¹Cyril Sturtz,¹Angela Limare,¹Stephen Tait,¹Édouard Kaminski
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2021JE007020]
¹Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, Paris, F-75005 France
Published by arrangement with John Wiley & Sons

This is the second of two companion papers that present a theoretical and experimental study of the thermal history of planetesimals in which heating by short-lived radioactive isotopes generates an internal magma ocean and the subsequent cooling and crystallization thereof. We study the conditions required to form and preserve basal cumulates and flotation crusts, and the implications for the thermal evolution of planetary bodies. Our model predicts that planetesimals larger than 30km can reach 1300oC and a melt fraction of 40 vol%, producing a solid-like to liquid-like rheological transition that triggers an internal magma ocean. In the magma ocean regime core-mantle differentiation occurs very quickly and the mantle convects under a relic of chondritic material whose thickness is controlled by the temperature of rheological transition. We show that the magma ocean episode is associated with time-dependent crystal segregation and no re-entrainment. Segregation of crystals is essentially constrained by their size and by their density difference with respect to the melt, the latter being fully determined by the planetesimal’s initial composition. Olivine cumulates are likely to form at the core-mantle boundary. Under certain particular conditions, a flotation crust can also form, which reduces the efficiency of heat evacuation by convection, thereby enhancing the magma ocean’s lifetime and the efficiency of crystal segregation. Two types of large-scale mantle structure are possible outcomes: a well-mixed upper mantle above an olivine cumulate, or a more finely layered ”onion-shell” structure.

¹Suyu Fu,²Stella Chariton,²Vitali B. Prakapenka,³Andrew Chizmeshya,¹Sang-Heon Shim
American Mineralogist 107, 2307-2314 Link to Article [http://www.minsocam.org/msa/ammin/toc/2022/Abstracts/AM107P2307.pdf]
¹School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, U.S.A.
²Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, U.S.
Copyright: The Mineralogical Society of America

Light elements alloying with metallic Fe can change the properties and therefore play a key role
in the structure and dynamics of planetary cores. Hydrogen and silicon are possible light elements in
the rocky planets’ cores. However, hydrogen storage in Fe-Si alloy systems remains unclear at high
pressures and high temperatures because of experimental difficulties. Taking advantage of pulsed laser
heating combined with high-energy synchrotron X‑ray diffraction, we studied reactions between FeSi
and H in laser-heated diamond-anvil cells (LHDACs) up to 61.9 GPa and 3500 K. We found that under
H-saturated conditions the amount of H alloying with FeSi (0.3 and <0.1 wt% for the B20 and B2 structures, respectively) is much smaller than that in pure Fe metal (>1.8 wt%). Our experiments also
suggest that H remains in the crystal structure of FeSi alloy when recovered to 1 bar. Further density
functional theory (DFT) calculations indicate that the low-H solubility likely results from the highly
distorted interstitial sites in the B20 and B2 structures, which are not favorable for H incorporation.
The recovery of H in the B20 FeSi crystal structure at ambient conditions could open up possibilities
to understand geochemical behaviors of H during core formation in future experiments. The low-H
content in FeSi alloys suggests that if a planetary core is Si-rich, Si can limit the ingassing of H into
the Fe-rich core.