Experimental investigation of the S and S-isotope distribution between H 2 O–S ± Cl fluids and basaltic melts during decompression Adrian Fiege a, ⁎ ,1 , François Holtz a,1 , Harald Behrens a,1 , Charles W. Mandeville b,1 , Nobumichi Shimizu c,1 , Lars S. Crede a,1 , Jörg Göttlicher d,1 a Leibniz Universität Hannover, Institut für Mineralogie, Callinstraße 3, 30167 Hannover, Germany b U.S. Geological Survey, 12201 Sunrise Valley Drive, Reston, MS 904, VA 20192, USA c Woods Hole Oceanographic Institution, 266 Woods Hole Rd., Woods Hole, MS 23, MA 02543-1050, USA d ANKA Synchrotron Radiation Facility, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany abstract article info Article history: Received 15 May 2014 Received in revised form 7 October 2014 Accepted 13 November 2014 Available online 21 November 2014 Editor: D.B. Dingwell Keywords: Sulfur Chlorine Sulfur fluid–melt distribution Sulfur isotope fractionation Magma degassing Basalt Decompression experiments (from 400 to 70 MPa) were conducted to investigate sulfur (S) distribution and S-isotope fractionation between basaltic melts and coexisting fluids. Volatile-bearing [~3 to ~ 7 wt.% water (H 2 O), ~300 to ~1200 ppm S, 0 to ~3600 ppm chlorine (Cl)] basaltic glasses were used as starting mate- rials. The MgO content in the melt was either ~1 wt.% (Mg-poor basalt) or ~10 wt.% (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 (fO 2 ; ranging from log(fO 2 /bar) ~ QFM 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 H 2 O contents in the melt decreased significantly during decompression, while the Cl contents remained almost constant. No changes in H 2 O and Cl content were observed with t A , while S concentrations decreased slightly with t A b 2 h; i.e., near-equilibrium fluid–melt conditions were reached within ~2 h after de- compression, even in experiments performed at the lowest T of 1050 °C. Thus, fluid–melt partitioning coefficients of S (D S fl/m ) were determined from experiments with t A ≥2 h. The MgO (~1 to ~10 wt.%), H 2 O (~3 to ~7 wt.%) and Cl contents (b 0.4 wt.%) in the melt have no significant effect on D S fl/m . Consistent with previous studies we found that D S fl/m decreased strongly with increasing fO 2 ; e.g., at ~1200 °C D S fl/m ≈ 180 at QFM + 1 and D S fl/m ≈ 40 at QFM + 4. A positive correlation was observed between D S fl/m and T in the range of 1150 to 1250 °C at both oxidizing (QFM + 4; D S fl/m = 52 ± 27 to 76 ± 30) and inter- mediate (QFM + 1.5; D S fl/m = 94 ± 20 to 209 ± 80) redox conditions. Data compiled at 1050 °C and relatively reducing conditions (~QFM; D S fl/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 (α fl–m ) were calculated via mass balance. At 1200 °C an average α fl–m of 0.9981 ± 0.0015 was determined for oxidizing conditions (~QFM + 4), while an average α fl–m of 1.0025 ± 0.0010 was found for fairly reducing conditions (~QFM + 1). Furthermore, at lower T (1050 °C) an average α fl–m of 1.0037 ± 0.0009 was determined for reducing conditions (~QFM). The data showed that equi- librium 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. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Sulfur is the third most abundant volatile in natural silicate melts (besides H 2 O and CO 2 ) and the highest concentrations are found in ba- saltic magmas which often contain ≫1000 ppm S (e.g., Perfit et al., Chemical Geology 393–394 (2015) 36–54 ⁎ Corresponding author. E-mail addresses: afiege@umich.edu (A. Fiege), f.holtz@mineralogie.uni-hannover.de (F. Holtz), h.behrens@mineralogie.uni-hannover.de (H. Behrens), cmandeville@usgs.gov (C.W. Mandeville), nshimizu@whoi.edu (N. Shimizu), crede@gmx.net (L.S. Crede), joerg.goettlicher@kit.edu (J. Göttlicher). 1 Tel.: +49 511 762 5281; fax: +49 511 762 3045. http://dx.doi.org/10.1016/j.chemgeo.2014.11.012 0009-2541/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo