Three-dimensional morphology of magmatic sulfides sheds light on ore formation and sulfide melt migration

Abstract

The morphology of magmatic sulfides in igneous cumulates is controlled by the wetting properties of sulfide liquids against silicates. The formation of nickel sulfide ores, the behavior of sulfide liquids during mantle melting, and potentially the segregation of the Earth's core, are all controlled by the ability of sulfide liquids to migrate through the pore space of partially molten silicates. Three-dimensional X-ray tomographic images of sulfide aggregates in komatiitic olivine cumulates indicate that sulfide liquids have a limited tendency to wet olivine crystals, forming interconnected networks only in the absence of silicate melt. Consequently, the ability of sulfide liquids to migrate through the pore space of olivine cumulates is limited. We conclude that disseminated sulfide ores in komatiites formed by accumulation of transported sulfide blebs a few millimeters in size, and not by settling of sulfide-olivine aggregates, and that sulfides accumulated in the proportions in which they are now found, rather than by percolation through cumulate pore space. It is unlikely that sulfide droplets can be entrained and carried from the mantle at low degrees of partial melting. Our results also support the hypothesis that segregation of the Earth's core took place from a magma ocean, rather than by percolation of sulfidic melt through partially molten mantle.