Volatile-depletion processing of the building blocks of Earth and Mars as recorded by potassium isotopes

Abstract

The stable potassium isotopic ratios (41K/39K) of Earth and Mars have been interpreted to reflect either nucleosynthetic isotope anomalies or volatility-driven K depletion. Chondrites comprise primordial materials from which planetary bodies are assembled, and thus are critical samples for this discussion. Here, we present high-precision K isotopic analyses (reported as δ41K) of 33 chondrites and two achondrites, which reveal unprecedented variation from −1.08 to 4.68‰. In addition, there is considerable overlap in δ41K values between carbonaceous and non-carbonaceous meteorites despite their contrasting nucleosynthetic isotope anomalies. These findings are inconsistent with the nucleosynthetic origin of 41K variations in meteorites. Instead, the δ41K values of chondrites correlate positively with the isotopic compositions of other moderately volatile elements (e.g., Rb, Cu, Zn, Sn, Ga, and Te). These correlations suggest that volatility-controlled fractionation is a common mechanism for mass-dependent isotopic variations in the Solar System. In particular, carbonaceous chondrites and the angrite parent body exhibit a trend of concomitant decreases in K and its heavier isotope due to incomplete K condensation. Earth and Mars also follow this trend, suggesting that their K depletion may reflect similar volatile-depleting processes that occurred with their respective precursors. That Mars is isotopically heavier than Earth is consistent with it having less K-depleted precursors, in addition to the previous suggestion of a later-stage K loss from proto-Mars during accretionary collisions.