Melting of the Fe-O-S system and reaction between olivine and iron melt at the lunar core conditions: Implications for the structure of the lunar core

Despite recent observations on the Moon from satellite sensing and analyses of Apollo-era seismic data, there are still several unsolved issues on the deep lunar interior. Recent studies suggest the presence of a solid inner core and liquid outer core in the Moon (1). If we could constrain the temperature and composition of the lunar outer core, this would help us to better understand the lunar interior. Here, we focused on the interaction between liquid iron-alloys and solid silicates, and revealed the nature of the core of the Moon. The lunar mantle is characterized by high FeO content (2). This implies that the Moon is more oxidizing compared to the Earth and thus the lunar core could be composed of Fe-O-S alloys. It is known that there is a liquid immiscibility in the Fe-O-S system, thus one of the aims of our study is to clarify whether the Moon has an immiscible liquid core or not. We performed oxygen partition experiments between the lunar silicate mantle and molten metal at 5 GPa and calculated the distribution coefficient, D, of oxygen between liquid metal and olivine crystals. Using this value, the amount of O in the lunar liquid core is calculated to be 4.45 at.% at 1000 °C and 5.56 at.% at 1400 °C when the mantle Mg number is 80, which is the assumed lunar core (2). This high amount of oxygen strongly indicates that the lunar core coexisting with the FeO-rich mantle is composed of the two immiscible liquids (FeO-rich melt and Fe-FeS melt). Estimations from the existing data on the density of FeO-rich melt and liquid Fe-FeS indicate that the liquid Fe-FeS is denser than the FeO-rich melt suggesting that the lunar liquid core is stratified to an upper FeO-rich layer and a lower Fe-FeS layer. The upper FeO-rich layer is reactive with the olivine rich lunar mantle to form magnesiowustite and could cause the partially molten low Q (high damping) region at the CMB of the Moon. (1) Weber, R.C., Lin. P., Garnero, E.J., Williams, Q., Lognonné, P., Science 331, 309-312, (2011) (2) Taylor, S.R., Solar system evolution, A new Perspective. Cambridge University Press, New York, 307 (1992)