[S-IT20] Deep Earth Sciences

[SIT20-12] A possible origin of dichotomic oxidation states between HED meteorites and angrite meteorites: Implications for planetesimals in the earliest inner solar system

★Invited Papers

*Hideharu Kuwahara¹, Damanveer Grewal^2,3,4, Kyusei Tsuno^4,5 (1.Geodynamics Research Center, Ehime University, 2.School of Molecular Sciences, Arizona State University, 3.School of Earth and Space Exploration, Arizona State University, 4.Facility for Open Research in a Compressed Environment (FORCE), Arizona State University, 5.Eying Materials Center, Arizona State University)

Keywords:Angrite parent body, Planetesimals in the earliest inner solar system, Mantle oxidation state

Characterizing planetesimals and planetary embryos in the earliest inner Solar System is a critical issue for understanding the origin of terrestrial planets. Achondrites, which likely originated from the disruption of differentiated rocky bodies, provide key constraints on planetesimals. Among achondrites, HED meteorites and angrite meteorites exhibit depletion of siderophile elements and early core-formation ages in their parent bodies, occurring around 1–2 Myr after the formation of Ca-Al-rich inclusions [1, 2]. The homogeneous oxygen isotopic compositions of HED and angrite meteorites also suggest the possible presence of a magma ocean in their parent bodies [3]. These results indicate that the mantles of the parent bodies of HED and angrite meteorites were likely reducing, below the iron-wüstite (IW) buffer [4, 5]. However, these two meteorite groups exhibit a dichotomy in oxygen fugacity.
Redox-sensitive chemical indicators in HED meteorites suggest that the oxygen fugacity of their mantle is approximately 1 log unit below the IW buffer (ΔIW-1), which is consistent with metal-silicate equilibration. Conversely, the oxygen fugacity of angrite meteorites is about ΔIW+1, which is inconsistent with the expected value based on metal-silicate equilibration (ΔIW-1 to -2) [2, 6]. The origin of this dichotomy in oxygen fugacity between these two bodies remains enigmatic and poorly understood.
Here we discuss two possible oxidation mechanisms for the angrite parent body (APB) mantle after the core formation:
1. Fe²⁺ disproportionation in a magma ocean (3Fe²⁺ → 2Fe³⁺ + Fe⁰) [7-9], which increases Fe³⁺/(Fe²⁺ + Fe³⁺) ratio of the mantle after the disproportionated Fe⁰ is segregated into the core.
2. Late accretion of water from beyond the snowline, which increases the intrinsic oxygen fugacity of the mantle after the escape of H₂ via the reaction: 2FeO + H₂O → Fe₂O₃ + H₂.
Our results demonstrate that the parent body larger than the Moon may explain the oxidized angrite meteorites if water content in the APB mantle does not significantly exceed previous estimates (< 1000 μg/g).

References: [1] Mittlefehldt, 2015, Chemie der Erde 75, 155-183. [2] Keil, 2012, Chemie der Erde 72, 191-218. [3] Greenwood et al., 2005, Nature 435, 916-918. [4] Steenstra et al., 2016, Geochimica et Cosmochimica Acta 177, 48-61. [5] Steenstra et al., 2017, Geochimica et Cosmochimica Acta 212, 62-83. [6] Tissot et al., 2022, Geochimica et Cosmochimica Acta 338, 278-301. [7] Armstrong et al., 2019, Science 365, 903-906. [8] Kuwahara et al., 2023, Nature Geoscience 16, 461-465. [9] Zhang et al., 2024, Science Advances 10, eadp1752.