Sulfur diffusion in dacitic melt at various oxidation states: Implications for volcanic degassing

7The diffusivity of S in a hydrous dacitic melt (4.5-6.0 wt.% H2O) has been investigated in the temperature (T) and pressure (P) range of 950 degrees C to 1100 degrees C and 200 to 250 MPa, respectively. Three series of experiments were conducted at relatively low oxygen fugacity (fO(2)) conditions [0.8 log units below fayalite-magnetite-quartz equilibrium (FMQ -0.8); referred to as low fO(2)] and high fO(2) conditions (FMQ + 2.5; referred to as high fO(2)) to determine if the diffusivity of S is affected by its oxidation state and speciation. Sulfur concentration profiles were measured by electron microprobe and the diffusion coefficient (D) was calculated by fitting these profiles. Sulfur diffusion is approximately one order of magnitude faster when S is dominantly present as sulfide species (low fO(2)) in comparison to the sulfate dominated experiments (high fO(2)). The following Arrhenian equations were obtained for high and low fO(2) conditions at 200 MPa: high fO(2) : D = 10(-5.92 +/- 0.86) * exp (-137.3 +/- 21.5 kJ/mol/RT) low fO(2) : D = 10(-5.18 +/- 1.39) * exp (-125.7 +/- 34.4 kJ/mol/RT) where D is the average diffusion coefficient in m(2) s(-1), R is the gas constant in 8.3144 J mol(-1) K-1 and T is the temperature in K. Our results demonstrate for the first time in natural melts that S diffusion is strongly sensitive to fO(2). Our S diffusivities under low fO(2) conditions are only slightly slower of those found for H2O, suggesting that S can be rather efficiently purged from reduced dacitic melts during volcanic eruptions. However, for more oxidized systems (e.g. subduction zones), S diffusion will be much slower and will hinder equilibrium syn-eruptive degassing during rapid decompression. Therefore, we conclude that the excess S measured during many explosive volcanic eruptions in arcs is dominantly derived from S-rich bubble accumulation in the eruptible portion of the magma reservoir. (C) 2018 The Authors. Published by Elsevier Ltd.