Investigation of chalcophile element fractionation processes in the oceanic crust by trace element analysis of magmatic sulphides

The fractionation of chalcophile trace elements during magmatic processes is strongly controlled by the segregation of immiscible sulphide liquids, which is a common feature in convergent and divergent plate margins [1-2]. To constrain the partitioning behaviour of these elements during magmatic processes, in-situ LA-ICP-MS analyses were performed on magmatic sulphide droplets and related fresh volcanic glass from mid-ocean ridges, back-arc basins, island arcs and the Troodos ophiolite on Cyprus. Minerals associated with the sulphide droplets are mainly olivine in mid-ocean ridge basalts and Fe-Ti oxides in more evolved island arc rocks, suggesting a different timing of sulphide saturation. Reduced mid-ocean ridge magmas seem to segregate a sulphide liquid in an early stage, whereas more oxidised magmas in subduction zones form a sulphide liquid in a later stage triggered by redox changes [3]. Surprisingly, systematic differences in trace element contents and partition coefficients between sulphide liquid and silicate melt were not observed, despite the different redox conditions and differences in the timing of sulphide segregation between these settings. However, within each sample, a great variance in the trace element contents of sulphide droplets is observed. Whether this is related to variations in the chalcophile element contents of the primary magmas between plate tectonic settings remains unknown [4].

[1] Patten, C. et al. (2013), Chemical Geology, 358, 170–188. [2] Keith, M. et al. (2017), Chemical Geology, 451, 67–77. [3] Jenner, F. E. et al. (2010), Journal of Petrology, 51, 2445-2464. [4] Lee, C. A. et al. (2012), Science, 336, 64-68.