Crystallisation history of magmatic sulphide droplets in intra-oceanic arcs

Abstract

Valuable understanding regarding the behaviour of chalcophile elements in magmatic systems can be gained by examining magmatic sulphide droplets, which are the solidified remnants of previously immiscible sulphide liquids [1-2]. We analysed the trace element composition of sulphide droplets by LA-ICP-MS, creating a comprehensive data set for volcanic and plutonic rocks from intra-oceanic arcs. We observed a significant enrichment of elements like Zn, Cd, Sn, Te, and Bi in sulphide droplets from lava samples compared to those hosted by gabbro xenoliths, which cannot be attributed to the fractionation of olivine, spinel, or magnetite [3-5]. These compositional differences are likely the result of changing sulphide droplet composition during cooling and solidification of the silicate melt. This process involves continuous sulphide segregation or re-equilibration with the silicate melt. A key aspect of our suggested model is sulphide droplets' resorption or partial remelting, particularly of the Cu-Fe-rich intermediate solid solution proportion. This process is driven by pressure decrease during magma ascent, leading to an increase in sulphur solubility in the silicate melt [6], which potentially liberates elements like Cu, Au, Zn, Bi, Te, and Ag from the sulphide droplet to the silicate melt. Our findings further suggest that subsequent magma stagnation and fractional crystallisation lead to a second stage of sulphide saturation, likely dominated by the elements previously liberated from the intermediate solid solution. The complex crystallisation history indicates that sulphide droplet formation during silicate melt evolution in subduction-related settings is a non-equilibrium process. We further propose that volatile saturation preceding the second stage of sulphide segregation from a silicate melt enriched in chalcophile elements liberated from intermediate solid solution could result in particularly metal-rich fluids (e.g., Cu, Au, Bi, Te) with a high ore-forming potential in magmatic-hydrothermal environments.   [1] Wood, B. J. and Kiseeva, E. S. (2015), Earth and Planetary Science Letters, 424, 280-294. [2] Patten, C. et al. (2013), Chemical Geology, 358, 170–188. [3] Distler, V. V. et al., (1983), Initial Reports of the Deep Sea Drilling Project, 69, 607-617. [4] Keith, M. et al. (2017), Chemical Geology, 451, 67–77. [5] Schäfer, W. et al., in prep. [6] Peach et al. (1990), Geochimica et Cosmochimica Acta, 12, 3379-3389.