Potassium isotope heterogeneity in the early Solar System controlled by extensive evaporation and partial recondensation

Abstract

Volatiles are vital ingredients for a habitable planet. Angrite meteorites sample the most volatile-depleted planetesimal in the Solar System, particularly for the alkali elements. They are prime targets for investigating the formation of volatile-poor rocky planets, yet their exceptionally low volatile content presents a major analytical challenge. Here, we leverage improved sensitivity and precision of K isotopic analysis to constrain the mechanism of extreme K depletion (>99.8%) in angrites. In contrast with the isotopically heavy Moon and Vesta, we find that angrites are strikingly depleted in the heavier K isotopes, which is best explained by partial recondensation of vaporized K following extensive evaporation on the angrite parent body (APB) during magma-ocean stage. Therefore, the APB may provide a rare example of isotope fractionation controlled by condensation, rather than evaporation, at a planetary scale. Furthermore, nebula-wide K isotopic variations primarily reflect volatility-driven fractionations instead of presolar nucleosynthetic heterogeneity proposed previously.

Rubidium isotopic compositions of angrites controlled by extensive evaporation and partial recondensation

“…These super-chondritic isotopic signatures may be attributed to volatile depletion caused by partial evaporation ( 12 , 34 , 36 ). However, extremely light K isotopic compositions have been observed in angrites ( 21 ), which contradict the excepted trend of increasing δ 41 K with enhanced volatile loss, if partial evaporation is considered as the sole cause. To explain the light K isotopic signature in angrites, a partial recondensation model was proposed ( 21 ).…”

“…However, extremely light K isotopic compositions have been observed in angrites ( 21 ), which contradict the excepted trend of increasing δ 41 K with enhanced volatile loss, if partial evaporation is considered as the sole cause. To explain the light K isotopic signature in angrites, a partial recondensation model was proposed ( 21 ). This contrasts with the traditional hypothesis of partial evaporation of K which is suggested for isotopically heavy bodies like Vesta and the Moon ( 19 , 37 ).…”

“…Isotopic analyses conducted on MVEs offer a window into the mechanisms and conditions associated with volatile depletion. Previous investigations employing isotopic systems such as Rb, K, Cu, and Zn have yielded a wealth of insights into understanding the mechanisms behind volatile depletion within the inner solar system ( 13 – 25 ).…”

“…Considering the geochemical and cosmochemical similarity between Rb and K, it is expected that evaporation and condensation would yield similar effects on these two elements. Consequently, observation of light Rb isotopic compositions in angrites is anticipated and can serve as a robust test for the partial recondensation model ( 21 ). Yet, the only reported Rb isotope data from an angrite sample (Sah99555, δ 87 Rb = +0.12‰) ( 16 ) aligns closely with that of chondrites, which scatter around zero ( 16 , 23 ).…”

“…This contradiction challenges the recondensation model proposed on the basis of K isotopes. It is important to consider that the reported value of Sah99555 might not reflect its pristine isotopic signature due to potential contamination from terrestrial materials ( 16 , 21 ). Therefore, analyzing a variety of angrites for Rb isotopes becomes essential to discern the effect of terrestrial contamination and effectively test the models of volatile depletion.…”