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The synchrotron technique of micro X-ray absorption near-edge structure spectroscopy at the sulfur K-edge (S-XANES) provides a unique opportunity to measure the proportion of different oxidation states of sulfur (S) in silicate glasses. Although applied extensively in the analysis of basaltic silicate glasses, few S-XANES studies have investigated variations in S oxidation states with fO2 in felsic silicate glasses. In addition, no study has systematically compared the S-XANES results obtained from the same samples at different photon flux densities to quantify the relationship between exposure time and changes in S speciation in silicate glass, as has been done for Fe and V. This study evaluates observed differences in S speciation measured in experimentally produced H2O-saturated dacitic glasses over a range of reducing to oxidizing conditions (from log fO2 = ΔFMQ-0.7 to ΔFMQ+3.3; FMQ is the fayalite-magnetite-quartz mineral redox buffer) and equilibrated at 1000 °C and 300 MPa. S-XANES spectra were collected at three different photon flux densities using three microspectroscopy beamlines. As is observed in S-XANES analyses of basaltic silicate glasses, beam-induced changes to the S6+/ΣS are observed as a function of photon flux density and beam exposure time. Our results demonstrate that silicate glasses of dacitic composition undergo beam-induced photo-reduction in samples equilibrated at ΔFMQ > +1.75 and photo-oxidation if equilibrated at ΔFMQ < +1. The time required to observe beam-induced changes in the spectra varies as a function of flux density, and our study establishes an upper photon density limit at ~1.0 × 1012 photons/μm2. The S6+/ΣS calculated from spectra collected below this absorbed photon limit at intermediate flux densities (~1–4 × 109 photons/s per μm2) are affected by beam damage, as no conditions were found to be completely free of beam-induced changes. However, the S6+/ΣS ratios calculated below the limit at intermediate flux densities are consistent with thermodynamic constraints, demonstrating that S6+/ΣS ratios calculated from S-XANES spectra can be considered reliable for estimating the oxygen fugacity. Our results carry important implications for the S budget of felsic magmas and dissolution mechanisms in evolved melts. While our results from all three flux densities show the presence of S4+ dissolved in relatively oxidized (ΔFMQ > +1.75) dacitic glass, even in the spectra exposed to the lowest photon densities, we are unable to rule out the possibility that the S4+ signal is the result of instantaneous X-ray irradiation induced beam damage using S-XANES alone. When our spectra are compared to S-XANES spectra from basaltic silicate glasses, important differences exist in the solubility of S2− and S6+ between dacitic silicate melts, pointing to differences in solubility mechanisms as melt composition changes. This study highlights the need for further investigation into beam damage systematics, presence of S4+, and the solubility mechanisms of different oxidation states of S as silicate melt composition changes. [ABSTRACT FROM AUTHOR]

Solubility experiments of MnN[b.sub.2][O.sub.6] and Mn[Ta.sub.2][O.sub.6] were conducted in two nominally dry to watersaturated pegmatitic melts with different amounts of Li, F, P, and B at 700 to 1000[degrees]C and 200 MPa to determine the maximum concentrations of Nb and Ta in pegmatitic melts. The [Li.sub.2]O, F, [B.sub.2][O.sub.3], and [P.sub.2][O.sub.5] contents in the melts were 1.16, 2.99, 1.78, and 1.55 wt% for melt composition PEG1 and 1.68, 5.46, 2.75, and 2.75 wt% for melt composition PEG2 and the resulting Al/(Na+K+Li) ratio for both melts is 0.92. The experimental data show that the solubility product of manganocolumbite increases by a factor of three upon increasing the water concentration from 0 to 4 wt%. Considering that pegmatitic melts at pressures above 50 to 100 MPa are hydrous (>4 wt% [H.sub.2]O), the increase in solubility by this magnitude, over the stated range of water concentration, is not significant for pegmatites. The data also point out that the solubility of MnN[b.sub.2][O.sub.6] and Mn[Ta.sub.2][O.sub.6] is strongly temperature dependent, increasing by a factor of 50 for manganocolumbite and 15-20 for manganotantalite from 700 to 1000[degrees]C under water-saturated conditions. The solubility also increases with increasing content of fluxing elements like Li, F, B, and E In the pegmatite melt containing the highest amount of fluxing elements, the maximum concentrations of Ta and Nb are higher by nearly one order of magnitude when compared to a subaluminous rhyolitic melt. Keywords: Pegmatitic melt, volatiles, solubility, columbite, tantalite DOI: 10.2138/am.2010.3157

Crystallization experiments were conducted at 200 MPa and water-undersaturated conditions to quantify the effect of small amounts of [H.sub.2]O on the crystallization temperature of olivine in basaltic melts (e.g., Mid-Oceanic Ridge Basalts, MORB).The [H.sub.2]O concentrations in the quenched glasses, determined by infrared spectroscopy and Karl-Fischer-Titration, ranged between 0.25 and 4.2 wt% [H.sub.2]O. The dry liquidus temperature was estimated from experiments at 0.1 MPa ([H.sub.2]O-free) and from the known pressure dependence of olivine crystallization temperature. The liquidus temperature depression can be predicted by the empirical equation ([T.sup.DRY]-[T.sup.WET]) = 39.69*[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (wt%). The comparison of the experimental results with available crystallization models and empirical methods shows that most of the predicted liquidus temperature depressions differ significantly from that observed in this study. Keywords: Crystallization, [H.sub.2]O, MORB, olivine, liquidus, FTIR, KFT

In the present work, we characterize the amphibole Na(NaMg)[Mg.sub.5][Si.sub.8][O.sub.22][(OH).sub.2] synthesized at 0.4 GPa and 750, 800, and 850 [degrees]C, and 0.5 GPa, 900 [degrees]C. Experiments at 800 and 900 [degrees]C yielded crystals suitable for single-crystal data collection. Structure refinement shows that synthetic Na(NaMg)[Mg.sub.5][Si.sub.8][O.sub.22][(OH).sub.2] has P[2.sub.1]/m symmetry at room T. The two non-equivalent tetrahedral double-chains differ in their degree of stretching and kinking. The infrared spectrum of synthetic Na(NaMg)[Mg.sub.5][Si.sub.8][O.sub.22][(OH).sub.2] has two well-defined absorption bands at 3742 and 3715 [cm.sup.-1] which can be assigned to O-H bands associated with the two independent anion sites (O3A and O3B) in the structure. The higher frequency band is assigned to the shorter O3B-H2 bond, and the lower tYequency band is assigned to the longer O3A-H1 bond. The broader shape of the 3743 [cm.sup.-1] band is consistent with a stronger interaction of the H2 atom with [sup.A]Na, which is confirmed by structure refinement. Increasing Tof synthesis causes a progressive departure from the ideal stoichiometry via the A[[cube root of].sub.1][sup.B][Mg.sub.1][sup.A][Na.sub.-1][sup.B][Na.sub.-1] substitution, as confirmed by EMPA, structure refinement, and FFIR spectroscopy.

A new calibration was performed for the molecular C[O.sub.2] band at 2349 [cm.sup.-1] in the infrared absorption spectra of rhyolitic glasses. Glasses with varying amounts of C[O.sub.2] (730-3900 ppm by weight) and [H.sub.2]O (0.5-7.0 wt%) were synthesized in an internally heated pressure vessel at 200-800 MPa and 1100 and 1200[degrees]C. The C[O.sub.2] content of the glasses was measured by coulometric C[O.sub.2]-titration after thermal extraction at 1200[degrees]C. It is shown that the entire C[O.sub.2] content cannot be extracted from a rhyolitic glass at this temperature. Using the Lambert-Beer law, and taking into account the residual C[O.sub.2] still present in the glasses after extraction, we have calculated a linear molar absorption coefficient of 1214 [+ or -] 78 L*[cm.sup.-1]/mol for the band at 2349 [cm.sup.-1]. Strictly, this value is a practical absorption coefficient because the IR band intensity is related to the total C[O.sub.2] content and not to the molecular C[O.sub.2] content. However, no direct evidence for other carbon species such as carbonate groups is found in the IR spectra. Hence, we suggest that the concentration of carbonate in the studied metaluminous rhyolite is negligible and the derived value is close to the real absorption coefficient for molecular C[O.sub.2]. The absorption coefficient does not vary noticeably with changing water content of the glasses. With the new calibration, C[O.sub.2] concentrations in rhyolitic glasses are 13% higher than data based on the previous calibration of Blank (1993) for water-poor glasses.

The influence of F substitution on the local structure of Al in the tetrahedral and octahedral sheets of synthetic Al-rich phlogopite samples with nominal gel compositions of K([Mg.sub.3-x][Al.sub.x] [[Al.sub.1+x][Si.sub.3-x][O.sub.10]] [(OH).sub.y][(F).sub.2-y] between 0.0 [less than or equal to] x [less than or equal to] 0.8 and 0.5 [less than or equal to] y [less than or equal to] 1.8, was studied by [sup.27]Al MAS, MQMAS, {[sup.1]H/[sup.19]F} [vector] [sup.27]Al 2D CPMAS (HETCOR) and {[sup.1]H/[sup.19]F} [sup.27]Al REDOR solid-state NMR an by IR spectroscopy. Changes in intensity of the absorption bands in the OH-stretching region of the IR spectra clearly indicate the incorporation of octahedral Al. Signais from the different phases can be separated in the [sup.27]Al MQMAS NMR spectra by generation of an isotropic dimension in F1. The [sup.27]Al quadrupolar parameters of the four phases were estimated from [sup.27]Al MAS NMR spectra obtained at 104.26 and 208.42 MHz. The quadrupolar coupling constant and isotropic chemical shift increases with increasing Al content for the [sup.IV]Al site in phlogopite. The [sup.VI]Al site shows a clear increase of the asymmetry parameter and [C.sub.Q] with increasing F content. The estimated [sup.27]Al signal areas show the lowest amount of impurity phases at high OH contents and a stabilization of [sup.VI]Al sites by hydroxyl groups. The {[sup.1]H} [vector] [sup.27]Al 2D CPMAS (HETCOR) NMR experiment at short contact rimes provides information about site neighborhoods of tetrahedral Al sites and [Mg.sub.3]OH as well as [Mg.sub.2]AlOH sites, whereas magnetization is only transferred to the octahedral Al sites from hydroxyl groups in [Mg.sub.2]AlOH sites. The {[sup.1]H/[sup.19]F} [vector] [sup.27]Al 2D CPMAS (HETCOR) NMR spectrum is dominated by [sup.IV]Al sites coupled to the [Mg.sub.3]F complex in phlogopite. Resonances from [Mg.sub.2]AlF complexes are not observed. Finally, the {[sup.1]H/[sup.19]F} [sup.27]Al REDOR experiments support the results of the 2D CPMAS (HETCOR) experiments.

The influence of F content on the formation and stability of Al-rich phlogopite has been investigated. Samples with varying OH/F-ratios and nominal gel compositions of K([Mg.sub.3-x][Al.sub.x])([Al.sub.1+x][Si.sub.3 -x][O.sub.10])[(OH).sub.y] [(F).sub.2-y] (0.0 [less than or equal to] x [less than or equal to] 0.8 and 0.5 [less than or equal to] y [less than or equal to] 1.8) were studied using [sup.29]Si, [sup.1]H, and [sup.19]F MAS NMR spectroscopies, powder X-ray diffraction, electron-probe microanalysis, and scanning electron microscopy. The synthetic phlogopites were synthesized from sol-gels in cold-seal pressure vessels at 1073 K, 2 kbar. The main phase (phlogopite) and three other impurity phases [corundum ([alpha]-[Al.sub.2][O.sub.3]), kalsilite (KalSi[O.sub.4]), and potassium aluminum hexafluoride ([K.sub.3]Al[F.sub.6]*0.5[H.sub.2]O)] were clearly identified by powder X-ray diffraction and electron-probe microanalysis. For phlogopite, the unit-cell parameters [a.sub.0] and [b.sub.0] decrease whereas [c.sub.0] increases with increasing Al-content (x). The average crystal size of phlogopite is about 1-2 [micro]m. The [sup.29]Si MAS NMR spectra show up to four resonances at approximately -91, -87, -83, and -80 ppm, which can be assigned as [Q.sup.3](n Al) signals with n = 0-3. The ideal [Si/.sup.IV] Al ratio calculated from the initial composition is always lower than that derived experimentally. Hydroxyl-rich compositions indicate an increased Al-content in the tetrahedral sheets, suggesting a stabilizing effect on the formation of Al-rich phlogopite. These conclusions are supported by [sup.1]H and [sup.19]F MAS NMR spectra. The [sup.1]H MAS NMR spectra show a water signal at 4.7 ppm, a signal due to [Mg.sub.2]AlOH at 1.8 ppm, and a signal due to [Mg.sub.3]OH at 0.7 ppm. The [sup.19]F MAS NMR spectra exhibit a signal for [Mg.sub.2]AlF at -150 ppm, one due to Al[F.sub.6] ([K.sub.3]Al[F.sub.6]*0.5[H.sub.2]O) at -157 ppm, and one for [Mg.sub.3]F at -174 ppm. Comparison of the [sup.1]H and [sub.19]F MAS NMR spectra for different Al- and F-contents reveals a non-statistical distribution of F- and OH-groups at the crystallographic sites in the octahedral sheets where F prefers sites coordinated by three Mg, and OH sites with Al in the next-nearest coordination sphere.

This study presents improvements of internally heated pressure vessels to realize high-pressure experiments at controlled f[o.sub.2] in low-viscosity systems such as basaltic ones. The new design is a combination of two experimental techniques: a hydrogen sensor membrane made of platinum to measure [f.sub.[H.sub.2]], and therefore [f.sub.[o.sub.2]], and a rapid-quench system to avoid crystallization of low-viscosity melts during quench. The experimental setup has been tested successfully at temperatures up to 1250[degrees]C and pressures up to 500 MPa. Basaltic melts containing up to 9.38 wt% water can be quenched as bubble-free and crystal-free glasses. The improvements allow synthesis of hydrated glass or partly crystallized samples with a large volume (for further studies) and to perform routine phase-equilibrium studies in basaltic systems at geologically relevant conditions. We used the new technique to determine the effect of [f.sub.[o.sub.2]] on water solubility in a melt with MORB composition. The results show that there is a small but significant decrease of water solubility with decreasing [f.sub.[o.sub.2]] from MnO-[Mn.sub.3][O.sub.4] to QFM buffer conditions in the pressure range 50-200 MPa. Kinetic problems in crystallization experiments in basaltic systems and the duration necessary to attain equilibrium [Fe.sup.2+]/[Fe.sup.3+] ratio in the charge are discussed.

A newly developed furnace assemblage helps perform long-duration and high temperature experiments in internally heated, Ar-medium pressure vessels working up to 10 kbar. This furnace enables the investigation of H2O solubility in granitic and SiO2 melts and the measurement of the viscosity of hydrous granitic melts. High-temperature equilibria and phase relationships in andesitic to basaltic systems are experimentally evaluated using this furnace.

A study was conducted on H2O influence on liquidus phase relations in the haplogranite system at two andfive kbar. Results showed an increase in normative quartz content at five kbar for the quartz-feldspar field boundary. At both pressures, liquidus temperatures increased whenever the H2O contend of the melt was reduced and minimum and eutectic compositions shifted toward the Qz-Or join. The results indicate a differential reactivity of Ab- and Or-forming units in the melts containing H2O.

Transport and deposition of copper in the Earth's crust are mainly controlled by the solubility of Cu-bearing phases and the speciation of Cu in magmatic-hydrothermal fluids. To improve our understanding of copper mobilization by hydrothermal fluids, we conducted an experimental study on the interaction between Cu-bearing phases (metallic copper, Cu2O, CuCl) and aqueous chloride solutions (H2O ± NaCl ± HCl; with Cl concentrations of 0 to 4.3 mol kg-1). The experiments were run in rapid heat/rapid quench cold-seal pressure vessels at 800 °C, 200 MPa, and logfO2 ~ NNO+2.3. Either Cu or Au capsules were used as containers. The reaction products were sampled in situ by the entrapment of synthetic fluid inclusions in quartz. Fluid composition was subsequently determined by analyzing individual fluid inclusions using a freezing cell and laser ablation inductively coupled plasma-mass spectrometry. Our results show that large isolated and isometric inclusions, free of late-stage modifications, can be preserved after the experiment even when using a high cooling rate of 25 K s-1. The obtained results demonstrate that: (1) reaction between native Cu, NaCl solution, and quartz (± silica gel) leads to the coexistence of fluid inclusions and Na-bearing silicate melt inclusions. Micrometer- to submicrometer-sized cuprite (Cu2O) crystals have been observed in both types of the inclusions, and they are formed most probably due to the dissociation of CuOH. (2) When Cu0 reacts with HCl and CuCl solutions, or Cu+ reacts with NaCl solution, nantokite (CuCl) formed due to oversaturation has been found in fluid inclusion. Copper concentration in the fluid shows a strong positive dependence on the initial chlorine content, with Cu/Cl molal ratios varying from 1:9 to 1:1 in case 1 and case 2, respectively. When Cl is fixed to 1.5 m, initial fluid acidity has a major control on the Cu content, i.e., 0.17 ± 0.09 and 1.29 ± 0.57 m Cu were measured in fluids of case 1 and case 2, respectively. Cu solubility in pure water and in 1.5 m NaCl solutions are 0.004 ± 0.002 and 0.16 ± 0.07 m, respectively. The main responsible Cu-bearing complexes are CuOH(H2O)x in water, NaCuCl2 in NaCl solutions and HCuCl2 in alkali-free solutions. These results provide quantitative constraints on the mobility of Cu in hydrothermal solutions and confirm that Cl is a very important ligand responsible for Cu transport. The first observation that silicate melt can be generated in the fluid-dominated and native-copper-bearing system implies that transitional thermosilicate liquids can coexist with metal-rich fluids and may enhance Cu mobility in magmatic-hydrothermal systems. This may have important implications for the formation of Cu deposits in the systems with low S activities. [ABSTRACT FROM AUTHOR]

Water solubility in silica melts was determined at 100-600 MPa, 1200-1350 [degrees] C, and at each temperature (T) was found to increase with pressure (P). At P [is less than or equal to] 250 MPa, the effect of T on water solubility in silica melts is small and within analytical precision ([+ or -] 0.15 wt% [H.sub.2]O). A positive correlation with T was observed at 400 MPa. Increasing solubility of water with increasing T was observed when large amounts of water are dissolved in silica and quartzofeldspathic melts (i.e., when molecular water is the dominant species in the glasses at room temperature), as already observed for feldspar melts. Change in water solubility (expressed in mol%) with decreasing Si[O.sub.2] content of the melt is nonlinear along the silica-albite join. In the compositional range [Ab.sub.100] to [Ab.sub.25] (100 to 25 mol% albite, respectively, compositions calculated on an eight-oxygen basis), the solubility of water at 200 MPa decreases only slightly with decreasing Ab content (-0.1 [+ or -] 0.01 mol% [H.sub.2]O per mol% albite). However, at Ab contents less than 25 mol%, water solubility decreases sharply with increasing Qz content. Similar behavior was observed at 500 MPa. These results suggest that two different incorporation mechanisms of water in quartzofeldspathic melts must be considered: one corresponding to an NaAl[Si.sub.3][O.sub.8]-[H.sub.2]O mechanism, the other to an Si[O.sub.2]-[H.sub.2]O mechanism.

Tapiolite [FeTa2O6] and columbite-group minerals [(Fe,Mn)(Ta,Nb)2O6] are common Nb-Ta-bearing accessory minerals in rare-element granites and pegmatites. Their compositional gap has inspired several experimental studies, but none of them have succeeded in reproducing the parameters that influence the compositional gap. In this study, tapiolite and columbite-group minerals (CGM) were crystallized from water-saturated, flux-rich granitic melts at various conditions of pressure, temperature, oxygen fugacity, and Ti contents. Crystals with a size as small as 500 nm were analyzed with a field emission gun (FEG) electron microprobe. The results show that temperature, pressure, and Ti content only slightly affect the compositional gaps between tapiolite and CGM, whereas high fO2 leads to complete solid solution between a rutile-structured component Fe3+TaO4 and (Fe,Mn)Ta2O6. The experimental CGM-tapiolite compositional gaps are compared with natural CGM-tapiolite pairs from rare-element granites and pegmatites worldwide. This study reveals that the crystallographic structure of tapiolite and CGM could be the dominant parameter that influences the position of the compositional gap. Order-disorder in CGM and tapiolite is tightly linked to disequilibrium crystallization triggered by supersaturation. Significant isothermal Nb-Ta fractionation is observed inside CGM crystals that grow at high degrees of supersaturation. The effect of supersaturation prevails over the solubility effect that is known to increase the Ta/(Ta+Nb) ratio in CGM and coexisting melts. Thus, even if global equilibrium in terms of the solubility of Nb-Ta-bearing minerals is attained, the Ta/(Nb+Ta) ratio in the crystals may differ significantly from equilibrium. It implies that Nb-Ta fractionation in Nb-Ta oxides is controlled by crystallization kinetics rather than equilibrium chemical fractionation (or any other processes such as F-complexing of Ta or fluid exsolution) in dynamic systems that can rapidly reach supersaturated conditions. These results have important implications for the understanding of crystallization processes in highly evolved and pegmatite-forming magmas. [ABSTRACT FROM AUTHOR]

We have investigated experimentally the partitioning of Au between solid and liquid sulfide phases and basaltic melts at 200 MPa, at redox conditions close to the sulfide-sulfate transition, over temperatures between 1050 and 1200 °C, which span the monosulfide solid solution (MSS) - sulfide liquid (SuL) solidus. The measured MSS/basalt partition coefficient of Au (DAuMSS-sil) is about 100-200, whereas the partition coefficient of sulfide liquid/basalt (DAuSuL-sil) is approximately 10 times larger at 2200. Although we find that temperature, pressure, and oxygen fugacity (fO2) exert relatively weak controls on Au partitioning, they exert major indirect influences on Au behavior by controlling the identity of the condensed sulfide phase and by affecting S solubility. These observations have important implications for the behavior of Au in the processes of partial melting in the mantle and magma crystallization in the crust. The occurrence of natural magmas with elevated concentrations of Au and presumably other highly siderophile and chalcophile elements requires predominance of MSS over SuL in the source or/and oxidizing conditions close to or above the sulfide-sulfate transition in the magma. [ABSTRACT FROM AUTHOR]

To investigate the processes that control Ta incorporation in zircon, two types of synthesis experiments were performed: (1) crystallization of zircon from an Li-Mo flux at 1 atm, and (2) crystallization of zircon (with or without coexisting tantalite) from a highly fluxed pegmatitic melt at 200 MPa and nearly water-saturated conditions. The first type of experiment is used to identify the influence of various doping elements (Hf, P, Al, and Mn) on Ta incorporation in zircon. These experiments reveal that P hinders the incorporation of Ta, whereas Al enhances Ta incorporation via charge balancing, and that Ta can be incorporated in the absence of any other doping element via the creation of vacancies in the zircon structure. Hafnium does not affect significantly Ta incorporation. Manganese and lithium do not enter the structure of zircon, except in the presence of P. Experiments with Nb show that the concentration of this element in zircon is nearly one order of magnitude lower than Ta (for similar Ta and Nb concentrations in the flux). The second type of experiments show how Ta is incorporated in zircon in natural granite and pegmatite systems. In those systems, P and Al have elevated concentrations, and P is preferentially incorporated in zircon via Al substitution to maintain charge balance. Below a P/Ta atomic ratio of ~10 in the melt, Ta competes with P for Al, and P is involved in a coupled substitution with Mn for charge balancing. Concentrations up to 3.7 wt% Ta2O5 (0.03 apfu calculated to four atoms of oxygen) were measured in these zircon samples. The partition coefficients of Ta between melt and zircon are around 1 at 800-900 °C at conditions close to tantalite saturation. These results show that zircon incorporates significant amounts of Ta from the melt in P-poor peraluminous granites with a P/Ta atomic ratio lower than ~10, which may ultimately affect the precipitation of Ta minerals. Such interactions between cations both in melts and in mineral structures are important to constrain in order to understand rare-metal enrichment in granitic systems. [ABSTRACT FROM AUTHOR]

Focuses on a study which investigated the influence of F substitution on the local structure of aluminum (Al) in the tetrahedral and octahedral sheets of synthetic Al-rich phlogopite with nominal compositions. Background to the study; Methods; Results and discussion.

Part I. Investigates the influence of fluorine content on the formation and stability of aluminum-rich phlogopite. Crystal structure of phlogopite; Determination of phlogopite lattice parameters; Behaviors of the curves for aluminum-rich phlogopites with different fluorine contents.