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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 fO2in 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.

Tin is a key strategic metal and indispensable in the high-tech industry. Constraining the source of the mineralizing fluids, their pathways, and subsequent ore-forming process is fundamental to optimizing tin exploration and efficient mining operations. Here, we present trace element analysis, LA-ICP-MS mapping, and the first systematic high-precision in situ Sn isotope analysis of cassiterite from several tin deposits (i.e., Weilasituo, Baiyinchagan, Maodeng Sn-polymetallic deposits) in northeast China using UV-fs-LA-ICP-MS. We show that the distribution of trace elements in cassiterite from these localities reflects crystallization under disequilibrium conditions with coexisting fluids or melts, and it suggests intense fluid-rock reactions. Among the three deposits, cassiterite from the Maodeng Sn-Cu deposit has the heaviest weighted mean Sn isotope composition, with δ124/117Sn values ranging from 0.11 ± 0.04‰ to 0.62 ± 0.08‰. The Baiyinchagan Sn-Ag-Pb-Zn deposit displays the lightest isotope composition with δ124/117Sn values ranging from –1.43 ± 0.06‰ to –0.50 ± 0.04‰. While the Weilasituo Sn-W-Li-polymetallic deposit shows the largest spread in δ124/117Sn values, ranging from –0.66 ± 0.05‰ to 0.59 ± 0.03‰. The Sn isotope variability in these natural cassiterites is attributed to Sn isotope fractionation associated with the diversity of Sn mineralization pathways and different physicochemical conditions. Furthermore, the δ124/117Sn values of cassiterite from the Maodeng and Baiyinchagan deposits gradually decrease from early to late mineralization stages, suggesting that they were generated by Rayleigh fractionation during progressive mineral precipitation from a hydrothermal fluid. In contrast, heavy Sn isotope values in late-stage Weilasituo cassiterites are likely a result of disequilibrium fluid-rock interaction with external, wall-rock-derived fluids. Our results reveal that liquid-vapor partitioning or fluid-rock interaction may have more influence on Sn isotope fractionation between cassiterite and evolving ore-forming fluids than do magmatic differentiation, pH, pressure, and temperature during the formation of tin deposits. According to the tin isotopic data obtained so far from this study and published previously, we observe no relationship between the Sn isotope composition of cassiterite and the age of mineralization or tectonic setting. However, cassiterite displays heavier Sn isotope compositions than coexisting stannite (Cu2FeSnS4) regardless of the deposit type and depth of emplacement, suggesting that the redox state may influence Sn isotope fractionation. More importantly, we first recognize a general shift toward light Sn isotope compositions in cassiterite associated with decreasing Ti/Zr ratios, suggesting that Sn isotopes can be a robust tool for identifying the source of the mineralization. Furthermore, based on our Sn isotope data together with previous studies of fluid inclusion, we propose that the dominant Sn(II) species occur in early ore mineralization systems, then shifts to the Sn(IV) species in late stage due to redox change or higher Cl–activity. Tin isotopes may be a robust tool to trace the mineralization center and fluid pathways and to ascertain the mechanisms of metal precipitation.

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.

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 fO2leads to complete solid solution between a rutile-structured component Fe3+TaO4and (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.

In this work, we have studied the relationships between mass concentration and unit formula of amphibole using 114 carefully selected high-quality experimental data, obtained by electron microprobe (EMP) + single-crystal X-ray structure refinement (SREF) ± secondary-ion mass spectrometry (SIMS) analyses, of natural and synthetic Li-free monoclinic species belonging to the Ca and Na-Ca subgroups, and 75 Li-free and Mn-free C2/mend-members including oxo analogs of Ca amphiboles. Theoretical considerations and crystal-chemical driven regression analysis allowed us to obtain several equations that can be used to: (1) calculate from EMP analyses amphibole unit-formulas consistent with SREF±SIMS data, (2) discard unreliable EMP analyses, and (3) estimate WO2–and Fe3+contents in Li-free C2/mamphiboles with relatively low Cl contents (≤1 wt%). The AMFORM approach mostly relies on the fact that while the cation mass in Cl-poor amphiboles increases with the content of heavy elements, its anion mass maintains a nearly constant value, i.e., 22O + 2(OH, F, O), resulting in a very well-defined polynomial correlation between the molecular mass and the cation mass per gram (R2= 0.998).

The strong dependence of vanadium partitioning between olivine and silicate melt (DVO1-M$\begin{array}{} D_{\text V}^{\text{O1-M}} \end{array} $) on redox conditions (fO2) can be used as sensitive oxybarometer in magmatic systems. Here we extend the experimental database on DVO1-M$\begin{array}{} D_{\text V}^{\text{O1-M}} \end{array} $, obtained so far at high temperatures (mainly above 1250 °C), to lower temperatures that are typical for island-arc basalts. Crystallization experiments were performed using a composition from Mutnovsky volcano (Kamchatka), and the investigated temperature, pressure, and oxygen fugacity ranges were 1025–1150 °C, 0.1 and 0.3 GPa, and ΔQFM of –0.5 to +3.2, respectively. The water content in melts ranged from 0.6 to ~6.5 wt% H2O. The data demonstrate a strong negative correlation between DVO1-M$\begin{array}{} D_{\text V}^{\text{O1-M}} \end{array} $and oxygen fugacity, similar to the behavior observed previously at higher temperatures and in MgO-rich compositions. The correlation between DVO1-M$\begin{array}{} D_{\text V}^{\text{O1-M}} \end{array} $and ΔQFM in the range from –0.5 to +3.2 is described for melts with MgO < 12 wt% and Na2O < 4 wt% at temperatures ≤1250 °C by the empirical equation: ΔQFM = −3.07−0.29+0.26log⁡DVO1-M−3.34−0.49+0.40$\begin{array}{} -3.07_{-0.29}^{+0.26}\,\log D_{\text V}^{\text{O1-M}}-3.34_{-0.49}^{+0.40} \end{array} $with the standard error (SE) as a function of log DVO1-M$\begin{array}{} D_{\text V}^{\text{O1-M}} \end{array} $: 2SE(ΔQFM) = –0.275log DVO1-M$\begin{array}{} D_{\text V}^{\text{O1-M}} \end{array} $+ 0.4.

One of the various problems faced in experimental petrology is the fact that most experimental products obtained by crystallization experiments are too small, making their accurate identification by electron microprobe and laser ablation analyses very difficult. This problem is magnified when a highly polymerized starting material is used for experiments at low temperature (e.g., 700–800 °C). In this study, we present the results of crystallization experiments performed using a rhyolitic starting glass in which we test the potential of temperature cycling and pre-hydrated starting material to increase crystal size and discuss the effect of those variables on the attainment of chemical equilibrium. Experiments were performed at different temperatures (725 to 815 °C) and pressures (1 and 2 kbar), under water-saturated conditions (aH2O= 1; with aH2Obeing the water activity). During the experiments, temperature was either constant or cycled to ±15 °C around the target temperature during the first half of the runs. We used either a pre-hydrated (7 wt% H2O) rhyolitic glass or a dry rhyolitic glass to which 7 wt% H2O was added during capsule preparation.

We present textures and halogen and trace-element compositions of apatites in intrusive rocks from the dike-gabbro transition zone of a fast-spreading mid-ocean ridge, which was formed at the East Pacific Rise and recently drilled by IODP Hole 1256D. These data are used to discuss the properties of parental magmas and seawater-derived hydrothermal fluids at the roof of the axial melt lens during the formation of oceanic crust. In general, zoning of apatites from three different lithologies, tonalites, diorites, and gabbros, is common and shows a consistent evolution trend with depletion in Cl and REEs from core to rim. The cores are usually homogenous in composition and interpreted as magmatic origin, whereas zones with lower Cl and REEs are disseminated with heterogeneous concentrations, indicating exchanges with hydrothermal fluids. The apatite cores in tonalites are rich in both F and Cl, with XApF$X^\text{F}_{\text{Ap}}$ (proportion of fluorapatite end-member) up to 0.5 and XApCl$X^\text{Cl}_{\text{Ap}}$ (proportion of chlorapatite end-member) up to 0.4. In contrast, the apatite cores in gabbros have high XApCl$X^\text{Cl}_{\text{Ap}}$ (up to 0.85) and very low XApF$X^\text{F}_{\text{Ap}}$ (<0.05). The two contrasting types of apatite cores are both observed in diorites, implying that magma mixing processes may have controlled the formation of the dioritic intrusives. The strong depletions in Cl and REESin some parts of the apatite crystals (mainly rim) can be explained by removal of these components via hydrothermal fluids. Based on available F-Cl-OH exchange coefficients for apatite-melt, the very high Cl/OH and Cl/F ratios and high Cl contents calculated for tonalitic melts cannot be reconciled with a formation of these felsic melts by partial melting of amphibole-bearing metabasalts, but indicate that an assimilation of high-Cl brines must have occurred. Similarly, the low-F chlorapatites in gabbros also imply an assimilation of high-Cl brines. The source of high-Cl fluids in the axial magmatic system may result from seawater-derived fluids, which may form immiscible vapor and brine at high temperatures as a result of hydrothermal boiling.

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 (DMSS-silAu) is about 100-200, whereas the partition coefficient of sulfide liquid/basalt (DMSS-silAu) is approximately 10 times larger at 2200. Although we find that temperature, pressure, and oxygen fugacity (ƒO2) 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.

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).

A new calibration was performed for the molecular CO2band at 2349 cm-1in the infrared absorption spectra of rhyolitic glasses. Glasses with varying amounts of CO2(730-3900 ppm by weight) and H2O (0.5-7.0 wt%) were synthesized in an internally heated pressure vessel at 200-800 MPa and 1100 and 1200 °C. The CO2content of the glasses was measured by coulometric CO2-titration after thermal extraction at 1200 °C. It is shown that the entire CO2content cannot be extracted from a rhyolitic glass at this temperature. Using the Lambert-Beer law, and taking into account the residual CO2still present in the glasses after extraction, we have calculated a linear molar absorption coefficient of 1214 ± 78 L⋅cm-1/mol for the band at 2349 cm-1. Strictly, this value is a practical absorption coefficient because the IR band intensity is related to the total CO2content and not to the molecular CO2content. 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 CO2. The absorption coefficient does not vary noticeably with changing water content of the glasses. With the new calibration, CO2concentrations in rhyolitic glasses are 13% higher than data based on the previous calibration of Blank (1993) for water-poor glasses.

A new calibration was performed for the molecular CO2band at 2349 cm-1in the infrared absorption spectra of rhyolitic glasses. Glasses with varying amounts of CO2(730-3900 ppm by weight) and H2O (0.5-7.0 wt%) were synthesized in an internally heated pressure vessel at 200-800 MPa and 1100 and 1200 °C. The CO2content of the glasses was measured by coulometric CO2-titration after thermal extraction at 1200 °C. It is shown that the entire CO2content cannot be extracted from a rhyolitic glass at this temperature. Using the Lambert-Beer law, and taking into account the residual CO2still present in the glasses after extraction, we have calculated a linear molar absorption coefficient of 1214 ± 78 L⋅cm-1/mol for the band at 2349 cm-1. Strictly, this value is a practical absorption coefficient because the IR band intensity is related to the total CO2content and not to the molecular CO2content. 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 CO2. The absorption coefficient does not vary noticeably with changing water content of the glasses. With the new calibration, CO2concentrations in rhyolitic glasses are 13% higher than data based on the previous calibration of Blank (1993) for water-poor glasses.

Water solubility in silica melts was determined at 100-600 MPa, 1200-1350 °C, and at each temperature (T) was found to increase with pressure (P). At P ≤ 250 MPa, the effect of T on water solubility in silica melts is small and within analytical precision (±0.15 wt% H2O). 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 SiO2content of the melt is nonlinear along the silica-albite join. In the compositional range Ab100to Ab25(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 ± 0.01 mol% H2O 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 NaAlSi3O8-H2O mechanism, the other to an SiO2-H2O mechanism.

Water solubility in silica melts was determined at 100-600 MPa, 1200-1350 °C, and at each temperature (T) was found to increase with pressure (P). At P ≤ 250 MPa, the effect of T on water solubility in silica melts is small and within analytical precision (±0.15 wt% H2O). 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 SiO2content of the melt is nonlinear along the silica-albite join. In the compositional range Ab100to Ab25(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 ± 0.01 mol% H2O 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 NaAlSi3O8-H2O mechanism, the other to an SiO2-H2O mechanism.