[S-GC38] Solid Earth Geochemistry, Cosmochemistry

[SGC38-04] Phosphorus host phases in rocky core of icy satellite: Insights from high-pressure experiments of chondrite under hydrous conditions

*Tetsu Kogiso¹, Shinnosuke Takeda¹, Yuri I. Fujii¹ (1.Kyoto University)

Keywords:extraterrestrial life, icy satellite, inner ocean, essential elements

Liquid oceans in icy satellites of gas giants could be viable environment for organisms. Circulation of essential elements between the ocean and rocky core in the satellite interior is one the most crucial processes to keep the oceans of icy satellites habitable for organisms. We here focus on phosphorus circulation in the satellite interior. Although it is difficult to estimate the chemical composition of rocky cores, a reasonable assumption is that rocky cores have chondritic composition. Chondrite contains small amounts of phosphorus (~0.1 wt.% order as P₂O₅), but the main host phase for phosphorus in chondritic rocks under high pressures is unknown, making it difficult to clarify whether the rocky cores of icy satellites can supply phosphorus to the liquid ocean. In this study we performed high-pressure experiments on synthetic chondritic material under hydrous conditions to investigate host phases of phosphorus in the rocky core of icy satellite. We prepared two starting materials from natural minerals and/or reagents. Both the starting materials have identical CI-chondrite composition approximated in the CaO-FeO-MgO-Al₂O₃-SiO₂-H₂O-P₂O₅ system. Experiments were conducted using the Au-Au double capsule technique with Ni-NiO oxygen-fugacity buffer at 700°C and 1.0 GPa for 7 days using a piston-cylinder apparatus. The run products consist of fine aggregates of olivine, orthopyroxene, magnetite, chlorite and apatite. The most abundant phase is olivine (about 50 vol.% or more). Olivine contains small amounts of P₂O₅ (typically less than 0.5 wt.%), whereas other silicate phases and magnetite are phosphorus free. Apatite can be regarded as a stable phase because one of the starting materials did not contain apatite but Ca hydrogen phosphate as a phosphorus source. These results suggest that phosphorus is mainly contained in olivine and apatite in the chondritic rocky core under high pressure. Since olivine and apatite are insoluble in neutral water, a chondritic rocky core can be the main supply source of phosphorus to the liquid ocean if the ocean water is strongly acid and/or there occur intense hydrothermal activities on the surface of rocky core.