Melting Experiments on Liquidus Phase Relations in the Fe‐S‐O Ternary System Under Core Pressures

Abstract

Melting experiments on the Fe‐S‐O ternary system were performed to 208 GPa in a laser‐heated diamond‐anvil cell. Compositions of liquids and coexisting solids in recovered samples were examined using a field‐emission‐type electron microprobe. The results demonstrate that the ternary eutectic point shifts toward the oxygen‐rich, sulfur‐poor side with increasing pressure, in accordance with changes in eutectic liquid compositions in the Fe‐O and Fe‐S binary systems. We also found that solid Fe crystallizing from liquid Fe‐S‐O does not include oxygen, while the partitioning of sulfur into solid Fe is enhanced with increasing pressure. These indicate that oxygen‐rich, sulfur‐poor liquid crystallizes Fe at the inner core boundary; however, it makes a large density difference between the liquid and solid core, which is inconsistent with observations. Alternatively, we found that a range of C‐bearing, S‐poor/O‐rich liquids account for the density and velocity in the outer core and the density in the inner core.

Partitioning of sulfur between solid and liquid iron under Earth’s core conditions: Constraints from atomistic simulations with machine learning potentials

“…6, which illustratively compares the evolutions of thermodynamic properties and structures in the simulations at 4000 K, 4.3893 cm 3 /mol and cS=0.20. (Kamada et al, 2010;Yokoo et al, 2019). For pressures lower than 250 GPa, we find remarkable pressure dependence of DS: its value decreases by over 40% from 250 GPa to 150 GPa.…”

“…With several decades' efforts of high-pressure experiments, current knowledge of the partitioning or more broadly phase behaviors of iron alloys has been significantly extended but is still far from adequate (Morard et al, 2014). For sulfur, the focus of this study, the eutectic melting phase relations have been determined by a number of experiments (Chen et al, 2008;Chudinovskikh and Boehler, 2007;Kamada et al, 2012;Kamada et al, 2010;Li et al, 2001;Morard et al, 2014;Morard et al, 2008;Mori et al, 2017;Stewart et al, 2007;Terasaki et al, 2011;Yokoo et al, 2019). Most of these experiments are done at pressures lower than 60 GPa and the data under the real core pressures (>140 GPa) are scarce.…”

“…Although the highest-pressure record in these experiments has reached GPa (Mori et al, 2017), it is still some distance away from true ICB condition (330 GPa). With these experimental constraints, people find a general trend of decreasing sulfur contents in the eutectic liquids and increasing solubility of sulfur in the solid solutions at higher pressures, which means an increasing tendency for the partition coefficients of sulfur (Kamada et al, 2012;Morard et al, 2014;Yokoo et al, 2019).…”

Fe Alloy Slurry and a Compacting Cumulate Pile Across Earth's Inner‐Core Boundary

“…Some theoretical calculations have suggested that O partitions more strongly from solid into liquid than Si and S (Alfè et al, ), so the solid inner core may be O poor upon freezing. High‐pressure melting experiments of Fe‐Si‐O and Fe‐S‐O alloys in diamond anvil cells found that O could be left in the liquid upon freezing of the Fe alloys (Hirose et al, ; Yokoo et al, ). The Fe‐Si‐O alloy may crystallize SiO 2 as it cools so that O and Si would partition from solid to liquid until one of the two is mostly depleted in the core (Hirose et al, ).…”

A Joint Experimental‐Modeling Investigation of the Effect of Light Elements on Dynamos in Small Planets and Moons

“…Cosmochemistry studies have suggested that carbon, hydrogen, and phosphorus are present in too small amounts (below 0.3 wt.%) to represent significant components of terrestrial cores (e.g., McDonough, 2003). In the Earth, several studies have pointed out that more than one element is likely present in the liquid outer core (e.g., Poirier, 1994; Alfé, 2002; Sanloup et al, 2004; Badro et al, 2007; Yokoo et al, 2019). Mercury's reducing conditions are consistent with a silicon‐rich, sulfur‐bearing core (e.g., Hauck et al, 2013; Namur et al, 2016; Cartier et al, 2020), while the more oxidizing conditions on Mars, the Moon, and possibly Ganymede are compatible with the presence of sulfur (e.g., Hauck et al, 2006; Kuskov & Belashchenko, 2016; Lodders & Fegley, 1997; Rückriemen et al, 2015) and oxygen in the core (Pommier et al, 2018; Tsuno et al, 2011).…”