Melting phase relations in the Fe-S and Fe-S-O systems at core conditions in small terrestrial bodies

Abstract

We report an experimental investigation of phase equilibria in the Fe-S and Fe-SO systems. Experiments were performed at high temperatures (1400-1850°C) and high pressure (14 and 20 GPa) using a multi-anvil apparatus. The results of this study are used to understand the effect of sulfur and oxygen on core dynamics in small terrestrial bodies. We observe that oxygen has little effect on the liquidus temperature and that the formation of solid FeO grains occurs at the Fe-S liquid Fe solid interface at high temperature (>1400°C at 20 GPa). Oxygen fugacities calculated for each O-bearing sample showed that redox conditions vary from)W-0.65 to 0. Considering the relative density of each phase and existing evolutionary models of terrestrial cores, we apply our experimental results to the cores of Mars and Ganymede. We suggest that the presence of FeO in small terrestrial planets tends to contribute to outer-core compositional stratification. Depending on the redox and thermal history of the planet, FeO may also help forming a transitional redox zone at the core-mantle boundary.

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

“…Therefore, this alloy consists of an Fe,S,O‐rich liquid that contains small amounts of Si and a few isolated SiO 2 grains, as illustrated by SEM‐EDS data (Figure 2). Increasing temperature increases the ability of the liquid to dissolve oxygen (e.g., Pommier et al, 2018), which is consistent with the absence of solid Fe 2 O 3 or FeO grains in the quenched samples.…”

“…Finally, we note that none of the models for the three cases in Figure 7 produce a snow zone as the core temperature remains above the liquidus temperature of Fe‐S‐O system for all time (Buono & Walker, 2015; Huang et al, 2010; Pommier et al, 2018; Terasaki et al, 2011; Urakawa et al, 1987).…”

“…Previous phase equilibria experiments in the Fe‐S‐O system under pressure and temperature conditions relevant to the cores of small terrestrial bodies have reported the presence of FeO grains coexisting with Fe grains as well as the liquid phase, and possibly forming a layer at the solid iron‐liquid interface (Pommier et al, 2018; Tsuno & Ohtani, 2009; Urakawa et al, 1987). Our quenched Fe‐S‐O(‐Si) samples do not show the presence of solid FeO coexisting with the liquid phase, which can be explained by the high quenching temperatures of the experiments (the Mg‐bearing samples were quenched at too low T to have a significant proportion of melt).…”

“…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).…”

Does Io Have a Magma Ocean?

“…Therefore, the induced magnetic field is sensitive to the conductivity of the layer and variations in the depth and thickness of the layer. MHD models incorporating a multi-layer representation of Io's interior, such as that used by Khurana et al (2011), can better constrain the conductivity and resulting melt fraction of a magma ocean by incorporating the results of recent experiments on the conductivity of Io-like materials (Pommier et al, 2018;Zhang and Pommier, 2017). Furthermore, it is unlikely that Io's magma ocean is perfectly spherically symmetric; future modeling should be capable of representing an ocean with large-scale topographies rising up from the ocean "floor" or descending from the crustal "cap".…”