Range Of Oxygen Fugacities Research Articles

Thermodynamic properties of alloys of gold-74/palladium-26 with variable amounts of iron and the use of Au-Pd-Fe alloys as containers for experimental petrology

Iron oxide-alloy equilibration experiments were conducted in H_2-CO_2 gas mixtures at 1 atm and 1125–1240 °C using strips of Au_(74)Pd_(26) (wt%) and produced Au-Pd-Fe alloys with 0.03–13 wt% iron. A thermodynamic calibration for the mixing of Au_(74)Pd_(26) with iron using an asymmetric regular solution leads to WG_(-Fe) = –45.0 ± 1.8 kJ/mol and WG_(-AuPd) = +19.5 ± 7.7 kJ/mol (1σ). Internal oxidation of iron was observed in a reversal experiment, suggesting that oxygen can be transferred across capsule boundaries during high-temperature experiments. This thermodynamic calibration is applicable to a wide range of oxygen fugacities and iron activities relevant to petrological and metallurgical applications at 1 atm and, as previous studies suggest excess volumes in this system are small, it can also be used to predict Fe activities in experiments at elevated pressure (up to 3 GPa). By pre-doping Au-Pd capsules to match Fe activities expected for the sample during an experiment, it is possible to maintain samples with little to no loss of iron. Pre-saturation of the capsule also provides a method for controlling the oxygen fugacity of samples if no formal oxygen buffer is available.

An experimental study of OH solubility in rutile at 500-900 C, 0.5-2 GPa, and a range of oxygen fugacities

The solubility of OH in pure synthetic rutile was experimentally constrained at 0.5-2.0 GPa and 500-900 °C, in equilibrium with four oxygen fugacity (ƒO₂) buffering mineral assemblages: hematite-magnetite (HM), nickel-nickel oxide (NNO), cobalt-cobalt oxide (CCO), and iron-wüstite (IW). The hydroxyl concentration ([OH], in parts per million H2O by weight) of equilibrated rutile crystals was characterized by FTIR spectroscopy. Measurements at 1 GPa at individual ƒO₂ buffers demonstrate that [OH] in rutile depends strongly on temperature: at HM, [OH] increases from 48 to 267 ppm as temperature rises from 500 to 900 °C, whereas at NNO, [OH] increases from 108 to 956 ppm over the same temperature range. The [OH] in rutile also increases strongly with decreasing ƒO₂ at any pressure and temperature, and exhibits a slight, linear, positive dependence on pressure at a given temperature and ƒO₂. The observed systematic dependences on pressure, temperature, and ƒO₂ indicate that hydrogen substitutes into rutile as hydroxyl, (OH), via forward progress of the reaction Ti4+O2 + ½H2O = Ti3+O(OH) + ¼O2. Our measured [OH] values are significantly greater than those determined in previous studies on finer-grained, polycrystalline rutile, which likely suffered diffusive loss of H during quenching. This is supported by our observation of narrow, OH-depleted rims on otherwise high-OH run products, pointing to minor but important diffusive H loss from crystal rims during quenching. Fitting of isothermal variations in composition with ƒO₂ at 1 GPa and temperature indicates nearly ideal, multi-site mixing of the TiO2-TiOOH solid solution. A fit to the entire data set suggests standard volume, enthalpy, and entropy of the hydration reaction of, respectively, 1.90 ± 0.48 cm3/mol, 219.3 ± 1.3 kJ/mol, and 19.9 ± 1.4 J/(mol∙K) (1σ uncertainty). These constraints form the basis for use of [OH] in rutile as a thermobarometer and oxybarometer in experimental and natural systems. The moderate to high [OH] in nominally anhydrous rutile at all investigated temperatures, pressures, and ƒO₂ values imply that Ti3+ may be higher than previously suspected in some terrestrial geologic settings.

The effect of fO2 on the partitioning and valence of V and Cr in garnet/melt pairs and the relation to terrestrial mantle V and Cr content

Chromium and vanadium are stable in multiple valence states in natural systems, and their distribution between garnet and silicate melt is not well understood. Here, the partitioning and valence state of V and Cr in experimental garnet/melt pairs have been studied at 1.8-3.0 GPa, with variable oxygen fugacity between IW-1.66 and the Ru-RuO2 (IW+9.36) buffer. In addition, the valence state of V and Cr has been measured in several high-pressure (majoritic garnet up to 20 GPa) experimental garnets, some natural megacrystic garnets from the western United States, and a suite of mantle garnets from South Africa. The results show that Cr remains in trivalent in garnet across a wide range of oxygen fugacities. Vanadium, on the other hand, exhibits variable valence state from 2.5 to 3.7 in the garnets and from 3.0 to 4.0 in the glasses. The valence state of V is always greater in the glass than in the garnet. Moreover, the garnet/melt partition coefficient, D(V), is highest when V is trivalent, at the most reduced conditions investigated (IW-1.66 to FMQ). The V2.5+ measured in high P-T experimental garnets is consistent with the reduced nature of those metal-bearing systems. The low V valence state measured in natural megacrystic garnets is consistent with fO₂ close to the IW buffer, overlapping the range of fo₂ measured independently by Fe2+/Fe3+ techniques on similar samples. However, the valence state of V measured in a suite of mantle garnets from South Africa is constant across a 3 log fo₂ unit range (FMQ-1.8 to FMQ-4.5), suggesting that the valence state of V is controlled by the crystal chemistry of the garnets rather than fo₂ variations. The compatibility of V and Cr in garnets and other deep mantle silicates indicates that the depletion of these elements in the Earth’s primitive upper mantle could be due to partitioning into lower mantle phases as well as into metal.

Complex Phenocryst Textures and Zoning Patterns in Andesites and Dacites: Evidence of Degassing-Induced Rapid Crystallization?

A detailed petrological study is presented of seven phenocryst-poor andesites and dacites erupted from short-lived, monogenetic, periph- eral vents in the Mexican volcanic arc. Despite low phenocryst (þ microphenocryst) abundances (55%), these magmas were each multiply saturated with five to seven mineral phases (plagio- clase þ orthopyroxene þ titanomagnetite þ apatite þ ilmeniteclino- pyroxenehornblendebiotite). Groundmass textures range from nearly complete glass to nearly complete microlite crystallization. Eruptive temperatures, on the basis of Fe^Ti two-oxide thermom- etry, range from 872 (� 13) to 1085 (� 16) 8C for the andesites and 760 (� 18) to 957 (� 19)8C for the dacites. Corresponding oxygen fugacities range from � 0· 9t oþ0·6 log units relative to the Ni^NiO buffer.With temperature known, the plagioclase^liquid hy- grometer was applied to the sparse plagioclase phenocrysts in each sample; the highest calculated water concentrations range from 6·4 to 2· 8w t %H 2O for the andesites and from 8· 3t o 4·1wt % H2O for the dacites. These results require that the magmas were fluid-saturated at depths 4 9a nd412 km, respectively, for the most hydrous andesite and dacite. The plagioclase population in each sample spans a wide range in composition (� 34 mol % An), which can be attributed to the effects of changing melt water concen- tration owing to degassing during magma ascent.The orthopyroxene population in each sample also spans a wide range in composition (� 17% Mg-number), and it is proposed on the basis of phase equi- librium experiments from the literature and data from the JANAF thermochemical tables that this also reflects variable melt water con- centrations, with higher water leading to more Fe-rich orthopyroxene. Degassing leads to a progressive increase in melt viscosity, and the phenocrysts crystallized over an interval of � 2·4^4· 3l og 10 Pa s in these samples. The phenocrysts often display diffusion-limited growth textures (e.g. skeletal and hopper crystals, large interior melt hollows) consistent with large undercoolings caused by degassing, which has produced a complicated petrography in these crystal-poor hydrous magmas, including reverse and patchy zoning in plagioclase and pyroxene.

CHEMICAL AND ISOTOPIC EVOLUTION OF THE ANORTHOSITIC PLUTONS OF THE LARAMIE ANORTHOSITE COMPLEX: EXPLANATIONS FOR VARIATIONS IN SILICA ACTIVITY AND OXYGEN FUGACITY OF MASSIF ANORTHOSITES

The Laramie anorthosite complex includes three anorthositic plutons: the Poe Mountain, Chugwater, and Snow Creek intrusions. The Poe Mountain and Chugwater bodies exhibit mappable magmatic stratigraphy that records fractionation processes in a magma chamber at or near the present level of exposure followed by doming and expulsion of residual liquid. The Snow Creek pluton is more poorly exposed, although layering is locally observed, and there is geochemical evidence for removal of interstitial liquid in the eastern portion of the body. The three plutons were emplaced in close succession. The U–Pb zircon and baddeleyite geochronology of anorthosite from the Chugwater and Snow Creek plutons, and a monzodiorite dike, indicates that the Chugwater is the oldest of the bodies, with a weighted average 207 Pb/ 206 Pb age of five samples of 1435.5 ± 0.3 Ma. The Snow Creek anorthosite intrudes the previously dated 1434 Ma Poe Mountain anorthosite and is, in turn, cut by the 1432.8 ± 2.4 Ma monzodiorite dike. The LAC and associated granites of the Sherman batholith suite were emplaced over a period no greater than 12 million years, and possibly in as little as three million years. Despite their similar ages, each of the LAC anorthosite plutons displays a distinctive assemblage of minerals. The Poe Mountain anorthosite is characterized by olivine, augite, low Ca-pyroxene, ilmenite and magnetite, and plagioclase in the range An 43–53 . The Chugwater anorthosite generally contains no olivine but is composed of augite, low Ca-pyroxene, ilmenite and magnetite, and iridescent plagioclase in the range An 50–56 . The Snow Creek anorthosite is characterized by a lack of olivine and magnetite and the presence of iridescent plagioclase of composition An 47 to An 56 ; it is commonly quartz-bearing. These assemblages record a range of silica activities and oxygen fugacities. Higher activities of silica and fugacities of oxygen are correlated with greater amounts of crustal assimilation, as indicated by Nd and Sr isotopic compositions and the presence of inherited components in zircon. We demonstrate that crustal assimilation can produce a range of mineral assemblages and Sr–Nd isotopic compositions in LAC anorthosites crystallized from common mantle-derived parental magmas, and that assimilation of crust is likely an important control on the compositional variations documented in other Proterozoic massif anorthosites.

Crystallization, melt inclusion, and redox history of a Martian meteorite: Olivine-phyric shergottite Larkman Nunatak 06319

The Larkman Nunatak (LAR) 06319 olivine-phyric shergottite is composed of zoned megacrysts of olivine (Fo 76–55 from core to rim), pyroxene (from core to rim En 70Fs 25Wo 5, En 50Fs 25Wo 25, and En 45Fs 45Wo 10), and Cr-rich spinel in a matrix of maskelynite (An 52Ab 45), pyroxene (En 30–40Fs 40–55Wo 10–25,), olivine (Fo 50), Fe–Ti oxides, sulfides, phosphates, Si-rich glass, and baddeleyite. LAR 06319 experienced equilibration shock pressures of 30–35 GPa based on the presence of localized shock melts, mechanical deformation of olivine and pyroxene, and complete transformation of plagioclase to maskelynite with no relict birefringence. The various phases and textures of this picritic basalt can be explained by closed system differentiation of a shergottitic melt. Recalculated parent melt compositions obtained from melt inclusions located in the core of the olivine megacrysts (Fo >72) resemble those of other shergottite parent melts and whole-rock compositions, albeit with a lower Ca content. These compositions were used in the MELTS software to reproduce the crystallization sequence. Four types of spinel and two types of ilmenite reflect changes in oxygen fugacity during igneous differentiation. Detailed oxybarometry using olivine-pyroxene-spinel and ilmenite-titanomagnetite assemblages indicates initial crystallization of the megacrysts at 2 log units below the Fayalite-Magnetite-Quartz buffer (FMQ – 2), followed by crystallization of the groundmass over a range of FMQ – 1 to FMQ + 0.3. Variation is nearly continuous throughout the differentiation sequence. LAR 06319 is the first member of the enriched shergottite subgroup whose bulk composition, and that of melt inclusions in its most primitive olivines, approximates that of the parental melt. The study of this picritic basalt indicates that oxidation of more than two log units of FMQ can occur during magmatic fractional crystallization and ascent. Some part of the wide range of oxygen fugacities recorded in shergottites may consequently be due to this process. The relatively reduced conditions at the beginning of the crystallization sequence of LAR 06319 may imply that the enriched shergottite mantle reservoir is slightly more reduced than previously thought. As a result, the total range of Martian mantle oxygen fugacities is probably limited to FMQ − 4 to − 2. This narrow range could have been generated during the slow crystallization of a magma ocean, a process favored to explain the origin of shergottite mantle reservoirs.

Identification and measurement of neutron-absorbing elements on Mercury’s surface

MESSENGER Neutron Spectrometer (NS) observations of cosmic-ray-generated thermal neutrons provide the first direct measurements of Mercury’s surface elemental composition. Specifically, we show that Mercury’s surface is enriched in neutron-absorbing elements and has a measured macroscopic neutron-absorption cross section of 45–81 × 10 −4 cm 2/g, a range similar to the neutron absorption of lunar basalts from Mare Crisium. The expected neutron-absorbing elements are Fe and Ti, with possible trace amounts of Gd and Sm. Fe and Ti, in particular, are important for understanding Mercury’s formation and how its surface may have changed over time through magmatic processes. With neutron Doppler filtering – a neutron energy separation technique based on spacecraft velocity – we demonstrate that Mercury’s surface composition cannot be matched by prior models, which have characteristically low abundances of Fe, Ti, Gd, and Sm. While neutron spectroscopy alone cannot separate the relative contributions of individual neutron-absorbing elements, these results provide strong new constraints on the nature of Mercury’s surface materials. For example, if all the measured neutron absorption were due to the presence of an Fe–Ti oxide and that oxide were ilmenite, then Mercury’s surface would have an ilmenite content of 7–18 wt.%. This result is in general agreement with the inference from color imaging and visible-near-infrared spectroscopy that Mercury’s overall low reflectance is consistent with a surface composition that is enriched in Fe–Ti oxides. The incorporation of substantial Fe and Ti in oxides would imply that the oxygen fugacity of basalts on Mercury is at the upper range of oxygen fugacities inferred for basalts on the Moon.

Study of oxygen fugacity influence on redox state of iron in granitoidic melts

A series of experiments has been carried out on two granitoid melt compositions under the wide range of oxygen fugacities. Room temperature 57Fe Mössbauer spectroscopy has been used to study the structural and oxidation state of Fe in granitoid composition glasses. Samples were produced in a series of melting experiments at temperatures from 1120 to 1420 °C and oxygen fugacities from 10−0.7 (air) to 10−12 (CCO buffer) bars. The analysis has shown that Fe3+/Fe2+ ratio in the melt considerably decreases with fO2 decreasing and/or temperature increasing. Dependence of Fe3+/Fe2+ ratio on fO2 is linear in logarithmic scale with slope 0.18–0.24. Ferrous iron occupies octahedral and probably five-fold positions and is regarded as a network-modifier. Ferric iron coordination depends on iron redox state. Its structural role changes from network-modifier to network-former.

Effect of oxygen fugacity and water on phase equilibria of a hydrous tholeiitic basalt

The influence of oxygen fugacity and water on phase equilibria and the link between redox conditions and water activity were investigated experimentally using a primitive tholeiitic basalt composition relevant to the ocean crust. The crystallization experiments were performed in internally heated pressure vessels at 200 MPa in the temperature range 940–1,220°C. The oxygen fugacity was measured using the H2-membrane technique. To study the effect of oxygen fugacity, three sets of experiments with different hydrogen fugacities were performed, showing systematic effects on the phase relations and compositions. In each experimental series, the water content of the system was varied from nominally dry to water-saturated conditions, causing a range of oxygen fugacities varying by ~3 log units per series. The range in oxygen fugacity investigated spans ~7 log units. Systematic effects of oxygen fugacity on the stability and composition of the mafic silicate phases, Cr–spinel and Fe–Ti oxides, under varying water contents were recorded. The Mg# of the melt, and therefore also the Mg# of olivine and clinopyroxene, changed systematically as a function of oxygen fugacity. An example of the link between oxygen fugacity and water activity under hydrogen-buffered conditions is the change in the crystallization sequence (olivine and Cr–spinel) due to a change in the oxygen fugacity caused by an increase in the water activity. The stability of magnetite is restricted to highly oxidizing conditions. The absence of magnetite in most of the experiments allows the determination of differentiation trends as a function of oxygen fugacity and water content, demonstrating that in an oxide-free crystallization sequence, water systematically affects the differentiation trend, while oxygen fugacity seems to have a negligible effect.

The Crystal/Melt Partitioning of V during Mantle Melting as a Function of Oxygen Fugacity Compared with some other Elements (Al, P, Ca, Sc, Ti, Cr, Fe, Ga, Y, Zr and Nb)

Vanadium exists in multiple valence states in silicate and oxide systems, namely V(2+), V(3+), V(4+) and V(5+). This special characteristic has been exploited in several ways to estimate the redox conditions of high-temperature planetary processes, such as partial melting and core formation. However, the use of V as a universal redox indicator (i.e. suitable for the entire range of oxygen fugacities found in the inner Solar System) requires precise knowledge of the partitioning of all of its several valence states. Here we report the results of a series of 1 atm (1300 degrees C) and high-pressure (1-3 GPa, 1315-1450 degrees C) experiments carried out over a range of redox conditions sufficiently large (from QFM-13 center dot 3 to QFM+11 center dot 4, where QFM is the quartz-fayalite-magnetite oxygen buffer) to constrain the full panoply of V chemical behaviour. Partition coefficients between the major upper-mantle minerals (olivine, clinopyroxene, orthopyroxene, spinel and garnet) and silicate melt were precisely measured with laser ablation inductively coupled plasma mass spectrometry for V and other selected heterovalent and homovalent elements (Al, P, Ca, Sc, Ti, Cr, Fe, Ga, Y, Zr and Nb). The particularly large range of redox conditions investigated here enabled concentrations of V(2+) and V(5+) to be constrained along with V(3+) and V(4+), allowing modelling of the change in bulk V partition coefficient with oxygen fugacity to be performed in a robust thermodynamic fashion. As in previous studies, we found a trend of increasing incompatibility from V(3+) to V(4+) to V(5+) for all phases. Partition coefficients for V(2+) can be either higher or lower than for V(3+), depending on the phase. Additionally, we found evidence for changes in the oxidation state of Cr (Cr(2+) Cr(3+)), Fe (Fe(2+) Fe(3+)) and Ti (Ti(3+) Ti(4+)). There is also indication that P may behave as a heterovalent element, occurring as a trivalent cation at very reducing conditions (P(3+) P(5+)). For all other trace elements, the oxidation state remained constant. The data presented here can be used to implement methods of estimating the redox state of mantle and mantle-derived planetary samples from bulk-rock concentration and crystal/melt partitioning with only minor extrapolation and bias, allowing better precision and accuracy than previously possible. The method of estimating the redox conditions of basalt suites from bulk-rock V concentrations relative to homovalent elements with similar compatibility (e.g. V/Sc and V/Ga) was tested using databases for mid-ocean ridge, ocean island and island arc basalts. Within the many assumptions involved in forward trace-element modelling (e.g. degree and style of melting, source composition, constant partition coefficients), we show that the redox states of the source regions of these different types of terrestrial basalts are indistinguishable from each other, having relative oxygen fugacities at similar to QFM +/- 1. The fact that arc magmas have higher Fe(3+)/Fe(2+) ratios than other types of basalts, making them appear to be more oxidized, may be due to late-stage processes rather than derivation from a more oxidized part of the asthenosphere.

High-precision determination of iron oxidation state in silicate glasses using XANES

Fe K-edge X-ray absorption near-edge structure (XANES) and Mössbauer spectra were collected on natural basaltic glasses equilibrated over a range of oxygen fugacity (QFM − 3.5 to QFM + 4.5). The basalt compositions and fO 2 conditions were chosen to bracket the natural range of redox conditions expected for basalts from mid-ocean ridge, ocean island, back-arc basin, and arc settings, in order to develop a high-precision calibration for the determination of Fe 3+/∑Fe in natural basalts. The pre-edge centroid energy, corresponding to the 1s → 3d transition, was determined to be the most robust proxy for Fe oxidation state, affording significant advantages compared to the use of other spectral features. A second-order polynomial models the correlation between the centroid and Fe 3+/∑Fe, yielding a precision of ± 0.0045 in Fe 3+/∑Fe for glasses with Fe 3+/∑Fe > 8%, which is comparable to the precision of wet chemistry. This high precision relies on a Si (311) monochromator to better define the Fe 2+ and Fe 3+ transitions, accurate and robust modeling of the pre-edge feature, dense fO 2-coverage and compositional appropriateness of reference glasses, and application of a non-linear drift correction. Through re-analysis of the reference glasses across three synchrotron beam sessions, we show that the quoted precision can be achieved (i.e., analyses are reproducible) across multiple synchrotron beam sessions, even when spectral collection conditions (detector parameters or sample geometry) change. Rhyolitic glasses were also analyzed and yield a higher centroid energy at a given Fe 3+/∑Fe than basalts, implying that major variations in melt structure affect the relationship between centroid position and Fe 3+/∑Fe, and that separate calibrations are needed for the determination of oxidation state in basalts and rhyolites.

Newania carbonatites, Western India: example of mantle derived magnesium carbonatites

The key mineralogical features of the Newania carbonatites, that illustrate their derivation from primary mantle melts (Gruau et al. Terra Nova, Abstract Suppl 1:336, 1995; Viladkar Petrology 6(3):272–283, 1998; Basu and Murty Abstracts of Goldschmidt Conference A40, 2006), are the presence of magnesite, graphite and Cr-rich magnetite. Magnesite is an early crystallizing phase. Cr-rich magnetite and graphite coexist with carbonatite minerals and precipitated from carbonate magma. Graphite, as well as gaseous CO2 and carbonate minerals such as dolomite and magnesite, can be stable in peridotite mantle. Coexistence of these minerals is controlled by fO 2 and PT-conditions. Mineral geothermometers for the Newania carbonatite give temperatures from 463 to 950°C. The parental source for Newania carbonatites was characterized by a relatively high log (fHF/fH2O) level which increased during the crystallization history of Newania. The estimated oxygen fugacity (for ilmenite–magnetite pairs) varies from −1.5 to +3.5 (log-bar unit deviation from FMQ buffer), which is supported by the presence of Fe-columbite, and the composition of phlogopite, amphibole and pyroxene that have an elevated concentration of Fe3+. However, the oxygen fugacity range represented by co-existing early-crystallized graphite and magnesite is below that of the FMQ buffer and lies on the CCO buffer.

Effect of pressure, temperature, and oxygen fugacity on the metal-silicate partitioning of Te, Se, and S: Implications for earth differentiation

We have measured liquid Fe metal–liquid silicate partitioning ( D i) of tellurium, selenium, and sulfur over a range of pressure, temperature, and oxygen fugacity (1–19 GPa, 2023–2693 K, fO 2 −0.4 to −5.5 log units relative to the iron-wüstite buffer) to better assess the role of metallic melts in fractionating these elements during mantle melting and early Earth evolution. We find that metal-silicate partitioning of all three elements decreases with falling FeO activity in the silicate melt, and that the addition of 5–10 wt% S in the metal phase results in a 3-fold enhancement of both D Te and D Se. In general, Te, Se, and S all become more siderophile with increasing pressure, and less siderophile with increasing temperature, in agreement with previous work. In all sulfur-bearing experiments, D Te is greater than D Se or D S, with the latter two being similar over a range of P and T. Parameterized results are used to estimate metal-silicate partitioning at the base of a magma ocean which deepens as accretion progresses, with the equilibration temperature fixed at the peridotite liquidus. We show that during accretion, Te behaves like a highly siderophile element, with expected core/mantle partitioning of >10 5, in contrast to the observed core/mantle ratio of ∼100. Less extreme differences are observed for Se and S, which yielded core/mantle partitioning 100- to 10 times higher, respectively, than the observed value. Addition of ∼0.5 wt% of a meteorite component (H, EH or EL ordinary chondrite) is sufficient to raise mantle abundances to their current level and erase the original interelement fractionation of metal-silicate equilibrium.

Geochronological and geochemical constraints on formation of the Tongling metal deposits, middle Yangtze metallogenic belt, east‐central China

The Tongling district is one of the most important non‐ferrous metal producers in China. The origin of Cu–Au deposits in the region is closely related to Late Mesozoic intermediate intrusions, which are mainly high‐K calc‐alkaline and shoshonitic series. Geochemical characteristics indicate that these granitic rocks are mixtures of more than two compositional end‐members, i.e. mantle‐derived melts and crust components incorporated through assimilation. Three important magmatic intrusions related to the Cu–Au deposits in the Tongling region – the Jiguanshi quartz monzodiorite, the Xishizishan quartz diorite and the Miaojia diorite porphyry – were selected for this study. Zircon U–Pb dating by LA ICP‐MS yielded two groups of ages (∼130–132 and 138–140 Ma) for these intrusions. Pyrite Re–Os age for the Xinqiao Cu–Fe–S deposit in the Tongling region is 126±11 Ma. Trace elements of zircon grains show that the earlier Cu–Au mineralization event was associated with adakitic rocks characterized by high positive Ce anomalies, produced at an elevated oxygen fugacity range. In contrast, later iron‐sulphur mineralization was closely related to low positive Ce anomalies, reflecting low oxygen fugacities. Considering that Pacific lithospheric subduction was the dominant factor that controlled major tectonic evolution in eastern China during the Early Cretaceous, the geochemical characteristics of these coeval ore‐forming intermediate intrusive rocks in the Tongling district were likely the result of Pacific plate underflow.

The behavior of Co and Ni in olivine in planetary basalts: An experimental investigation

Cobalt and nickel in olivine from lunar, martian, and terrestrial basalts show systematic differences that appear to be related to planetary parentage. We present results from a series of 1 atm experiments designed to examine the partitioning of Co and Ni in olivine over a range of conditions relevant to planetary basalts. Our results confirm the suggestions of previous workers that the partitioning of Co is such that an increase in D Cool with crystallization is balanced by a decrease in Co concentration in the melt, resulting in constant Co in olivine, either within olivine showing otherwise normal igneous zoning, or within a suite of basalts related by fractionation. It is evident that the partitioning of Co and Ni into planetary olivine is influenced by such intensive variables as temperature, oxygen fugacity, and composition. Although separating these effects is not possible, our data demonstrate that over the range of oxygen fugacity of lunar, martian, and terrestrial basalts, oxygen fugacity has no significant effect on Co and Ni partitioning into olivine. We propose that an increase in melt polymerization with decreasing temperature, coupled with the preference of Co and Ni for the octahedral sites in olivine, is the mechanism by which D Cool and D Niol increase with crystallization. With our experimental results as a baseline, we interpret data from Co and Ni in olivine from lunar, martian, and terrestrial basalts and conclude that the cause of increases in Co concentrations in olivine, either core-to-rim or within a co-genetic suite and observed primarily in martian and terrestrial basalts, is enrichment of the melt as crystallization progresses. We hypothesize that this enrichment is attributable either to crystallization of a phase in which Co is incompatible (such as plagioclase) or replenishment of the igneous system with Co-enriched melt.

Oxygen fugacity, temperature reproducibility, and H2O contents of nominally anhydrous piston-cylinder experiments using graphite capsules

aB stra C t The Pt-graphite double-capsule technique is a very commonly used method in high-temperature, high-pressure experimental petrology, particularly for anhydrous experiments relevant to primitive basaltic magmas and mantle melting. We have performed a series of experiments that place better constraints on the range of oxygen fugacity imposed by this capsule material, on the Fe 3+ /Fe 2+ ratios in experimentally produced melts and minerals, and on the temperature reproducibility in Pt-graphite capsules. Oxygen fugacity in our piston-cylinder experiments using Pt-graphite capsules is CCO-0.7 (IW+1.5, QFM-2.2) at 1.5 GPa and 1360 °C. Comparison with other estimates and thermodynamic calculations indicate that a value of CCO-0.8 ± 0.3 can be used as a first approximation at least over the P-T range relevant for MORB and OIB magma generation (0.5–3.0 GPa, 1100–1500 °C). Under those conditions, the amount of Fe 3+ in silicate phases (pyroxenes, olivine, glass) and spinel is negligible (Fe 3+ / ΣFe < 0.05) and would not significantly affect thermodynamic properties. Significantly higher values of fO2 cannot be achieved using Pt-graphite or graphite only capsules, but fO2 can be tuned to lower values by using small pieces of PtFe alloys. The potential range of fO2 that can be reached in graphite or Pt-graphite capsules is CCO to CCO-4. Temperature reproducibility in piston-cylinder experiments has been examined and can be as low as ±10 °C. Finally, unless capsules are dried overnight at 400 °C before the experiment, small amounts of H 2O are always present in nominally dry experiments. These small amounts of H 2O should not, however, significantly change phase relations.

Phase equilibria in a volatile-free kimberlite at 0.1 MPa and the search for primary kimberlite magma

Phase relations at 0.1 MPa over a range of oxygen fugacities ( fO 2) from 1130 to 1350 °C on a volatile-free synthetic kimberlite analog composition show the crystallization sequence of olivine, monticellite, akermanite, perovskite, clinopyroxene, ulvopsinel and kirschteinite. At low pressures kimberlite liquids show limited change in Ca, Si, Al and Ti, but a large variation in Fe/Mg from the liquidus to ~ 90% crystallization. We observe low Kd FeMg ol/liq for kimberlite liquids (0.15 to 0.25). There is limited low pressure differentiation of kimberlites, affecting mostly only FeO and MgO by olivine fractionation or accumulation. The low Kd FeMg ol/liq for kimberlite liquids require they crystallize higher Mg# olivine compared to most other mantle-derived magmas. Olivine phenocrysts in kimberlite are dominantly more magnesian (Fo 92–94) than typical mantle olivine (Fo 90) and with a low Kd FeMg ol/liq necessitates the large amounts of this phase in kimberlites precipitated and accumulated at low pressure by decompression induced degassing of H 2O or CO 2. The comparison of experimental with natural data suggest that kimberlites can be viewed as essentially carbonate liquids with only 10 to 25% SiO 2 (nearly carbonatites) that have accumulated olivine.

Redox Conditions on Small Bodies, the Moon and Mars

Research Article| January 01, 2008 Redox Conditions on Small Bodies, the Moon and Mars Meenakshi Wadhwa Meenakshi Wadhwa School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, U.S.A., wadhwa@asu.edu Search for other works by this author on: GSW Google Scholar Reviews in Mineralogy and Geochemistry (2008) 68 (1): 493–510. https://doi.org/10.2138/rmg.2008.68.17 Article history first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Meenakshi Wadhwa; Redox Conditions on Small Bodies, the Moon and Mars. Reviews in Mineralogy and Geochemistry 2008;; 68 (1): 493–510. doi: https://doi.org/10.2138/rmg.2008.68.17 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search Abstract The current state of knowledge regarding redox conditions of rocks from asteroidal bodies (as represented by various classes of meteorites), the Moon and Mars is discussed here. In the case of the differentiated meteorite parent bodies, the redox conditions range over at least six orders of magnitude (from ~5 log units below the Iron-Wüstite buffer to slightly above it) and are determined in large part by the compositions of the undifferentiated precursor materials that accreted to form these parent bodies. Lunar basalts record oxygen fugacities ranging from close to the Iron-Wüstite (IW) buffer to ~2 log units below it. The current best estimate of the oxygen fugacity of the lunar mantle is ~1 log unit below IW. Martian crustal rocks represented by the Shergottite-Nakhlite-Chassignite group of meteorites record a wide range of oxygen fugacities. The basaltic shergottites range from slightly below the IW buffer to ~2 log units above it, whereas the cumulate nakhlites (and chassignites) are relatively oxidized (~3–4 log units above the IW buffer). Following early metal-silicate and crust-mantle differentiation on Mars, the depleted martian mantle is likely to have been reduced (close to the IW buffer or slightly lower). Metasomatism and secondary (hydrous) alteration are likely to have produced silicate reservoirs on Mars that are relatively more oxidized (most likely ≥3 log units above the IW buffer). The redox conditions on the other terrestrial planets (Mercury and Venus) are not well constrained. Based on the limited information from remote spacecraft and telescopic observations of surface rocks on these planets, it is inferred that silicate reservoirs on Mercury are highly reduced; those on Venus are likely to be somewhat more oxidized than on Mercury, possibly similar to the lower mantle of Earth. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Mineral chemistry, crystallization conditions and magma mixing‐mingling at Orduzu volcano (Malatya), Eastern Anatolia, Turkey

AbstractThe Middle Miocene Orduzu volcanic suite, which is a part of the widespread Neogene Yamadağ volcanism of Eastern Anatolia, consists of a rhyolitic lava flow, rhyolitic dykes, a trachyandesitic lava flow and basaltic trachyandesitic dykes. Existence of mafic enclaves and globules in some of the volcanic rocks, and microtextures in phenocrysts indicate that magma mingling and mixing between andesitic and basaltic melts played an important role in the evolution of the volcanic suite. Major and trace element characteristics of the volcanic rocks are similar to those formed in convergent margin settings. In particular, incompatible trace element patterns exhibit large depletions in high field strength elements (Nb and Ta) and strong enrichments in both large ion lithofile elements (Ba, Th and U) and light rare earth elements, indicating a strong subduction signature in the source of the volcanic rocks. Furthermore, petrochemical data obtained suggest that parental magmas of rhyolite lava and dykes, and trachyandesite lava and basaltic trachyandesite dykes were derived from subduction‐related enriched lithospheric mantle and metasomatized mantle (± asthenosphere), respectively. A detailed mineralogical study of the volcanic suite shows that plagioclase is the principal phenocryst phase in all of the rock units from the Orduzu volcano. The plagioclase phenocrysts are accompanied by quartz in the rhyolitic lava flows and by two pyroxenes in the trachyandesitic lava flows and basaltic trachyandesitic dykes. Oxide phases in all rocks are magnetite and ilmenite. Calculated crystallization temperatures range from 650°C to 800°C for plagioclase, 745°C–1054°C for biotite, 888°C–915°C for pyroxene and 736°C–841°C for magnetite–ilmenite pairs. Calculated crystallization pressures of pyroxenes vary between 1.24–5.81 kb, and oxygen fugacity range from −14.47 to −12.39. The estimates of magmatic intensive parameters indicate that the initial magma forming the Orduzu volcanic unit began to crystallize in a high‐level magma chamber and then was stored in a shallow reservoir where it underwent intermediate‐mafic mixing. The rhyolitic lava flow and dykes evolved in relatively shallower crustal magma chambers. Copyright © 2007 John Wiley &amp; Sons, Ltd.

A Mössbauer effect study of Fe environments in impact glasses

A room temperature 57Fe Mössbauer effect investigation is reported for four impact glasses: Aouelloul glass, Darwin glass, an Irghizite and Wabar glass. Resulting spectra are suitably fit to two components using the two-dimensional extended Voigt-based fitting method. The present results show that the Fe environments in impact glasses show a well defined distribution of Fe oct 2 + sites similar to that seen in tektites with mean values of the center shift and quadrupole splitting of about +1.0 mm/s (relative to α-Fe) and 1.9 mm/s, respectively. However, unlike tektites there is no clearly defined Fe tet 2 + sites and no clear distinction, in general between Fe tet 2 + and Fe 3+ sites. The non- Fe oct 2 + sites in impact glasses show both a broad distribution within each sample as well as considerable variation between samples. These features are characteristic of a wider range of pressures, temperatures and oxygen fugacities that were present during formation these materials. The clear differences between the Mössbauer spectra of impact glasses and tektites may form an effective method of distinguishing between these classes of materials.