Iron meteorites: formation under oxidised conditions and relatively far from the sun
Understanding the early stages of solar system formation is an important challenge that the scientific community has been trying to solve for many years. One proposal suggests that during its formation the solar system was zoned into areas with differing oxidation-reduction conditions. The areas near the sun were depleted of oxygen and were therefore more reduced than the external areas.
Iron meteorites represent the nucleus of planetesimals, tiny planetary bodies with the potential to become full-sized planets. These formed early in the history of the Solar System from the segregation of metal during planetary differentiation, similar to the formation of the Earth’s core. The conditions in which they were formed, however, remain enigmatic. Determining these conditions can tell us about the process of metal segregation and also about the planetesimals formation zone.
Former Oxford Postdoctoral Research Assistant Pierre Bonnand and Professor Alex Halliday, who recently left Oxford to head up Columbia University’s Earth Institute, investigated the history of Iron meteorite formation by measuring the elementary and isotopic variations of chromium (Cr) in these objects. Cr is an element whose siderophile character (which accompanies iron during the formation of the nucleus) is strongly dependent on the oxidation-reduction conditions prevailing during metal segregation. The concentration of Cr in the metal liquid increases in reduced conditions. This property makes the concentration of Cr in the metal a very good tool for estimating initial redox conditions in planetesimals. Because the Cr concentration in iron meteorites is strongly influenced by fractional crystallization during cooling of the metallic liquid, the initial concentration in the liquid is not directly accessible, so in this study, Bonnand and Halliday developed a new method while they were working together in Oxford to recalculate this initial concentration.
The study of isotopic (mass-dependent) variations in Cr reveals a systematic isotopic fractionation in Iron meteorites: the latter are generally heavier isotopically than Silicate Earth (Figure 1). The modeling of the elementary and isotopic compositions makes it possible to determine the initial concentration of Cr in the metallic liquid and thus to estimate the redox conditions during its formation. The results show that the metal segregation in the planetesimals from which the iron meteorites originated took place under relatively oxidized conditions, comparable to those determined for ordinary chondrites. Contrary to what was advanced in previous studies, these planetesimals formed in a zone located beyond the planet Mars.
Understanding how the parent bodies of iron meteorites were formed not only helps constrain the conditions at the beginning of the solar system, but also sheds light on the formation of the Earth’s core. Such samples are often used as a comparison, but the findings from this study suggest that the planetesimals’ cores formed under more oxidised conditions, and therefore further away from the sun than Earth.
Paper: Oxidized conditions in iron meteorite parent bodies by P. Bonnand & A. N. Halliday was published in Nature Geoscience volume 11, pages401–404 (2018). DOI: doi.org/10.1038/s41561-018-0128-2
Main image: Rowton IIIAB iron meteorite BM50062, sample and image courtesy of the Natural History Museum.