Signatures of vapor evolution in turbulent convection cells recorded by K and Rb isotopes in Chicxulub impact spherules

The Cretaceous–Paleogene boundary (~66 Ma) records the Chicxulub asteroid impact, a pivotal event in Earth's history that affected much of the Earth's surface. While products of this impact have been widely studied, the thermodynamics of the plume and formation mechanisms of certain impact materials remain unclear. This study investigates conditions within the intermediate fraction of the Chicxulub impact plume (pyrocloud) using K and Rb isotope signatures of Gorgonilla Island impact spherules with published δ 25 Mg data to trace evaporation and condensation processes during ejecta dispersion. The δ 41 K and δ 87 Rb signatures vary from −1.18 ± 0.04‰ to 0.43 ± 0.09‰ and − 0.35 ± 0.01‰ to 0.07 ± 0.03‰ relative to the UCC, respectively. Combined with δ 25 Mg, these variations indicate evaporation and condensation in the pyrocloud. A fractional condensation model involving Mg and K suggests recondensation of spherules between ~2520–2130 K, with a K- and Rb-saturated vapor at the end of the condensation sequence. The δ 41 K, δ 87 Rb, and δ 25 Mg data imply that spherules formed mainly through recondensation and underwent secondary evaporation due to depressurization and fluctuating temperatures in turbulent convection cells. Decoupled recondensation and evaporation of K and Rb from Mg are required to explain their isotopic signatures. While Mg isotopes trace overall spherule formation, K and Rb isotopes reflect fine-scale physicochemical processes at the end of their formation. These results highlight the thermodynamically complex, yet key effects of turbulent convection cell environments in impact materials and emphasize the need to pair MVE isotope data with refractory element isotopes to better understand the formation of various impacts materials. • δ 41 K & δ 87 Rb reveal evaporation & recondensation processes in Chicxulub spherules. • δ 25 Mg reject splash–melt origin & support recondensation origin. • Decoupled δ 25 Mg, δ 41 K, δ 87 Rb show secondary evaporation at late-stage formation. • δ 25 Mg, δ 41 K, δ 87 Rb behavior imply spherule formation in turbulent convection cells. • Mg traces primary formation, K & Rb trace fine-scale processes in impact plumes.