40AR/39AR DATING OF CHICXULUB IMPACT SPHERULES FROM ISLA GORGONILLA, COLOMBIA

Glass spherules are a common occurrence around the Cretaceous-Paleogene (KPG) boundary in the stratigraphic record throughout the Caribbean and North Atlantic and are widely thought to represent tektites produced by the Chicxulub impact [1,2]. A new example, which contains some of the largest individual spherules yet found (typically 0.5–1.5 mm in diameter), was recently discovered on Isla Gorgonilla, off the Pacific coast of Colombia [3]. Here, the tektites occur in 2-3 discontinuous layers, within alternating sequences of mudstones and turbidites, and show varying degrees of minor, post-depositional alteration. Based on fossil assemblages, the ages of the tektite layers are thought to closely approximate the KPG, but the size and composition of the spherules invites ⁴⁰Ar/³⁹Ar dating to constrain their ages more precisely. Here we present the results of preliminary laser step-heating experiments on roughly 30 individual spherules undertaken at both Arizona State University and the New Mexico Institute of Mining and Technology. Step-heating experiments at both facilities yield similar release spectra that are characterized by young dates for low-temperature steps, that climb to either formally defined plateaus or near-plateaus for the bulk of the gas released at higher temperatures. We presently infer that the young, low-temperature dates reflect variable amounts of post-depositional alteration and consequent ⁴⁰Ar loss. Such alteration also may be responsible for the fact that the plateau dates from both laboratories are slightly over dispersed, that is, slightly too variable to be solely explained by analytical imprecision alone. At present, while we cannot infer a highly precise age for the impact event (or events) responsible for these tektites, the developing dataset indicates an age within no more than a few hundred thousand years of current estimates for the KPG boundary. The preliminary data are encouraging and indicate that further work may hold valuable constraints on the timing of Chicxulub impact and its temporal relation to the biostratigraphic record.

[1] Keller et al., 2013. Geol Mag 150, 885-907. [2] Mateo et al., 2016. Palaeogeogr Palaeoclimatol Palaeoecol 441, 96-115. [3] Bermudez et al., 2016. Terra Nova, 28, 83-90.