In situ LA-ICP-MS apatite and zircon U–Pb geochronology of the Nicholson Lake impact structure, Canada: Shock and related thermal effects

Abstract

In situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been deployed to determine the U–Pb ages of impact metamorphosed apatite and zircon associated with the ∼12.5 km diameter Nicholson Lake impact structure, Northwest Territories, Canada. The dated phases occur within both impact melt-bearing breccias and clast-laden impact melts. A total of 84 laser ablation analyses from 57 apatite grains within seven rock samples yield a minimum refined lower intercept at 387 ± 5 Ma (MSWD = 0.87, 2σ, n=26), and maximum age with lower intercept of ∼1740 Ma. A total of 90 laser ablation analyses on 52 zircon grains from two rock samples yield a minimum intercept age of 384 ± 8 Ma (MSWD = 1.3, 2σ, n=22), with a maximum upper intercept age of 2679 ± 14 Ma, and a second discordia with a ∼1740 Ma upper intercept age. The results are consistent with the target rocks comprising Archean Snow Island Suite (∼2.7 Ga) and Paleoproterozoic Nueltin plutonic suite (∼1.74 Ga), with the impact event occurring at approximately 385 Ma. The degree of resetting of inherited apatite can be related to its proximity to impact melt (now partly devitrified glass) within the host impact melt-bearing breccias and clast-laden impact melt rocks. Those grains in direct contact with melt bodies are reset, while those occurring as inclusions in lithic or mineral clasts partly or wholly retain their original isotopic compositions. The youngest (impact-reset) apatite ages are most closely associated with the highest shock levels (i.e., granular textured and thermally dissociated zircons), which we relate to juxtaposition of apatite with superheated melt. Due to accumulated radiation damage prior to impact, many of the relic zircons are metamict, which facilitated enhanced Pb diffusion and their resetting to the impact age. Granular zircons record impact ages while those exhibiting planar fractures or experiencing negligible shock record basement ages. We link apatite and zircon geochronology to the textural and structural states of ZrSiO4 and its associated shock levels via field emission scanning electron microscopy and micro-Raman spectrometry.