Beyond the Urey-Craig diagram: a ternary framework for iron redox evolution during aqueous alteration of carbonaceous chondrites
Damanveer S. Grewal, Zhongtian Zhang
Geochimica et Cosmochimica Acta, In Press, Journal Pre-proof, Available online 3 September 2026
“Water-rock reactions during aqueous alteration were critical in establishing the redox state of early Solar System planetesimals. The Urey-Craig (UC) diagram, which plots “reduced Fe” (metallic Fe + sulfide Fe) against “oxidized Fe” (silicate- and oxide-bound Fe), is the canonical framework for interpreting redox variations among chondrites. Historical CI chondrites, assumed to be metal-free with their sulfur residing in sulfate, were assigned no “reduced Fe” and long regarded as the oxidized endmember. However, the finding that the returned Ryugu and Bennu samples, along with fresh CI material, host sulfides rather than sulfates, revises their position in the UC diagram, placing them closer to the more reduced CR, L, and LL chondrites. This shift has been interpreted as evidence that CIs are more reduced, challenging their long-standing role as the oxidized endmember.
This interpretation, however, conflates two distinct Fe reservoirs. Under the low-Eh conditions of planetesimal alteration, metallic Fe oxidizes readily whereas sulfides persist, so only the non-sulfide fraction of bulk Fe is accessible to water-driven oxidation. Because bulk S content increases with matrix fraction, the proportion of non-sulfide Fe decreases, making matrix-rich CIs the group with the smallest “oxidizable” Fe reservoir. By quantifying the mean valence state and oxygen associated with the non-sulfide Fe, we show that CI chondrites, although they appear reduced in the revised UC diagram, record pervasive oxidation of the non-sulfide Fe pool, comparable to or greater than that of the most altered CM lithologies. This convergence occurs despite mineralogically distinct alteration pathways on the CM and CI parent bodies, consistent with the endpoint Fe valence being set by the redox potential of the alteration fluid rather than by specific reaction pathways. The lower total O uptake by Fe in CIs reflects not reduced conditions but a smaller initial reservoir of “oxidizable” Fe, itself limited by high bulk S. We introduce a Fe0-FeS-Feox (metallic Fe-sulfide Fe-oxidized Fe) ternary framework that reconciles extensive oxidation in CIs with their misleading UC position and provides a more accurate basis for tracing aqueous redox pathways in carbonaceous chondrite parent bodies”
