Absence of Fe isotope fractionation in Cretaceous-Paleogene impact spherules: insights into the thermodynamic processes in the Chicxulub impact plume

Stable Fe isotopic variations recorded in impact spherules provide insights in the evolution of the impact plume generated by a hypervelocity impact. This study reports the first high-precision Fe isotope ratio of both proximal and distal impact spherules, originating from the Chicxulub impact. A total of 47 impact spherules, formed as the
result of melting and condensation, are investigated from different localities at different distances and directions from the source crater. The major challenge of studying 66 million years old impact spherules lies in the extensive alteration and diagenesis processes that could affect their original Fe signatures. Proximal and distal
impact spherules show a comparable mean δ56Fe value of -0.036 ± 0.28 ‰ (n = 40). This Fe isotope signature, identical to the mean value for the Earth crust, shows that Fe did not significantly fractionate in the plume generated by the Chicxulub impact event. Only a few impact spherules display light isotopic composition, with δ56Fe values down to -3.01 ± 0.07 ‰, due to their high degree of alteration. The lack of Fe fractionation in the Chicxulub impact spherules likely reflects the thermal conditions within the impact generated plume. The rate of temperature change in the Chicxulub impact plume is assumed to be slower than the evaporation and condensation timescales (seconds-minutes), allowing the temperature to remain above 1300 K for a sufficient period to enable re-equilibration of the Fe isotopic system.