High-pressure experimental constraints of partitioning behavior of Si and S at the Mercury's inner core boundary

The partitioning of light elements between liquid and solid at the inner core boundary (ICB) governs compositional difference and density deficit between the outer and inner core. Observations of high S and low Fe concentration on the surface of Mercury from MESSENGER mission indicate that Mercury is formed under much more reduced conditions than other terrestrial planets, which may result in a Si and S-bearing metallic Fe core. In this study, we conducted high-pressure experiments to investigate the partitioning behavior of Si and S between liquid and solid in the Fe-Si-S system at 15 and 21GPa, relevant to Mercury's ICB conditions. Experimental results show that almost all S partitions into liquid. The partitioning coefficient of Si (D-Si) between liquid and solid is strongly correlated with the S content in liquid (X-S(liquid)) as: log(10)(D-Si) = 0.0445 + 5.9895 * log(10)(1 - X-S(liquid)). Within our experimental range, pressure has limited effect on the partitioning behavior of Si and S between liquid and solid. For Mercury with an Fe-Si- S core, compositional difference between the inner and outer core is strongly dependent on the S content of the core. The lower S content is in the core, the smaller compositional difference and density deficit between the liquid outer core and solid inner core should be observed. For a core with 1.5wt% bulkS, amodel ICB temperature would intersect with the melting curve at similar to 17GPa, corresponding to an inner core with a radius of similar to 1600km. (C) 2021 Elsevier B.V. All rights reserved.