Smith Lecture: Kinetics of Sulfur Degassing During Decompression of Silicate Melts
Sulfur (S) is a major volatile in magmatic systems and large amounts of S can be released to the atmosphere during volcanic eruptions as well as passively between two eruptions. Hence, a good knowledge of the S degassing mechanisms in magmatic systems is a pre-requisite to allow one to interpret volcanic gas signatures and, thus, to improve monitoring of active volcanic systems. However, data on S fluid-melt distribution are rare and, until recently, no experimental data on the kinetics of S degassing during decompression of silicate melts were published.
We conducted isothermal decompression experiments with volatile-bearing (H2O-S±Cl) andesitic and basaltic melts at temperatures (T) of 1030 to 1250°C and various oxygen fugacities fO2 (log(fO2/bar) = QFM to QFM+4; QFM: quartz-fayalite-magnetite buffer). In these experiments, pressure (p) was released continuously from ~400 to ~70 MPa at a constant rate of ~0.1 MPa/s. The samples were annealed for tA = 0 to 72 h at final p-T conditions after decompression to allow us to investigate the kinetics of S degassing.
Experiments with andesitic composition conducted under oxidizing conditions (>QFM+3) and at ~1030°C revealed a strong decrease of the S(fluid)/S(melt) ratio with increasing tA (S(fluid) = wt% S in the fluid; S(melt) = wt% S in the melt). The data show that fluid-melt near-equilibrium conditions were achieved shortly after decompression; i.e., within ~5 h. Such a kinetically controlled transient release of large amounts of S from a melt to a fluid phase during fast decompression was not observed at lower fO2 (~QFM+1 to ~QFM+1.5) in andesitic systems and was not detected at all (i.e., independent of fO2) in basaltic systems.