Sulphide melt distribution in partially molten silicate aggregates: implications to core formation scenarios in terrestrial planets
European Journal of Mineralogy Volume 25 Number 3 (2013), p. 267 - 277
published: Jun 1, 2013
Open Access (paper can be downloaded for free)
The early differentiation of the Earth and terrestrial planets resulted in the formation of metallic cores surrounded by silicate mantles. In the past, models of core formation were typically built on experimentally based constraints from the geochemical behaviour of siderophile and chalcophile elements in combination with their geo- and cosmochemical abundances. As core-mantle differentiations occur by physical separation of core forming metal and proto-mantle silicate phases insights from textural equilibria of the most likely coexisting phases, i. e. metallic liquid, molten and/or solid silicates, have to be combined with geochemical constraints to derive models for core formation scenarios. In the present study textural equilibria of iron sulphide liquids in partly molten silicate aggregates were studied at various pressures, and the influence of silicate melt fraction as well as sulphide melt fraction on the two-phase dihedral angles and three-phase interfacial angles of the coexisting phases were investigated. In combination with literature data, the results of the present study do not support models of core formation that are based on separation of reduced metallic (Fe–Ni–S–O) melts from partially molten silicate or on separation of these liquid metals from solid silicate. Efficient core formation by a percolation mechanism seems only feasible when conditions are very oxidizing or a large fraction of silicate partial melt or a completely molten silicate is present. These findings fit perfectly to the requirements of the multi-stage model of heterogeneous accretion described by Rubie et al. (2011) with (1) an initial accretion (60–70% of the Earth's mass) from highly-reduced (oxygen-poor) material, (2) a final accretion (30–40% of the Earth's mass) from more oxidised material, and (3) a late-stage disequilibrium where differentiated impactor cores (only a few percent of the Earth's mass) fail to re-equilibrate completely with the Earth's mantle silicate as they segregate to the Earth's proto-core.