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

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. s 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. 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). MR 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. served 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.