The partitioning of sulfur and chlorine between andesite melts and magmatic volatiles and the exchange coefficients of major cations

Andesite melts were equilibrated with an H-O-S-bearing volatile phase to determine the partition coefficients for S and Cl as a function of melt composition and oxygen fugacity. The experiments were conducted in rapid-quench MHC vessel assemblies at 200 MPa and 1000 degrees C, and over a range of imposed fO(2) between NNO-1.2 and NNO+1.8. High fluid/melt mass ratios (similar to 15) were employed, allowing precise and accurate partition coefficients to be obtained by mass balance calculations. Chlorine exhibits Henrian behavior at ClO-0.5 activities typical for arc magmas, with D-Cl(volatile/melt) = 1.36 +/- 0.06 (1 sigma) below 0.2 wt.% Cl in the melt; at higher ClO-0.5 activities, D-Cl(volatile/melt) increases linearly to 2.11 +/- 0.02 at 1 wt.% Cl in the melt. In the volatile phase: FeCl2 similar to NaCl > KCl similar to HCl. The determination of cation exchange coefficients for major cations yielded: K-K,Na(volatile/melt) = 1.23 +/- 0.10 (1 sigma) and *K-Fe,Na(volatile/melt) = D-Fe(volatile/melt)/D-Na(volatile/melt) = 1.08 +/- 0.16 (1 sigma). Under these conditions, the concentration of HCl in the vapor is negatively correlated with the (Na + K)/(Al + Fe3+) ratio in the melt. Reduced sulfur (S2-) appears to obey Henry's law in andesite melt-volatile system at fH(2)S below pyrrhotite saturation. The partition coefficient for S at fO(2) = NNO-0.5 correlates negatively with the FeO concentration in the melt, changing from 254 +/- 25 at 4.0 wt.% FeO to 88 +/- 6 at 7.5 wt.% FeO. Pyrrhotite saturation is reached when approximately 3.2 mol% S is present in the volatile phase at fO(2) = NNO-0.5. At the sulfide/sulfate transition, the partition coefficient of S drops from 171 + 23 to 21 +/- 1 at a constant FeO content of similar to 6 wt.% in the melt. At fO(2) = NNO+1.8, anhydrite saturation is reached at similar to 3.3 mol% S present in the volatile phase. Aqueous volatiles exsolving from intermediate to mafic magmas can efficiently extract S, and effect its transfer to sites of magmatic-hydrothermal ore deposit formation. (C) 2012 Elsevier Ltd. All rights reserved.