Rubidium and potassium isotopic variations in chondrites and Mars: Accretion signatures and planetary overprints

As moderately volatile elements, isotopes of Rb and K can trace volatilization processes in planetary bod-ies. Rubidium isotopic data are however very scarce, especially for non-carbonaceous meteorites. Here, we report combined Rb and K isotopic data (d87/85Rb and d41/39 Kappa) 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 T. for Rb and-0.26 +/- 0.05 T. for K, bulk OCs-0.120.15-0.24 T. for Rb and-0.720.28 -0.41 T. for K, bulk ECs 0.020.29-0.26 T. for Rb and-0.330.37-0.23 T. for K. The bulk K isotopic compositions of subgroup OCs are estimated to be-0.720.26-0.55 T. for H chondrites,-0.710.23 -0.39T.for L chondrites, and-0.770.63-0.30 T. 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 nucle-osynthetic anomalies).Individual ordinary and enstatite chondrites show large Rb and K isotopic variations (-1.02 to +0.29 T. for Rb and-0.91 to-0.15 T. for K). Samples of lower metamorphic grades display correlated elemental and isotopic fractionations 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 iso-topes 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 metamor-phism (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 pro-cesses 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 neg-ligible. 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.(c) 2023 Elsevier Ltd. All rights reserved.

Automated summary

This article reports the Rb and K isotopic data of 7 ordinary, 6 enstatite, and 4 Martian meteorites to understand the causes of variations in volatile abundances and isotopic compositions. The study finds a broad correlation between the Rb and K isotopic compositions of planetary bodies, suggesting volatility-controlled mass-dependent isotope fractionation. The large isotopic variations in non-carbonaceous meteorites are most likely due to diffusive redistribution of K and Rb during metamorphism and/or shock-induced heating and vaporization. Rating: 8/10.