¹Maree McGregor,²Erin L.Walton,³Christopher R.M.McFarlane,¹John G.Spray
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.01.052]
¹Planetary and Space Science Centre, University of New Brunswick, Fredericton, NB E3B 5A3, Canada
²Department of Physical Sciences, MacEwan University, Edmonton, AB, T5J 4S2, Canada
³Department of Earth Sciences, University of New Brunswick, Fredericton, NB E3B 5A3, Canada
Copyright Elsevier

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 < 850°C. Apatite yields a refined lower intercept age of 141 ± 4 Ma, which we interpret as the best estimate 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 ages that control this lower intercept are complicated by combinations of common-Pb incorporation and evidence for recent Pb loss associated with granularized and radiation-damaged 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.

¹Eleanor C.McIntosh,¹James M.D.Day,²Yang Liu,¹Courtney Jiskoot
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.01.051]
¹Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093, USA
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
Copyright Elsevier

Impactors striking the Moon since the formation of its crust have left an indelible imprint on the lunar surface, in the guise of craters and associated impact rocks. The lunar crust has low intrinsic abundances of the highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Rh, Pd, Au), at greater than 3000 times lower than in chondrite meteorites. Consequently, during impact, bolides with chondritic or differentiated iron-rich compositions should impart elevated HSE signatures to the lunar crust. Here we examine glassy lunar impact melt coats (IMC) from the outside rims of Apollo 16 cataclastic anorthosites (60015, 65325) and breccias (65035), as well as both fragments and powders of Antarctic anorthositic regolith breccia (ARB) meteorites (Miller Range 090034/36/70/75 and MacAlpine Hills 88105) for their petrography, mineral chemistry and bulk-rock compositions. The HSE concentrations for IMC range from ∼0.001 to 0.1 × CI chondrite, with measured ¹⁸⁷Os/¹⁸⁸Os between 0.1189 and 0.1366. Anorthositic regolith breccia meteorites, which have components with 2.6-4.1 Ga ages, have similar HSE concentrations to IMC, but typically have lower ¹⁸⁷Os/¹⁸⁸Os (0.1164-0.1284). These latter Os ratios are generally less radiogenic than those measured in ∼3.8-3.9 Ga Apollo impact melt breccias. The Apollo 16 IMC are not well-dated, but their KREEPy trace-element signatures and associated ages of 3.7 to 3.8 Ga for Apollo 16 glasses might imply, at least in part, an origin from the Imbrium or Serenitatis basin-forming impacts. Within the IMC, metal-schreibersite-troilite assemblages record significant inter-element HSE fractionation which is also reflected in bulk HSE patterns for both IMC and ARB meteorites. Variations in relative and absolute HSE compositions directly reflect the control of metal and sulfide segregation within and between impact melt and breccia lithologies. Collectively, IMC and ARB meteorites exhibit approximately 50% of the variation in Ru/Ir and ¹⁸⁷Os/¹⁸⁸Os observed in lunar impact melt breccias. These results imply that significant variations in inter-element compositions can occur during impact brecciation and melting and so some impact melt rock HSE compositions may not record the compositions of impactors that struck the Moon with fidelity. Nonetheless, the generally low ¹⁸⁷Re/¹⁸⁸Os of lunar impact melt rocks means that osmium isotope ratios provide evidence for impact composition, and a change from ordinary to carbonaceous-like impactors either with time – or location – striking the Moon.