Multiphase U-Pb geochronology of sintered breccias from the Steen River impact structure, Canada: Mixed target considerations for a Jurassic-Cretaceous boundary event

Abstract

In situ U-Pb geochronology has been conducted using laser ablation inductively coupled mass spectrometry (LA-ICP-MS) on shocked and thermally metamorphosed apatite, titanite, and zircon grains from the Steen River impact structure, Canada. The dated relict mineral phases occur within impact melt-bearing breccias that underwent post-deposition sintering at 450 °C > T < 800 °C. Apatite yields a refined lower intercept age of 141 ± 4 Ma, which we interpret as the best estimate age for the Steen River impact event. Titanite was only partially reset; yielding a lower intercept age of 113 ± 41 Ma. Zircon yields a lower intercept impact age of 120 ± 14 Ma, which is considered a minimum best-estimate impact age. The most reset zircon analyses that control this lower intercept are complicated by combinations of common-Pb incorporation and recent Pb loss associated with granularized and radiation-damaged (metamict) domains. All three phases preserve Paleoproterozoic crystalline basement ages, with a single concordant 206Pb/238U age of 1914 ± 39 Ma from apatite, an upper intercept age of 1882 ± 11 Ma from zircon, and an upper intercept age of 1842 ± 9 Ma from titanite. For apatite, the degree of isotopic resetting is largely thermally controlled, with the extent of reset closely associated with textural setting (degree of grain armouring, melt proximity and sample temperature) and, to a lesser extent, by shock/thermally generated microstructures (i.e., planar fractures, micro-vesicles, and recrystallized domains). While titanite records an impact age that falls within error of apatite and zircon, the majority of grains experienced only partial isotopic resetting, which we attribute to incomplete Pb loss associated with rapid cooling and post-depositional sintering of the breccia matrix below the titanite closure temperature (∼800 °C). In zircon, ancient (impact) Pb loss was facilitated along defect-related, fast-diffusion pathways within pre-impact metamict domains, shock defects, and via recrystallization. These same domains were also subject to recent (post-impact) Pb loss and common Pb contamination, significantly compromising the reliability of zircon ages. The distribution of U-Pb ratios in apatite and titanite is unlike those obtained in crystalline targets, a feature we interpret to be characteristic of impact structures developed in mixed (sedimentary – crystalline) targets, such as Steen River. In this case, disaggregated melt systems create thermal regimes distinct from those derived from predominantly igneous/metamorphic targets. With an age of 141 ± 4 Ma, Steen River joins the Dellen (Sweden), Gosses Bluff (Australia), Mjølnir (Barents Sea), and Morokweng (South Africa) impact structures in being formed at, or close to, the Jurassic-Cretaceous boundary.