Updating the Urey-Craig diagram: The iron redox states in the building blocks of the outer solar system

The iron speciation and valence state of meteorites reflect the origin and evolution of primitive bodies in the solar system. This idea is illustrated in an iconic plot in planetary science, the so-called Urey-Craig diagram (UC diagram), where the amount of iron accommodated in sulfides and metallic alloys is plotted against that found in oxides and silicates. Hydrous carbonaceous chondrites (CI and CM) are usually considered the oxidized end-member despite the minor presence of sulfides. Here, we reevaluate the position of CIs inspired by the recent space missions that sampled CI-like materials and from a fresh CI meteorite (OC 002). The same approach was undertaken for C2-ungrouped, CM, and CR chondrites with varying proportions of phyllosilicates based on X-ray diffraction and Mössbauer spectroscopy. As a result, all fresh CI materials contain a considerable amount of iron sulfides and therefore plotted in the central part of the UC diagram. Tagish Lake and Tarda accommodate up to ~10 % of the iron in sulfides, and a contribution of kamacite is detected in less altered CM samples. The updated UC diagram suggests that the formation environments of carbonaceous chondrites are much less oxidizing than previously assumed, and possibly not so different from those of non-carbonaceous chondrites. Furthermore, the CM clan defines a trend that the more Fe2+-rich in phyllosilicates, the less metal in the samples. This CM trend excludes CI and C2-ungrouped samples, suggesting their distinct redox environments. This work demonstrates that CIs are never an oxidized end-member in the UC diagram. Instead, it most certainly marks the position of the dust present in the outer part of the protoplanetary disk before secondary processes occurred at parent bodies.