Effect of sulphur on the structure of silicate melts under oxidizing conditions

- Sulphate dissolution in silicate melts induces a polymerization of the melt network.
- Sulphate affects only the silicon environments in the melt.
- Oxidized sulphur is suggested to increase melt viscosity on the contrary to water.

In magmatic systems, sulphur is an important volatile element after C, H and O. Under oxidizing conditions, S dissolves in aluminosilicate melt as sulphate groups (SO42 −). The way SO42 − groups dissolve in the melt is currently poorly understood. We present experimental results of the effect of SO42 − dissolution on the aluminosilicate melt structure.Glasses of haplogranitic (HPG) and anorthite-diopside eutectic (An-Di) compositions were synthesized at 300 MPa and 1250 °C and under oxidizing conditions (ΔFMQ + 1.7 to + 3.0). Starting compositions were equilibrated under fluid-saturated conditions with a mixture of S (0 to 5 wt.%) and H2O (5 wt.%). The S and H2O contents of the recovered glasses were determined with EPMA and FTIR, respectively. Solid state NMR was used to investigate the glass structure. Micro-Raman analyses were performed to identify S species present in glass and coexisting fluid phases.The S content determined in glasses changes from 0 to 979 ppm and 0 to 7519 ppm for HPG and An-Di, respectively. S is present in the glasses as Mn +SO42 − groups (Mn + is possibly Ca2 + in An-Di and Na+ in HPG).29Si NMR analyses show important changes in the An-Di glass structure upon S dissolution. High S content in An-Di glasses induces a strong polymerization of the glass which we explain by the presence of non-bridging oxygen (NBO) promoting the dissolution of S as SO42 − groups. On the contrary, the dissolution of S within HPG melt does not produce visible changes in the silicate melt structure due to the low concentration in NBO in this melt composition.The observed structural changes suggest that An-Di melt physical properties might be affected by S dissolution. S might produce large changes in melt viscosity, but opposite, as compared to H2O for slightly depolymerized aluminosilicate melts.