Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
8910823 | Geochimica et Cosmochimica Acta | 2018 | 60 Pages |
Abstract
The diffusivity of S in a hydrous dacitic melt (4.5-6.0â¯wt.% H2O) has been investigated in the temperature (T) and pressure (P) range of 950â¯Â°C to 1100â¯Â°C and 200 to 250â¯MPa, respectively. Three series of experiments were conducted at relatively low oxygen fugacity (fO2) conditions [0.8 log units below fayalite-magnetite-quartz equilibrium (FMQ â0.8); referred to as “low fO2”] and high fO2 conditions (FMQ +2.5; referred to as “high fO2”) 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 fO2) in comparison to the sulfate dominated experiments (high fO2). The following Arrhenian equations were obtained for high and low fO2 conditions at 200â¯MPa:highfO2:D=10-5.92±0.86âexp-137.3±21.5kJ/molRTlowfO2:D=10-5.18±1.39âexp-125.7±34.4kJ/molRTwhere D is the average diffusion coefficient in m2 sâ1, R is the gas constant in 8.3144â¯Jâ¯molâ1 Kâ1 and T is the temperature in K. Our results demonstrate for the first time in natural melts that S diffusion is strongly sensitive to fO2. Our S diffusivities under low fO2 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.
Keywords
Related Topics
Physical Sciences and Engineering
Earth and Planetary Sciences
Geochemistry and Petrology
Authors
Matthias Bernhard Lierenfeld, Zoltán Zajacz, Olivier Bachmann, Peter Ulmer,