Carbon solubility and liquid–liquid immiscibility in Fe–C–S ternary system up to 6 GPa: implications for the core of small planetary bodies
Abstract
High-pressure, high-temperature experiments were conducted in a multi-anvil press using seven different starting compositions to investigate the phase relations of Fe–C–S liquids at pressures up to 6 GPa and temperatures up to 2000 K. Quenched samples revealed immiscible C-rich and S-rich metallic liquids at 2 and 4 GPa, manifesting as either large immiscible zones or emulsified smaller droplets. On the contrary, no immiscibility was observed at 5 or 6 GPa. At these higher pressures, the carbon solubility limit, significantly reduced by increasing sulfur content in the ternary liquid, prevents the formation of a liquid phase rich in both C and S. Our findings thus indicate that super-liquidus immiscibility in Fe-rich compositions does not occur at pressures above 5 GPa, due to the differential pressure dependence of C solubility and miscibility. The limited C solubility in S-rich liquids at higher pressures effectively suppresses immiscibility. Based on these results, terrestrial planetary bodies with fluid cores at pressures exceeding 5 GPa are unlikely to experience liquid–liquid immiscibility, even when both carbon and sulfur are abundant, as sulfur limits carbon solubility and promotes its exsolution. For the Moon, immiscibility-induced core stratification is not expected, though local emulsification could occur near the core–mantle boundary (CMB). In smaller bodies, a two-liquid Fe–C–S core may form at high temperatures, but progressive cooling may lead to graphite crystallization and the potential development of a graphitic crust.
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BibTeX
@article{zhao2026carbon,
title = {{Carbon solubility and liquid–liquid immiscibility in Fe–C–S ternary system up to 6 GPa: implications for the core of small planetary bodies}},
author = {Bin Zhao and Guillaume Morard and Geeth Manthilake and Paraskevas Parisiades and Jing Yang and Yingwei Fei and Yann Le Godec and Nozomi Kondo and Takashi Yoshino and Daniele Antonangeli},
journal = {Progress in Earth and Planetary Science},
year = {2026},
month = sep,
doi = {10.1186/s40645-026-00846-3},
url = {https://hal.science/hal-05756586v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Carbon solubility and liquid–liquid immiscibility in Fe–C–S ternary system up to 6 GPa: implications for the core of small planetary bodies
AU - Bin Zhao
AU - Guillaume Morard
AU - Geeth Manthilake
AU - Paraskevas Parisiades
AU - Jing Yang
AU - Yingwei Fei
AU - Yann Le Godec
AU - Nozomi Kondo
AU - Takashi Yoshino
AU - Daniele Antonangeli
PY - 2026
DA - 2026/09/20
JO - Progress in Earth and Planetary Science
DO - 10.1186/s40645-026-00846-3
UR - https://hal.science/hal-05756586v1
AB - High-pressure, high-temperature experiments were conducted in a multi-anvil press using seven different starting compositions to investigate the phase relations of Fe–C–S liquids at pressures up to 6 GPa and temperatures up to 2000 K. Quenched samples revealed immiscible C-rich and S-rich metallic liquids at 2 and 4 GPa, manifesting as either large immiscible zones or emulsified smaller droplets. On the contrary, no immiscibility was observed at 5 or 6 GPa. At these higher pressures, the carbon solubility limit, significantly reduced by increasing sulfur content in the ternary liquid, prevents the formation of a liquid phase rich in both C and S. Our findings thus indicate that super-liquidus immiscibility in Fe-rich compositions does not occur at pressures above 5 GPa, due to the differential pressure dependence of C solubility and miscibility. The limited C solubility in S-rich liquids at higher pressures effectively suppresses immiscibility. Based on these results, terrestrial planetary bodies with fluid cores at pressures exceeding 5 GPa are unlikely to experience liquid–liquid immiscibility, even when both carbon and sulfur are abundant, as sulfur limits carbon solubility and promotes its exsolution. For the Moon, immiscibility-induced core stratification is not expected, though local emulsification could occur near the core–mantle boundary (CMB). In smaller bodies, a two-liquid Fe–C–S core may form at high temperatures, but progressive cooling may lead to graphite crystallization and the potential development of a graphitic crust.
SN - 2197-4284
ER -