¹Addi Bischoff, ^1,2Maximilian Schleiting, ¹Markus Patzek
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13208]
¹Institut für Planetologie, University of Münster, Münster, Germany
²Institut für konstruktiven Ingenieurbau, Universität Kassel, Kassel, Germany
Published by arrangement with John Wiley & Sons

The brecciation and shock classification of 2280 ordinary chondrites of the meteorite thin section collection at the Institut für Planetologie (Münster) has been determined. The shock degree of S3 is the most abundant shock stage for the H and LL chondrites (44% and 41%, respectively), while the L chondrites are on average more heavily shocked having more than 40% of rocks of shock stage S4. Among the H and LL chondrites, 40–50% are “unshocked” or “very weakly shocked.” Considering the petrologic types, in general, the shock degree is increasing with petrologic type. This is the case for all meteorite groups. The main criteria to define a rock as an S6 chondrite are the solid‐state recrystallization and staining of olivine and the melting of plagioclase often accompanied by the formation of high‐pressure phases like ringwoodite. These characteristics are typically restricted to local regions of a bulk chondrite in or near melt zones. In the past, the identification of high‐pressure minerals (e.g., ringwoodite) was often taken as an automatic and practical criterion for a S6 classification during chondrite bulk rock studies. The shock stage classification of many significantly shocked chondrites (>S3) revealed that most ringwoodite‐bearing rocks still contain more than 25% plagioclase (74%). Thus, these bulk chondrites do not even fulfill the S5 criterion (e.g., 75% of plagioclase has to be transformed into maskelynite) and have to be classified as S4. Studying chondrites on typically large thin sections (several cm2) and/or using samples from different areas of the meteorites, bulk chondrites of shock stage S6 should be extremely rare. In this respect, the paper will discuss the probability of the existence of bulk rocks of S6.

^1,2Maree McGregor, ²Christopher R.M.McFarlane, ^1,2John G.Spray
Earth and Planetary Science Letters 504, 185-197 Link to Article [https://doi.org/10.1016/j.epsl.2018.10.006]
¹Planetary and Space Science Centre, Canada
²Department of Earth Sciences, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada
Copyright Elsevier

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.