Rubidium and Potassium Isotopic Variations in Chondrites and Mars: Accretion Signatures and Planetary Overprints

1,2Nicole X.Nie,3,4Xin-Yang Chen,1Zhe J.Zhang,5Justin Y.Hua,2Weiyi Liu,2Francois L. H. Tissot,3Fang-Zhen Teng,6Anat Shahar,1Nicolas Dauphas
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2023.01.004]
1Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA
2The Isotoparium, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
3Isotope Laboratory, Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA
4Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu, China
5Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
6Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC 20015, USA
Copyright Elsevier

As moderately volatile elements, isotopes of Rb and K can trace volatilization processes in planetary bodies. Rubidium isotopic data are however very scarce, especially for non-carbonaceous meteorites. Here, we report combined Rb and K isotopic data (δ87/85Rb and δ41/39Κ) for 7 ordinary, 6 enstatite and 4 Martian meteorite falls to understand the causes for the variations in volatile abundances and isotopic compositions. Bulk Rb and K isotopic compositions of planetary bodies are estimated to be (Table 1): Mars +0.10±0.03 ‰ for Rb and −0.26±0.05 ‰ for K, bulk OCs ‰ for Rb and ‰ for K, bulk ECs + ‰ for Rb and ‰ for K. The bulk K isotopic compositions of subgroup OCs are estimated to be ‰ for H chondrites, ‰ for L chondrites, and ‰ for LL chondrites. A broad correlation between the Rb and K isotopic compositions of planetary bodies is observed. The correlation follows a slope that is consistent with kinetic evaporation and condensation processes, suggesting volatility-controlled mass-dependent isotope fractionation (as opposed to nucleosynthetic anomalies).

Individual ordinary and enstatite chondrites show large Rb and K isotopic variations (−1.02 to +0.29 ‰ for Rb and −0.91 to −0.15 ‰ for K). Samples of lower metamorphic grades display correlated elemental and isotopic fractionation between Rb and K, while samples of higher metamorphic grades show great scatter, suggesting that chondrite parent-body processes have decoupled the two elements and their isotopes at the sample scale. Several processes could have contributed to the observed isotopic variations of Rb and K, including (i) chondrule “nugget effect”, (ii) volatilization during parent-body thermal metamorphism (heat-induced vaporization and gas transport within parent bodies), (iii) thermal diffusion during parent-body metamorphism, and (iv) impact/shock heating. Quantitative modeling of the first two processes suggests that neither of them could produce isotopic variations large enough to explain the observed isotopic variations. Volatilization during parent-body thermal metamorphism [the scenario (ii)], which has been commonly invoked to explain the isotopic variations of volatile elements, is gas transport-limited and its effect on isotopic fractionations of moderately volatile elements should be negligible. Modeling of diffusion processes suggests that (iii) could produce K isotopic variation comparable to the observed variation. The large isotopic variations in non-carbonaceous meteorites are thus most likely due to diffusive redistribution of K and Rb during metamorphism and/or shock-induced heating and vaporization.

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