U-Pb isotope systematics of shocked and thermally overprinted zircon and apatite from the Pilot Lake impact structure, Canada

Abstract

In situ U-Pb geochronology has been performed via laser ablation inductively coupled mass spectrometry on relict zircon and apatite from the ~ 8 km-diameter Pilot Lake impact structure, Canada. Zircon results, filtered to mitigate the effects of post-impact Pb loss, yield an upper intercept date of 1900 ± 22 Ma (MSWD = 0.9). This corresponds to the age of the Slave granites of the Taltson Magmatic Zone. A lower intercept date of 420.4 ± 19.2 Ma (MSWD = 0.9) is interpreted to record the impact event. Apatite results yield a triangular isotopic array with an older regression intercepting concordia at 1830.7 ± 8 Ma (MSWD = 0.59) and a younger regression with an intercept date of 420 ± 13 Ma (MSWD = 0.14). The older date is coincident with the age of the regional Sparrow Dyke Swarm and/or Hudsonian metamorphism. The younger date is synchronous with the zircon lower intercept. With a slightly improved error, apatite provides the best-estimate age for the Pilot Lake impact event. While shock has caused important structural effects in zircon and apatite, it has primarily been the subsequent thermal overprinting and its duration that have controlled the U-Pb systematics. Laser analyses of zircon and apatite juxtaposed with hotter (melt-bearing) areas have been reset to the impact age, while cooler settings have preserved target rock ages.