Résumé
The effect of sulphur on phosphorus solubility in rhyolitic melt and the sulphur distribution between apatite, + or - anhydrite, melt and fluid has been determined at 200 MPa and 800-1,100 (super degrees ) C via apatite crystallization and dissolution experiments. The presence of a small amount of sulphur in the system (0.5 wt.% S) under oxidizing conditions increases the solubility of phosphorus in the melt, probably due to changing calcium activity in the melt as a result of the formation of Ca-S complexing cations. Apatite solubility geothermometers tend to overestimate T in Ca-poor, S-bearing system at oxidizing conditions. In crystallization experiments, the sulphur content in apatite decreases with decreasing T and also with decreasing sulphur content of the melt. The sulphur partition coefficient between apatite and rhyolitic melt increases with decreasing T (Kd (sub s) (super apatite/melt) = 4.5-14.2 at T= 1,100-900 (super degrees ) C) under sulphur unsaturated conditions (no anhydrite). The sulphur partition coefficient is lower in anhydrite-saturated melt ( approximately 8 at 800 (super degrees ) C) than in anhydrite-unsaturated melt, suggesting that Kd (sub s) (super apatite/melt) depends not only on the T but also on the sulphur content of the melt. These first results indicate that the sulphur content in apatite can be used to track the evolution of sulphur content in a magmatic system at oxidizing conditions.