^1,2Kyusei Tsuno, ³Hideharu Kuwahara, ¹Varun Manilal, ²Axel Wittmann, ^2,4Kurt Leinenweber, ³Tetsuo Irifune, ¹Damanveer S Grewal
Earth and Planetary Science Letters (in Press) Link to Article [DOI: 10.1016/j.epsl.2026.120246]
¹Department of Earth and Planetary Sciences, Yale University, New Haven, CT, 06511, United States
²Eyring Materials Center, Arizona State University, Tempe, AZ, 85287, United States
³Geodynamic Research Center, Ehime University, Matsuyama, 790-8577, Japan
⁴School of Molecular Sciences, Arizona State University, Tempe, AZ, 85287, United State
Copyright Elsevier

Geochemical constraints imply that a solid silicate layer existed between the base of the Martian magma ocean (∼14 GPa) and the core-mantle boundary (∼18–20 GPa) during early differentiation. Both S-poor metallic melts segregated during core formation and S-rich sulfide melts exsolved upon subsequent magma ocean cooling must have percolated through this layer to the core, but the efficiency of this process is poorly constrained. At ∼18 GPa, this layer comprises roughly equal proportions of ringwoodite and majorite garnet, yet no dihedral angles in majorite garnet have been reported. We conducted experiments at 18 GPa and 1723–2200 K to determine dihedral angles between Fe(-Ni)-S-O alloy melts (25–46 mol% S+O) and both ringwoodite and majorite garnet. Dihedral angles decrease with increasing temperature, S+O content, and oxygen fugacity, while Ni has no effect. Dihedral angles in majorite garnet are systematically ∼10° lower than in ringwoodite under comparable conditions. Despite this, all dihedral angles (89°-126°) remain above the 60° threshold for melt interconnection, so the entire mineral assemblage acts as a percolation barrier. Because our experimental S+O contents exceed those of the S-poor core-forming alloy (∼15 mol% S), the measured angles represent a lower bound; the barrier for core-forming metal was even more severe. Theoretical models predict that for such angles, a few (> ∼1–2) vol.% of melt remains trapped as isolated pockets upon network disconnection. Such melts constitute a hidden deep mantle reservoir of highly siderophile elements (HSEs) and siderophile volatiles (C, N), explaining their abundances in bulk silicate Mars without requiring a late veneer.

¹Fabrice Gaillard, ²Yves Marrocchi, ¹Gregory Rogerie, ³Mohamed A. Bouhif, ¹Camille Bernard, ⁴Mathieu Roskosz
Earth and Planetary Science Letters, 692, 120245 Link to Article [DOI: 10.1016/j.epsl.2026.120245]

¹Institut Des Sciences de la Terre d’Orléans, CNRS/Université d’Orléans/BRGM, 1a Rue de la Férollerie 2, Orléans, 45071 CEDEX, France
²Université de Lorraine, CNRS, CRPG, Nancy, F-54000, France
³Laboratoire Magmas et Volcans, Université Clermont Auvergne, CNRS, IRD, OPGC, Clermont-Ferrand, F-63000, France
⁴IMPMC, MNHN, CNRS, UMR 7590, Muséum National d’Histoire Naturelle, Sorbonne Universités, CP 52, 57 rue Cuvier, Paris, F-75231, France
Copyright Elsevier

The Mercury-Venus-Earth-Mars-Vesta planetary suite exhibits large variations in oxidation state as defined by the Fe to FeO ratio (i.e. core to silicate ratio), with increasingly oxygen-depleted bodies toward the centre of the solar system. As undifferentiated materials (i.e., chondrites) likely display a similar heliocentric FeO-gradient, planetary and chondritic oxidation states should be related in this respect. We develop an approach wherein, the equilibrium oxygen redistribution during gas – silicate melt – molten metal alloy during differentiation is resolved for bodies of various compositions and sizes. As a case study, three chondritic end-members were considered: enstatite, ordinary, and carbonaceous. A broad range of planetary oxidation states are obtained that encompass the above-mentioned planetary suite. The oxidation state during the growth of small bodies (<2000 km in radius) of constant bulk composition is affected by metal-vapour carbon redistribution, whereas on larger bodies, the incorporation of hydrogen, oxygen and silicon in the core prevails, causing the convergence toward a putative universal magma ocean FeO content. A dual regime is observed for the water content in the silicate magma ocean, which increases up to a planetary radius of ∼3000 km, whereas in larger bodies, hydrogen incorporation into the core brings about dehydration of the complementary silicate mantles. The accretion of ordinary chondrites perfectly matches the oxidation state of Mars and produces a core with C-H-S-N contents matching the Martian core density as suggested by the Insight missions. Finally, Vesta’s oxidation state seemingly requires an H2O-rich oxidizing component during the formation of planetesimals.