Range Of Oxygen Fugacities Research Articles

INFOREX-3.0: A database on experimental studies of phase equilibria in igneous rocks and synthetic systems: II. Data description and petrological applications

The purpose of the paper is to provide details of the application of the INFOREX-3.0 database, a package designed to store, retrieve, and process phase equilibria information. This most recent release of the system accesses data of 162 experimental studies, conducted from 1962 to 1994, including a total of 6174 experiments with 5188 addressed to natural igneous rocks and 986 runs carried out in synthetic systems, mostly CMAS. The total database was divided into 3893 “dry” conditions experiments, and 2281 runs performed in the presence of H 2O and/or CO 2: 1618 of the “wet” runs represented are water saturated. The number of 1 atm experiments (3750) is greater than the number of high-pressure runs (2474). The INFOREX database contains 8311 coexisting phase compositions: 3197 for glasses, 1247—olivine, 1429—pyroxenes, 501—spinels, 842—plagioclase. One block of the INFOREX information includes 298 liquid compositions where the Fe 3+ Fe 2+ ratio was determined. Data for sulfur and water solubility experiments also have been systematized. The INFOREX data management system allows users to find and print out data on a specific set of mineral-melt or two mineral equilibrium experiments requested for a given range of temperatures, pressures, oxygen fugacities, and compositions in a matter of a few seconds. In addition, one can use subsets of the data to develop mineral-melt geothermometers for equilibria including olivine, plagioclase, pyroxenes, and spinels for any specific system type. Two examples illustrate the use of INFOREX for testing empirical equations proposed for the calculation of water solubility and Fe 3+ Fe 2+ ratio in basic to acid melts.

Core Formation in Earth's Moon, Mars, and Vesta

Stimulated by new experimental results on metal/silicate partitioning of elements at elevated temperatures and pressures, we have revisited the question of core formation in Earth's Moon, Mars, and Vesta (the probable source of the eucritic meteorites). Earlier studies suggested metal/silicate equilibrium in Mars, but led to the paradox that Mars accreted homogeneously while the Earth accreted heterogeneously. Using new elevated pressure and temperature metal/silicate partition coefficients, we show that abundances of the moderately siderophile elements in the mantles of the Moon, Mars and Vesta are consistent with early magma oceans on these bodies. The most successful model for explaining the lunar mantle siderophile element abundances requires a core of 5% of the mass of the Moon (500-km radius). Siderophile element abundances in Mars are consistent with intermediate pressure metal–silicate equilibrium, as has also been recently suggested for the Earth. The most successful model for explaining the martian siderophile element abundances requires a bulk planetary composition that has greater than CI chondritic abundances of the moderately siderophile elements, and a core of 30% of the mass of Mars. Siderophile element abundances in the mantle of Vesta are consistent with low pressure liquid metal–liquid silicate equilibrium, as expected for an asteroid-sized body, and a core of 10% of the mass of Vesta.Comparison of our best-fit oxygen fugacities for Vesta with thermodynamic calculations of oxygen fugacity for silicate-bearing iron meteorites indicates that parts of the inner solar system were homogeneous with respect to redox state at 4.5 Ga, approximately 2 log fO2units below the Fe–FeO buffer—much higher than estimates for the solar nebula. Similar comparisons for Mars and the Earth indicate that these bodies have undergone oxidation since 4.5 Ga, because the oxygen fugacities associated with metal–silicate equilibrium in both Mars and the Earth–Moon system are much lower than those recorded in martian and terrestrial basaltic and periodotitic samples. The oxidation on Mars is most likely due to atmospheric effects, whereas Earth's much wider range of oxygen fugacities must be due to both atmospheric and plate tectonic effects.

Quaternary minettes and associated volcanic rocks of Mascota, western Mexico: a consequence of plate extension above a subduction modified mantle wedge

Pleistocene-Holocene volcanism in the Jalisco block of western Mexico is confined to two conspicuous grabens, where potassic eruptives range from absarokites (48–52% SiO2) and minettes (49–54% SiO2) through basaltic andesites (53–57% SiO2), the most voluminous type, to andesites and their lamprophyric equivalent spessartite (58–62% SiO2); there are no contemporary rhyolitic rocks. This suite has high concentrations of Mg, Cr (<550 ppm) and Ni (<450 ppm) accompanied by large concentrations of K, P, Ba (<4000 ppm) and Sr (<5000 ppm) and elements such as LREE and Zr (<600 ppm). No combination of crystal fractionation and/or crustal contamination can reproduce the compositional range of these magmas, which nevertheless are believed to be genetically related because of their proximity in time and space. Hydrous minerals in the lamprophyres and the typical absence of plagioclase phenocrysts in both basaltic andesites and andesites reflect the relatively high concentrations of water in the magmas, which suppressed the crystallisation of feldspar. Experimental verification of the minimal amounts of water required to reproduce the phenocryst assemblages in selected rocks range from 3.5 to 6%. During ascent in a volcanic conduit, andesitic magma may lose water and consequently precipitate plagioclase, or it may ascend more rapidly, retaining more of its initial water, which stabilises phenocrysts of hornblende at the expense of plagioclase. Our estimates of water concentrations, which are consistent with the various low pressure phenocryst assemblages, will be minimal for the magmas in their source regions, and the process of magmatic dewatering on ascent may be typical in well established volcanic conduits. In accord with the compositions of phenocrystic olivine in the basaltic andesites and the minettes, the values of FeO and Fe2O3 of the bulk lavas and scoriae are demonstrably pristine. As a consequence, there are two characteristic features of the Mascota suite: the high range of relative oxygen fugacities (ΔNNO=1–5) and the high Mg# (MgO/MgO+FeO) that ranges from 0.70 to 0.91 (with only one andesite as low as 0.66). From the evidence of phlogopite phenocrysts, a partial melt involving phlogopite would have a higher Mg# than one from olivine (Fo90) and pyroxene alone. As the Mascota series shows a correlation between K2O and Mg#, we conclude that it was generated by partial fusion of the mantle wedge, with a variable contribution of phlogopite and apatite from veins throughout the lherzolitic assemblage. In conformity with an origin by varying increments of partial fusion of a phlogopite-bearing mantle, all incompatible elements vary linearly with Ti (or K) as if phlogopite (+apatite) in the source dominated their contribution to the partial melts. Fluids from dehydration of the subducting slab presumably deposit hydrous and other minerals in veins in the mantle wedge and also increase its redox state. As the Mascota volcanism occurs in grabens closer to the trench than the main andesite arc, it is concluded that the eruption of these small volumes of hydrous magmas require the tectonically favored ascent paths offered by the extensional grabens to reach the surface from their mantle sources.

Magnesite-bearing inclusion assemblage in natural diamond

A significant mineral assemblage has been found as an inclusion in a natural diamond from the Finsch kimberlite pipe of South Africa: a euhedral rhombohedron-shaped magnesite (MgCO 3) crystal (d ∼ 30 μm) co-exists with several idiomorphic olivine [(Mg 1.86Fe 0.14)SiO 4] grains (d ∼ 80 μm). Many tiny anatase (TiO 2) particles (d ∼ 2–5 μm) and microcrystallites (d < 1 μm) of diamond and disordered graphite are attached to the surface of the magnesite grain. Structural and compositional characterization of the inclusion phases was achieved by micro-Raman spectroscopy and electron microprobe analysis. The occurrence of this syngenetic multiphase inclusion assemblage in a natural diamond provides unambiguous evidence for the existence in the Earth's mantle of magnesite, which has been proposed as a major carbon reservoir in most of the mantle. Both the formation and preservation aspects of the assemblage have been investigated. The mineralogy of the assemblage indicates that carbonated peridotite formed the surrounding petrologic environment. The inclusion assemblage suggests two reactions involving the decomposition of carbonates in mantle peridotite during decompression, which, in part, may explain the paucity of magnesite that has been found in other mantle rocks. The chemical inertness and low compressibility of the host diamond must have been critical to the preservation of this magnesite-bearing assemblage. The incorporation of a pure TiO 2 phase in a peridotitic diamond inclusion and its occurrence in the anatase structural form further emphasize the unusual conditions that allowed both the formation and preservation of this multiphase inclusion. The P-T-fO 2 conditions defined by the inclusion assemblage are represented by the intersection of the graphite-diamond transition curve and the enstatite-magnesite-olivine-diamond buffer. The oxygen fugacity range represented by the inclusion assemblage is below that of the quartz-fayalite-magnetite and CCOCO 2 buffers in the P-T range common to most diamonds. However, the co-existence of diamond, graphite, magnesite, olivine and anatase in this inclusion assemblage represents the highest oxygen fugacity at which olivine could be stable with both diamond and magnesite; that is, the highest oxidation state under which a mantle diamond can be stabilized in a peridotite environment. The diamond-carbonate-silicate co-existence wedge is relatively restricted in P-T-fO 2 space. Therefore, the P-T-fO 2 conditions implied by this and other diamond inclusion assemblages lead to two significant implications for mantle petrology: (1) the conditions for diamond formation are very limited if carbonates are major carbon sources for diamonds; (2) given the low- fO 2 conditions inferred for portions of the Earth's mantle, carbonates may rarely occur in peridotites, and much of the carbon in the mantle may be locked in reduced phases.

Solubility of tungsten in a haplobasaltic melt as a function of temperature and oxygen fugacity

The solubility of tungsten (W) in a haplobasaltic melt has been determined as a function of oxygen fugacity in the temperature range 1300–1500°C using the mechanically assisted equilibrium technique of Dingwell et al. (1994), and at 1600–1700°C by the wire loop method. Quenched samples were analysed for W by using ICP-AES as well as INAA, and sample major element compositions were checked by electron microprobe. W concentrations ranged from 20 ppm to 17 wt%, and the solution of WO 2 in the melt may be described by Henry's Law up to remarkably high concentrations (e.g., 14 wt% at 1500°C). W dissolves in the melt with a quadrivalent (4+) formal oxidation state over the entire range of oxygen fugacity and temperature investigated. The solubility of W decreases strongly with increasing temperature at constant oxygen fugacity. The solubility data have been used to calculate trace distribution coefficients for W between Fe-rich metal and silicate melt, using literature values for the activity coefficient of W in liquid Fe. Comparison of our data with the distribution coefficients for Mo calculated from the analogous Mo solubility data of Holzheid et al. (1994) shows that the ratio of the metal-silicate distribution coefficient D Mo met/sil/ D W met/sil remains very high (~10 3) at all T-fO 2 conditions. However, in the Earth's mantle, Mo is relatively more depleted than W only by a factor of three. The relative abundances of W and Mo in the Earth's mantle cannot, therefore, be explained by core formation from a homogeneously accreted Earth whatever the temperature at which metal/silicate equilibrium may have ocurred might have been. Their abundances may be quantitatively accounted for by a heterogeneous accretion model such as that of O'Neill (1991).

The influence of FeO on the solubilities of cobalt and nickel in silicate melts

The effect of iron on the solubilities of Co and Ni in silicate melts was determined at a wide range of oxygen fugacities (from 10 −8.1 to 10 −12.6 atm) and at temperatures ranging from 1305–1600°C. Four different systems were studied. In system A an FeO-free silicate of anorthite diopside eutectic composition was equilibrated with pure Co- or pure Ni-metal. In system B the same silicate starting material was equilibrated with FeCoNi-alloys. In systems C and D, FeO-containing silicates (basaltic komatiite) were equilibrated with pure Co and pure Ni metal (system C) and with FeCo- and FeNi-alloys (system D). Experiments at temperatures below 1450°C were performed by equilibrating metal loops with silicates; at higher temperature well mixed metal powder and silicates were equilibrated in alumina crucibles. Concentrations of Ni and Co in the silicate and in some of the alloys were determined by neutron activation analysis; most of the alloy compositions were analyzed by electron microprobe. At constant temperature (1400°C) but variable oxygen fugacities, Co, Ni, and Fe metal/silicate partition coefficients calculated from metal solubilities increase with decreasing oxygen fugacities. The extent of the increase suggests that all three metals are dissolved as oxides (CoO, NiO, FeO) in the silicate melt. No evidence for the presence of metallic Co, Ni, or Fe as major species in silicate melt was found within the oxygen fugacity range covered by the experiments, even at oxygen fugacities three log units below the Fe-FeO equilibrium (IW-3). A minor fraction of metallic Ni (below 20%) may be present in the most reducing experiments. No difference in Ni and Co solubility was found for FeO-free and FeO-containing silicates. Depending on temperature all metal/silicate partition coefficients of Co and Ni decrease slightly with increasing temperature along the oxygen fugacity appropriate for core formation, 2.3 log units below the Fe-FeO equilibrium (IW-2.3). The experiments confirm that the relatively high present-day upper mantle concentrations of Co, Ni, and Fe and in particular the chondritic Ni/Co ratio cannot be explained by equilibrium between coreforming metal and silicates of the Earth's mantle in a low pressure environment even at temperatures as high as 4000°C, thus supporting models of inhomogeneous accretion of the Earth. Accumulation of differentiated planetesimals, i.e., with metal cores, as major building blocks of the Earth can also be excluded. In these bodies metal-silicate equilibration would have occurred at low pressures and equilibration of metal cores of accreting planetesimals with the Earth's mantle is unlikely. Homogeneous accretion models may only be possible if equilibration occurred between lower mantle minerals and liquid core forming metal during differentiation of the Earth, and if the relevant Ni and Co partition coefficients are sufficiently high. The Ni and Co partition coefficients between liquid metal and Mg-wüstite and Mg-perovskite are not well known.

Experimental determination of the solubility of Au in silicate melts

We report here new data on the solubility of Au in silicate melts of anorthite-diopside eutectic composition at a wide range of oxygen fugacities, from pure oxygen to 10−8 atm, and at a temperature range of 1300 °C to 1480 °C. Because experiments were done with metal loops at temperatures above the Au-melting temperature, PdAu-metal-alloys had to be used. Pd-solubility data derived from the same set of experiments agree with earlier data obtained from experiments with pure Pd-metal (Borisov et al., 1994a). The results of the present experiments show that Pd-solubilities are by a factor of 2 to 6 higher than Au-solubilities. Both, Au and Pd solubilities decrease with decreasing oxygen fugacity. At oxygen fugacities below the iron-wiistite buffer (IW) Au solubility increases with decreasing fO2 probably reflecting formation of Au-silicides at such reducing conditions. Compared to Pd, Au has higher activity coefficients in Fe-metal and lower solubility in silicate melts. This leads to similar metal-silicate partition coefficients for both elements. At a temperature of 1350 °C and an oxygen fugacity corresponding to IW-2 DAu (met/sil) is about 2.5 · 107 and DPd (met/sil) about 1.6 · 107. Thus similar behavior is expected during metal separation in planetary bodies including core formation in the Earth. The metal/silicate partition coefficient of Ir is, however, by several orders of magnitudes higher (Borisov and Palme, 1995a). Equilibration with chondritic metal will therefore lead to grossly non-chondritic Pd/Ir or Au/Ir ratios in coexisting silicate phases. Chondritic ratios are thus indicative of the presence of unfractionated meteoritic components. Samples from the upper mantle of the Earth, for example, reflect the admixture of a late unfractionated (chondritic) veneer (e.g.,Kimura et al., 1974;Jagoutz et al., 1979).

Native Copper Deposits and the Disposal of High-Level Waste

Highly radioactive, heat-producing waste contains radionuclides and generates daughter products that have half-lives as long as millions of years. This high-level waste must be sealed from the biosphere for a very long period in stable containers stored in deep geological repositories. Copper has been seriously considered as a container material in several waste-disposal projects. Native copper deposits may serve as natural analogues for a chemically stable container-repository system. These deposits formed in terrestrial basalt-conglomerate sequences at temperatures of approximately 100 to 200°C. The hydrothermal systems possessed oxygen fugacities below the stability limit of cuprite and at least 3 log units above the magnetite-hematite buffer at 100° C, and at least 1 log unit above this buffer at 200 ° C. This corresponds to an oxygen fugacity range of ≤10 log units. The fluids were characterized by low total sulfur activities and high Cu-Cl ratios. Similiar conditions can be expected to exist in a th...

The solubility of iridium in silicate melts: New data from experiments with Ir 10Pt 90 alloys

Solubilities of Ir in silicate melts of anorthite-diopside eutectic composition were determined by equilibrating IrPt-loops with silicates at a wide range of oxygen fugacities, from air to about 1.7 log units below the iron-wdstite buffer (IW-1.7) at 1300 and 1480°C. Instead of pure Ir metal, Ir 10Pt 90-alloys were used to avoid formation of tiny Ir nuggets in the silicate glass. Although nugget formation was less severe than in experiments with pure Ir metal, some samples were still contaminated as indicated by sporadically high Ir contents. These samples were subdivided and the separates were analysed again. Samples with high Ir were eliminated. The procedure was repeated several times until compositional uniformity of subsamples was achieved. In a log (Ir solubility) vs. log (F O 2 )-diagram the most reliable data plot along a line with a slope of about 1 4 suggesting Ir 1+ as the formal Ir species in the melt, similar to earlier findings for Pd. The Ir solubility was found to be temperature independent within the range of temperatures used in the present experiments. The results with Ir 10Pt 90-alloys were recalculated to solubilities for pure Ir metal. These solubilities are significantly lower than earlier data (e.g., 1.52 ppb at IW-1 and 1478°C). As a result, calculated metal/silicate melt partition coefficients are extremely high (10 8−10 12 at an oxygen fugacity corresponding to IW-2). Because of the uncertainty introduced by the presence of Ir nuggets it cannot be completely excluded that some of the solubility data are upper limits only. This would, however, only increase Ir metal/silicate partition coefficients and thus make arguments against a global core-mantle equilibrium stronger.

The P-T-fO 2 stability of deerite, Fe 12 2+ Fe 6 3+ [Si12O40](OH)10

New equilibrium experiments have been performed in the 20–27 kbar range to determine the upper thermal stability limit of endmember deerite, Fe 12 2+ Fe 6 3+ [Si12O40](OH)10. In this pressure range, the maximum thermal stability limit is represented by the oxygen-conserving reaction: deerite(De)=9 ferrosilite(Fs)+3 magnetite(Mag)+3 quartz(Qtz)+5 H2O(W) (1). Under the oxygen fugacities of the Ni-NiO buffer the breakdown-reduction reaction: De=12 Fs+2 Mag+5 W+1/2 O2 (10) takes place at lower temperatures (e.g. ΔT=63° at 27 kbar). The experimental brackets can be fitted using thermodynamic data for ferrosilite, magnetite and quartz from Berman (1988) and the following 1 bar, 298 K data for deerite (per gfw): Vo=55.74 J.bar-1, So=1670 J.K-1, ΔH f o =-18334 kJ, α=2.5x10-5K-1, β=-0.18x10-5 bar-1. Using these data in conjunction with literature data on coesite, grunerite, minnesotaite, and greenalite, the P-T stability field of endmember deerite has been calculated for P s=P H 2O. This field is limited by 6 univariant oxygenconserving dehydration curves, from which three have positive dP/dT slopes, the other three negative slopes. The lower pressure end of the stability field of endmember deerite is thus located at an invariant point at 250±70°C and 10+-1.5 kbar. Deerite rich in the endmember can thus appear only in environments with geothermal gradients lower than 10°C/km and at pressures higher than about 10 kbar, which is in agreement with 4 out of 5 independent P-T estimates for known occurrences. The presence of such deerite places good constraints on minimum pressure and maximum temperature conditions. From log f O 2-T diagrams constructed with the same data base at different pressures, it appears that endmember deerite is, at temperatures near those of its upper stability limit, stable only over a narrow range of oxygen fugacities within the magnetite field. With decreasing temperatures, deerite becomes stable towards slightly higher oxygen fugacities but reaches the hematite field only at temperatures more than 200°C lower than the upper stability limit. This practically precludes the coexistence deerite-hematite with near-endmember deerite in natural environments.

Solubility of palladium in silicate melts: Implications for core formation in the Earth

Palladium solubilities in silicate melts of anorthite-diopside-eutectic composition were determined at a wide range of oxygen fugacities, from pure O 2 to f o2 slightly below the iron-wüstite buffer and at temperatures ranging from 1343 to 1472°C. Experiments were performed by heating palladiumloops with silicates inside a gas controlled furnace. Palladium concentrations were determined by neutron activation analysis. Repeated analyses of the glasses after removal of the outer layers and several reversed experiments with initially high Pd in the glass showed that equilibrium was attained in the experiments. At 1350°C concentrations of Pd in silicate melts range from 428 ppm to 1.2 ppm with decreasing palladium content at decreasing oxygen fugacities. The dependence of log Pd on log f o 2 indicates a change in valence of the dominant palladium species in the silicate melt. The data can be explained by the presence of complexes containing Pd 2+ and Pd 0. Alternatively, a good fit is obtained by assuming mixtures of Pd 2+, Pd 1+and Pd 0 in the melt with increasing contributions of the lower valence species at increasingly reducing conditions. Solubilities increase with temperature at fixed oxygen fugacities independent of the absolute fugacity. This is an unexpected result. From the solubility data, metal/silicate partition coefficients were calculated using known activity coefficients of Pd in Fe-metal. Extrapolations were made to higher temperatures and lower oxygen fugacities. A palladium metal/silicate partition coefficient of 1.6 · 10 7 is inferred for 1623 K and IW-2. Extrapolation to 3500 K leads to a partition coefficient of 3.8 · 10 3. From earlier data on Ir solubilités, a metal/silicate partition coefficient of 2 · 10 8 was estimated for the same conditions. The high absolute metal/silicate partition coefficients for Pd and Ir and the large difference between the two partition coefficients are not compatible with a global core/mantle equilibrium as a source of the highly siderophile elements in the Earth mantle. The data favour models invoking the accretion of a late chondritic veneer after core formation without further metal segregation.

High-temperature creep of olivine crystals from four localities

To compare the rheologies of olivine samples from various tectonic settings, an experimental investigation of the high-temperature creep behavior of crystals from China (Fo 89–91), Norway (Fo 93) and the Red Sea (Fo 90) was carried out. The results were examined in the context of the well-documented flow data for olivine from the USA (Fo 89–93). Both the major element and the minor element compositions differed depending on locality. Creep experiments were performed in an 1 atm deformation rig at temperatures from 1200 to 1500°C and compressive stresses from 15 to 175 MPa. Oxygen fugacity was varied over the full stability field of olivine. During deformation along three orientations, the samples remained in contact with orthopyroxene. The mechanical results for samples from all three localities can be summarized as follows: For the [110] c orientation, three rate-controlling creep mechanisms were observed; each can be described by a power-law relationship. Activation energies of 160 kJ mol −1, 1100 kJ mol −1 and 380 kJ mol −1 and oxygen fugacity exponents of 0.37, 0.04 and 0.13 were obtained for the low-, intermediate- and high-temperature creep regimes, respectively. For the [101] c orientation, activation energies of 270 kJ mol −1 and 660 kJ mol −1 and oxygen fugacity exponents of 0.30 and 0.05 were measured for the low- and high-oxygen fugacity creep regimes, respectively. For the [011] c orientation, an activation energy of 510 kJ mol −1 was obtained over the full range of oxygen fugacity; however, oxygen fugacity exponents of 0.02 and 0.31 were determined for the low- and high-oxygen fugacity creep regimes, respectively. The stress exponent is approximately 3.5 for all creep regimes. Within experimental error, the creep results for olivine from all three localities are the same as those reported for San Carlos olivine single crystals. Therefore, moderate differences in forsterite content (from Fo 89 to Fo 93) and in the concentrations of minor elements (such as Ni, Cr and Mn) do not significantly affect the creep behavior of olivine, Consequently, rheological data obtained for olivine samples from specific localities can be reasonably applied to model plastic deformation processes in the upper mantle.

Metal-silicate partitioning of nickel and cobalt: The influence of temperature and oxygen fugacity

We report new metal-silicate melt partitioning experiments for Co and Ni. One atmosphere, gas-mixing (H 2CO 2 or CO-CO 2) experiments were run at 1300, 1425, and 1550°C and at oxygen fugacities between air and near or below the iron-wüstite (IW) buffer. Bulk melt composition, at the diopsideanorthite-forsterite eutectic, was constant in all runs. Metals in these experiments were platinum alloys permitting a large oxygen fugacity range to be investigated while maintaining fixed bulk melt composition. Since activity-composition relations of Pt-Ni and Pt-Co alloys are reasonably well known, we calculated equilibrium constants for the metal-silicate partitioning reactions. Variations of the equilibrium constants as a function of experimental parameters allowed us to monitor the chemical behavior of Ni and Co in the silicate melt. Nickel and Co behave as divalent species in the silicate melt over the range of conditions investigated. However, differences in solubility between low and high oxygen fugacity could not be entirely explained by a simple oxidation-reduction equilibrium constant. Differences in melt solubility were correlated with which gas-mixing system was used, H 2-CO 2 gave larger NiO solubilities at low oxygen fugacity than CO-CO 2. The difference in melt solubility was attributed to minor amounts of melt-dissolved volatile species, perhaps H 2O. Along the IW buffer, Fe metal-silicate partition coefficients for Ni and Co decrease with increasing temperature, but strengths of the temperature dependencies, even in this simple system, appear to vary with experimental conditions. Such differences highlight the danger of extrapolation to conditions far beyond the experimental regime. For example, extrapolation to magma ocean temperatures (3000–4000°C) results in uncertainties larger than the order of magnitude difference in siderophility between Ni and Co.

Application of a solid-electrolyte oxygen fugacity sensor to high-pressure experiments

To monitor the relative oxidation state, a calcia-stabilized zirconia solid-electrolyte oxygen fugacity sensor was used in high-pressure experiments in a multi-anvil type apparatus. The sensor was tested at high pressures using several combinations of common oxygen buffers, and was shown to work up to at least 7 GPa and 1350°C over a wide range of oxygen fugacities between the iron-wüstite (IW) and the magnetite-hematite (MH) oxygen buffers. Rapid responses of the e.m.f. of the sensor to the changes in P- T conditions allow us to carry out in situ oxygen fugacity measurement during high-pressure experiments. The sensor was also used to measure the relative oxygen fugacity prevailing within graphite sample capsules at conditions from 2 to 7 GPa and from 1000 to 1350°C. The redox condition within the graphite capsule is as follows: (1) with increasing temperature, relative to the wüstite-magnetite (WM) oxygen buffer, graphite becomes more reducing at constant pressure, and this causes the stable iron oxide phase in the capsule to change from magnetite to wüstite; (2) this temperature dependence is, however, affected by pressure—below 3 GPa, graphite becomes more oxidizing relative to WM with increasing pressure at constant temperature, whereas above 3 GPa, it becomes less oxidizing relative to WM with increasing pressure, and this results in an enlargement of the field of graphite-wüstite coexistence. Some other applications of the sensor to high-pressure research are briefly discussed, including the in situ detection of phase transformation and pressure calibration at elevated temperature.

Cassiterite solubility, tin speciation, and transport in a magmatic aqueous phase

The solubility of SnO 2 in chloride + or - fluoride-bearing fluids in equilibrium with a number of crystalline, silicate acid buffers, has been measured at 2.0 kbars, as a function of temperature (700 degrees -800 degrees C), oxygen fugacity (QFM to QFM + 1.5 log units), and fluid chemistry. Silicate buffers included sanidine-sillimanite-quartz, analbite-sillimanite-quartz, anorthite-sillimanite-quartz, talc-quartz, and (ortho)enstatite-quartz. Hundreds to tens of thousands of parts per million of tin can be dissolved in dilute, chloride-bearing acid solutions. Fluoride was found to have no significant influence on the solubility of SnO 2 at 700 degrees C. In alkali chloride-bearing, acid solutions, across the entire range of oxygen fugacities examined, tin transport is effected by a complex series of stannous chloride-bearing species. These include simple chloride, mixed ligand (hydroxy chloride), and alkali-bearing chloride and hydroxy chloride stannous complexes (SnOHCl, SnCl 2 , KSnOHCl 2 , KSnCl 3 , K 2 SnCl 4 , K 3 SnCl 5 , K 4 SnCl 6 , and NaSnOHCl 2 , NaSnCl 3 , etc). Apparent equilibrium constants for the predicted aqueous tin species have been calculated and species distribution plots derived for different solution compositions. The solubility of SnO 2 is favored by increasing fluid acidity (m (sub HCl, aq ) ), elevated aqueous alkali/acid and K/Na ratios, and reducing conditions, but appears to be only weakly enhanced by temperature increases in the 500 degrees to 750 degrees C range.

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 behavior of tin in granitoid magmas

The spatial and temporal relationship between tin mineralization and granitoid rocks is well documented. A great deal of work, including mineralogic, geochemical, fluid inclusion, and stable and radiogenic isotope studies, has been undertaken on numerous cassiterite-bearing deposits in order to resolve the problem of their genesis. Despite this exhaustive research, the association between tin mineralization and felsic magmatism remains poorly understood. Experimental investigations into the behavior of tin in melts have been undertaken to evaluate the physical and chemical constraints on the mobilization of tin from magmas.The concentrations of tin in equilibrium with SnO 2 -saturated, synthetic, peraluminous granitoid melts have been examined as a function of temperature (700 degrees -800 degrees C), pressure (2.0-3.0 kbars), oxygen fugacity (from 0.7 log units more reduced to 1.5 log units more oxidized than the quartz-fayalite-magnetite buffer) and granitoid composition (haplogranite, K feldspar-quartz, and albite-quartz). Melts were found to contain between 400 and 2,500 ppm tin. The solvent capacity of the granitoid melts is increased by decreasing oxygen fugacity and by increasing the alkali/alkali + aluminum ratio (ALK/AL), i.e., K 2 O + Na 2 O/K 2 O + Na 2 O + Al 2 O 3 (Naski and Hess, 1985) and the Na/K ratio of a melt. Changes in temperature or pressure do not exert a significant influence over the concentration of tin in melts. Nuclear gamma resonance analysis of haplogranite glasses indicates that both Sn (super +2) (sub (melt)) and Sn (super +4) (sub (melt)) are stable over the range of oxygen fugacities typically attending the crystallization of granitoids. The coordination and likely speciation of Sn (super +2) and Sn (super +4) in melts is discussed. The basicity of a melt (activity of free oxygens, a (sub O (super -2) ) ), which is often strongly correlated with the ALK/AL of a melt (cf. Hess, 1980; Mysen, 1986), is likely to be very important in influencing the acidity of a coexisting fluid phase. Peraluminous melts (low ALK/AL) will generally display low melt basicities and therefore will buffer the acidity of a magmatic aqueous phase at far higher values than a peralkaline melt.These results indicate that granitoid melts derived from common crustal sources will be markedly SnO 2 undersaturated. Consequently, alternative processes must be sought to explain the elevated tin concentrations in many plutons. The complex distribution of tin in any batholith will be largely controlled by the redox conditions accompanying the crystallization of assemblages capable of accommodating tin and the physical processes which can influence their distribution.

Oxygen fugacity stratification of a magma chamber revealed by Mössbauer spectroscopy: evidence from the 1875 Askja eruption, N. Iceland

Mossbauer spectra of volcanic glasses from the 1875 Askja eruption reveals that the dacitemagma chamber was stratified in oxygen fugacity. The eruption was triggered by basaltic magma which was intruded into the base of the chamber. The observed oxidation state falls within the range of oxygen fugacities of H2O/CO2 mixtures at the liquidus temperature of the magma. It is concluded that oxygen fugacity of the chamber was effected by volcanic gases. The source of the gases was the degassing of superheated dacite at the dacite/basalt interface along the bottom of the chamber.

Redox Systematics in Model Glass Compositions from West Valley

ABSTRACTAt a processing temperature of 1150°C for model West Valley glass compositions, the prescribed range of oxygen fugacities needed to achieve an [Fe2+]/[Fe3+] of 0.1 to 0.5 is 10−4 to 10−7 atm. Establishment of the Fe2+-Fe0 equilibrium, resulting in metal precipitation from the melt, occurs at oxygen fugacities lower than 10−11 atm at this temperature. The target processing range as defined by the iron redox ratio is equally valid at both lower and higher temperatures (±100°C). Elevations of the concentrations of redox-active components to I wt% Cr2O3, 1 wt% NiO, 1 wt% CuO, 1 wt% CeO2, and 4 wt% Mn2O3 in the waste glass will not affect the redox limits as established by the iron redox ratio of 0.1 to 0.5; these limits provide sufficiently large margins of safety to assure no stabilization of reduced or oxidized forms of these elements.

ZrO 2 oxygen sensors: An evaluation of behavior at temperatures as low as 300°C

Y 2O 3-stabilized ZrO 2 sensors with simple Pt wire and foil electrodes permit accurate oxygen sensing as low as 300°C over an oxygen fugacity range of as much as 10 log units. These cells are non-Nernstian at least below 600°C, but they have been demonstrated to be reproducible enough to allow iterative calibration correction procedures based on known redox calibrants.