THE PARTITIONING OF SULFUR BETWEEN AQUEOUS FLUIDS AND BASALTIC MELTS

Previous experimental studies on the partitioning of sulfur between hydrous fluids and silicate melts (D-S fl/m) have been performed to better understand magma degassing and its influence on igneous petrogenesis, volcanic eruptions, and the formation of ore deposits. Although sulfur is more abundant in mafic magmatic systems, the behavior of sulfur in basaltic magmas is still poorly understood due to experimental problems. A series of experiments were performed with a sample of a natural MORB (with ̴800 ppm S) equilibrated with a H₂O fluid phase to determine the partition coefficient of S. The experiments were conducted in a piston cylinder apparatus at 500 MPa, 1150 ˚C, and at log ƒO₂NNO+1.8 (Nickel-nickel oxide buffer+1.8 log units) to simulate island arc oxygen fugacities. All experiments were done at sulfate and sulfide undersaturated conditions. Experiments were quenched isobarically and analyzed by electron microprobe. Although all successful experiments contained quench crystals we were able to obtain reliable analyses of the quenched melt that were combined with the water concentration in melt modelled following Papale et al. (1999, Am. Mineral.). These values were used to calculate the concentration of S in the fluid by mass balance.

The results yield a D-S fl/m of ̴2.4 +/-0.3. This D-S fl/m was independent of experimental duration and provides evidence of equilibrium in the experiments. Our results complement those of Lesne et al. (2011, J. Petrol.) and Zajacz et al. (2013, GCA) for basalts at 200 MPa. Although we have evidence that the D-S fl/m decreases with increasing pressure, our results show that D is more sensitive to changes in oxygen fugacity than to changes in temperature and pressure. At crustal pressures the D-S fl/m is high, ~80 (Lesne et al.; Zajacz et al.), at fO2's between NNO-0.8 to NNO+1, whereas D-S fl/m decreases at oxygen fugacities above NNO +1 to values between 2 (this study) and 6 at NNO+2.4 (Lesne et al.).