Experimental study of the influence of SiO<Subscript>2</Subscript> on the solubility of cobalt and iron in silicate melts

The solubility of cobalt and iron in silicate melts with variable SiO₂ content was experimentally determined under controlled oxygen fugacity. It was shown that, independent of temperature and oxygen fugacity, the solubility of the two metals reaches a maximum (minimum of CoO and FeO activity coefficients) in melts of intermediate compositions. The analysis of available published data demonstrated that the γ_MeO values of at least four metals (Ni, Co, Fe, and Cr) dissolving in melts as divalent oxides show a minimum in melts with \(X_{SiO_2 } \) ≈ 57 ± 2 mol %. The position of the minimum is essentially independent of the element, melt temperature, and oxide concentration (from a few ppm to 13 wt%). The extremes of iron solubility (γ_FeO) in Fe-rich MgO-free melts may shift toward significantly lower \(X_{SiO_2 } \) values, although this inference requires additional experimental verification. Using a numerical example, some problems were discussed in the use of experimental data obtained in different laboratories for the development of a general model for the γ_MeO dependence on melt composition.     

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.     

Solubility of platinum and palladium in silicate melts under high water pressure as a function of redox conditions

The solubility of platinum and palladium in a silicate melt of the composition Di ₅₅ An ₃₅ Ab ₁₀ was determined at 1200°C and 2 kbar pressure in the presence of H₂O-H₂ fluid at an oxygen fugacity ranging from the HM to WI buffer equilibria. The influence of sulfur on the solubility of platinum in fluid-bearing silicate melt was investigated at a sulfur fugacity controlled by the Pt-PtS equilibrium at 1200°C and a pressure defined in such a way that the \(f_{H_2 O} \) and \(f_{O_2 } \) values were identical to those of the experiments without sulfur. The experiments were conducted in a high pressure gas vessel with controlled hydrogen content in the fluid. Oxygen fugacity values above the NNO buffer were controlled by solid-phase buffer mixtures using the two-capsule technique. Under more reducing conditions, the contents of H₂O and H₂ were directly controlled by the argon to hydrogen ratio in a special chamber. The hydrogen fugacity varied from 5.2 × 10^?2 bar (HM buffer) to 1230 bar (\(X_{H_2 } \) = 0.5). Pt and Pd contents were measured in quenched glass samples by neutron activation analysis. The results of these investigations showed that the solubility of Pt and Pd increases significantly in the presence of water compared with experiments in dry systems. The content of Pd within the whole range of redox conditions and that of Pt at an oxygen fugacity between the HM to MW buffer reactions are weakly dependent on \(f_{O_2 } \) and controlled mainly by water fugacity. This suggests that, in addition to oxide Pt and Pd species soluble at the ppb level in haplobasaltic melts, much more soluble (ppm level) hydroxide complexes of these metals are formed under fluid-excess conditions. Despite a decrease in water fugacity under reducing conditions, Pt solubility increases sharply near the MW buffer. It was shown by electron paramagnetic resonance spectrometry that, in contrast to dry melts, fluid-saturated silicate melts do not contain a pure metal phase (micronuggets). Therefore, the increase in Pt solubility under reducing conditions can be explained by the formation of Pt hydride complexes or Pt-fluid-silicate clusters. At a sulfur fugacity controlled by the Pt-PtS equilibrium, the solubility of Pt in iron-free silicate melts as a function of redox conditions is almost identical to that obtained in the experiments without sulfur at the same water and oxygen fugacity values. These observations also support Pt dissolution in iron-free silicate melts as hydroxide species.     

Oxygen fugacity control in piston-cylinder experiments: a re-evaluation

Jakobsson (Contrib Miner Petrol 164(3):397–407, 2012) investigated a double capsule assembly for use in piston-cylinder experiments that would allow hydrous, high-temperature, and high-pressure experiments to be conducted under controlled oxygen fugacity conditions. Using a platinum outer capsule containing a metal oxide oxygen buffer (Ni–NiO or Co–CoO) and H₂O, with an inner gold–palladium capsule containing hydrous melt, this study was able to compare the oxygen fugacity imposed by the outer capsule oxygen buffer with an oxygen fugacity estimated by the AuPdFe ternary system calibrated by Barr and Grove (Contrib Miner Petrol 160(5):631–643, 2010). H₂O loss or gain, as well as iron loss to the capsule walls and carbon contamination, is often observed in piston-cylinder experiments and often go unexplained. Only a few have attempted to actually quantify various aspects of these changes (Brooker et al. in Am Miner 83(9–10):985–994, 1998; Truckenbrodt and Johannes in Am Miner 84:1333–1335, 1999). It was one of the goals of Jakobsson (Contrib Miner Petrol 164(3):397–407, 2012) to address these issues by using and testing the AuPdFe solution model of Barr and Grove (Contrib Miner Petrol 160(5):631–643, 2010), as well as to constrain the oxygen fugacity of the inner capsule. The oxygen fugacities of the analyzed melts were assumed to be equal to those of the solid Ni–NiO and Co–CoO buffers, which is incorrect since the melts are all undersaturated in H₂O and the oxygen fugacities should therefore be lower than that of the buffer by 2 log $a_{{{\text{H}}_{ 2} {\text{O}}}}$ .     

Nitrogen solubility in basaltic melt. Part I. Effect of oxygen fugacity

The role of the oxygen fugacity on the incorporation of nitrogen in basaltic magmas has been investigated using one atmosphere high temperature equilibration of tholeiitic-like compositions under controlled nitrogen and oxygen partial pressures in the [C-N-O] system. Nitrogen was extracted with a CO₂ laser under high vacuum and analyzed by static mass spectrometry. Over a redox range of 18 oxygen fugacity log units, this study shows that the incorporation of nitrogen in silicate melts follows two different behaviors. For log fO₂ values between −0.7 and −10.7 (the latter corresponding to IW − 1.3), nitrogen dissolves as a N₂ molecule into cavities of the silicate network (physical solubility). Nitrogen presents a constant solubility (Henry’s) coefficient of 2.21 ± 0.53 × 10⁻⁹ mol g⁻¹ atm⁻¹ at 1425°C, identical within uncertainties to the solubility of argon. Further decrease in the oxygen fugacity (log fO₂ between −10.7 and −18 corresponding to the range from IW − 1.3 to IW − 8.3) results in a drastic increase of the solubility of nitrogen by up to 5 orders of magnitude as nitrogen becomes chemically bounded with atoms of the silicate melt network (chemical solubility). The present results strongly suggest that under reducing conditions nitrogen dissolves in silicate melts as N³⁻ species rather than as CN⁻ cyanide radicals. The nitrogen content of a tholeiitic magma equilibrated with N₂ is computed from thermochemical processing of our data set as

     

Platinum solubility in a haplobasaltic melt at 1250°C and 0.2 GPa: The effect of water content and oxygen fugacity

Haplobasaltic melts with a 101 kPa dry eutectic composition (An₄₂Di₅₈) and varying water contents were equilibrated with their platinum capsule at 1523 K and 200 MPa in an internally heated pressure vessel (IHPV) equipped with a rapid quench device. Experimental products were inclusion-free glasses representative of the Pt-saturated silicate melts at the experimental conditions. Platinum concentrations were determined using an isotope dilution multicollector inductively coupled plasma mass spectrometer and water contents and distribution by Karl Fischer titration and Fourier transform infrared spectroscopy, respectively.The water content of the melt has no intrinsic effect on platinum solubility, for concentrations between 0.9 wt.% and 4.4 wt.% H₂O (saturation). Platinum solubility increases with increasing water content, but this effect is an indirect effect because increasing water content at fixed f_H2 (imposed by the IHPV) increases the oxygen fugacity of the experiment.The positive oxygen fugacity dependence of Pt solubility in a hydrous silicate melt at 200 MPa is identical to that in anhydrous melts of the same composition determined in previous studies at 101 kPa. This study extends the range of platinum solubilities to oxygen fugacities lower than was previously possible. Combining the data of this and previous studies, Pt solubility is related to oxygen fugacity (in bar) at 1523 K by the equation:

[Pt]_total(ppb)=1389×f_O2+7531×(f_O2)^1/2     

The role of acidity–basicity in evaporating refractory inclusions in chondrites

When melts of Ca–Al inclusions in chondrites, which are dominated by the oxides SiO₂, MgO, CaO, and Al₂O₃, evaporate at high temperatures, the SiO₂ and MgO fugacities are inverted: SiO₂, which is more volatile than MgO, becomes less volatile when melts rich in refractory CaO and Al₂O₃ evaporate. This fugacity inversion can be realistically explained within the framework of D.S. Korzhinskii’s theory of acid–base interaction between components in silicate melts. According to this theory, an increase in CaO concentration in the melt increases its basicity, and this, in turn, increases the activity (and hence, also fugacity) of MgO and decreases those of SiO₂. In the real compositions of the Ca–Al inclusions in chondrites, the MgO/SiO₂ ratio systematically decreases with an increase in the CaO concentration under the effect of acid–base interaction.     

Oxygen fugacity control in piston-cylinder experiments

The main goal of this study was to develop and test a capsule assembly for use in piston-cylinder experiments where oxygen fugacity could be controlled in the vicinity of the QFM buffer without H₂O loss or carbon contamination of the sample material. The assembly consists of an outer Pt-capsule containing a solid buffer (Ni–NiO or Co–CoO) plus H₂O and an inner AuPd-capsule, containing the sample, H₂O and a Pt-wire. No H₂O loss is observed from the sample, even after 48 h, but a slight increase in H₂O content is found in longer runs due to oxygen and hydrogen diffusion into the AuPd-capsule. Oxygen fugacity of runs in equilibrium with the Ni–NiO (NNO) and Co–CoO (CoCO) buffers was measured by analyzing Fe dissolved in the Pt-wire and in the AuPd-capsule. The second method gives values that are in good agreement with established buffer values, whereas results from the first method are one half to one log units higher than the established values.     

Ferric/ferrous ratio in silicate melts: a new model for 1 atm data with special emphasis on the effects of melt composition

The effect of MgO and total FeO on ferric/ferrous ratio in model multicomponent silicate melts was investigated experimentally in the temperature range 1300–1500 °C at 1 atm total pressure in air. We demonstrate that the addition of these weak network modifier cations results in an increase of Fe³⁺/Fe²⁺ ratio in both mafic and silicic melts. Based on present and published experimental data, a new empirical equation is proposed to predict the ferric/ferrous ratio as a function of oxygen fugacity, temperature and melt composition. In contrast to previous equations, the compositional effect of melts on the Fe³⁺/Fe²⁺ ratio is not only modeled by the sum of the molar fraction of the individual oxide components. Additional interactions terms have also been incorporated. The main advantage of the proposed model is its applicability for a wide compositional range. However, its application to felsic melts (>?68 wt% SiO₂) is not recommended. Other advantages of this equation and differences when compared with previous models are discussed.     

Solubility of cassiterite in evolved granitic melts: effect of T, fO2, and additional volatiles

The aim of this experimental study was to determine the solubility of cassiterite in natural topaz- and cassiterite-bearing granite melts at temperatures close to the solidus. Profiles of Sn concentrations at glass–crystal (SnO₂) interface were determined following the method of (Harrison, T.M., Watson, E.B., 1983. Kinetics of zircon dissolution and zirconium diffusion in granitic melts of variable water content. Contributions to Mineralogy and Petrology 84, 66–72). The cassiterite concentration calculated at the SnO₂–glass interface is the SnO₂ solubility. Experiments were performed at 700–850 °C and 2 kbar using a natural F-bearing peraluminous granitic melt with 2.8 wt.% normative corundum. Slightly H₂O-undersaturated to H₂O-saturated melt compositions were chosen in order to minimize the loss of Sn to the noble element capsule walls. At the nickel–nickel oxide assemblage (Ni–NiO) oxygen fugacity buffer, the solubility of cassiterite in melts containing 1.12 wt.% F increases from 0.32 to 1.20 wt.% SnO₂ with an increasing temperature from 700 to 850 °C. At the Ni–NiO buffer and a given corundum content, SnO₂ solubility increases by 10% to 20% relative to an increase of F from 0 to 1.12 wt.%. SnO₂ solubility increases by 20% relative to increasing Cl content from 0 to 0.37 wt.% in synthetic granitic melts at 850 °C. We show that Cl is at least as important as F in controlling SnO₂ solubility in evolved peraluminous melts at oxygen fugacities close to the Ni–NiO buffer. In addition to the strong effects of temperature and fO₂ on SnO₂ solubility, an additional controlling parameter is the amount of excess Al (corundum content). At Ni–NiO and 850 °C, SnO₂ solubility increases from 0.47 to 1.10 wt.% SnO₂ as the normative corundum content increases from 0.1 to 2.8 wt.%. At oxidizing conditions (Ni–NiO +2 to +3), Sn is mainly incorporated as Sn⁴⁺ and the effect of excess Al seems to be significantly weaker than at reducing conditions.     

Activities and volatilities of trace components in silicate melts: a novel use of metal–silicate partitioning data

Ian Carmichael spent 45 years thinking about and working on the activities of components in silicate melts and their use to estimate physicochemical conditions at eruption and in the source regions of igneous rocks. These interests, principally in major components such as SiO₂, led us to think about possible ways of determining the complementary activity coefficients of trace components in silicate melts. While investigating the conditions of accretion and differentiation of the Earth, a number of authors have determined the partitioning of trace elements such as Co, Ni, Mo and W between liquid Fe metal and liquid silicate. These data have the potential to provide activity information for a large number of trace components in silicate melts. In order to turn the partitioning measurements into activities, however, we need to know the activity coefficient of FeO, γ_FeO in the silicate. We obtained γ_FeO as a function of melt composition by fitting a simple model to 83 experimental data for which the authors had measured the FeO content of the silicate melt in equilibrium with metal (Fe-bearing alloy) at known f_O2. The compositional dependence of γ_FeO is weak, but, when calculated in the system Diopside–Anorthite–Forsterite, it decreases towards the Forsterite apex. A similar approach for Ni, for which twice as many data are available, leads to similar composition dependence of activity coefficient and confirms the suggestion that γ_NiO/γ_FeO is almost constant over a wide range of silicate melt composition. The activity coefficients for FeO were used in conjunction with measured Mo and W partitioning between Fe-rich metal and silicate melt to estimate activity coefficients for trace MoO₂ and WO₃ dissolved in silicate melt. When combined with data on Mo- and W-saturated silicate melts a strong dependence of activity coefficient is observed. Calculated in the system Diopside–Anorthite–Forsterite, both MoO₂ and WO₃ exhibit similar behaviour to FeO and NiO in that activity coefficients decrease as Forsterite content increases. The effect is much larger for Mo and W, however, γ_MoO2 and γ_WO3 varying by factors of 20 and nearly 100, respectively, in this system. In order to illustrate the potential applications of the metal–silicate partitioning approach to determine the activity coefficients of volatile elements, we used it to determine activity coefficients of PbO, CuO_0.5 and InO_1.5 in a silica-saturated melt at 1,650 °C. We find values of 0.22, 3.5 and 0.02, respectively, indicating a strong dependence on cation charge. The value for CuO_0.5 is in excellent agreement with experimental data of Holzheid and Lodders (Geochim Cosmochim Acta 65:1933–1951, 2001), which shows that the method is viable. When combined with thermodynamic data on the gas species, we find that Pb is the most volatile of the 3 elements under ‘normal’ terrestrial conditions of oxygen fugacity but that In should become the most volatile under strongly reducing conditions such as those of the solar nebula. The oxygen fugacity dependence of volatility has implications for the high relative abundance of In in silicate Earth. We conclude that metal–silicate partitioning experiments are a viable means for determining activities of trace components in silicate melts and are particularly useful if the metal of the element is unstable or volatile at igneous temperatures.     

Experimental Study on the Electrical Conductivity of Orthopyroxene atHigh Temperature and High Pressure under Different Oxygen Fugacities

1 Introduction recognized and accepted by more and more experts engaged in experimental research at high temperature and In-situ laboratory measurement of the electricity of high pressure. This method has been regardedgeological materials at high temperature and high pressure internationally as the most advanced one for the in-situis an important approach to revealing the composition, laboratory measurement of the electric properties ofstructure and properties of materials in the deep interior…     

Redox potential of mantle magmatic systems

A new variant of the olivine-clinopyroxene-spinel oxygen barometer was developed on the basis of the equilibrium 3CaMgSi₂O₆(Cpx) + 2Fe₃O₄(Spl) = 3CaMgSiO₄(Ol) + 3Fe₂SiO₄(Ol) + O₂. Oxygen fugacity was estimated for the mineral assemblages of meymechites, olivine-bearing rocks of the Guli intrusion, and olivine and clinopyroxene microphenocrysts from interstitial glasses in mantle xenoliths containing metal alloys from Sal Island, Cape Verde Archipelago. It was shown that oxygen fugacity may vary in mantlederived magmatic systems by 7–8 orders of magnitude. Thermodynamic analysis showed that the low water activity in the lower part of the subcratonic lithosphere prevents the formation of hydrocarbons even at the presence of elemental carbon and low oxygen fugacity. The most probable mechanism of diamond formation is the reduction of carbonate components in the composition of near-solidus melts coming into the lithosphere from ascending mantle plumes.     

Copper partitioning between granitic silicate melt and coexisting aqueous fluid at 850 °C and 100 MPa

Experiments on the partitioning of Cu between different granitic silicate melts and the respective coexisting aqueous fluids have been performed under conditions of 850 °C, 100 MPa and oxygen fugacity（f O₂） buffered at approaching Ni–Ni O（NNO）. Partition coefficients of Cu（DCu= cfluid/cmelt） were varied with different alumina/alkali mole ratios [Al₂O₃/（Na₂O·K₂O）, abbreviated as Al/Alk], Na/K mole ratios, and Si O₂ mole contents. The DCu increased from 1.28 ± 0.01 to 22.18 ± 0.22 with the increase of Al/Alk mole ratios（ranging from 0.64 to 1.20）and Na/K mole ratios（ranging from 0.58 to 2.56）. The experimental results also showed that DCuwas positively correlated with the HCl concentration of the starting fluid.The DCuwas independent of the Si O₂ mole content in the range of Si O₂ content considered. No DCuvalue was less than 1 in our experiments at 850 °C and 100 MPa, indicating that Cu preferred to enter the fluid phase rather than the coexisting melt phase under most conditions in the melt-fluid system, and thus a significant amount of Cu could be transported in the fluid phase in the magmatichydrothermal environment. The results indicated that Cu favored partitioning into the aqueous fluid rather than themelt phase if there was a high Na/K ratio, Na-rich, peraluminous granitic melt coexisting with the high Cl-fluid.     

Etude expérimentale du comportement de l'uranium dans les magmas: États d'oxydation et coordinance

Depending upon oxygen fugacity, uranium exists in three different oxidation states in magmatic silicate liquids. The hexavalent state, present as the uranyl group, UO²⁺₂, is stable under highly oxidizing conditions, but can still be detected in the presence of the NiNiO buffer. Under the same conditions the pentavalent state forms about 30–40% of total uranium and is also characteristic of relatively high oxygen fugacities. Optical absorption spectra obtained on granitic and basaltic glasses synthesized in the presence of the NiNiO buffer are very different: this is interpreted as being due to the presence of UO⁺₂ complexes in the former and 6-coordinated U(V) in the latter. The tetravalent state is the most stable under reducing conditions: at the FeFeO buffer, it is the only one present. An 8-coordinated U(IV) species seems the most probable, by comparison of the spectra with those of crystallized U(IV) compounds. The trivalent state was not detected, even under the most reducing conditions. Interpretation of the spectra obtained in the glasses in terms of coordination and bonding is however difficult, due to the lack of knowledge of 5f-systems in iono-covalent systems such as oxide glasses. The presence of the pentavalent state must be taken into account in discussing partition coefficients of uranium and trans-uranium compounds in natural and synthetic systems (because of the effect of oxygen fugacity and oxide ion activity on the U(IV) U(V) system). During postmagmatic hydrothermal processes U(V) is destroyed, resulting in the early precipitation of U(IV) containing minerals and possible migration of uranyl ions.     

The thermodynamics of water in quartz

The thermodynamic model of Doukhan and Trepied (1985) for water solubility in quartz, based on the (4H)_Si substitution, has been developed to take into account the variation in oxygen fugacity under different buffering conditions, as well as the solubility of the quartz in the water. An evaluation of the thermodynamic parameters has then been made using analogous data on grossular and hydrogrossular. This evaluation, although approximate, leads to predicted solubilities that are lower than some published experimental values but are consistent with other observations indicating that the solubility is relatively low, less than 100H/10⁶ Si at most, even at the highest temperatures and pressures. The predictions give a monotonically increasing solubility with increasing temperature at given pressure, and an increase in solubility with increasing pressure at given temperature up to a maximum, beyond which the solubility decreases with further increase in pressure. The variation of solubility with oxygen fugacity at fixed pressure and temperature broadly mirrors the observations of Ord and Hobbs (1986) although not predicting the finer details. An interstitial H₂O model is also considered and shown to be probably less important as a mechanism for water solubility than the (4H)_Si mechanism. Hydrogen/alkali exchange depends on hydrogen fugacity rather than water fugacity and is predicted to be more important at lower oxygen fugacities.     

Water solubility in haplogranitic melts coexisting with H2O-H2 fluids

The water solubility in haplogranitic melts (normative composition Ab₃₉Or₃₂Qz₂₉) coexisting with H₂O-H₂ fluids at 800 and 950 °C and 1, 2 and 3 kbar vapour pressure has been determined using IR spectroscopy. The experiments were performed in internally heated pressure vessels and the hydrogen fugacity (f _H2) was controlled using the double capsule technique and oxygen buffer assemblages (WM and IW). Due to the limited lifetimes of these oxygen buffers the water solubility was determined from diffusion profiles (concentration-distance profiles) measured with IR spectroscopy in the quenched glasses. The reliability of the experimental strategy was demonstrated by comparing the results of short- and long-duration experiments performed with pure H₂O fluids. The water solubility in Ab₃₉Or₃₂Qz₂₉ melts equilibrated with H₂O-H₂ fluids decreases progressively with decreasing f _H2O, as f _H2 (or X _H2) increases in the fluid phase. The effect of H₂ on the evolution of the water solubility is similar to that of CO₂ or another volatile with a low solubility in the melt and can be calculated in a first approximation with the Burnham water solubility model. Recalculation of high temperature water speciation for AOQ melts coexisting with H₂O-H₂ fluids at 800 °C, 2 kbar suggests that the concentrations of molecular H₂O are proportional to f _H2O (calculated using available mixing models), indicating Henrian behaviour for the solubility of molecular H₂O in haplogranitic melts. Received: 29 June 1998 / Accepted: 10 March 1999     

Water solubility in a calcium aluminosilicate melt

We have measured the water solubility between 1 atmosphere and 5 kilobars for a calcium aluminosilicate melt of molar composition CaO 0.28, Al₂O₃ 0.06, SiO₂ 0.66 (An₉Wo₃₈Qz₅₃). The water contents were measured via thermogravimetric analysis of isobarically quenched glasses, and range from 0.121 wt% H₂O near 1 aim to 9.25 wt% H₂O at 5 kilobars. The molar water solubility lies between those of SiO₂ and albite melts below around three kilobars, and crosses the albite solubility curve above this pressure. The present results are compared with data in the literature on related calcium aluminosilicate melts. There seems to be little variation of water solubility with composition for calcium aluminosilicate melts, unlike analogous alkali aluminosilicate compositions. Examination of the data suggests that there may be a maximum in molar water solubility along the albite-anorthite join.     

Gold solubility in oxidized and reduced, water-saturated mafic melt

We have performed experiments to evaluate Au solubility in natural, water-saturated basaltic melts as a function of oxygen fugacity. Experiments were carried out at 1000 °C and 200 MPa, and oxygen fugacity was controlled at the fayalite-magnetite-quartz (FMQ) oxygen fugacity buffer and FMQ + 4. All experiments were saturated with a metal-chloride aqueous solution loaded initially as a 10 wt% NaCl eq. fluid. The stable phase assemblage at FMQ consists of basalt melt, olivine, clinopyroxene, a single-phase aqueous fluid, and metallic Au. The stable phase assemblage at FMQ + 4 consists of basalt melt, clinopyroxene, magnetite-spinel solid solution, a single-phase aqueous fluid, and metallic Au. Silicate glasses (i.e., quenched melt) and their contained crystalline material were analyzed by using both electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Measured Au concentrations in the quenched melt range from 4.8 μg g⁻¹ to 0.64 μg g⁻¹ at FMQ + 4, and 0.54 μg g⁻¹ to 0.1 μg g⁻¹ at FMQ. The measured solubility of Au in olivine and clinopyroxene was consistently below the LA-ICP-MS limit of detection (i.e., 0.1 μg g⁻¹). These melt solubility data place important limitations on the dissolved Au content of water-saturated, Cl- and S-bearing basaltic liquids at geologically relevant fO₂ values. The new data are compared to published, experimentally-determined values for Au solubility in dry and hydrous silicate liquids spanning the compositional range from basalt to rhyolite, and the effects of melt composition, oxygen fugacity, pressure and temperature are discussed.     

The mobility of U and Th in subduction zone fluids: an indicator of oxygen fugacity and fluid salinity

The solubility of U and Th in aqueous solutions at P-T-conditions relevant for subduction zones was studied by trapping uraninite or thorite saturated fluids as synthetic fluid inclusions in quartz and analyzing their composition by Laser Ablation-ICPMS. Uranium is virtually insoluble in aqueous fluids at Fe-FeO buffer conditions, whereas its solubility increases both with oxygen fugacity and with salinity to 960 ppm at 26.1 kbar, Re-ReO₂ buffer conditions and 14.1 wt% NaCl in the fluid. At 26.1 kbar and 800°C, uranium solubility can be reproduced by the equation: log\textU = 2.681 + 0.1433logf\textO₂ + 0.594\textCl, \log {\text{U}} = 2.681 + 0.1433\log f{\text{O}}_{2} + 0.594{\text{Cl,}} where fO₂ is the oxygen fugacity, and Cl is the chlorine content of the fluid in molality. In contrast, Th solubility is generally low (<10 ppm) and independent of oxygen fugacity or fluid salinity. The solubility of U and Th in clinopyroxene in equilibrium with uraninite and thorite was found to be in the order of 10 ppm. Calculated fluid/cpx partition coefficients of Th are close to unity for all conditions. In contrast, D^fluid/cpx for uranium increases strongly both with oxygen fugacity and with salinity. We show that reducing or NaCl-free fluids cannot produce primitive arc magmas with U/Th ratio higher than MORB. However, the dissolution of several wt% of oxidized, saline fluids in arc melts can produce U/Th ratios several times higher than in MORB. We suggest that observed U/Th ratios in arc magmas provide tight constraints on both the salinity and the oxidation state of subduction zone fluids.