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Abstract: Partitioning experiments between a basaltic melt from Mt. Etna and a low-density hydrous fluid or vapor containing H2O or H2O–CO2 were performed at 1200–1260 °C, at pressures between 1 and 200 MPa, either near the nickel–nickel oxide (NNO) buffer or at two log units above it (NNO+2), and with different chloride concentrations. Most of the experiments were done at chloride-brine-undersaturated conditions, although at the highest Cl concentrations explored brine saturation might have been reached. The average partition coefficients (D Cl fluid/melt) over the range of Cl concentrations were derived on a weight basis by plotting the calculated concentrations of Cl in the fluid phase versus the measured ones in the melt. For H2O–Cl experiments in which the Cl concentration in the melt was ≤0.4 wt.%, a negative dependence between D Cl f/m and pressure is observed. D Cl fluid/melt in H2O+Cl-bearing experiments ranges between 11–14 at 1 and 25 MPa to 6 at 200 MPa at NNO; and between 4 at 50 MPa and 13 at 100 MPa at ΔNNO≥2. Addition of CO2 at NNO yields lower partition coefficients than in CO2-free conditions over the pressure range investigated. The negative pressure dependence observed for H2O–Cl experiments disappears when CO2 is present in the system. Overall, once CO2 is introduced in the system, Cl fugacity in the silicate melt tends to increase, thus resulting in a decrease of D Cl f/m. Application to Mt. Etna shows that the composition of the volcanic plume in terms of Cl records very shallow pressures of magma equilibration with its exsolved fluid. [Copyright &y& Elsevier]

Abstract: With a combined approach using Solid-State 13C-MAS NMR and Laser Raman Microspectroscopy, we investigated the CO2–CO gas composition (X(CO2)) in fluid inclusions synthesized under high pressure (200–300 MPa), high temperature (1225–1250 °C) and reducing conditions (17< P(H2)<62 bars). Fluid inclusions are entrapped in a volatile-bearing basaltic glass which was characterized by FTIR for determining the water solubility (H2Om). 13C-MAS NMR is used as a standard analysis for determining the X(CO2). The Raman quantification factors between 13CO2 and 13CO are determined from peak area (F-factor), peak height (G-factor) and according to the Placzek''s polarizability theory. The calibration is derived for both CO2 Fermi diad resonances: 2ν 2 and ν 1. We obtain similar values for the main CO2 resonance (2ν 2) with 1.956 and 1.809 for F and G respectively. Results are consistent with the fact that peak height and area will measure the same quantity. For ν 1, multiple calibration trends are observed. The different trends are explained by the different 13C/12C ratio observed in between the samples. However, we suggest that such resonance is not suitable for determining the fluid inclusion compositions. We extended the 13C results for calibrating the F- and G-factors for 12CO2–12CO gas mixture in the fluid inclusions and for the main CO2 resonance. For 12CO2–12CO mixture, F and G values are 1.856 and 1.756 which is in the same order as the derived values for 13C species. Thus, we propose that no significant 13C/12C fractionation occurs in the fluid phase and both isotopes will behave in a similar way. Using the derived calibration for 12C and 13C species, the X(CO2) in the fluid phase was recalculated. Results are similar for both isotopes witnessing the similar behaviour of 12C and 13C fluid species during the experiments. The log f(O2) experienced by each sample has been calculated through a thermodynamic approach using 2 independent methods. The log f(O2) calculated from the H2Om in the glass and the X(CO2) in the fluid phase are in good agreement. Large discrepancy is observed for low H2Om content which gives lower log f(O2) value than expected from experimental conditions. Large uncertainties on the H2Om measurements will induce a very approximate value for the log f(O2). This method may not be accurate enough at low H2Om and using the X(CO2) in the fluid phase would therefore provide a better estimate of the log f(O2). [Copyright &y& Elsevier]