Two phases of sulphide saturation in Réunion magmas: Evidence from cumulates

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The behaviour of chalcophile elements in magmas is controlled both by degassing and by partitioning into a sulphide melt. While sulphide melts are rarely observed in erupted volcanic products, their high densities ensure that they should be concentrated in the plutonic roots of active volcanoes. The eruptions of the Bellecombe ash member on Réunion included plentiful sulphide-bearing ultramafic cumulate nodules and sulphides were also observed in oxide bearing gabbro nodules. However, Réunionmelt inclusions also show that S is affected by degassing. We have combined analyses of cumulates, sulphides and melt inclusions with an investigation of published data to determine the roles of sulphide saturation and magmatic degassing at Réunion. It is possible to identify two sulphide saturation events during magmatic evolution. The first relates to S becoming saturated during fractional crystallisation and formation of sulphide melts prior to partitioning into a gas phase. The second is more obvious in the more differentiated magmas of Piton des Neigesand is prompted by the crystallisation of titanomagnetite which reduces the sulphur concentration at sulphide melt saturation. Sulphide melts saturation and segregation is likely to be a common feature at many volcanoes and should be taken into account when using trace metal concentrations to track gas transfer processes or when using the petrologic method to estimate loss of volatile or chalcophile elements such as S and Cu to the atmosphere. At Réunionloss of sulphur by degassing could be over estimated by 20–90% when sulphide saturation is not taken into account. The effect of sulphide melt formation and immiscibility on gas compositions in volcanic systems has important implications for both ore exploration and atmospheric monitoring.