U-Pb geochronology of apatite crystallized within a terrestrial impact melt sheet: Manicouagan as a geochronometer test site

Abstract

The Manicouagan impact event has been the subject of multiple age determinations over the past ~50 yr, providing an ideal test site for evaluating the viability of different geochronometers. This study highlights the suitability of Manicouagan’s essentially pristine impact melt body as a medium for providing insight into the U-Pb isotope systematics of geochronometers in the absence of shock-related overprinting. We performed in situ laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb geochronology on apatite and zircon, both of which crystallized as primary phases. This study is the first application of U-Pb geochronology to apatite crystallized within a terrestrial impact melt sheet. U-Pb analyses were obtained from 200 melt-grown apatite grains (n = 222 spots), with a data subset providing a lower-intercept age of 212.5 ± 8.0 Ma. For melt-grown zircon, a total of 30 analyses from 28 grains were obtained, with a subset of the data yielding a lower-intercept age of ± 1.6 Ma. The lower precision (±8.0 Ma; ±3%) obtained from apatite is a consequence of low U and a high and variable common-Pb composition. This resulted from localized Pb*/PbC heterogeneity within the impact melt sheet that was incorporated into the apatite crystal structure during crystallization (where Pb*/PbC is the ratio of radiogenic Pb to common Pb). While considered a limitation to the precision obtainable from melt-grown apatite, its ability to record local-scale isotopic variations highlights an advantage of U-Pb studies on melt-grown apatite. The best-estimate ages from zircon and apatite overlap within error and correlate with previously determined ages for the Manicouagan impact event. An average formation age from the new determinations, combined with previous age constraints, yields a weighted mean age of 214.96 ± 0.30 Ma for the Manicouagan impact structure.