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 (SO₄ ²⁻). The way SO₄ ²⁻ groups dissolve in the melt is currently poorly understood. We present experimental results of the effect of SO₄ ²⁻ dissolution on the aluminosilicate melt structure. Glasses of haplogranitic (HPG) and anorthite–diopside eutectic (An–Di) compositions were synthesized at 300MPa 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 5wt.%) and H₂O (5wt.%). 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 979ppm and 0 to 7519ppm for HPG and An–Di, respectively. S is present in the glasses as Mⁿ⁺SO₄ ²⁻ 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 SO₄ ²⁻ 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.