The Chicxulub impact (in the northern Yucatan Penninsula, Mexico) marks the Cretaceous-Paleogene (K-Pg) boundary and is implicated in one of the five major extinctions. Researchers have examined ejecta from the Chicxulub impact, and most recently a drill core from the crater itself, yet the processes and chemical reactions occurring in the impact vapor plume are poorly constrained. Rounded carbonate particles, identified as accretionary lapilli, have been found thousands of kilometers from the impact crater and may be a unique record of plume conditions. We present carbon (δ13C), oxygen (δ18O), and clumped (Δ47) isotope ratios of lapilli from the Brazos River, Texas (USA), as well as from foraminifera and a mudstone. Unaltered lapilli δ13C and δ18O values covary, ranging from −9.38‰ to −2.10‰ and from −7.72‰ to −5.36‰, respectively, and they are distinct from mudstones, foraminifera, and secondarily altered lapilli in the same section. Clumped isotope temperatures [T(Δ47)] from the lapilli range from 66 °C to 539 °C and average 155 ± 46 °C (1 standard deviation), with sedimentary and fossil carbonates recording clement, shallow ocean–like T(Δ47). These data are consistent with petrography and hypothesized vapor plume formation, and we argue that the δ13C and δ18O values result from target rock decarbonation. Atmospheric temperatures >100 °C extending >1800 km from the Chicxulub crater imply an uninhabitable zone within seconds to minutes of the impact that was 10× larger in diameter than the crater itself.