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

Decompression experiments (from 400 to 70 MPa) were conducted Lo invesligale sulfur (5) dislaibulion and 5-isotope fraclionalion between basaltic melts and coexisling fluids. Volaffle-bearing [similar to 3 to similar to 7 wt.% water (H2O), similar to 300 to similar to 1200 ppm S,0 to similar to 3600 ppm chlorine (Cl)] basaltic glasses were used as slailing mareLials. The MgO conlent in the melt was effher similar to 1 wt.% (Mg poor basalt.) or similar to 10 wt.% (alkali basalt.) toinvesligale he possible role of compositional changes in basaltic sysLems on fluid -melt distribution of S and S-isoLopes. The experiments were performed in internally heated pressure vessels (IHPV) at 1050"C to 1250 C, variable oxygen fugacities fO(2); ranging from log(fO(2)/bar) similar to QFM to similar to QFM + 4. QFM = quartz-fayalite-magnetite buffer) and at a constant decompression rate (r) of 0.1 MPa/s.The annealing time (t(A)) at final pressure (p) and temperature (T) after decompression was varied from 0 to 5.5 h to study the fluid-melt equilibration process. Sulfur and H2O contents in the melt decreased significantly during decompression, while the Cl contents I emained almost constant. No changes in H2O and Cl content were observed with t(A), while S concentrations decreased slightly with t(A) <2 h; i.e., near-equilibrium fluid-melt conditions were reached within similar to 2 h after decompression, even in experiments performed at the lowest Tot 1050 degrees C. Thus, fluid-melt partitioning coefficients of S (D-S(fl/m)) were determined from experiments with t(A) >= 2 h. The MgO (similar to 1 to similar to 10 wt.%), H2O (similar to 3 to similar to 7 wt.%) and Cl contents (<0.4 wt.') in the melt have no significant effect on LA7-/"'. Consistent with previous studies we found that e" decreased strongly with increasing f02; e.g., at similar to 1200 degrees C D-s(fl/m) approximate to 180 at QTM + 1 and D-s(fl/m) 40 at (2FM 4. A positive correlation was observed between and Tin the range of 1150 to 1250 C at both oxidizing (QEM + 4; DDsfl/m = 52 +/- 27 to 76 +/- 30) and intermediate (QTM + 1.5; DDsfl/m 94 +/- 20 to 209 +/- 80) reclox conditions. Data compiled at 1050 C and relatively reducing conditions ( -QTM; DDsfl/m 58 +/- 18) indicate that the trends may be extrapolated to lower T, at least for intermediate to reducing conditions (-(2FM + 1.5 to -WM). The S-isotope composition in glasses of selected samples was measured by secondary ion mass spectrometry (SIMS). Gas-melt isotopic fractionation factors (an r) were calculated via mass balance. At 1200 degrees C an average r of 0.9981 - 0.0015 was determined for oxidizing conditions (-QFM + 4), while an average an r of 1.0025 0.0010 was found for fairly reducing conditions (-QFM + 1). Furthermore, at lower T (1050 C) an average ciql r, 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 + LI& in relatively reduced systems and of about 2% in relatively oxidized systems. The reported experimental results are valuable for the interpretation of S and 5-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. (c) 2014 Elsevier B.V. All rights reserved.