Experimental investigation of the S and S-isotope distribution between H2O–S ± Cl fluids and basaltic melts during decompression

Abstract

Decompression experiments (from 400 to 70MPa) were conducted to investigate sulfur (S) distribution and S-isotope fractionation between basaltic melts and coexisting fluids. Volatile-bearing [~3 to ~7wt.% water (H2O), ~300 to ~1200ppmS, 0 to ~3600ppm chlorine (Cl)] basaltic glasses were used as starting materials. The MgO content in the melt was either ~1wt.% (Mg-poor basalt) or ~10wt.% (alkali basalt) to investigate the possible role of compositional changes in basaltic systems on fluid-melt distribution of S and S-isotopes. The experiments were performed in internally heated pressure vessels (IHPV) at 1050°C to 1250°C, variable oxygen fugacities (fO2; ranging from log(fO2/bar)~QFM to ~QFM+4; QFM=quartz–fayalite–magnetite buffer) and at a constant decompression rate (r) of 0.1MPa/s. The annealing time (tA) at final pressure (p) and temperature (T) after decompression was varied from 0 to 5.5h to study the fluid–melt equilibration process.Sulfur and H2O contents in the melt decreased significantly during decompression, while the Cl contents remained almost constant. No changes in H2O and Cl content were observed with tA, while S concentrations decreased slightly with tA <2h; i.e., near-equilibrium fluid–melt conditions were reached within ~2h after decompression, even in experiments performed at the lowest T of 1050°C. Thus, fluid–melt partitioning coefficients of S (DSfl/m) were determined from experiments with tA ≥2h.The MgO (~1 to ~10wt.%), H2O (~3 to ~7wt.%) and Cl contents (<0.4wt.%) in the melt have no significant effect on DSfl/m. Consistent with previous studies we found that DSfl/m decreased strongly with increasing fO2; e.g., at ~1200°C DSfl/m≈180 at QFM+1 and DSfl/m≈40 at QFM+4. A positive correlation was observed between DSfl/m and T in the range of 1150 to 1250°C at both oxidizing (QFM+4; DSfl/m=52±27 to 76±30) and intermediate (QFM+1.5; DSfl/m=94±20 to 209±80) redox conditions. Data compiled at 1050°C and relatively reducing conditions (~QFM; DSfl/m=58±18) indicate that the trends may be extrapolated to lower T, at least for intermediate to reducing conditions (~QFM+1.5 to ~QFM).The S-isotope composition in glasses of selected samples was measured by secondary ion mass spectrometry (SIMS). Gas–melt isotopic fractionation factors (αfl–m) were calculated via mass balance. At 1200°C an average αfl–m of 0.9981±0.0015 was determined for oxidizing conditions (~QFM+4), while an average αfl–m of 1.0025±0.0010 was found for fairly reducing conditions (~QFM+1). Furthermore, at lower T (1050°C) an average αfl–m of 1.0037±0.0009 was determined for reducing conditions (~QFM). The data showed that equilibrium fractionation effects during closed-system degassing of basaltic melts at T relevant for magmatic systems (1050 to 1250°C) can induce a S-isotope fluid–melt fractionation of about +4‰ in relatively reduced systems and of about −2‰ in relatively oxidized systems.The reported experimental results are valuable for the interpretation of S and S-isotope signature in magmatic systems (e.g., in volcanic gasses or melt inclusions) and will help to elucidate, for instance, volatile transport processes across subduction zones and Earth's S cycle.