Pressure dependence of the speciation of dissolved water in rhyolitic melts

Water speciation in rhyolitic melts with dissolved water ranging from 0.8 to 4 wt% under high pressure was investigated. Samples were heated in a piston-cylinder apparatus at 624-1027 K and 0.94-2.83 GPa for sufficient time to equilibrate hydrous species (molecular H₂O and hydroxyl group, H₂O_m + O ? 2OH) in the melts and then quenched roughly isobarically. The concentrations of both hydrous species in the quenched glasses were measured with Fourier transform infrared (FTIR) spectroscopy. For the samples with total water content less than 2.7 wt%, the equilibrium constant (K) is independent of total H₂O concentration. Incorporating samples with higher water contents, the equilibrium constant depends on total H₂O content, and a regular solution model is used to describe the dependence. K changes with pressure nonmonotonically for samples with a given water content at a given temperature. The equilibrium constant does not change much from ambient pressure to 1 GPa, but it increases significantly from 1 to 3 GPa. In other words, more molecular H₂O reacts to form hydroxyl groups as pressure increases from 1 GPa, which is consistent with breakage of tetrahedral aluminosilicate units due to compression of the melt induced by high pressure. The effect of 1.9 GPa (from 0.94 to 2.83 GPa) on the equilibrium constant at 873 K is equivalent to a temperature effect of 49 K (from 873 K to 922 K) at 0.94 GPa. The results can be used to evaluate the role of speciation in water diffusion, to estimate the apparent equilibrium temperature, and to infer viscosity of hydrous rhyolitic melts under high pressure.     

Water speciation and diffusion in haploandesitic melts at 743-873 K and 100 MPa

Water is an important volatile component in andesitic eruptions and deep-seated andesitic magma chambers. We report an investigation of H₂O speciation and diffusion by dehydrating haploandesitic melts containing ?2.5 wt.% water at 743-873 K and 100 MPa in cold-seal pressure vessels. FTIR microspectroscopy was utilized to measure species [molecular H₂O (H₂O_m) and hydroxyl group (OH)] and total H₂O (H₂O_t) concentration profiles on the quenched glasses from the dehydration experiments. The equilibrium constant of the H₂O speciation reaction H₂O_m+O?2OH, K = (X_OH)²/(X_H2OmX_O) where X means mole fraction on a single oxygen basis, in this Fe-free andesite varies with temperature as ln K = 1.547-2453/T where T is in K. Comparison with previous speciation data on rhyolitic and dacitic melts indicates that, for a given water concentration, Fe-free andesitic melt contains more hydroxyl groups. Water diffusivity at the experimental conditions increases rapidly with H₂O concentration, contrary to previous H₂O diffusion data in an andesitic melt at 1608-1848 K. The diffusion profiles are consistent with the model that molecular H₂O is the diffusion species. Based on the above speciation model, H₂O_m and H₂O_t diffusivity (in m²/s) in haploandesite at 743-873 K, 100 MPa, and H₂O_t ? 2.5 wt.% can be formulated as

     

Water diffusion in dacitic melt

H₂O diffusion in dacitic melt was investigated at 0.48-0.95 GPa and 786-893 K in a piston-cylinder apparatus. The diffusion couple design was used, in which a nominally dry dacitic glass makes one half and is juxtaposed with a hydrous dacitic glass containing up to ∼8 wt.% total water (H₂O_t). H₂O concentration profiles were measured on quenched glasses with infrared microspectroscopy. The H₂O diffusivity in dacite increases rapidly with water content under experimental conditions, similar to previous measurements at the same temperature but at pressure <0.15 GPa. However, compared with the low-pressure data, H₂O diffusion at high pressure is systematically slower. H₂O diffusion profiles in dacite can be modeled by assuming molecular H₂O (H₂O_m) is the diffusing species. Total H₂O diffusivity D_H2Ot within 786-1798 K, 0-1 GPa, and 0-8 wt.% H₂O_t can be expressed as: where D_H2Ot is in m²/s, T is temperature in K, P is pressure in GPa, K = exp(1.49 − 2634/T) is the equilibrium constant of speciation reaction (H₂O_m+O?2OH) in the melt, X = C/18.015/[C/18.015 + (100 − C)/33.82], C is wt.% of H₂O_t, and 18.015 and 33.82 g/mol correspond to the molar masses of H₂O and anhydrous dacite on a single oxygen basis. Compared to H₂O diffusion in rhyolite, diffusivity in dacite is lower at intermediate temperatures but higher at superliquidus temperatures. This general H₂O diffusivity expression can be applied to a broad range of geological conditions, including both magma chamber processes and volcanic eruption dynamics from conduit to the surface.     

A new method for synthesizing fluid inclusions in fused silica capillaries containing organic and inorganic material

Considerable advances in our understanding of physicochemical properties of geological fluids and their roles in many geological processes have been achieved by the use of synthetic fluid inclusions. We have developed a new method to synthesize fluid inclusions containing organic and inorganic material in fused silica capillary tubing. We have used both round (0.3 mm OD and 0.05 or 0.1 mm ID) and square cross-section tubing (0.3 × 0.3 mm with 0.05 × 0.05 mm or 0.1 × 0.1 mm cavities). For microthermometric measurements in a USGS-type heating-cooling stage, sample capsules must be less than 25 mm in length. The square-sectioned capsules have the advantage of providing images without optical distortion. However, the maximum internal pressure (P; about 100 MPa at 22 °C) and temperature (T; about 500 °C) maintained by the square-sectioned capsules are less than those held by the round-sectioned capsules (about 300 MPa at room T, and T up to 650 °C).The fused silica capsules can be applied to a wide range of problems of interest in fluid inclusion and hydrothermal research, such as creating standards for the calibration of thermocouples in heating-cooling stages and frequency shifts in Raman spectrometers. The fused silica capsules can also be used as containers for hydrothermal reactions, especially for organic samples, including individual hydrocarbons, crude oils, and gases, such as cracking of C₁₈H₃₈ between 350 and 400 °C, isotopic exchanges between C₁₈H₃₈ and D₂O and between C₁₉D₄₀ and H₂O at similar temperatures. Results of these types of studies provide information on the kinetics of oil cracking and the changes of oil composition under thermal stress.When compared with synthesis of fluid inclusions formed by healing fractures in quartz or other minerals or by overgrowth of quartz at elevated P-T conditions, the new fused-silica method has the following advantages: (1) it is simple; (2) fluid inclusions without the presence of water can be formed; (3) synthesized inclusions are large and uniform, and they are able to tolerate high internal P; (4) it is suitable for the study of organic material; and (5) redox control is possible due to high permeability of the fused silica to hydrogen.     

Experimental determination of the solubility of natural wollastonite in pure water up to pressures of 5 GPa and at temperatures of 400-800 °C

The solubility of natural, near-end-member wollastonite-I (>99.5% CaSiO₃) has been determined at temperatures from 400 to 800 °C and pressures between 0.8 and 5 GPa in piston-cylinder apparatus with the weight-loss method. Chemical analysis of quench products and optical monitoring in a hydrothermal diamond anvil cell demonstrates that no additional phases form during dissolution. Wollastonite-I, therefore, dissolves congruently in the pressure-temperature range investigated. The solubility of CaSiO₃ varies between 0.175 and 13.485 wt% and increases systematically with both temperature and pressure up to 3.0 GPa. Above 3.0 GPa wollastonite-I reacts rapidly to the high-pressure modification wollastonite-II. No obvious trends are evident in the solubility of wollastonite-II, with values between 1.93 and 10.61 wt%. The systematics of wollastonite-I solubility can be described well by a composite polynomial expression that leads to isothermal linear correlation with the density of water. The molality of dissolved wollastonite-I in pure water is then

log(m_woll)=2.2288-3418.23×T^-1+671386.84×T^-2+logρ_H2O×(5.4578+2359.11×T^-1).     

Solubilities of corundum, wollastonite and quartz in H2O-NaCl solutions at 800 °C and 10 kbar: Interaction of simple minerals with brines at high pressure and temperature

Solubilities of corundum (Al₂O₃) and wollastonite (CaSiO₃) were measured in H₂O-NaCl solutions at 800 °C and 10 kbar and NaCl concentrations up to halite saturation by weight-loss methods. Additional data on quartz solubility at a single NaCl concentration were obtained as a supplement to previous work. Single crystals of synthetic corundum, natural wollastonite or natural quartz were equilibrated with H₂O and NaCl at pressure (P) and temperature (T) in a piston-cylinder apparatus with NaCl pressure medium and graphite heater sleeves. The three minerals show fundamentally different dissolution behavior. Corundum solubility undergoes large enhancement with NaCl concentration, rising rapidly from Al₂O₃ molality (m_Al2O3) of 0.0013(1) (1σ error) in pure H₂O and then leveling off to a maximum of ∼0.015 at halite saturation (X_NaCl ≈ 0.58, where X is mole fraction). Solubility enhancement relative to that in pure H₂O, , passes through a maximum at X_NaCl ≈ 0.15 and then declines towards halite saturation. Quenched fluids have neutral pH at 25 °C. Wollastonite has low solubility in pure H₂O at this P and T(m_CaSiO3=0.0167(6)). It undergoes great enhancement, with a maximum solubility relative to that in H₂O at X_NaCl ≈ 0.33, and solubility >0.5 molal at halite saturation. Solute silica is 2.5 times higher than at quartz saturation in the system H₂O-NaCl-SiO₂, and quenched fluids are very basic (pH 11). Quartz shows monotonically decreasing solubility from m_SiO2=1.248 in pure H₂O to 0.202 at halite saturation. Quenched fluids are pH neutral. A simple ideal-mixing model for quartz-saturated solutions that requires as input only the solubility and speciation of silica in pure H₂O reproduces the data and indicates that hydrogen bonding of molecular H₂O to dissolved silica species is thermodynamically negligible. The maxima in for corundum and wollastonite indicate that the solute products include hydrates and Na⁺ and/or Cl⁻ species produced by molar ratios of reactant H₂O to NaCl of 6:1 and 2:1, respectively. Our results imply that quite simple mechanisms may exist in the dissolution of common rock-forming minerals in saline fluids at high P and T and allow assessment of the interaction of simple, congruently soluble rock-forming minerals with brines associated with deep-crustal metamorphism.     

Temperature dependence of sulfide and sulfate solubility in olivine-saturated basaltic magmas

The sulfur concentration at pyrrhotite- and anhydrite-saturation in primitive hydrous basaltic melt of the 2001-2002 eruption of Mt. Etna was determined at 200 MPa, T = 1050-1250 °C and at log fO₂ from FMQ to FMQ+2.2 (FMQ is Fayalite-Magnetite-Quartz oxygen buffer). At 1050 °C Au sample containers were used. A double-capsule technique, using a single crystal olivine sample container closed with an olivine piston, embedded in a sealed Au₈₀Pd₂₀ capsule, was developed to perform experiments in S-bearing hydrous basaltic systems at T > 1050 °C. Pyrrhotite is found to be a stable phase coexisting with melt at FMQ-FMQ+0.3, whereas anhydrite is stable at FMQ+1.4-FMQ+2.2. The S concentration in the melt increases almost linearly from 0.12 ± 0.01 to 0.39 ± 0.02 wt.% S at FeS-saturation and from 0.74 ± 0.01 to 1.08 ± 0.04 wt.% S at anhydrite-saturation with T ranging from 1050-1250 °C. The relationships between S concentration at pyrrhotite and/or anhydrite saturation, MgO content of the olivine-saturated melt, T, and log fO₂ observed in this study and from previous data are used to develop an empirical model for estimating the magmatic T and fO₂ from the S and MgO concentrations of H₂O-bearing olivine-saturated basaltic melts. The model can also be used to determine maximum S concentrations, if fO₂ and MgO content of the melt are known. The application of the model to compositions of melt inclusions in olivines from Mt. Etna indicates that the most primitive magmas trapped in inclusions might have been stored at log fO₂ slightly higher than FMQ+1 and at T = 1100-1150 °C, whereas more evolved melts could have been trapped at T ? 1100 °C. These values are in a good agreement with the estimates obtained by other independent methods reported in the literature.     

Chlorine-rich metasomatic H2O-CO2 fluids in amphibole-bearing peridotites from Injibara (Lake Tana region, Ethiopian plateau): Nature and evolution of volatiles in the mantle of a region of continental flood basalts

Petrological and geochemical study of volatile bearing phases (fluid inclusions, amphibole, and nominally anhydrous minerals) in a spinel lherzolite xenolith suite from Quaternary lavas at Injibara (Lake Tana region, Ethiopian plateau) shows compelling evidence for metasomatism in the lithospheric mantle in a region of mantle upwelling and continental flood basalts. The xenolith suite consists of deformed (i.e., protogranular to porphyroclastic texture) Cl-rich pargasite lherzolites, metasomatized (LILE and Pb enrichment in clinopyroxene and amphibole) at T ? 1000 °C. Lherzolites contain chlorine-rich H₂O-CO₂ fluid inclusions, but no melt inclusions. Fluid inclusions are preserved only in orthopyroxene, while in olivine, they underwent extensive interaction with the host mineral. The metasomatic fluid composition is estimated: X_CO2 = 0.64, X_H2O = 0.33, X_Na = 0.006, X_Mg = 0.006, X_Cl = 0.018, (salinity = 14-10 NaCl eq. wt.%, a_H2O = 0.2, Cl = 4-5 mol.%). Fluid isochores correspond to trapping pressures of 1.4-1.5 GPa or 50-54 km depth (at T = 950 °C). Synchrotron sourced micro-infrared mapping (ELECTRA, Trieste) shows gradients for H₂O-distribution in nominally anhydrous minerals, with considerable enrichment at grain boundaries, along intragranular microfractures, and around fluid inclusions. Total water amounts in lherzolites are variable from about 150 up to 400 ppm. Calculated trace-element pattern of metasomatic fluid phases, combined with distribution and amount of H₂O in nominally anhydrous minerals, delineate a metasomatic Cl- and LILE-rich fluid phase heterogeneously distributed in the continental lithosphere. Present data suggest that Cl-rich aqueous fluids were important metasomatic agents beneath the Ethiopian plateau, locally forming a source of high water content in the peridotite, which may be easily melted. High Cl, LILE, and Pb in metasomatic fluid phases suggest the contribution of recycled altered oceanic lithosphere component in their source.     

Quartz-bearing C-O-H fluid inclusions diamond: Retracing the pressure-temperature path in the mantle using calibrated high temperature IR spectroscopy

Infrared spectra of C-O-H micro-inclusions were collected from a micro-inclusion bearing diamond during step-heating and freezing experiments to examine fluid speciation as a function of pressure and temperature. The inclusions contain H₂O, CO₂, carbonate, apatite, quartz and mica, which together represent the oxidising remnant mantle fluid composition after diamond crystallisation. The internal pressure of the inclusions, measured from calibrated shifts of the quartz peaks, increases from 1.3 GPa at ambient temperature, to approximately 4-5 GPa at 737 °C, close to the conditions of crystallisation of the host diamond in the mantle.     

The Fe-rich liquidus in the Fe-FeS system from 1 bar to 10 GPa

The composition and evolution of a metallic planetary core is determined by the behavior with pressure of the eutectic and the liquidus on the Fe-rich side of the Fe-FeS eutectic. New experiments at 6 GPa presented here, along with existing experimental data, inform a thermodynamic model for this liquidus from 1 bar to at least 10 GPa. Fe-FeS has a eutectic that becomes more Fe-rich but remains constant in T up to 6 GPa. The 1 bar, 3 GPa, and 6 GPa liquidi all cross at a pivot point at 1640 ± 5 K and FeS_37 ± 0.5. This liquid/crystalline metal equilibrium is T-x-fixed and pressure independent through 6 GPa. Models of the 1 bar through 10 GPa experimental liquidi show that with increasing P there is an increase in the T separation between the liquidus and the crest of the metastable two-liquid solvus. The solvus crest decreases in T with increasing P. The model accurately reproduces all the experimental liquidi from 1 bar to 10 GPa, as well as reproducing the 0-6 GPa pivot point. The 14 GPa experimental liquidus ( [Chen et al., 2008a] and Chen et al., 2008b) deviates sharply from the lower pressure trends indicating that the 0-10 GPa model no longer applies to this 14 GPa data.     

Ammonium in aqueous fluids to 600 °C, 1.3 GPa: A spectroscopic study on the effects on fluid properties, silica solubility, and K-feldspar to muscovite reactions

The behavior of ammonium, NH₄⁺, in aqueous systems was studied based on Raman spectroscopic experiments to 600 °C and about 1.3 GPa. Spectra obtained at ambient conditions revealed a strong reduction of the dynamic three-dimensional network of water with addition of ammonium chloride, particularly at small solute concentrations. The differential scattering cross section of the ν₁-NH₄⁺ Raman band in these solutions was found to be similar to that of salammoniac.The Raman band of silica monomers at ∼780 cm⁻¹ was present in all spectra of the fluid at high temperatures in hydrothermal diamond-anvil cell experiments with H₂O ± NH₄Cl and quartz or the assemblage quartz + kyanite + K-feldspar ± muscovite/tobelite. However, these spectra indicated that dissolved silica is less polymerized in ammonium chloride solutions than in comparable experiments with water. Quantification based on the normalized integrated intensity of the H₄SiO₄⁰ band showed that the silica solubility in experiments with H₂O + NH₄Cl was significantly lower than that in equimolal NaCl solutions. This suggests that ammonium causes a stronger decrease in the activity of water in chloridic solutions than sodium.The Raman spectra of the fluid also showed that a significant fraction of ammonium was converted to ammonia, NH₃, in all experiments at temperatures above 300 °C. This indicates a shift towards acidic conditions for experiments without a buffering mineral assemblage. The estimated pH of the fluid was ∼2 at 600 °C, 0.26 GPa, 6.6 m initial NH₄Cl, based on the ratio of the integrated ν₁-NH₃ and ν₁-NH₄⁺ intensities and the HCl⁰ dissociation constant. The NH₃/NH₄⁺ ratio increased with temperature and decreased with pressure. This implies that more ammonium should be retained in K-bearing minerals coexisting with chloridic fluids upon high-P low-T metamorphism. At 500 °C, 0.73 GPa, ammonium partitions preferentially into the fluid, as constrained from infrared spectroscopy on the muscovite and from mass balance.The conversion of K-feldspar to muscovite proceeded much faster in experiments with NH₄Cl solutions than in comparable experiments with water. This is interpreted as being caused by enhancement of the rate-limiting alumina solubility, suggesting complexation of Al with NH₄. Nucleation and growth of mica at the expense of K-feldspar and NH₄⁺/K⁺ exchange between fluid and K-feldspar occurred simultaneously, but incorporation of NH₄⁺ into K-feldspar was distinctly faster than K-feldspar consumption.     

Petroleum and aqueous inclusions from deeply buried reservoirs: Experimental simulations and consequences for overpressure estimates

Synthetic hydrocarbon and aqueous inclusions have been created in the laboratory batch reactors in order to mimic inclusion formation or re-equilibration in deeply buried reservoirs. Inclusions were synthesized in quartz and calcite using pure water and Mexican dead oil, or n-tetradecane (C₁₄H₃₀), at a temperature and pressure of 150 °C and 1 kbar. One-phase hydrocarbon inclusions are frequently observed at standard laboratory conditions leading to homogenization temperatures between 0 and 60 °C. UV epifluorescence of Mexican oil inclusions is not uniform; blue and green-yellow colored inclusions coexist; however, no clear evidence of variations in fluid chemistry were observed. Homogenization temperatures were recorded and the maxima of T_h plotted on histograms are in good agreement with expected T_h in a range of 6 °C. Broad histograms were reconstructed showing non-symmetrical T_h distributions over a 20 °C temperature range centered on the expected T_h. This histogram broadening is due to the fragility of the fluid inclusions that were created by re-filling of pre-existing microcavities. Such T_h histograms are similar to T_h histograms recorded on natural samples from deeply buried carbonate reservoirs. T_h values lower than those expected were measured for hydrocarbon inclusions in quartz and calcite, and for aqueous inclusions in calcite. However, the results confirm the ability of fluid inclusions containing two immiscible fluids to lead to PT reconstructions, even in overpressured environments.     

A model for the viscosity of rhyolite as a function of H2O-content and pressure: A calibration based on centrifuge piston cylinder experiments

The Newtonian viscosity of synthetic rhyolitic liquids with 0.15-5.24 wt% dissolved water was determined in the interval between 580 and 1640 °C and pressures of 1 atm and 5-25 kbar. Measurements were performed by combining static and accelerated (up to 1000g) falling sphere experiments on water-bearing samples, with high temperature concentric cylinder experiments on 0.15 wt% H₂O melts. These methods allowed viscosity determinations between 10² and 10⁷ Pa s, and cover the complete range of naturally occurring magmatic temperatures, pressures, and H₂O-contents for rhyolites.Our viscosity data, combined with those from previous studies, were modeled by an expression based on the empirical Vogel-Fulcher-Tammann equation, which describes viscosities and derivative properties (glass transition temperature T_g, fragility m, and activation volume of viscous flow Va) of silicic liquids as a function of P-T-X_(H2O). The fitted expressions do not account for composition-dependent parameters other than X_(H2O) and reproduce the entire viscosity database for silicic liquids to within 3.0% average relative error on log η (i.e. std. error of estimate of 0.26 log units).The results yield the expected strong decrease of viscosity with temperature and water content, but show variable pressure dependencies. Viscosity results to be strongly affected by pressure at low pressures; an effect amplified at low temperatures and water contents. Fragility, as a measure for the deviation from Arrhenian behavior, decreases with H₂O-content but is insensitive to pressure. Activation volumes are always largely negative (e.g., less than −10 cm³/mol) and increase strongly with H₂O-content. Variations in melt structure that may account for the observed property variations are discussed.     

Effect of oxygen fugacity on the H2O storage capacity of forsterite in the carbon-saturated systems

High pressure experiments have been performed in the systems Mg₂SiO₄-C-O-H and Mg₂SiO₄-K₂CO₃-C at 6.3 GPa and 1200 to 1600 °C using a split-sphere multi-anvil apparatus. In the Mg₂SiO₄-C-O-H system the composition of fluid was modeled by adding different amounts of water and stearic acid. The fO₂ was controlled by the Mo-MoO₂ or Fe-FeO oxygen buffers. Several experiments in the Mg₂SiO₄-C-O-H system and all experiments in the Mg₂SiO₄-K₂CO₃-C system have been conducted without buffering the fO₂. Forsterite in the system Mg₂SiO₄-K₂CO₃-C does not reveal OH absorption bands in the IR spectra, while forsterite coexisting with carbon-bearing fluid and silicate melt at logfO₂ from FMQ-2 to FMQ-5 (from 2 to 5 log units below fayalite-magnetite-quartz oxygen buffer) contains 800-1850 wt. ppm H₂O. The maximum concentrations were detected at 1400 °C and FMQ-3.5. We observed an increase in the solidus temperature in the system Mg₂SiO₄-C-O-H from 1200 to above 1600 °C with log fO₂ decreasing from FMQ-2 to FMQ-5. The increase of the solidus temperature and the broadening of the stability field of the H₂O-H₂-CH₄ subsolidus fluid phase at 1400-1600 °C explain the high H₂O storage capacity of forsterite relative to that crystallized from carbon-free, oxidized, hydrous, silicic melt. At temperatures above 1400 °C liquidus forsterite precipitated along with diamond from oxidized (FMQ-1) carbonate-silicate melt and from silicate melt dissolving the moderately reduced C-O-H fluid (from FMQ-2 to FMQ-3.5). Formation of diamond was not detected under ultra-reduced conditions (FMQ-5) at 1200-1600 °C. Olivine co-precipitating with diamond from dry carbonate-silicate or hydrous-silicic fluid/melt can provide information on the H₂O contents and speciation of the diamond-forming media in the mantle. The conditions for minimum post-crystallization alteration of olivine and its hydrogen content are discussed.     

Quantification of water content and speciation in natural silicic glasses (phonolite, dacite, rhyolite) by confocal microRaman spectrometry

The determination of total water content (H₂O_T: 0.1-10 wt%) and water speciation (H₂O_molecular/OH) in volcanic products by confocal microRaman spectrometry are discussed for alkaline (phonolite) and calcalkaline (dacite and rhyolite) silicic glasses. Shape and spectral distribution of the total water band (H₂O_T) at ∼3550 cm⁻¹ show systematic evolution with glass H₂O_T, water speciation and NBO/T. In the studied set of silicic samples, calibrations based on internal normalization of the H₂O_T band to a band related to vibration of aluminosilicate network (TOT) at ∼490 cm⁻¹ vary with glass peraluminosity. An external calibration procedure using well-characterized glass standards is less composition-dependent and provides excellent linear correlation between total dissolved water content and height or area of the H₂O_T Raman band. Accuracy of deconvolution procedure of the H₂O_T band to quantify water speciation in water-rich and depolymerized glasses depends on the strength of OH hydrogen bonding. System confocal performance, scattering from embedding medium and glass microcrystallinity have a crucial influence on accuracy of Raman analyses of water content in glass-bearing rocks and melt inclusions in crystals.     

Structure of H2O-saturated peralkaline aluminosilicate melt and coexisting aluminosilicate-saturated aqueous fluid determined in-situ to 800 °C and ∼800 MPa

The structure of H₂O-saturated silicate melts and of silicate-saturated aqueous solutions, as well as that of supercritical silicate-rich aqueous liquids, has been characterized in-situ while the sample was at high temperature (to 800 °C) and pressure (up to 796 MPa). Structural information was obtained with confocal microRaman and with FTIR spectroscopy. Two Al-bearing glasses compositionally along the join Na₂O•4SiO₂-Na₂O•4(NaAl)O₂-H₂O (5 and 10 mol% Al₂O₃, denoted NA5 and NA10) were used as starting materials. Fluids and melts were examined along pressure-temperature trajectories of isochores of H₂O at nominal densities (from PVT properties of pure H₂O) of 0.85 g/cm³ (NA10 experiments) and 0.86 g/cm³ (NA5 experiments) with the aluminosilicate + H₂O sample contained in an externally-heated, Ir-gasketed hydrothermal diamond anvil cell.Molecular H₂O (H₂O°) and OH groups that form bonds with cations exist in all three phases. The OH/H₂O° ratio is positively correlated with temperature and pressure (and, therefore, fugacity of H₂O, f_H2O) with (OH/H₂O°)^melt > (OH/H₂O°)^fluid at all pressures and temperatures. Structural units of Q³, Q², Q¹, and Q⁰ type occur together in fluids, in melts, and, when outside the two-phase melt + fluid boundary, in single-phase liquids. The abundance of Q⁰ and Q¹ increases and Q² and Q³ decrease with f_H2O. Therefore, the NBO/T (nonbridging oxygen per tetrahedrally coordination cations), of melt is a positive function of f_H2O. The NBO/T of silicate in coexisting aqueous fluid, although greater than in melt, is less sensitive to f_H2O.The melt structural data are used to describe relationships between activity of H₂O and melting phase relations of silicate systems at high pressure and temperature. The data were also combined with available partial molar configurational heat capacity of Qⁿ-species in melts to illustrate how these quantities can be employed to estimate relationships between heat capacity of melts and their H₂O content.     

High-pressure melting relations in Fe-C-S systems: Implications for formation, evolution, and structure of metallic cores in planetary bodies

We present new high-pressure temperature experiments on melting phase relations of Fe-C-S systems with applications to metallic core formation in planetary interiors. Experiments were performed on Fe-5 wt% C-5 wt% S and Fe-5 wt% C-15 wt% S at 2-6 GPa and 1050-2000 °C in MgO capsules and on Fe-13 wt% S, Fe-5 wt% S, and Fe-1.4 wt% S at 2 GPa and 1600 °C in graphite capsules. Our experiments show that: (a) At a given P-T, the solubility of carbon in iron-rich metallic melt decreases modestly with increasing sulfur content and at sufficiently high concentration, the interaction between carbon and sulfur can cause formation of two immiscible melts, one rich in Fe-carbide and the other rich in Fe-sulfide. (b) The mutual solubility of carbon and sulfur increases with increasing pressure and no super-liquidus immiscibility in Fe-rich compositions is likely expected at pressures greater than 5-6 GPa even for bulk compositions that are volatile-rich. (c) The liquidus temperature in the Fe-C-S ternary is significantly different compared to the binary liquidus in the Fe-C and Fe-S systems. At 6 GPa, the liquidus of Fe-5 wt% C-5 wt% S is 150-200 °C lower than the Fe-5 wt% S. (d) For Fe-C-S bulk compositions with modest concentration of carbon, the sole liquidus phase is iron carbide, Fe₃C at 2 GPa and Fe₇C₃ at 6 GPa and metallic iron crystallizes only with further cooling as sulfur is concentrated in the late crystallizing liquid. Our results suggest that for carbon and sulfur-rich core compositions, immiscibility induced core stratification can be expected for planets with core pressure less than ∼6 GPa. Thus planetary bodies in the outer solar system such as Ganymede, Europa, and Io with present day core-mantle boundary (CMB) pressures of ∼8, ∼5, and 7 GPa, respectively, if sufficiently volatile-rich, may either have a stratified core or may have experienced core stratification owing to liquid immiscibility at some stage of their accretion. A similar argument can be made for terrestrial planetary bodies such as Mercury and Earth’s Moon, but no such stratification is predicted for cores of terrestrial planets such as Earth, Venus, and Mars with the present day core pressure in the order ?136 GPa, ?100 GPa, and ?23 GPa. (e) Owing to different expected densities of Fe-rich (and carbon-bearing) and sulfur-rich metallic melts, their settling velocities are likely different; thus core formation in terrestrial planets may involve rain of more than one metallic melt through silicate magma ocean. (f) For small planetary bodies that have core pressures <6 GPa and have a molten core or outer core, settling of denser carbide-rich liquid or flotation of lighter, sulfide-rich melt may contribute to an early, short-lived geodynamo.     

CO2 solubility in Martian basalts and Martian atmospheric evolution

To understand possible volcanogenic fluxes of CO₂ to the Martian atmosphere, we investigated experimentally carbonate solubility in a synthetic melt based on the Adirondack-class Humphrey basalt at 1-2.5 GPa and 1400-1625 °C. Starting materials included both oxidized and reduced compositions, allowing a test of the effect of iron oxidation state on CO₂ solubility. CO₂ contents in experimental glasses were determined using Fourier transform infrared spectroscopy (FTIR) and Fe³⁺/Fe^T was measured by Mössbauer spectroscopy. The CO₂ contents of glasses show no dependence on Fe³⁺/Fe^T and range from 0.34 to 2.12 wt.%. For Humphrey basalt, analysis of glasses with gravimetrically-determined CO₂ contents allowed calibration of an integrated molar absorptivity of 81,500 ± 1500 L mol⁻¹ cm⁻² for the integrated area under the carbonate doublet at 1430 and 1520 cm⁻¹. The experimentally determined CO₂ solubilities allow calibration of the thermodynamic parameters governing dissolution of CO₂ vapor as carbonate in silicate melt, K_II, (Stolper and Holloway, 1988) as follows: , ΔV⁰ = 20.85 ± 0.91 cm³ mol⁻¹, and ΔH⁰ = −17.96 ± 10.2 kJ mol⁻¹. This relation, combined with the known thermodynamics of graphite oxidation, facilitates calculation of the CO₂ dissolved in magmas derived from graphite-saturated Martian basalt source regions as a function of P, T, and f_O2. For the source region for Humphrey, constrained by phase equilibria to be near 1350 °C and 1.2 GPa, the resulting CO₂ contents are 51 ppm at the iron-wüstite buffer (IW), and 510 ppm at one order of magnitude above IW (IW + 1). However, solubilities are expected to be greater for depolymerized partial melts similar to primitive shergottite Yamato 980459 (Y 980459). This, combined with hotter source temperatures (1540 °C and 1.2 GPa) could allow hot plume-like magmas similar to Y 980459 to dissolve 240 ppm CO₂ at IW and 0.24 wt.% of CO₂ at IW + 1. For expected magmatic fluxes over the last 4.5 Ga of Martian history, magmas similar to Humphrey would only produce 0.03 and 0.26 bars from sources at IW and IW + 1, respectively. On the other hand, more primitive magmas like Y 980459 could plausibly produce 0.12 and 1.2 bars at IW and IW + 1, respectively. Thus, if typical Martian volcanic activity was reduced and the melting conditions cool, then degassing of CO₂ to the atmosphere may not be sufficient to create greenhouse conditions required by observations of liquid surface water. However, if a significant fraction of Martian magmas derive from hot and primitive sources, as may have been true during the formation of Tharsis in the late Noachian, that are also slightly oxidized (IW + 1.2), then significant contribution of volcanogenic CO₂ to an early Martian greenhouse is plausible.     

Pressure effects on the reduced partition function ratio for hydrogen isotopes in water

We have developed a simple, yet accurate theoretical method for calculating the reduced isotope partition function ratio (RIPFR) for hydrogen of water at elevated pressures. This approach requires only accurate equations of state (EOS) for pure isotopic end-members (H₂O and D₂O), which are available in the literature. The effect of pressure or density on the RIPFR of water was calculated relative to that of ideal-gas water at infinitely low pressure for the temperature range from 0 to 527 °C. For gaseous and low-pressure (ca. ?15 MPa) supercritical phases of water, the RIPFR increases slightly (1-1.3‰) with pressure or density in a fashion similar to those of many other geologic materials. However, in liquid and high-pressure (>20 MPa) supercritical phases, the RIPFR of water decreases (0.5-6‰) with increasing pressure (or density) to 100 MPa. This rather unique phenomenon is ascribed to the inverse molar volume isotope effects (MVIE) of liquid and high-density supercritical waters, V (D₂O) > V (H₂O), while other substances including minerals show the normal MVIE. These theoretical predictions were experimentally confirmed by Horita et al. [Horita, J., Cole, D.R., Polyakov, V.B., Driesner, T., 2002. Experimental and theoretical study of pressure effects on hydrogen isotope fractionation in the system brucite-water at elevated temperatures. Geochim. Cosmochim. Acta66, 3769 - 3788.] for the system brucite-water. Although the P-T ranges for the EOS of normal and heavy waters are rather limited, our modeling indicates that the RIPFR of water continues to decrease with pressure above 100 MPa. The method developed here can be applied to any other geologic fluids, if accurate EOS for their isotopic end-members is available. These results have important implications for the interpretation of high-pressure isotopic partitioning in the Earth, the outer planets, and their moons.     

Experimental investigation of the solubility of albite and jadeite in H2O, with paragonite + quartz at 500 and 600 °C, and 1-2.25 GPa

The solubilities of the assemblages albite + paragonite + quartz and jadeite + paragonite + quartz in H₂O were determined at 500 and 600 °C, 1.0-2.25 GPa, using hydrothermal piston-cylinder methods. The three minerals are isobarically and isothermally invariant in the presence of H₂O, so fluid composition is uniquely determined at each pressure and temperature. A phase-bracketing approach was used to achieve accurate solubility determinations. Albite + quartz and jadeite + quartz dissolve incongruently in H₂O, yielding residual paragonite which could not be retrieved and weighed. Solution composition fixed by the three-mineral assemblage at a given pressure and temperature was therefore bracketed by adding NaSi₃O_6.5 glass in successive experiments, until no paragonite was observed in run products. Solubilities derived from experiments bounding the appearance of paragonite thus constrain the equilibrium fluid composition. Results indicate that, at a given pressure, Na, Al, and Si concentrations are higher at 600 °C than at 500 °C. At both 500 and 600 °C, solubilities of all three elements increase with pressure in the albite stability field, to a maximum at the jadeite-albite-quartz equilibrium. In the jadeite stability field, element concentrations decline with continued pressure increase. At the solubility maximum, Na, Al, and Si concentrations are, respectively, 0.16, 0.05, and 0.48 molal at 500 °C, and 0.45, 0.27, and 1.56 molal at 600 °C. Bulk solubilities are 3.3 and 10.3 wt% oxides, respectively. Observed element concentrations are everywhere greater than those predicted from extrapolated thermodynamic data for simple ions, monomers, ion pairs, and the silica dimer. The measurements therefore require the presence of additional, polymerized Na-Al-Si-bearing species in the solutions. The excess solubility is >50% at all conditions, indicating that polymeric structures are the predominant solutes in the P-T region studied. The solubility patterns likely arise from combination of the large solid volume change associated with the albite-jadeite-quartz equilibrium and the rise in Na-Al-Si polymerization with approach to the hydrothermal melting curves of albite + quartz and jadeite + quartz. Our results indicate that polymerization of Na-Al-Si solutes is a fundamental aspect of fluid-rock interaction at high pressure. In addition, the data suggest that high-pressure metamorphic isograds can impose unexpected controls on metasomatic mass transfer, that significant metasomatic mass transfer prior to melting should be considered in migmatitic terranes, and that polymeric complexes may be an important transport agent in subduction zones.