[S-IT18] Planetary cores: Structure, formation, and evolution

[SIT18-10] High temperature separation of the oxygen-rich big Martian core

*Eiji Ohtani¹, William F McDonough² (1.Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, 2.Department of Geology, University of Maryland)

Keywords:Martian core, Oxygen, Sulfur, High temperature, radiogenic energy, liquid immiscibility

Recent geophysical observations revealed existence of a large Martian core with a low mean density around 6 g/cm³ [1]. This discovery of a large Martian core provided a new constraint for the properties and composition of the Martian core. Based on this new observation, there are several reports on the composition of the Martian core based on the metal-silicate partitioning experiments of oxygen and sulfur at high pressure and temperature [e.g., 1, 2]. These estimations favor the sulfur rich Martian core (14~19 wt.%S and 1.3~3.5 wt.%O) assuming the core separation occurred at the temperatures around solidus/liquidus of peridotite (1960~2300K). However, the core formation temperature in the Martian magma ocean with the solidus/liquidus temperature of Martian mantle may be underestimated. McDonough et al. [3] proposed a possible energy inventory due to radiogenic energy from the decay of ²⁶Al and ⁶⁰Fe isotopes provides significant energy, comparable to the gravitational energy of the core separation to raise the temperature of the early Mars if fractionation and core formation of Mars occurred in the first 10 million yeas of solar system history resulting in super-liquidus temperature during the core separation stage of Mars.
Thermodynamic calculations of oxygen solubility due to the metallic-silicate partitioning of oxygen and sulfur suggest a high concentration of oxygen 11-13 wt% in the inner core with a limited amount of sulfur around 5-8.5 wt% when the core separation process occurs at the super-liquidus temperature (2500~3000 K), which is 400-600 K higher than the solidus temperature of the silicate mantle. The high oxygen content in Martian core is consistent with the recent estimation of the sulfur content of the bulk Mars [4] suggesting a limited sulfur content of Martian inner core with 5-8.5 wt%.
Phase relations of Fe-O, Fe-O-S, and Fe-Ni-O-S systems [5, 6, 7] indicated existence of a large region of the liquid immiscibility at least to 27 GPa. The bulk Martian core composition estimated above locates in the field of a liquid immiscibility coexisting with an oxygen-rich ionic and metallic iron liquids. Therefore, the Martian core may have a stratification of the oxygen-rich liquid outer core and small metallic inner core separated during cooling through the liquid immiscibility region. The early Martian dynamo [8] might have been generated by thermal convection of the miscible liquid core. However, the dynamo activity may have ceased by cooling and gravitational stratification of the core by formation of the O-rich ionic liquid outer core with a low thermal and electrical conductivities. The present model of the Fe-S-O Martian core reveals that the change from miscible to immiscible liquid in the Martian core during cooling provided a strong effect for formation and disappearance of the Marian magnetic field in the early Martian history.

References
[1] Stähler et al. (2021) Science, 373, 433-448 (2021).
[2] Gendre et al., Geochemical Prospective Letter, 21, 42-46, 2022
[3] McDonough et al., Geochemistry, Geophysics, Geosystems, 21, e2019GC008865.
[4] Yoshizaki and McDonough, Geochim Cosmochim. Acta 273, 137-162 (2020).
[5] Ohtani et al., Earth Planetary Sci. Lett., 71, 94-103 (1984).
[6] Urakawa et al. TERRAPUB/AGU, Tokyo/Washington, D.C., pp. 95–111 (1987).
[7] Tsuno and Ohtani, Phys Chem Minerals, 36:9–17 (2009)
[8] Mittelholz et al., Sci. Adv. 6: eaba0513 (2020)