on thermodynamics and partially empirical relationships. The results from the experiments with non-graphite capsules revealed that the effects of oxygen fugacity are roughly consistent with Ni and Co being divalent ions in liquid silicate. Within the experimental pressure and temperature ranges, both Ni and Co become less siderophile with increasing pressure and temperature. The effect of pressure is more pronounced for Ni while the effect of temperature is more pronounced for Co. Furthermore, both effects become smaller at higher pressures and higher temperatures. Nickel and Co are less soluble in liquid silicate with higher degree of polymerization. The presence of S in liquid Fe-alloy enhances the siderophile behavior of Ni and Co. However, for a given relative proportion of Fe, Ni and Co in the Fe-alloy, D Ni LA/LS and D Co LA/LS are larger for the Fe-alloy with a smaller S content. Applying the observed effects of pressure, temperature, oxygen fugacity and composition on D Ni LA/LS and D Co LA/LS, we found that the observed partitioning of Ni and Co between core and mantle ( D Ni core/mantle and D Co core/mantle) can be matched at high pressures and high temperatures. The pressure-temperature range corresponds to that in the current midmantle (about 1200–1450 km in depth). Therefore, the observed high abundances and near-chondritic ratio of Ni and Co in the Earth’s upper mantle can be explained by chemical equilibrium between liquid Fe-Ni-S-alloy and liquid silicate in a deep magma ocean.

Water solubility and D/H fractionation in the system basaltic andesite–H 2O at 1250°C and between 0.5 and 3 kbars

Water solubilities in melt of basaltic andesite composition have been measured in the range 500 to 3000 bars, 1200–1250°C. The saturation curve obey an empirical relationship very close to the square root of pressure law of Hamilton et al. [Hamilton, D.L., Burnham, C.W., Osborn, E.F., 1964. The solubility of water and effects of oxygen fugacity and water content on crystallisation in mafic magmas. J. Petrol., 5, 21–39]. The best fit is obtained by a power law H 2O wt.%= aP n ( P in b), where a=0.05±0.01 and n=0.63; the departure from n=0.5 is mainly due to the increasing importance of dissolved molecular water at high pressures. Hydrogen isotopic fractionation between water vapor and total water dissolved in the melt (ΔD v–m) has been measured at 0.5, 2 and 3 kb. Δ D v–m decreases with pressure from 32 to 20±2‰. This variation is related to the increase in molecular water proportions relative to hydroxyl group when water solubility increases. The variation of the D/H ratios in water dissolved in such melts thus appears as a possible indirect measurement of the original OH/H 2O ratio of total dissolved water, not subject to variation during quenching as the later is. This correlation shows that the Δ D v–m isotopic fractionation at very high water vapor pressure tend toward a limit of 16±3‰ for basic magmas.

Self diffusion of europium, neodymium, thorium, and uranium in haplobasaltic melt: The effect of oxygen fugacity and the relationship to melt structure

We report new measurements of self diffusion coefficients ( D) for Mg, Nd, Eu, Th, and U in a haplobasaltic (Fo 15Di 40An 45) melt at 1 atm, 1400–1500°C, and oxygen fugacities corresponding to air and the FeFeO buffer. Diffusion couples consisted of isotopically distinct melts of the same chemical composition, and isotopic concentration profiles in quenched couples were measured with an ion probe. The valence state distributions of Eu and U were determined from absorption spectroscopy and model calculations, which demonstrate a shift from Eu 3+ and U 5.5+ in air to Eu 2.5+ and U 4+ at FeFeO. D Mg, D Nd, and D Th are independent of oxygen fugacity and agree well with our previous measurements. D Eu = D Nd in air and increases by 42% at FeFeO, while D U = D Th in air and shows a possible small increase of ∼20% at FeFeO. The change in D Eu with oxygen fugacity matches the established ionic radius and charge dependence for Mg, Ca, Ba, Nd, Yb, Ti, and Zr, while diffusion coefficients for Zr, Th, U 4+, and U 5.5+ are independent of ionic radius and charge. Activation energies for all cations are approximately equal, independent of oxygen fugacity, and approximately match the activation energy for viscous flow. In addition, activation energies and diffusion coefficients recently measured for O and Si in basalt agree well with the present values. The good agreement between the various activation energies and between network modifier and network former diffusivities is consistent with a model in which diffusion of network modifying cations in low viscosity melts is controlled largely by the extrinsic influence of the melt network reorganization, with an additional influence from the intrinsic mobilities of the individual cations. The constant diffusion coefficient defined by the high ionic radius and charge elements is interpreted to represent the characteristic network diffusivity for this composition, which dominates over the intrinsic diffusivities for these elements. Elements with faster intrinsic diffusivities still display a small ionic radius and charge dependence. Diffusion coefficients in high viscosity melts are expected to be decoupled from the network, and thus may display a much greater dependence on ionic radius and charge.

Ferric iron dependence of the electrical conductivity of the Earth´s lower mantle material

We measured the DC electrical conductivity of samples of magnesiowustite-perovskite assemblage, prepared by laser heating at 400 kbar in a diamond-anvil cell, from San Carlos olivine equilibrated at various oxygen partial pressures. The conductivity was measured as a function of temperature between room temperature and 400 °C. The conductivity obeys the Arrhenius law, compatible with a conduction by small polarons. The activation energy is about 0.3 eV atm -1 , and decreases with increasing pressure and increasing oxygen fugacity of equilibration. The effect of the oxygen fugacity on the pre-exponential factor is negligible. We suggest that the effect of oxygen fugacity is essentially due to the compression of the lattice accompanying the increase in trivalent iron ions content. The effect of oxygen partial pressure is therefore comparable to the effect of pressure. The present results confirm our previous estimate that the electrical conductivity of the lower part of the mantle is of the order of 1-10 S m -1 .

Effect of oxygen fugacity on trace-element partitioning between immiscible silicate melts at atmospheric pressure: A proton and electron microprobe study

The partitioning behaviour for the trace elements Rb, Sr, Ba, Th, U, Zr, Hf, Nb, Ta, La, Ce, Sm, Ho, Lu and Y between immiscible Fe-rich, Si-poor and Fe-poor, Si-rich silicate liquids in the system K 2O-Al 2O 3-SiO 2-FeO-Fe 2O 3-P 2O 5 at 1265°C, 1 atm was determined as a function of oxygen fugacity in the range of log f O2 from −8.7 to −5.7. The miscibility gap for all major elements is enlarged and the partition coefficients for Sr, Ba and U increase and Rb decreases with increasing f O2. The partition coefficients for the remaining trace elements stay nearly constant. Thus it is predicted that the partitioning of Sr, Ba, U and Rb between crystals and melt will show increasing dependence on melt composition as f O2 is increased, but the compositional dependence of the crystal/melt partition coefficients for the other elements will remain unchanged.

The effect of oxygen fugacity and temperature on solubilities of nickel, cobalt, and molybdenum in silicate melts

Solubilities of Ni, Co and Mo in silicate melts of anorthite-diopside eutectic composition were determined at reducing conditions, with ƒ O 2 values ranging from 10 −8.6 to 10 −12.6 atm and at temperatures of around 1400°C. In a log (solubility) vs. log ( ƒ O 2 ) diagram, Ni and Co data plot along straight lines with slopes of 0.45 ± 0.01 for Ni and 0.52 ± 0.01 for Co, approximately corresponding to a valence state in the melt of +2 for both metals. No indications for the presence of metallic Ni and/or Co were found, even at the lowest ƒ O 2 (IW-3) reached with gas mixing. This result is in agreement with independent experimental data for Ni solubilities in silicate melts by Dingwell et al. (1994) but is in conflict with recent experimental results by two other groups ( Colson, 1992; Ehlers et al., 1992). The experimental data for Mo suggest a change in valence slightly below the IW-1-buffer, from +6 to +4. This is the first reliable determination of the valence of Mo in silicate melts. No effect of the composition of gas mixtures used for fixing the ƒ O 2 was found; i.e., CO 2-CO-N 2 and CO 2-H 2-N 2 mixtures defining the same ƒ O 2 gave the same results. Temperature dependences for Ni, Co, and Mo solubilities were derived from experiments at temperatures ranging from 1349 to 1438°C. Solubilities of all three metals decrease with increasing temperature at constant oxygen fugacities. Metal/silicate partition coefficients were calculated from solubilities. Although Ni and Co metal/ silicate partition coefficients decrease with increasing temperature at ƒ O 2 appropriate for core formation, temperature extrapolated Ni-metal/silicate partition coefficients are much higher than corresponding Co-metal/silicate partition coefficients. Thus, the nearly chondritic Ni Co ratio of the mantle is difficult to reconcile with a global mantle-core equilibrium. In addition, although the metal/silicate partition coefficient of Mo decreases with temperature at ƒ O 2 values appropriate for core formation, the decrease is less steep than that of Ni and Co leading to a Ni Mo concentration ratio in a hypothetical magma ocean ten times above the Ni Mo ratio characteristic of the Earth's mantle.

The effect of oxygen fugacity on the partitioning of nickel and cobalt between olivine, silicate melt, and metal

Partitioning of nickel and cobalt between olivine, silicate melt, and metal has been determined in the system olivine-albite-anorthite-CaO at 1350°C and 1 atm over the oxygen fugacity ( ƒ O 2 ) range 10 −5.5 to 10 −12.6 atm. This range of ƒ O 2 , from one log unit above Nickel-Nickel Oxide (NNO + 1) to two log units below the Iron-Quartz-Fayalite (IQF-2) buffers, spans the range relevant for crystal/liquid processes in terrestrial planets and meteorite parent bodies. There is no effect of oxygen fugacity on D Ni Ol gls over the range of ƒ O 2 from 10 −5.5 to 10 −9.4. Over the range of ƒ O 2 from 10 −9.4to 10 −10.3, D Ni Ol gls shows an apparent decrease from a value of 6.8 at logs ƒ O 2 = −9.4 to a value of 5.5 at log ƒ O 2 = −10.3 , a factor of 1.2. At the lowest value of ƒ O 2 = 10 −12.6 the value for D Ni Ol gls is 3.4, a factor of 2 lower than that measured under oxidizing conditions. Experimentally produced melts are metal saturated over the range of ƒ O 2 from 10 −10.3 to 10 −12.6. Partition coefficients for Co are constant ( D Co Ol gls = 2.8) over the range of ƒ O 2 from 10 −6.6 to 10 −10.6. The Co partition coefficient decreases to a value of 1.65 at log ƒ O 2 = −12.2 . The variations in D Ni Ol gls are examined using equilibrium constants that account for the effects of variable silicate melt composition on Ni partitioning. Measured values of D Ni Ol gls and values estimated from equilibrium constants are identical for the experiments carried out under oxidizing conditions. The predicted values of D Ni Ol gls for the low ƒ O 2 experiments are higher than the measured values. These changes in Ni and Co partitioning behavior at low ƒ O 2 may indicate that Ni and Co are present as Ni 2+ and Ni 0 and Co 2+ and Co 0 in the silicate melt. We estimate that approximately 30% of the Ni may be present as Ni 0 in the low ƒ O 2 experiments.

The effect of oxygen fugacity on the solubility of carbon-oxygen fluids in basaltic melt

The solubility of CO 2 CO fluids in a mid-ocean ridge basalt ( morb) has been measured at 1200°C, 500–1500 bar, and oxygen fugacities between NNO and NNO-4. High oxygen fugacities, and thus CO 2-rich fluids, were produced by using a starting material equilibrated at NNO, and Ag 2C 2O 4 as the fluid source. Low oxygen fugacities were achieved by using graphite capsules, and MgCO 3 as the fluid source. These graphite-saturated fluids have the lowest possible C/O 2CO ratio for a given pressure and temperature. Experiments were run in a rapid-quench internally heated pressure vessel. Fluid compositions were measured using a simple vacuum technique and by Raman spectroscopy of fluid inclusions. The two techniques yielded comparable results. Fourier transform micro-infrared spectroscopy was used to identify and measure concentrations of dissolved volatiles in double-polished wafers of the quenched glasses. Carbonate was the only carbon-bearing species identified. Raman spectroscopic analysis of inclusion-free areas of glass confirmed the absence of dissolved molecular CO 2, CO and carbon. The measured concentrations of dissolved CO 2 in the glasses were proportional to the fugacity of CO 2 during the experiments, calculated from the measured fluid compositions. The data were fit to the equation X CO 2 melt(ppm)= 0.492 fCO 2 (bar). The insolubility of CO, compared to CO 2, may be related to the fact that dissolution of CO requires reduction of another species in the melt, whereas dissolution of CO 2 does not. Due to the fact that CO will be an important component of natural C O fluids at low pressures and low oxygen fugacities, equilibrium dissolved CO 2 contents will be less than calculated assuming pure CO 2 fluids, but as the C/O 2CO ratio in a pure C O fluid at fixed pressure and temperature is a direct function of oxygen fugacity, measurement of the oxygen fugacity of quenched glasses or trapped fluids in natural samples should allow saturation concentrations to be calculated. Dissolved CO 2 contents of some morb are less than expected if they were in equilibrium with pure CO 2. These samples must, therefore, have been more reduced than average if they were fluid-saturated. Together with results from other studies of CO 2 and H 2O solubilities in basalt, the results of this study provide a comprehensive framework for modelling CO 2 solution in morb.