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

Abstract

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 200MPa and 1000°C, and over a range of imposed fO2 between NNO−1.2 and NNO+1.8. High fluid/melt mass ratios (∼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 DClvolatile/melt=1.36±0.06 (1σ) below 0.2wt.% Cl in the melt; at higher ClO−0.5 activities, DClvolatile/melt increases linearly to 2.11±0.02 at 1wt.% Cl in the melt. In the volatile phase: FeCl2∼NaCl>KCl∼HCl. The determination of cation exchange coefficients for major cations yielded: KK,Navolatile/melt=1.23±0.10 (1σ) and ∗KFe,Navolatile/melt=DFevolatile/melt/DNavolatile/melt=1.08±0.16 (1σ). 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 fH2S below pyrrhotite saturation. The partition coefficient for S at fO2=NNO−0.5 correlates negatively with the FeO concentration in the melt, changing from 254±25 at 4.0wt.% FeO to 88±6 at 7.5wt.% FeO. Pyrrhotite saturation is reached when approximately 3.2mol% S is present in the volatile phase at fO2=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 ∼6wt.% in the melt. At fO2=NNO+1.8, anhydrite saturation is reached at ∼3.3mol% 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.