High P-T experimental perspective on Cr isotopic fractionation during planetary core formation

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 Crmetal-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 Crmetal-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.

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