R3: High-pressure science and deep Earth’s material

[R3-P-11] Experimental constrains on the presence of a Fe-enriched molten silicate layer above Mars’ core

*Ryota Shimizu¹, Jiejun Jing¹, Yoshihiro Inoue¹, Mizuho Sakurahara¹, Rémy Pierru², Lianje Man², Steeve Gréaux¹ (1. Ehime Univ. GRC, 2. Bayerisches Geoinstitut)

Keywords:

high-pressure,partial melting,multi-anvil apparatus,Martian mantle,basal magma layer

NASA’s InSight mission on Mars was launched in 2018 and from early 2019 its SEIS broadband seismometer has continuously recorded the planet’s seismic activity, operating until late 2022. These unprecedented data have shed new lights on the planet’s internal composition and structure. Among the many discoveries, the presence of a thick layer of basal magma existing on top of the Martian core (Khan+ 2023, Samuel+ 2023) is intriguing the science community. The Martian mantle is thought to be richer in Fe (Taylor, 2013) but colder than Earth's mantle (Huang+ 2022). Current solidus models suggest mantle melting at the P and T conditions of the top of the Martian core may be difficult (Duncan+ 2018), and only partial melting may not be sufficient to form of a thick melt layer atop the core. The presence of a gravitationally stable basal melt layer would thus require other factors, such as a chemically distinct melt composition and/or higher T and oxygen fugacity (fO₂) at the core-mantle boundary. However, most experimental work on the Martian mantle solidus has been conducted under relatively reducing conditions, leaving the effects of more oxidizing conditions largely unaddressed.Here we present high P and high T melting experiments on a Martian mantle aggregate at 20 GPa and 2000 ºC using the Kawai-type multianvil press (ORANGE2000) at the Geodynamics Research Center (Ehime Univ.). High-P experiments were conducted using a double capsule technique, with the sample placed in either metallic (Mo, Re or Pt) or graphite inner capsule while fine-grained oxides (MoO₂, ReO₂ or Fe₂O₃/Fe₃O₄) was placed in the outer capsule to influence different fO₂. The fO₂ was determined from analyzing Fe-content of a Pt ball (Medard+ 2008) placed in a nearby identical double capsule. Electron microprobe and X-ray diffraction analyses showed the recovered samples consisted of Ringwoodite, Majorite, Magnesiowüstite, Stishovite, and a silicate melt containing 20.3 to 29.7 wt.% FeO. The melt proportions determined by mass balance calculations however showed only up to 29.3 vol.% melt would be generated at FMQ -2, which is about the fO₂ expected in the Martian mantle (Nicklas+ 2021). These results suggest more Fe-enriched mantle and basal magma compositions are required to explain the characteristics of the melt layer atop Mars’ core.