Constraints on Magma Self-oxidation in Angrites and the Role of Parent-body SizeOPEN ACCESS 

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Hideharu Kuwahara, Damanveer S. Grewal and Kyusei Tsuno

The Planetary Science Journal, Volume 7, Number 8

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“Earth, and potentially Venus, formed through giant impacts involving early-formed planetesimals and planetary embryos in the inner solar system, but the size distribution of these bodies remains poorly constrained. Angrites sample one of the earliest differentiated bodies in the inner solar system and preserve a marked redox contrast with HEDs: evolved angrites record oxygen fugacity (fO2) near-iron-wüstite (IW) or slightly supra-IW conditions, whereas HEDs are ∼1–2 log units more reduced. This difference has been interpreted to indicate a comparatively oxidized mantle for an angrite parent body (APB), but most fO2 estimates are derived from evolved magmas that may not track deep-mantle conditions. Here, we explore whether internal magma self-oxidation driven by Fe3+ stabilization in a deep magma ocean followed by fractional crystallization can account for the redox state of evolved angrites and what this implies for APB size. We combine parameterizations of Fe3+/ΣFe in silicate melts with fractional crystallization models of Group 1–2 angrites, starting from metal-saturated conditions consistent with primitive angrites. We find that magma self-oxidation reproduces the fO2 inferred for evolved angrites only if core–mantle equilibration pressures exceed several gigapascals. Such pressures imply a body substantially larger than asteroid Vesta. For plausible APB compositions and starting redox states, radii of ∼1250–2500 km are required, comparable to or exceeding the size of the Moon and overlapping independent geobarometric estimates. Rather than uniquely determining APB size, our results provide a redox-based lower bound on its size and demonstrate that internal oxidation offers a viable mechanism for generating oxidized signatures in evolved angrites.”

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