Controlled Oxygen Fugacity Research Articles

New Technique to In situ Control Oxygen Fugacity in Water-free High Pressure System.

Through inserting an oxygen-specific electrolyte disc inside and exerting a DC (direct current) voltage outside between the sample and an oxygen buffer, oxygen can be poured into or pumped from the sample at a fixed temperature and pressure. By measurement of the voltage between the sample and a reference buffer, the oxygen fugacity in the sample is simultaneously monitored. Therefore, the oxygen fugacity in the sample at high temperature and high pressure can be willfully and in situ adjusted and controlled independently of temperature and pressure.

Partial melting below the magmatic arc in the central Andes deduced from geoelectromagnetic field experiments and laboratory data

Magnetotelluric and geomagnetic deep soudings in northern Chile revealed a pronounced high conductivity zone (HCZ). Below the Western Cordillera, which constitutes the present magmatic arc with active volcanism of the South American continental margin, conductivities in the range of 1 S/m are observed. The anomalously high conductivities in a broad depth range from approximately 20 km to at least 60 km, are interpreted in terms of partial melting. Other geophysical observations, such as a zone of low seismic velocities (LVZ) at similar depths, high heat flow values (> 100 mW/m 2) and a pronounced negative anomaly in the residual gravity field, are also considered. Impedance spectroscopic laboratory experiments up to and in the temperature range of partial melting were performed under controlled oxygen fugacities. At sub-solidus temperatures, electrical behavior is described by defect electrons with an activation energy of 1.34 eV and a conductivity of 2.5 mS/m at 900°C. Model calculations using a modified-brick-layer model (MBL) were compared with experimental observations. A good agreement between calculations and experiments is achieved with an electrical resistivity of the melt phase of 7 S/m at 1250°C assuming an activation energy of 1 eV. The same MBL model is used to calculate melt proportions beneath the Western Cordillera. Between 14 and 27 vol.% of interconnected melt are necessary to explain the observed HCZ. The stability of the melt rich crust is explained by a dynamic melting-crystallisation behavior during crustal anatexis and by magma filled dikes.

Grain boundary diffusion in enstatite

We have investigated grain boundary diffusion rates in enstatite by heating single crystals of quartz packed in powdered San Carlos olivine (Mg0.90Fe0.10)2SiO4 at controlled oxygen fugacities in the range 10−5.7 to 10−8.7 atm and temperatures from 1350° to 1450 °C for times from 5 to 100 h at 1 atm total pressure. Following the experiments, the thickness of the coherent polycrystalline reaction rim of pyroxene that had formed between the quartz and olivine was measured using backscatter scanning imaging in the electron microprobe. Quantitative microprobe analysis indicated that the composition of this reaction phase is (Mg0.92Fe0.08)2Si2O6. The rate of growth of the pyroxene increases with increasing temperature, is independent of the oxygen fugacity, and is consistent with a parabolic rate law, indicating that the growth rate is controlled by ionic diffusion through the pyroxene rim. Microstructural observations and platinum marker experiments suggest that the reaction phase is formed at the olivine-pyroxene interface, and is therefore controlled by the diffusion of silicon and oxygen. The parabolic rate constants determined from the experiments were analyzed in terms of the oxide activity gradient across the rim to yield mean effective diffusivities for the rate-limiting ionic species, assuming bulk transport through the pyroxene layer. These effective diffusivities are faster than the lattice diffusivities for the slowest species (silicon) calculated from creep experiments, but slower than measured lattice diffusivities for oxygen in enstatite. Thus, silicon grain boundary diffusion is most likely to be the rate-limiting process in the growth of the pyroxene rims. Also, as oxygen transport through the pyroxene rims must be faster than silicon transport, diffusion of oxygen along the grain boundaries must be faster than through the lattice. The grain boundary diffusivity for silicon in orthopyroxenite is then given by D¯gbSiδ=(3.3±3.0)×10−9f0.0O2e−400±65/RT m3s−1, where the activation energy for diffusion is in kJ/mol, and δ is the grain boundary width in m. Calculated growth rates for enstatite under these conditions are significantly slower than predicted by an extrapolation from similar experiments performed at 1000 °C under high pressure (hydrous) conditions by Yund and Tullis (1992), perhaps due to water-enhancement of diffusion in their experiments.

Thermochemistry of sulfide liquids. I. the system O-S-Fe at 1 bar

Recrystallized globules representing former immiscible sulfide liquids are found in a variety of igneous environments. Relatively little is known about the physical properties and thermochemistry of sulfide liquids, despite their importance in igneous systems. This study presents results of a series of experiments designed to calibrate a thermodynamic model for sulfide liquids in the system O-S-Fe at one atmosphere pressure. Sulfide liquids were equilibrated under controlled oxygen and sulfur fugacities at temperatures between 1100 and 1350 ° C in equilibrium with a silica mineral and a silicate melt. Experiments were quenched in a high-speed double-roller “splat” quencher in order to assure that measured compositions were as close to equilibrium liquid values as possible. Sulfide liquids are not stable in equilibrium with a silica-saturated silicate melt at log10(fO2) > FMQ-1 at 1250 °C and log10(fS2)=−3. Iron content of the sulfide changes little with variations in oxygen and sulfur fugacity at a given temperature. Consequently, oxygen and sulfur contents are inversely correlated in these liquids. Sulfur is present entirely as sulfide. Iron appears to be present in both its ferric and ferrous states. Data from this study were combined with data compiled from the literature to calibrate an asymmetric regular solution thermodynamic mixing model for O-S-Fe liquids. This model reproduces miscibility gaps and data from this study quite well, but exhibits minor but systematic errors at the O-Fe binary. The observed inverse correlation between sulfur and oxygen is reflected in the predicted free-energy surface by a sharp energy valley running along a line of constant Fe content.

Calcium self-diffusion in natural diopside single crystals

We have measured the diffusion coefficient of 44Ca along and perpendicular to c direction in natural Fe-bearing (∼2 at.%) diopside single crystals. Specimens were annealed at temperatures ranging from 1000 to 1250°C, with controlled oxygen fugacity. Diffusion profiles were analysed by Rutherford Back-Scattering Spectrometry (RBS) of α-particles. The diffusion of Ca is isotropic along c and b directions. In addition, the results clearly show two distinct diffusional regimes for the natural diopside, revealed by silica precipitates occurrence in the diopside matrix when T ≥ 1150°C. In this case the oxygen partial pressure pO2 does not influence the self-diffusion coefficient which is characterized by the activation energy E = 396 ± 38 kJ/mol. For T ≤ 1100°C the diffusional process has a lower activation energy (E = 264 ± 33 kJ/mol) and varies as (pO2)−0.14±0.01 in the investigated range (from 10−16 atm to 10−6 atm). These results are consistent with previously reported results on electrical conductivity (Huebner and Voigt, 1988) and high temperature plastic deformation of natural diopside single crystals (Jaoul and Raterron, 1994). According to the point defects model, elaborated by Jaoul and Raterron (1994), the diffusional mechanism of Ca should be essentially interstitial. Furthermore, this mechanism should be the same for different diopside samples with iron content ranging from 0.4 to 2.42 at.%. Indeed, for Ca diffusion in synthetic diopside (0.4 at.% Fe) the activation enthalpy is very similar (281 ± 26 kJ/mol, Dimanov and Ingrin, 1995). On the other hand, the Fe content indoubtly influences the preexponential factor. The present paper reports Ca self-diffusion in diopside as a function of T, pO2 crystallographic orientation, and Fe content.In fact, among all diffusion coefficients previously reported in diopside, but Si, DCa is the lowest. Thereby, Ca should be a kinetically limiting species for diffusion-controlled processes such as plastic deformation and cation exchanges. For instance, Ca self-diffusion controls Ca‐Mg exchanges between pyroxenes. Then, our results could be helpful to better understand the closure behaviour (Dodson, 1973, 1976) of the Clinopyroxene-Orthopyroxene geothermometer.

High-temperature creep in fine-grained polycrystalline CaTiO 3, an analogue material of (Mg, Fe)SiO 3 perovskite

High-temperature creep in fine-grained polycrystalline CaTiO 3 perovskite was studied to get some insight into the rheology of the lower mantle. Polycrystalline specimens with grain-size ranging from 5 to 63 μm were prepared by hot-pressing and subsequently deformed under room pressure or at 300 MPa confining pressure, temperatures of T = 1453–1573K, and stresses of σ = 6–100 MPa under the controlled oxygen fugacity of fO 2 = 0.24 × 10 5−10 −10 Pa. Under all the tested conditions, the strain rate increases nearly linearly with stress and is inversely proportional to the second power of grain-size, a result consistent with the volume diffusion creep model. The comparison of the creep data with the diffusion coefficients and point defect models suggests that the diffusion of titanium or calcium ions through vacancy mechanism may be the rate-controlling process and that impurities play an important role. Both creep and grain-growth kinetics are shown to be significantly enhanced in the temperature range of T = 1515–1525K, presumably related to a structural phase transformation. A deformation mechanism map is constructed based on our data on diffusion creep and on the data on dislocation creep in coarse-grained polycrystals. Conditions under which diffusion creep dominates in CaTiO 3 perovskite are wide which is presumably due to the role of twin boundaries to suppress dislocation creep but not diffusion creep. Assuming that the creep properties of (Mg, Fe)SiO 3 perovskite are similar to CaTiO 3 when compared at the same homologous temperature, the present results suggest that diffusion creep may dominate in a significant portion of the lower mantle and therefore significant rheological softening may occur associated with grain-size reduction after the transformation of spinel to perovskite + magnesiowustite. Also the structural phase transformation from the orthorhombic to higher symmetry structures (such as the tetragonal structure) in perovskite may reduce the creep strength.

Melting of a chromite-bearing harzburgite and generation of boninitic melts at low pressures under controlled oxygen fugacity

Compositions of melts and coexisting olivine, chromite and Ca-poor pyroxene have been determined in the temperature range 1242 °–1405 °C at 1 atm and at 10 −10–10 −5 atm oxygen fugacity (fO 2). Experiments were also conducted at 5 kbar to examine the effect of pressure on the composition of the melts. Starting materials are mixtures of olivine, enstatite, diopside and chromite separated from peridotites and a chromitite. The compositions of melts coexisting with olivine, Ca-poor pyroxene and chromite are relatively silica- and MgO-rich (SiO 2, 53–57; MgO, 11–20 wt.%) and are similar to those of some boninites. Melts formed at 5 kbar from the same starting material are also boninitic, although the silica contents are slightly lower than those formed at 1 atm. The effect of fO 2 on the composition of melts is small; the total iron content in the melts slightly increases with increasing fO 2. The Cr 2O 3 contents in the melts saturated with chromite range from 0.34–1.82 wt.%. The experimental results suggest that boni nitic magmas can be formed even under anhydrous conditions, if magmas are equilibrated with harzburgite at shallow levels. Some boninitic lavas reported in ophiolite complexes may have been formed under such conditions. The composition of chromite coexisting with olivine and Ca-poor pyroxene becomes more iron-rich with increasing fO 2. Magnetite component in chromite increases from 0.4–2.2 mole% at 10 −10 atm fO 2 to 8–10% at 10 −5 atm. fO 2 at temperatures between 1240 and 1350 °C. The Cr (Cr + Al) ratio of chromite varies from 0.52 at about 1250 °C to 0.79 at about 1400 °C. The partition coefficient for the Mg-Fe +2 distribution between coexisting chromite and olivine [( Fe 2+ Mg ) chr ( Fe 2+ Mg ) ol] has a range of 3.25–4.03 in the temperature range of the present experiments and varies as a function of temperature and Cr (Cr + Al) ratio of chromite. The isotherms for 1250 °, 1300 °, 1350 ° and 1400 °C are drawn from the present experimental results. These results combined with those of the previous experiments are applicable to the estimation of temperature and oxygen fugacity of the crystallization of chromite in mafic magmas.

Structure of (VO) 2P 2O 7

The crystal structure of the vanadium (IV) pyrophosphate that shows the remarkable ability to selectively catalyze the oxidation of butane to maleic anhydride has been accurately determined for the first time. This was facilitated by the growth of high quality (VO) 2P 2O 7 crystals under controlled oxygen fugacity. Single crystal X-ray diffraction indicated monoclinic symmetry rather than the orthorhombic symmetry assumed in earlier studies. The space group is P2 1 with a = 7.28 A ̊ , b = 16.588 A ̊ , c = 9.58 A ̊ , β = 89.975 Å, and Z = 4. A final R of 3.1% was obtained. Typical vanadyl environments are found for all eight crystallographically distinct vanadium atoms, and normal bond angles and distances are found for the P 2O 7 groups.

M�ssbauer spectroscopic and x-ray powder diffraction studies of synthetic micas on the join annite KFe3AlSi3O10(OH)2-phlogopite KMg3AlSi3O10(OH)2

Micas of the composition K(Fe3−x Mg x )AlSi3 O10(OH)2 (x=0.6, 1.2, 1.8, 2.4 and 3.0, corresponding to ann80phl20, ann60phl40, ann40phl60, ann20phl80 and ann0phl100) were synthesized hydrothermally under controlled oxygen fugacity conditions. Lattice parameters a 0 and b 0 show a distinct linear decrease with increasing Mg content. With increasing ferric iron content a deviation from this linear trend is observed especially within iron rich samples. The tetrahedral rotation angle α increases smoothly from 0° in annite to 9.1° in phlogopite. Mossbauer spectra show Fe2+ and Fe3+ on the octahedral M1 and M2 sites and partially also Fe3+ on the tetrahedral site. There is a smooth increase of the quadrupole splitting on both the M1 and the M2 site going from annite to phlogopite, probably due to changes in the lattice contribution to the electric field gradient, assuming a positive correlation between quadrupole splitting and distortion. Fe3+ contents, as determined by Mossbauer spectroscopy, versus oxygen fugacity shows that, depending on the composition of the micas, minimum amounts of Fe3+ are present. For ann80phl20 this minimum amount of Fe3+ is about 8% decreasing to about 1–2% Fe3+ for ann20phl80. The molar volume of each solid solution member has been estimated from the determined relations of the molar volume versus % Fe3+ contents, extrapolated back to 0% Fe3+. Plotting these volumes as a function of Xphl shows that negative excess volume occur in the annitephlogopite join, with the maximum deviation from ideality around X phl=0.3. Margules volume parameters have been constrained as: Wv, AnnPhl=0.018±0.016 J/(bar.mol) and Wv, PhlAnn=-0.391±0.025 J(bar.mol) (three site basis).

Kinetics and equilibria of redox systems at temperatures as low as 300°C

ZrO 2 oxygen sensors, gas mixtures, and conventional solid buffers have been used for decades to either control or measure oxygen fugacity (ƒ O 2) at high temperatures. In dry systems below ca. 700°C these techniques were used cautiously, if at all, due to doubt that there was any equilibration at lower temperatures. We have re-investigated these three types of redox systems in a study where each system (two different Y 2O 3ZrO 2 cells, four different gas mixtures, and four different dry solid buffers) was simultaneously cross-checked with the other to temperatures below 300°C and compared to JANAF data, extrapolated down to low temperatures. Steady and reproducible readings were observed down to T ≤ 300°C, from which we infer fast kinetics for all three systems. Specifically, we find equilibration of various CO 2H 2 gas mixtures over the entire temperature range and to much lower temperature than previously predicted. We assign the reactivity (decomposition) of CO 2 at low T to the catalytic action of Pt, whereby chemisorption of H 2 on the platinum surface enhances the reactivity with CO 2. This catalytic reactivity is diminished over time due to a long-term irreversible reaction of Pt with H 2. Subsequent embrittling and aging after prolonged exposure to H 2 explains erroneously high emf readings. Oxygen sensing of ZrO 2 cells is linear in 1/ T-log ƒ O 2 space and Nernstian at high temperatures. However, for cells with a specific and complex trace element chemistry, one may observe a non-Nernstian behavior in the low T range, i.e., below 470° or lower, probably caused by partially blocked O 2− migration, dependent on the H 2 content in the gas mixture. Linearity and reproducibility of this deviation still allows, however, a useable calibration. Solid buffers of the metal-metal oxide type are known to alloy with noble metals and we therefore used AgPd electrodes, for consistency in all studies, including (IW), (IM), (FMQ), and (NNO). Whereas (IW) and (IM) can be used in the temperature range of consideration, (FMQ) and (NNO) react sluggishly. Complex defect structure of (FMQ) and age alteration of Ni surfaces by chemisorption of oxygen and/or AgNi alloying of (NNO) may be the reason. Fast kinetics and successful redox sensing of CO 2H 2 gas mixtures, of ZrO 2 cells and of at least some solid buffers are therefore promising for future research on low- T redox equilibria.

Fractionation of volatile elements in the early solar system: evidence from heating experiments on primitive meteorites

Compared to the average composition of the solar nebula (as determined by solar photospheric abundances), most meteorites and planets are depleted in volatile elements. The only exceptions are CI chondrites and perhaps interstellar dust particles and comets. The depletion of volatile elements is one of the most important fractionation processes that has affected solid matter in the early solar system. Loss of volatile elements may have occurred either by evaporation during heating of material of solar composition or by incomplete condensation in the early solar nebula. To test the evaporation hypothesis heating experiments on chunks of the Allende CV3 chondrite were performed under controlled oxygen fugacities ( fO 2), from air to log fO 2 = −16.5, and at temperatures ranging from 1050 to 1300°C. After removal from the furnace, residues were analyzed by instrumental neutron activation analysis, spark source mass spectrometry and with a C,S analyzer. Experiments in closed quartz vials, with solid fO 2 buffers (FeFeO, etc.), were found to be problematic because of strong preference of quartz for alkalis and Ga. Most experments were therefore made in an open system with fO 2 controlled by COCO 2 gas mixing. The volatile element pattern of heated samples from the Murchison CM meteorite is very similar to that of heated Allende. Since Murchison and Allende are different in chemical composition (in particular in volatile element contents), texture and mineralogy, the results of the heating experiments do not strongly depend upon these parameters. Most experiments were therefore confined to Allende. The results show for most elements a strong dependence of apparent volatilities on fO 2. The elements Os, Re, Au and As are stable at reducing (low fO 2) and volatile under oxidizing conditions (high fO 2). The transition occurs for Re, Au and As between the FeFeO and NiNiO buffers. Volatilization of Os requires oxygen fugacities several orders of magnitude above NiNiO. Alkali elements (Na, K), Cu, Ga and Zn show the apposite behavior, losses in experiments at reducing and retention at oxidizing conditions. The element Mn shows no depletion, even in the 1300°C experiments, while the major fraction of Se is lost in all experiments, reflecting loss of sulfide, which is confirmed by the results of S analyses. Lead was found to be similarly volatile to Se and S. Since the host phases of most of the volatile elements are similar in CM, CV and ordinary chondrites, the results obtained in the present study should be applicable to carbonaceous and ordinary chondrites. Metamorphism at temperatures above 1050°C and at oxygen fugacities above the FeFeO buffer would thus lead to large losses of Au, As and Re, which is observed neither in carbonaceous nor in ordinary chondrites. Ratios of volatile to non-volatile metals, e.g. Au Ir , As Ir and Re Ir ratios, are “normal” in these meteorites (i.e. minor depletion in Au and As and chondritic Re Ir ratios). Apparently, carbonaceous and ordinary chondrites were never exposed to such conditions. The strong depletion of Ga and Zn in experiments at low oxygen fugacities excludes metamorphism of chondritic meteorites under these conditions. This is confirmed by earlier results of Matza, S. D. and Lipschutz, M. E. ( Proc. 8th Lunar Sci. Conf., pp. 161–176, 1977), who found that in reducing conditions losses of Zn are observed at temperatures as low as 700°C and Ga losses at 900°C. A detailed comparison of the volatile element patterns in residues of heating experiments with volatile element patterns of CV3 meteorites shows that samples produced during heating at the IW buffer (iron-wüstite buffer) have volatile element patterns approximately resembling the CV3 pattern, although there are still significant differences, making the origin of such patterns by thermal metamorphism of CI material unlikely. Additional arguments are presented suggesting that heating of meteoritic material to produce fractionated volatile element patterns is unlikely. The observed element patterns are better explained by incomplete condensation, whereby solar gas is dissipated during condensation of volatile elements.

Transient electrical response of San Quintin Dunite as a function of oxygen fugacity changes: Information about charge carriers

The electrical conductivity (σ) of San Quintin dunite (SQD) measured between 950 and 1150°C at controlled oxygen fugacity (fO2) within the olivine stability field shows transients in response to changes in fO2. Such behavior has not been reported previously for measurements made under similar conditions on either olivine single crystals or polycrystalline samples (dunites and lherzolites) in which olivine is the major phase. In general for olivine, an increase in fO2 results in an increase in σ. The transient is manifested as a change in σ from a stable equilibrium value at a specific fO2 to a quickly established subsequent value, the direction of which is opposite that of the final value that will be attained for the fO2 change. This transient may be caused by a changing population of electrons produced by oxygen vacancies, the grain boundaries, or a short‐lived defect. We postulate that the transient is observed in this particular dunite because of its large surface area to volume ratio. The transient is more pronounced at relatively high fO23 and temperatures between 950 and 1100°C. At relatively low fO2s and temperatures higher than 1100°C, the effect is diminished. The Seebeck coefficient (S), at 1200°C, is slightly smaller than that reported for single‐crystal olivine, and at 1100°C is similar to that reported for single crystal olivine.

Transient creep in natural and synthetic, iron-bearing olivine single crystals: Mechanical results and dislocation microstructures

This study examines transient creep of single crystals of both natural and synthetic iron-bearing olivine under uniaxial compression (0.1 MPa confining pressure and loads of 25–30 MPa) at high temperature (1650 K) and controlled oxygen fugacity. Natural samples were obtained from San Carlos, Arizona and synthetic crystals were grown at Lawrence Livermore National Laboratory. Samples were deformed in the [110] c and [101] c orientations, corresponding to the softest and intermediate strength orientations, respectively, as determined from steady-state creep tests. Dislocation microstructures were examined for samples unloaded at 0%, 0.1%, 0.5% and 5% strain. At 0% strain the dislocation density and morphology were lower and less complex in the synthetic olivine than in San Carlos samples. Nearly all microstructures initially present in undeformed material were overwritten by 0.1% strain. With further straining, little change in microstructure occurred to the 5% strain limit tested here. Dislocation microstruc- tures in [101] c samples were consistent with the activation of the [100](001) and [001](100) slip systems. Microstruc- tures formed in [110] c samples matched those expected from activation of a single slip system, [100](010). These slip systems are the same as those identified as responsible for steady-state creep under similar temperature, oxygen fugacity and stress conditions. Both natural and synthetic crystals deformed under constant stress in [101] c showed normal strain hardening with initial strain rates about an order of magnitude higher than those at 5% strain. After an initial high strain rate that was roughly equal in both sample types, the synthetic samples deformed at higher rates than the natural crystals. Crystals in the [110] c orientation deformed in a strikingly different manner. San Carlos olivine showed inverse or strain-softening creep in the first 1–2% strain, after which there is weak evidence suggesting a change to strain-hardening behavior. The temporal strain behavior of synthetic olivine in [110] c is strongly sigmoidal. An inflection point at 1% strain marks the change from inverse to normal transient creep. In the [110] c orientation, steady-state creep was not attained for synthetic samples in the 5% strain limit tested here. These results imply that anisotropic transient creep may exist in the upper mantle, complicating rheological models of post-glacial rebound. The transient creep observed in the [110] c orientation illustrates that the strain- hardening Burgers body model is not universally applicable. The (100)[010] slip system is not always the softest system in the transient regime. Strain may initially be accommodated primarily in the [100](001), [001](100) duplex system. Finally, the transient regime has been shown to extend to several percent strain (in the [110] c. orientation), making transient creep potentially important in modelling initial post-glacial rebound.

Electrical conductivity of polycrystalline olivine containing a silicate glass

The complex impedance of a polycrystalline sample of San Carlos olivine including 5 vol% of a synthetic silicate glass containing sodium, magnesium, and iron has been measured as a function of temperature under controlled oxygen fugacity at a.c. frequencies of 10², 10³, and 104 Hz. Below ∼800°C, the conductivity of the sample is at least 5 to 10 times higher than that of single crystal and polycrystalline San Carlos olivine, respectively, and has an activation energy half that of the single crystal. The sample conductivity increases dramatically as a function of the measurement frequency. Hysteresis and time‐dependent changes in sample conductivity are observed above the liquidus of the glass (∼900°C). The data suggest that the electrical conductivity of partially molten samples is characterized by parallel conduction of the solid and melt phases and is a sensitive indicator of melt distribution as well as the kinetics of melt re‐equilibration.

Dislocation recovery in olivine under deep upper mantle conditions: Implications for creep and diffusion

The kinetics of dislocation recovery in olivine have been studied up to 10 GPa and 1500°C using a multianvil apparatus. Dislocations in the (010)[100] slip system were generated by high‐temperature deformation (at room pressure), and then the specimens were statically annealed at high pressures and temperatures. The rate of dislocation recovery was measured as a function of pressure at controlled oxygen fugacity and oxide activity under anhydrous conditions. Microstructural observations indicate that dislocation recovery occurs through the climb of edge dislocations and hence is controlled by diffusion. A small activation volume (V* = 6 ± 1 cm3/mol) was obtained, which is interpreted to be due to an interstitial mechanism of diffusion for the relevant transport‐limiting species (oxygen or silicon). Combined with the previous observation of a small activation volume for Mg‐Fe diffusion, we conclude that ionic diffusion in the olivine lattice in general has small activation volumes and therefore will not vary significantly with depth in the deep portions of the upper mantle. We have estimated the relative importance of dislocation and diffusion creep in the upper mantle using the present result and the published results on creep parameters (such as stress exponents, grain‐size exponents and activation volume for dislocation creep). The results suggest that the dominant creep mechanism may change from dislocation creep in the shallow upper mantle to diffusion creep in the deep upper mantle.

Reduced chromium in olivine grains from lunar basalt 15555: X-ray Absorption Near Edge Structure (XANES)

The oxidation state of Cr in 200 μm regions within individual lunar olivine and pyroxene grains from lunar basalt 15555 was inferred using X-ray Absorption Near Edge Structure (XANES). Reference materials had previously been studied by optical absorption spectroscopy and included Crbearing borosilicate glasses synthesized under controlled oxygen fugacity and Cr-doped olivines. The energy dependence of XANES spectral features defined by these reference materials indicated that Cr is predominantly divalent in the lunar olivine and trivalent in the pyroxene. These results coupled with the apparent f O 2 -independence of partitioning coefficients for Cr into olivine imply that the source magma was dominated by divalent Cr at the time of olivine crystallization.

Composition of chromite and co‐existing Pt‐Fe alloy at magmatic temperatures

The composition of Pt‐Fe alloy in equilibrium with chromite and basaltic melt has been determined in experiments under controlled oxygen fugacities at temperatures of 1200–1400°C. The composition of the alloy found in the experiments is on average 0.02 higher in Fe/(Fe+Pt) than the alloy composition calculated from the composition of chromite and the fo2 of the experiments using published thermodynamic data and solid solution models of alloy and chromite. The composition of Pt‐Fe alloy in equilibrium with the composition of chromite found associated with alloy in layered intrusions was calculated assuming a temperature of 1200°C and a log fo2 of ‐8.41 (FMQ). The calculated composition of alloy under these conditions was found to be Fe/(Fe+Pt) = 0.20–0.25.

FeMg interdiffusion in single crystal olivine at very high pressure and controlled oxygen fugacity: technological advances and initial data at 7 GPa

The interdiffusion of iron and magnesium in natural single crystals of San Carlos olivine (Fo 90) has been determined using a 2000 ton uniaxial split-sphere apparatus (USSA-2000) and Rutherford back-scattering spectrometry (RBS). Advances in the microanalysis technique made it possible to perform high-pressure experiments for short-duration runs at low temperatures in a controlled chemical and mechanical environment. Olivine crystals were coated with a thin film of fayalite, inserted in an Fe capsule and placed inside an 18 mm pyrophyllite octahedral cell assembly. After experiments at 7 GPa and 900°C for 6 h at pO 2 ≈ 10 −14 bar, the specimens were recovered with very few fractures and chemically unaltered. Analysis of RBS spectra from these crystals yields a value of D FeMg = 10 −13.7 cm 2 s −1. Comparison of this value with previous data at atmospheric pressure suggests that the activation volume for FeMg interdiffusion is V FeMg ∗ ≈ 2.2 ± 0.9 cm 3 mol −1 which, if proven correct, has important implications for electrical conductivity and creep in the upper mantle.

Seismic wave dispersion and attenuation in Åheim dunite: an experimental study

A machine has been developed which provides for the study of both shear modulus dispersion and internal friction in geological materials through the observation of forced torsional oscillations of low frequency (10-1000 mHz) and strain amplitude (< 10-6) under conditions of high pressure (to 300 MPa) and temperature (to > 1000°C). The eventual goal is an understanding of the dispersion and attenuation of seismic shear waves in the Earth's upper mantle. Measurements have been made on cylindrical specimens of an olivine-rich rock from Åheim, Norway which } contains, in addition to olivine, about 10 per cent pyroxene and 5-10 per cent of hydrous silicate phases dominantly clinochlore, serpentine and talc. In order to separate aspects of the mechanical behaviour associated with the olivine aggregate from those attributable to the hydrous phases and/or their dehydration products, the specimens were either previously fired under controlled oxygen fugacity at 1200 °C for 24 hours in order to effect complete dehydration within the olivine stability field, or simply oven-dried at 110 °C. Linearity of the mechanical behaviour has been demonstrated by the amplitude independence of the results for the strain amplitude range 10-8-10-6, and by the quantitative consistency between the observed modulus dispersion and that calculated from the measured internal friction through the Kramers-Krönig relations of linear theory. Both the pressure dependence at room temperature, and the temperature dependence at 300 MPa, of the shear modulus and internal friction have been investigated. Much of the variation of the shear modulus is attributed to changes of crack porosity associated with changes in pressure and temperature and with in situ dehydration. Temporal evolution of Q-1 towards a lower asymptotic value over periods of hours of exposure to given conditions of high pressure and temperature is tentatively attributed to the gradual diminution of enhanced anelastic relaxation associated with regions of decaying stress concentration at asperities on cracks and/or grain boundaries. At the highest pressures and temperatures achieved in this study, 300 MPa and 1000 °C, marked dispersion of the shear modulus, amounting to 5 per cent between periods of 3 and 100 s, and concomitant strong internal friction, varying with oscillation period To approximately as 0.011To1/6, are observed. The energy dissipation appears to be concentrated within, rather than at the boundaries between, the olivine grains. These results provide the clearest indication yet that solid-state, probably intragranular, anelastic relaxation in ultramafic rocks gives rise to losses comparable with those observed seismologically in the Earth's upper mantle, although the mechanistic basis for the observed anelasticity remains to be established.

Oxygen Fugacity Control in Nonflowing Atmospheres: II, Theoretical Model

Experimentally observed deviations from ideal isothermal behavior of solid electrolyte sensors are explained on the basis of gas‐phase diffusion in the furnace atmosphere. The presence of small amounts of CO/CO2 or other oxygenbearing gas species is essential to the theoretical explanation of these effects. Solution of the basic transport equations for limiting conditions appropriate to the control of a stagnant atmosphere by a sensor/pump combination indicates that stable fugacity fronts may exist, separating regions of relatively uniform oxygen fugacity which differ from each other by many orders of magnitude. The position of the front depends on the oxygen fugacity difference between the two regions of the furnace and can be moved through the furnace by electrochemically pumping oxygen from one end of the furnace. The relative output of two sensors separated by some finite distance in the direction in the direction of oxygen transport will depend on the position of this front, and as the front passes between the sensors a plateau in the curve of one sensor emf versus the other is predicted.