Si isotope fractionation between Si-poor metal and silicate melt at pressure-temperature conditions relevant to metal segregation in small planetary bodies

Experimental investigations of Si isotope fractionation between Si-bearing metal alloy and silicate phases have to date been limited to high pressure (1-7 GPa) and high temperature (1800-2200 °C) conditions at highly reducing conditions, to optimize applicability of results to early core formation processes in the Earth. Here, we assess the extent and mechanism of Si isotopic fractionation at conditions relevant to metal segregation in small (km-scale) planetary bodies, using samples obtained from an industrial-scale blast furnace of Tata Steel (IJmuiden, the Netherlands).During the low-pressure, high-temperature process of steelmaking inhomogeneous blast furnace burden consisting of pre- and untreated iron ore, iron silicates and coke is reduced to oxygen fugacities near the C-CO buffer, resulting in the segregation of a metal phase containing only ∼0.3 wt% Si. Seven sample sets, each comprising a metal alloy and a silicate slag, were taken during tapping of the blast furnace at tapping temperatures between 1400 °C and 1600 °C. We find large isotopic mass fractionation between metal and silicate, with Δ30Sisilicate-metal varying between 0.7‰ and 1.6‰, values that are as high as previously obtained in high-pressure, highly reduced experiments. A model for metal-silicate Si isotope fractionation in blast furnaces can explain both the sense and magnitude of fractionation, if the presence of SiO-bearing vapour is explicitly taken into account. Our data indicate that significant Si isotope fractionation can occur between metal and silicate at low-pressure, high-temperature and only mildly reducing conditions for which Si solubility in molten Fe-rich metal is low. This suggests an important role for SiO at low confining pressures. Our data can be applied to models of aubrite meteorite formation through high-temperature differentiation of an enstatite chondrite parent body. Our calculations suggest a far larger degree of rehomogenisation during differentiation than previously thought on the basis of metal-silicate Si isotopic fractionation measured in natural meteorites that re-equilibrated at low temperature.

► We analysed Si isotopes in metal and silicate phases from an industrial blast furnace. ► Sign and sense of our data are in good agreement with existing literature data. ► Results indicate that high-pressure is not required to fractionate Si isotopes. ► We suggest that Si isotope fractionation is not a single-stage equilibrium process.