The effect of oxygen fugacity on ionic conductivity in olivine

The oxygen fugacity(f_O2) may affect the ionic conductivity of olivine under upper mantle conditions because Mg vacancies can be produced in the crystal structure by the oxidization of iron from Fe²⁺ to Fe3+. Here we investigated olivine ionic conductivity at 4 GPa, as a function of temperature, crystallographic orientation, and oxygen fugacity, corresponding to the topmost asthenospheric conditions. The results demonstrate that the ionic conductivity is insensitive to f_O2 under relatively reduced conditions(f_O2 below Re-ReO₂ buffer), whereas it has a clear f_O2-dependence under relatively oxidized conditions(f_O2 around the magnetite-hematite buffer). The ionic conduction in olivine may contribute significantly to the conductivity anomaly in the topmost asthenosphere especially at relatively oxidized conditions.     

Hydration of mantle olivine under variable water and oxygen fugacity conditions

The incorporation of H into olivine is influenced by a significant number of thermodynamic variables (pressure, temperature, oxygen fugacity, etc.). Given the strong influence that H has on the solidus temperature and rheological behavior of mantle peridotite, it is necessary to determine its solubility in olivine over the range of conditions found in the upper mantle. This study presents results from hydration experiments carried out to determine the effects of pressure, temperature, and the fugacities of H₂O and O₂ on H solubility in San Carlos olivine at upper mantle conditions. Experiments were carried out at 1–2 GPa and 1,200 °C using a piston-cylinder device. The fugacity of O₂ was controlled at the Fe⁰–FeO, FeO–Fe₃O₄, or Ni⁰–NiO buffer. Variable duration experiments indicate that equilibration is achieved within 6 h. Hydrogen contents of the experimental products were measured by secondary ion mass spectrometry, and relative changes to the point defect populations were investigated using Fourier transform infrared spectroscopy. Results from our experiments demonstrate that H solubility in San Carlos olivine is sensitive to pressure, the activity of SiO₂, and the fugacities of H₂O and O₂. Of these variables, the fugacity of H₂O has the strongest influence. The solubility of H in olivine increases with increasing SiO₂ activity, indicating incorporation into vacancies on octahedral lattice sites. The forsterite content of the olivine has no discernible effect on H solubility between 88.17 and 91.41, and there is no correlation between the concentrations of Ti and H. Further, in all but one of our experimentally hydrated olivines, the concentration of Ti is too low for H to be incorporated dominantly as a Ti-clinohumite-like defect. Our experimentally hydrated olivines are characterized by strong infrared absorption peaks at wavenumbers of 3,330, 3,356, 3,525, and 3,572 cm^?1. The heights of peaks at 3,330 and 3,356 cm^?1 correlate positively with O₂ fugacity, while those at 3,525 and 3,572 cm^?1 correlate with H₂O fugacity.     

Experimental determination of Ni diffusion coefficients in olivine and their dependence on temperature, composition, oxygen fugacity, and crystallographic orientation

Diffusion couple experiments were carried out with San Carlos olivine (Fo₉₀) and NiFe alloys (Ni₁₀₀, Ni₉₇Fe₃, Ni₉₀Fe₁₀) or other olivine compositions (Fo₁₀₀, Fo₂₅) in order to determine the dependence on temperature, oxygen fugacity, composition and crystallographic orientation of Ni diffusion coefficient (D_Ni) in olivine. Experiments at 1 atmosphere total pressure cover a temperature range of 900-1445°C with run durations from 48 to 2155 h at different oxygen fugacities. In an Arrhenius plot the best fit for all data for Fo₉₀ yields an activation energy (E_D) of 220 ± 14 kJ/mol and an fO₂ dependence of (1/4.25)·Δ log fO₂ = Δ log D_Ni. The relationship between diffusion coefficients along different crystallographic axes at 1200°C is given by D_[001] ≈ 6·D_[100] ≈ 6·D_[010]. D_Ni depends strongly on the major element (i.e. Fe/Mg) composition of olivine and decreases by about 1 order of magnitude as the olivine composition changes from Fo₃₅ to Fo₉₀. Thus, experimental investigations in Fe-free systems cannot be applied to natural samples. For calculation of residence times or cooling rates the present Ni data yield shorter timescales compared to those obtained using diffusion data published until now.In addition to Ni diffusion coefficients, Fe-Mg, Mn and Ca diffusion data were obtained from some of the same diffusion couples (Fo₉₀-Fo₁₀₀). It is found that the activation energies, E_D[Ni] ≅ E_D[Fe-Mg] ≅ E_D[Mn] ≤ E_D[Ca]. All diffusion coefficients are strongly dependent on the major element composition of olivine.     

Calcium zoning in olivine and its relationship to silica activity and pressure

Microprobe analyses of olivine phenocrysts from nephelinite and basanite lavas show strong zoning toward calcium enrichment. Tholeiitic and other more siliceous lavas show little or no such calcium enrichment, but more extensive magnesium to iron zoning. Consideration of reactions between clinopyroxene and olivine indicate that silica activity as well as pressure is an important variable. Zoning trends toward calcium enrichment in olivine could be interpreted as a response to pressure release during crystallization. A normal, magnesium to iron, trend with no increase in calcium would, then, reflect crystallization with stable pressure conditions.     

Hydroxyl in mantle olivine xenocrysts from the Udachnaya kimberlite pipe

The incorporation of hydrogen in mantle olivine xenocrysts from the Udachnaya kimberlite pipe was investigated by Fourier-transform infrared spectroscopy and secondary ion mass spectrometry (SIMS). IR spectra were collected in the OH stretching region on oriented single crystals using a conventional IR source at ambient conditions and in situ at temperatures down to −180°C as well as with IR synchrotron radiation. The IR spectra of the samples are complex containing more than 20 strongly polarized OH bands in the range 3,730–3,330 cm⁻¹. Bands at high energies (3,730–3,670 cm⁻¹) were assigned to inclusions of serpentine, talc and the 10 Å phase. All other bands are believed to be intrinsic to olivine. The corresponding point defects are (a) associated with vacant Si sites (3,607 cm⁻¹ E || a, 3,597 E || a, 3,571 cm⁻¹ E || c, 3,567 E || c, and 3,556 E || b), and (b) with vacant M1 sites (most of the bands polarized parallel to a). From the pleochroic behavior and position of the OH bands associated with the vacant M1 sites, we propose two types of hydrogen—one bonded to O1 and another to O2, so that both OH vectors are strongly aligned parallel to a. The O2–H groups may be responsible for the OH bands at higher wavenumbers than those for the O1–H groups. The multiplicity of the corresponding OH bands in the spectra can be explained by different chemical environments and by slightly different distortions of the M1 sites in these high-pressure olivines. Four samples were investigated by SIMS. The calculated integral molar absorption coefficient using the IR and SIMS results of 37,500±5,000 L mol H₂O cm⁻² is within the uncertainties slightly higher than the value determined by Bell et al. (J Geophys Res 108(B2):2105–2113, 2003) (28,450±1,830 L mol H₂O cm⁻²). The reason for the difference is the different distributions of the absorption intensity of the spectra of both studies (mean wavenumber 3,548 vs. 3,570 cm⁻¹). Olivine samples with a mean wavenumber of about 3,548 cm⁻¹ should be quantified with the absorption coefficient as determined in this study; those containing more bands at higher wavenumber (mean wavenumber 3,570 cm⁻¹) should be quantified using the value determined by Bell et al. (J Geophys Res 108(B2):2105–2113, 2003).

The effect of silica activity on the diffusion of Ni and Co in olivine

The diffusion of Ni and Co was measured at atmospheric pressure in synthetic monocrystalline forsterite (Mg₂SiO₄) from 1,200 to 1,500 °C at the oxygen fugacity of air, along [100], with the activities of SiO₂ and MgO defined by either forsterite + periclase (fo + per buffer) or forsterite + protoenstatite (fo + en buffer). Diffusion profiles were measured by three methods: laser-ablation inductively-coupled-plasma mass-spectrometry, nano-scale secondary ion mass spectrometry and electron microprobe, with good agreement between the methods. For both Ni and Co, the diffusion rates in protoenstatite-buffered experiments are an order of magnitude faster than in the periclase-buffered experiments at a given temperature. The diffusion coefficients D _M (M = Ni or Co) for the combined data set can be fitted to the equation:

$$\log \,D_{\text{M}} \,\left( {{\text{in}}\,{\text{m}}^{2} \,{\text{s}}^{ - 1} } \right) = - 6.77( \pm 0.33) + \Delta E_{\text{a}} (M)/RT + 2/3\log a_{{SiO_{2} }}$$

with E_a(Ni) = ? 284.3 kJ mol^?1 and E_a(Co) = ? 275.9 kJ mol^?1, with an uncertainty of ±10.2 kJ mol^?1. This equation fits the data (24 experiments) to ±0.1 in log D _M. The dependence of diffusion on \(a_{{{\text{SiO}}_{2} }}\) is in agreement with a point-defect model in which Mg-site vacancies are charge-balanced by Si interstitials. Comparative experiments with San Carlos olivine of composition Mg_1.8Fe_0.2SiO₄ at 1,300 °C give a slightly small dependence on \(a_{{{\text{SiO}}_{2} }}\), with D \(\propto\) (\(a_{{{\text{SiO}}_{2} }}^{0.5}\)), presumably because the Mg-site vacancies increase with incorporation of Fe³⁺ in the Fe-bearing olivines. However, the dependence on fO₂ is small, with D \(\propto\) (fO₂)^0.12±0.12. These results show the necessity of constraining the chemical potentials of all the stoichiometric components of a phase when designing diffusion experiments. Similarly, the chemical potentials of the major-element components must be taken into account when applying experimental data to natural minerals to constrain the rates of geological processes. For example, the diffusion of divalent elements in olivine from low SiO₂ magmas, such as kimberlites or carbonatites, will be an order of magnitude slower than in olivine from high SiO₂ magmas, such as tholeiitic basalts, at equal temperatures and fO₂.

     

地幔氧逸度的时空变化

地幔氧逸度是反映地幔氧化还原程度的参量,由温度、压力、岩石化学成分、矿物结构等共同作用控制.目前对上地幔氧逸度的研究主要针对镁橄榄石-磁铁矿-石英体系、含角闪石的橄榄岩体系和玄武岩(熔体)体系,通过实验岩石学方法进行.地幔氧逸度在垂直深度上随深度增加而减小受到普遍认可.天然样品和理论研究认为,岩石圈地幔底部的软流圈氧逸...     

Results of study of crystallographic orientation of olivine and diamond from Udachnaya kimberlite pipe,Yakutia

The crystallographic orientation of three diamonds and 19 olivine inclusions from Udachnaya kimberlite pipe was studied using monocrystal X-ray diffractometry. No epitaxial olivine inclusions were found.     

FTIR spectroscopy of OH in olivine: A new tool in kimberlite exploration

Our study of olivines from Canadian kimberlites shows that the application of FTIR spectroscopy significantly improves the reliability of olivine as a kimberlite indicator mineral (KIM). We have developed an algorithm that yields the water concentration and the normalized intensity of the OH IR absorption band at 3572 cm⁻¹ from unpolished olivine grains of unknown thickness. For 80% of kimberlitic olivines these two parameters are significantly higher than those for olivines from non-kimberlitic magmas and consequently, olivines with water concentrations >60 ppm and a strong absorption band at 3572 cm⁻¹ can be reliably classified as being kimberlitic.We have identified two major spectral features in the OH absorption bands of kimberlitic olivines that allow for a more detailed classification: (a) the presence of three types of high-requency OH absorption bands (Group 1A, 1B and 1C) and (b) the proportion of low-frequency OH absorption bands (Group 2) relative to high-frequency bands (Group 1). Comparison of our results with experimental studies suggests that differences within Group 1 OH absorption bands are due to different pressures of crystallization or hydrogenation. The three identified types of Group 1 OH absorption bands approximately correspond to high (P > 2 GPa, Group 1A), moderate (2-1 GPa, Group 1B), and low (<1 GPa, Group 1C) pressures of hydrogenation. Group 2 OH IR absorption bands in olivines with NiO > 3500 ppm are interpreted to reflect olivine-orthopyroxene equilibria and hence are indicative of xenocrystic olivine derived from lherzolitic or harzburgitic mantle sources. Interaction of xenocrystic olivine with hydrous kimberlitic melts with low silica activity likely will cause a gradual increase in Group 1 absorption bands. Therefore, FTIR spectra of olivine can be used to obtain qualitative estimates of the duration of interaction between mantle material and a kimberlitic melt.In addition to applications in kimberlite and diamond exploration, FTIR spectra of olivine phenocrysts, combined with mineral chemical data, may also provide insights into kimberlite evolution. Our data suggest that in some instances the ascent of kimberlitic magmas could have been interrupted at or near the Moho, followed by olivine crystallization and exsolution of aqueous fluids.     

An image analysis method to determine crystal size distributions of olivine in kimberlite

Crystal size distributions (CSDs) are a standard method of describing populations of crystals within magmatic rocks. Olivine is the dominant phase in kimberlite (∼40–50% by volume) and features a diverse range of sizes, shapes and origins. CSDs of olivine provide a logical means of semi-quantitatively characterising kimberlite. The CSDs can then be used to distinguish or correlate between kimberlite bodies or to investigate processes related to ascent, emplacement and eruption. In this paper, we present an automatic image analysis technique that provides efficient quantification of olivine CSDs within digital images of polished slabs of kimberlite. This technique relies on a combination of algorithms for detecting regions of interest (ROI) and for segmentation of ROIs in order to identify individual olivine crystals that are used for size distribution datasets. The detection process identifies regions expected to be olivine using a model-based colour detection technique using Mahalanobis distance combined with texture analysis based on local standard deviation and greyscale foreground enhancement techniques. The segmentation process separates adjacent domains to identify individual crystals using an iterative marker-based watershed algorithm to separate adjoined structures of varying sizes. We demonstrate the utility of automatic image analysis by comparing CSDs for olivine derived from this method versus results from manual digitisation of olivine grains. The automatic detection system correctly identified ∼86% of the manually detected olivine domains; ∼88% of the automatically detected regions correctly correlate to manually defined olivine grains. Discrepancies between the two methods are mostly the result of oversimplification of crystal margins (i.e. rounding) by manual tracing whereas automatic boundary recognition shows clear advantages in identifying irregularities in crystal edges. Closer examination of the results shows that both methods suffer from under-representation of smaller crystals due to: (1) human subjectivity and error in manual tracing and (2) noise removal processes in automatic detection. Automatic detection of olivine grains is much more efficient than conventional manual tracing; manual detection requires ∼6 h per sample versus ∼1 min for automatic analysis of the same sample.     

C-O-H-S fluid composition and oxygen fugacity in graphitic metapelites

Abstract C-O-H fluid produced by the equilibration of H₂O and excess graphite must maintain the atomic H/O ratio of water, 2:1. This constraint implies that all thermodynamic properties of the fluid are uniquely determined at isobaric-isothermal conditions. The O₂, H₂O and CO₂ fugacities (fo₂, f_H2O and f_CO2) of such fluids have been estimated from equations of state and fit as a function of pressure and temperature. These fugacities can be taken as characteristic for graphitic metamorphic systems in which the dominant fluid source is dehydration, e.g. pelitic lithologies. Because there are no compositional degrees of freedom for graphite-saturated fluids produced entirely by dehydration, the variance of the dehydration process is not increased in comparison with that in non-graphitic systems. Thus, compositional ‘buffering’of C-O-H fluids by dehydration equilibria, a common petrological model, requires that redox reactions, decarbonation reactions or external, H/O ± 2, fluid sources perturb the evolution of the metamorphic system. Such perturbations are not likely to be significant in metapelitic environments, but their tendency will be to increase the f_O2 of the fluid phase. At high metamorphic grades, pyrite desulphidation reactions may cause a substantial reduction of f_H2O and slight increases in f_O2 and f_CO2 relative to sulphur-free fluid. At low metamorphic grade, sulphur solubility in H/O ± 2 fluids is so low that pyrite decomposition must occur by sulphur-conserving reactions that cause iron depletion in silicates, a common feature of sulphidic pelites. With increasing temperature and sulphur solubility, pyrite desulphidation may be driven by dehydration reactions or infiltration of H₂O-rich fluids. The absence of magnetite and the assemblages carbonate + aluminosilicate or pyrite + pyrrhotite + ilmenite from most graphitic metapelites is consistent with an H/O = 2 model for GCOH(S) fluid. For graphitic rocks in which such a model is inapplicable, a phase diagram variable that defines the H/O ratio of GCOH(S) fluid is more useful than the conventional f_O2 variable.     

Crystal chemistry of amphiboles: implications for oxygen fugacity and water activity in lithospheric mantle beneath Victoria Land,Antarctica

Amphibole is the hydrous metasomatic phase in spinel-bearing mantle xenoliths from Baker Rocks, Northern Victoria Land, Antarctica. It occurs in veins or in disseminated form in spinel lherzolites. Both types derive from reaction between metasomatic melts and the pristine paragenesis of the continental lithospheric mantle beneath Northern Victoria Land. To determine the effective role of water circulation during the metasomatic process and amphibole formation, six amphibole samples were fully characterized. Accurate determination of the site population and the state of dehydrogenation in each of these amphiboles was carried out using single-crystal X-ray diffraction, electron microprobe and secondary ion mass spectroscopy on the same single crystal. The Fe³⁺/ΣFe ratio was determined by X-ray absorption near edge spectroscopy on amphibole powder. The degree of dehydrogenation determined by SIMS is 0.870–0.994 O3(O^2?) a.p.f.u., primary and ascribed to the Ti-oxy component of the amphibole, as indicated by atom site populations; post-crystallization H loss is negligible. Estimates of aH₂O (0.014–0.054) were determined from the dehydration equilibrium among end-member components assuming that amphiboles are in equilibrium with the anhydrous peridotitic phases. A difference up to 58 % in determination of aH₂O can be introduced if the chemical formula of the amphiboles is calculated based on 23 O a.p.f.u. without knowing the effective amount of dehydrogenation. The oxygen fugacity of the Baker Rocks amphibole-bearing mantle xenoliths calculated based upon the dissociation constant of water (by oxy-amphibole equilibrium) is between ?2.52 and ?1.32 log units below the fayalite–magnetite–quartz (FMQ) buffer. These results are systematically lower and in a narrow range of values relative to those obtained from anhydrous olivine–orthopyroxene–spinel equilibria (fO₂ between ?1.98 and ?0.30 log units). A comparative evaluation of the two methods suggests that when amphibole is present in mantle peridotites, the application of oxy-amphibole equilibrium is preferred, because ol–opx–sp oxy-calibrations are not “sensitive” enough in recording the effects (if any) of amphibole in the peridotite matrix. Amphibole acts as the main H acceptor among the peridotite minerals and may prevent fluid circulation and buffer oxygen fugacity. The important conclusion of this study is that amphibole within the lithospheric mantle does not always means high water activity and oxidizing conditions.     

中酸性侵入岩的氧逸度计算

氧逸度是岩石物理化学的1个重要参数,对岩浆演化、岩石成因和岩浆热液成矿具有明显的控制作用。含有变价元素的矿物常被用来计算成岩与成矿过程中的氧逸度,但是不同方法之间的对比研究较少,各种方法的适用性还不明晰。作者对晋东北黑狗背花岗岩体的氧逸度进行了较为系统的研究,通过黑云母、角闪石、磁铁矿-钛铁矿矿物对和锆石的Ce异常的氧逸度计算,发现不同方法的估算结果存在较大的差异,甚至会得出完全相左的结论。其中,基于Fe~(3+)/Fe~(2+)比值的黑云母和磁铁矿-钛铁矿氧逸度计计算结果吻合度较高,可适用于中酸性侵入岩的氧逸度研究,而基于Fe/(Fe+Mg)比值的氧逸度计算方法可能受到母岩浆化学成分的影响,不适合应用于低镁侵入岩的氧逸度估算。     

Lamproite and kimberlite of the Sayany area: Composition,sources, and diamond potential

Doklady Earth Sciences -     

The magmatic oxygen fugacity of an ultrabasic dyke

The magmatic oxygen fugacity (fO₂) of a thirty foot wide feldspathic peridotite dyke has been determined using the experimental method of Fudali (1965). Determinations were made on samples from both the marginal and central portions of the dyke and a difference of approximately one order of magnitude in fO₂ was observed. This difference is attributed to the increase in the H₂O content of the liquid as crystallization proceeded and to diffusion of hydrogen out of the dyke. It is concluded that the dyke was emplaced with an fO₂ between 10^–8 and 10^–9 atmospheres. Data on the absorbtion of Fe by the silver/palladium sample containers during the experiments are given in an appendix.     

A computational study of oxygen diffusion in olivine

Atomistic modelling techniques are used to study the rate-determining steps that limit diffusion of oxygen in forsterite. The activation energies for diffusion parallel to all three crystallographic axes by the vacancy and interstitial mechanisms are calculated. The activation energy for extrinsic vacancy diffusion is predicted to be isotropic with a barrier height of 119 kJ mol^–1. Conversely, in the interstitial case it is found to be anisotropic, with extrinsic activation energies that range between 94 and 178 kJ mol^–1. The effect of intrinsic defects and two typical impurities, iron and hydrogen, upon diffusion is also considered. We find that the migration energy is slightly higher in iron-rich fayalite compared with forsterite and that the presence of hydrogen defects will not affect the diffusion mechanism. These observations lead us to reinterpret existing experimental results on oxygen diffusion in natural olivine. We suggest that at low oxygen partial pressure the mechanism observed is a vacancy mechanism, while at high oxygen partial pressure the mechanism is interstitial. We believe that this change in mechanism is mediated by iron redox reactions. Taking this process into account, we derive activation energies in excellent agreement with those found experimentally in natural samples of olivine. The anisotropy of activation barriers and hence the change in diffusion rates with temperature could be used to distinguish between the two mechanisms in future experimental work.     

Armalcolite stability as a function of pressure and oxygen fugacity

The stability of synthetic armalcolite of composition (Fe_0.5Mg_0.5Ti₂O₅ was studied as a function of total pressure up to 15 kbar and 1200°C and also as a function of oxygen fugacity (?O₂) at 1200°C and 1 atm total pressure. The high pressure experiments were carried out in a piston-cylinder apparatus using silver-palladium containers. At 1200°C, armalcolite is stable as a single phase at 10 kbar. With increasing pressure, it breaks down ($\text{dT}\text{dP}\text{= 20°C/}\text{kbar}$), to rutile, a more magnesian armalcolite, and ilmenite solid solution. At 14 kbar, this three-phase assemblage gives way ($\text{dT}\text{dP}\text{= 30°C/}\text{kbar}$) to a two-phase assemblage of rutile plus ilmenite solid solution.A zirconian-armalcolite was synthesized and analyzed; 4 wt % ZrO₂ appears to saturate armalcolite at 1200°C and 1 atm. The breakdown of Zr-armalcolite occurs at pressures of 1–2 kbar less than those required for the breakdown of Zr-free armalcolite. The zirconium partitions approximately equally between rutile and ilmenite phases.The stability of armalcolite as a function of ?O₂ was determined thermogravimetrically at 1200°C and 1 atm by weighing sintered pellets in a controlled atmosphere furnace. Armalcolite, (Fe_0.5Mg_0.5)-Ti₂O₅, is stable over a range ?O₂ from about 10^?9.5to 10^?10.5 atm. Below this range to at least 10^?12.8 atm, ilmenite plus a reduced armalcolite are formed. These products were observed optically and by Mössbauer spectroscopy, and no metallic iron was detected; therefore, some of the titanium must have been reduced to Ti³⁺. This reduction may provide yet another mechanism to explain the common association of ilmenite rims around lunar armalcolites.     

Geochronology and magma oxygen fugacity of Ehu S-type granitic pluton in Zhe-Gan-Wan region,SE China

In this paper, we determined the U-Pb isotopic and trace element compositions of zircons from the Ehu S-type granite in the Zhe-Gan-Wan region, SE China, using in-situ laser ablation (LA) ICP-MS. The weighted mean ²⁰⁶Pb/²³⁸U age of 132.0 ± 0.6 Ma for the Ehu granite indicates that the pluton was formed in the Early Cretaceous and during the Late Mesozoic Cu-Mo mineralization quiescence in Zhe-Gan-Wan region. The calculated logarithmic magma oxygen fugacities for Ehu granite range from −19.19 to −11.43 with an average magma oxidation state of FMQ-0.29, which is much lower than those of Cu-Mo bearing granites in the Zhe-Gan-Wan region. Since Ehu granite was derived from partial melting of metasedimentary basement without fractional crystallization and mantle-derived magma contamination, the low oxidation state of this granite suggests that the assimilation of metasedimentary basement component may not significantly increase the oxidation state of reduced melts from asthenospheric mantle and could not generate oxidized magmas that are favorable for Cu-Mo mineralization.     

Solubility of CO2 in a Ca-rich leucitite: effects of pressure,temperature, and oxygen fugacity

The solubility of carbon dioxide in a Ca-rich leucitite has been investigated as a function of pressure (0.1–2.0 GPa), temperature (1200–1600°C), and oxygen fugacity. The experiments were done in a rapid-quench internally-heated pressure vessel (0.1 GPa) and a piston cylinder (0.5–2.0 GPa). The leucitite glass, previously equilibrated at NNO, and silver oxalate were loaded in Fe-doped Pt capsules (oxidized conditions) and graphitelined Pt capsules (reduced conditions). Secondary Ion Mass Spectrometry and bulk carbon analyses were used to determine the amount of dissolved carbon. Speciation of carbon was characterized by Fourier transform microinfrared spectroscopy. At oxidized conditions, only CO₃ ^2- is observed as a dissolved species. The solubility is high with CO₂ contents in the melt attaining 6.2 wt% at 2.0 GPa and 1350°C. The solubility increases with pressure and shows a significant negative temperature dependence. An excellent correlation is obtained when the data are fit to a model, based on the simplified solubility reaction CO₂ ^(vapor)+O^2-(melt)CO₃ ^2-(melt), which describes the solubility of CO₂ as a function of pressure, temperature and fCO₂. At reduced conditions, the amount of carbon dissolved is significantly lower, and CO₃ ^2- is still the only species present in the melt. If the solubility model established at oxidized conditions is applied, the carbon dissolved appears to be essentially a function of fCO₂ alone although divergence increases in a consistent manner with pressure and temperature. This could suggest a low but significant solubility of CO with a positive temperature dependence or a departure of the calculated fluid compositions determined by the equation of state from the actual ones. The strong preferential solubility of carbon in its oxidized C⁴⁺ form, even at reduced conditions, implies that ascending melts with high CO₂ solubility can experience significant oxidation through degassing. This could reconcile the oxidized nature of some Ca-rich alkaline magmas with more reduced mantle source regions.