40AR-39AR AGES OF THE PALEOCENE-EOCENE BOUNDARY EXTRATERRESTRIAL IMPACT SPHERULES (Invited Presentation)

We report ⁴⁰Ar-³⁹Ar step-heating ages of P-E boundary impact spherules from Atlantic Margin coastal plain and open ocean sites. We test the hypothesis that the P-E spherules are reworked from an earlier event (e.g., K-Pg impact at ~66 Ma), which predicts a cooling age discordant from their depositional age of 55.93 ± 0.05 Ma at the P-E boundary. Isochrons from the step heating analyses yield ⁴⁰Ar-³⁶Ar intercepts in excess of the modern in most cases, indicating that the spherules have inherited radiogenic Ar (⁴⁰Ar*), typical of impact glasses incompletely degassed before solidification. The weighted mean of the isochron-corrected plateau age is 54.1 ± 3.4 Ma (1s), and their isochron age is 55.4 ± 4.0 Ma, both indistinguishable from their P-E depositional age, not supporting the K-Pg reworking hypothesis. This is consistent with all other stratigraphic and geochemical evidence for an impact at the P-E boundary and ejecta distribution by air-fall.