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

The Cretaceous-Paleogene boundary (similar to 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 delta Mg-25 data to trace evaporation and condensation processes during ejecta dispersion. The delta K-41 and delta Rb-87 signatures vary from -1.18 +/- 0.04 parts per thousand to 0.43 +/- 0.09 parts per thousand and - 0.35 +/- 0.01 parts per thousand to 0.07 +/- 0.03 parts per thousand relative to the UCC, respectively. Combined with delta Mg-25, these variations indicate evaporation and condensation in the pyrocloud. A fractional condensation model involving Mg and K suggests recondensation of spherules between similar to 2520-2130 K, with a K- and Rb-saturated vapor at the end of the condensation sequence. The delta K-41, delta Rb-87, and delta Mg-25 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.