Solubility of C-H-O-S Volatiles in Basaltic Melts.
Résumé
Arc volcanism is known for his dangerousness because of the high contents H$_{2}$O, CO$_{2}$ and S (H$_{2}$S or SO$_{2}$ depending on the fO$_{2}$ of the system). The behaviour of the volatiles C, H, O and S in basaltic melts is poorly known, yet the knowledge of the solubility of these volatiles is critical to understand volcanic degassing. A significant amount of work has been done to define the solubility laws of H$_{2}$O, CO$_{2}$ an S in silicic melts. However, such data are still scarce for basaltic liquids. To remedy this gap, we are conducting experiments on basaltic liquids at $1200\deg$C, at pressures varying between 250 and 2000 bar and at high fO$_{2}$ (NNO+2). Basaltic compositions from Vesuvius, Etna and Stromboli are equilibrated with an H$_{2}$O+S and H$_{2}$O+CO$_{2}$+S rich fluid phase. After rapid quench, contents of H$_{2}$O and CO$_{2}$ in glasses are determined by using KFT and near/mid infrared spectroscopic measurements. Major elements and sulphur contents are determined by electron microprobe analyses. Microprobe analyses show that, at near H$_{2}$O saturation, sulphur contents increase with pressure (from 2500 ppm at 250 bar, to 6700 ppm at 2000 bar for Etna and Stromboli compositions). The melt composition has an influence on the sulphur contents, in particular iron, as shown in previous work, and alkalies. In particular, potassium seems to have a negative role on sulphur: for similar pressure, temperature, oxygen fugacity, and bulk S content, a difference of 3% in alkalinity induces a difference of 1000 to 2000 ppm of S contents. Results for basaltic melts equilibrated with H$_{2}$O+CO$_{2}$+S fluid phase will be presented at the meeting. Experimental results on H$_{2}$O, CO$_{2}$ and S solubilities in basaltic liquids will constitute a data base for subsequent experiments on degassing processes.