CONSTRAINING THE AGE OF KENTLAND CRATER (INDIANA, USA) WITH APATITE (U-TH)/HE THERMOCHRONOLOGY

Apatite (U-Th)/He dating is a method of low temperature thermochronology that can be used to quantify the time since a sample has cooled below ~70 ºC. This is useful in the context of dating small impact craters that do not experience complete resetting of high-temperature chronometers. The Kentland dome in east-central Indiana is the stump of a 6-7 km diameter crater that formed via a meteorite impact. The crater has experienced weathering, erosion, and exhumation such that the original melt sheet is no longer present. Stratigraphic relations provide broad constraints that indicate the Kentland event occurred between ~300 and 1 m.y. ago, and a recent paleomagnetic study is consistent with a Jurassic age. A previous study utilized apatite fission track (AFT) thermochronology of Paleozoic rocks (St. Peter Sandstone) in the central uplift in an attempt to further refine the age of the Kentland impact event. However, samples within and distal to the impact structure yielded statistically similar AFT ages indicative of a regional exhumation event at ~185 Ma, and therefore did not constrain the age of the crater. Using a similar sample suite, we separated additional apatite grains for (U-Th)/He measurements, which are currently underway. If the crater-forming impact predates the regional exhumation event, we hypothesize that the (U-Th)/He ages from both proximal and distal samples will record the same regional exhumation event as the AFT data. However, if the impact postdates the regional exhumation event, we hypothesize that the (U-Th)/He ages of the proximal samples will be younger than those of the distal samples and record the timing of the impact.