Behavior of Sulfur during Core-Mantle Differentiation of the Earth

Differentiation of the Earth to form its metallic core and silicate mantle is the most important chemical fractionation event of Earth's history. All elements have been distributed between the mantle and the core following their respective chemical affinities for silicate vs. iron. This process is modeled using the metal-silicate partition coefficient of element i (D_i) which is a function of variables such as pressure (P), temperature (T) and chemical composition of both metal and silicate phases. Sulfur is a powerful element when it comes to understanding processes that occurred during the formation and differentiation of the Earth. Being both volatile and siderophile, it can provide important constraints on the delivery of volatile elements to the Earth and on the conditions of core formation. Several models have been proposed in the past few years to explain the abundance of sulfur in the Bulk Silicate Earth, leading to different and sometimes contradictory conclusions. Here, we combined all pertinent data published in the literature (1.5-91 GPa, 1838-4100K) together with 49 new experimental data obtained over a wide range of P-T conditions (3-19 GPa, 2223-2693 K) in order to develop a simple model of sulfur partitioning as a function of P, T, silicate composition and oxygen and silicon abundances in core-forming metal. Based on about 100 experimental results and covering a wide range of physical and chemical conditions, we propose new regression parameters that allow sulfur partitioning to be modeled reliably up to a pressure of 91GPa. This parametrization has been incorporated in a state-of-the-art multi-stage core formation model that is coupled with N-body accretion simulations (Rubie et al., 2015, 2016). We show that the only way to attain the current abundance of sulfur in the mantle is to invoke a pervasive segregation of iron sulphide liquid (the "Hadean Matte") during magma ocean cooling and crystallization. We will discuss the consequences of this process for the HSE budget of Earth's mantle.

Rubie et al. (2015), Icarus 248, 89-108

Rubie et al. (2016), Science 353 (6304), 1141-1144