11:15 〜 11:30
[SCG48-03] Effects of light elements on the core-mantle partitioning of Ni and Cr in Mars
キーワード：火星、ニッケル、クロム
Seismological observations by the Insight mission have revealed that the density of the Martian core is lower than previously thought, indicating the presence of high abundance of light elements (Khan et al., 2023; Samuel et al., 2023). While S is a major candidate for the light elements in the Mars’ core, the estimations of core S concentration are dependent on the assumed bulk composition of Mars, ranging from ~4 wt% to more than 20 wt% (Steenstra and van Westrenen, 2018; Khan et al., 2022). In addition, significant amounts of other light elements, such as H and C, may have also been incorporated into the core during the core formation because they are siderophile at high-pressure and -temperature conditions (Tagawa et al., 2021; Blanchard et al., 2022). However, the bulk Mars and core abundances of C and H are not clear in part because they are highly volatile elements.
On the other hand, the abundance of refractory elements in the Martian mantle has been estimated from the compositions of Martian meteorites (Yoshizaki and McDonough, 2020). The depletion in siderophile elements in the mantle relative to lithophile elements records the conditions of the core-mantle partitioning. While pressure, temperature, and oxygen fugacity of the core-forming conditions can be estimated from the metal-silicate partitioning of siderophile elements (Rai and van Westrenen, 2013), partition coefficients may also depend on the composition of the core-forming metals. Indeed, the metal-silicate partition coefficients of siderophile elements are affected by the presence of sulfur (Wood et al., 2014).
In this study, we investigated the effects of light elements on the partition coefficients of Ni and Cr under the Martian core pressures based on melting experiments in a diamond-anvil cell. We performed metal/silicate partitioning experiments of Ni and Cr in the presence of S, C, and/or H. Chemical analyses of recovered samples and in-situ X-ray diffraction measurements to determine H contents in metal showed that the partition coefficient of Ni increases (becomes more siderophile) and that of Cr decreases (becomes more lithophile) in the presence of S, C, and H. We discuss the possible Martian core composition that reproduces the depletion in Ni and the overabundance of Cr in the Martian mantle based on their core-mantle partitioning.
On the other hand, the abundance of refractory elements in the Martian mantle has been estimated from the compositions of Martian meteorites (Yoshizaki and McDonough, 2020). The depletion in siderophile elements in the mantle relative to lithophile elements records the conditions of the core-mantle partitioning. While pressure, temperature, and oxygen fugacity of the core-forming conditions can be estimated from the metal-silicate partitioning of siderophile elements (Rai and van Westrenen, 2013), partition coefficients may also depend on the composition of the core-forming metals. Indeed, the metal-silicate partition coefficients of siderophile elements are affected by the presence of sulfur (Wood et al., 2014).
In this study, we investigated the effects of light elements on the partition coefficients of Ni and Cr under the Martian core pressures based on melting experiments in a diamond-anvil cell. We performed metal/silicate partitioning experiments of Ni and Cr in the presence of S, C, and/or H. Chemical analyses of recovered samples and in-situ X-ray diffraction measurements to determine H contents in metal showed that the partition coefficient of Ni increases (becomes more siderophile) and that of Cr decreases (becomes more lithophile) in the presence of S, C, and H. We discuss the possible Martian core composition that reproduces the depletion in Ni and the overabundance of Cr in the Martian mantle based on their core-mantle partitioning.