^1,2Bing Yang,¹Jiuxing Xiad,^1,2Xuan Guo,^1,Huaiwei Ni,³Anat Shahar,³Yingwei Fei,³Richard W.Carlson,^1,2Liping Qin
Earth and Planetary Science Letters 595, 117701 Link to Article [https://doi.org/10.1016/j.epsl.2022.117701]
¹CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
²CAS Center for Excellence in Comparative Planetology, China
³Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC, USA
⁴Institute of Geology and Geophysics, School of Earth Sciences, Zhejiang University, Hangzhou, Zhejiang 310027, China
Copyright Elsevier

Core formation may modify the stable isotopic signatures for both the mantles and cores of differentiated planetary bodies. We performed high P-T experiments with a piston-cylinder apparatus at 1 GPa and 1873-2073 K to determine the Cr isotopic fractionation factor during metal-silicate segregation. Experimental results consistently indicate that the metal phase is isotopically heavier than the coexisting silicate phase, with Cr_{metal-silicate} up to 0.3‰ at the investigated experimental conditions. Oxygen fugacity, silicate composition, and S content in the metal phase do not have significant effects on the Cr isotopic fractionation factor. By contrast, increasing Ni content in the metal increases the Cr_{metal-silicate} value, implying that the Ni content of the core could influence planetary isotopic signatures. We conclude that heavier Cr isotopes enter the core preferentially during planetary core formation. The Cr value of the terrestrial mantle could be lowered by up to ∼0.02‰ by core formation, despite that this is within current analytical uncertainty of chondritic Cr isotopic composition. For smaller bodies such as the Moon, Mars, and Vesta, the lower core formation temperatures could potentially generate a resolvable core-mantle Cr isotopic fractionation. However, the Moon’s small core size would limit the change in the Cr isotopic composition of the lunar mantle compared to chondritic. For Vesta and Mars, core formation could lower the Cr values of their mantles by ∼0.01-0.02‰, which is trivial relative to the analytical uncertainty. On the other hand, core formation could increase the Cr values of the cores of the parent bodies of iron meteorites by up to ∼0.2‰ at 1873 K. Therefore, the significantly heavy Cr isotopic composition (up to 2.85‰) of iron meteorites cannot be explained by equilibrium fractionation between the core and the mantle of the parent bodies of iron meteorites.

^1,2Jan Render,^1,2Gregory A.Brenneck,¹Christoph Burkhardt,^1,3Thorsten Kleine
Earth and Planetary Science Letters 595, 117748 Link to Article [https://doi.org/10.1016/j.epsl.2022.117748]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Straße 10, Münster, 48149 Germany
²Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Ave., Livermore, CA, USA
³Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
Copyright Elsevier

Nucleosynthetic isotope signatures in meteorites provide key insights into the structure and dynamics of the solar protoplanetary disk and the accretion history of the planets. We present high-precision Zr isotopic data of a comprehensive suite of non-carbonaceous (NC) and carbonaceous (CC) meteorites, and find that various meteorite groups, including enstatite chondrites, exhibit ⁹⁶Zr enrichments, whereas there is no resolved ⁹¹Zr and ⁹²Zr variability. These new Zr isotope data reveal the same fundamental NC-CC dichotomy observed for several other elements, where CC meteorites are more anomalous compared to NC meteorites and are shifted towards the isotopic composition of Ca-Al-rich inclusions (CAIs). For Zr and other elements, the CC composition is reproduced as a mixture of materials with CAI-like and NC-like isotopic compositions in approximately constant proportions, despite these elements exhibiting disparate nucleosynthetic origins or different cosmo- and geochemical behaviors. These constant mixing proportions are inconsistent with an origin of the dichotomy by thermal processing or selective dust-sorting in the disk but indicate mixing of isotopically distinct materials with broadly solar chemical compositions. This corroborates models in which the NC-CC dichotomy reflects time-varied infall from an isotopically heterogeneous molecular cloud. Among NC meteorites, the isotope anomalies in Zr are linearly correlated with those of other elements, which likewise reflects primordial mixing. Lastly, the new Zr isotope data reinforce the notion that Earth incorporated s-process enriched material from the innermost Solar System, which is not represented by known meteorites. By contrast, contributions to Earth and Mars from outer Solar System CC-like materials were limited, indicating that these planets did not form by pebble accretion, which would have led to high CC fractions.