Late Silurian zircon U–Pb ages from the Ludlow and Downton bone beds, Welsh Basin, UK

The Ludlow Bone Bed (Welsh Basin) is a critical stratigraphic horizon and contains a rich assemblage of fish scales. Units above provide insights into the early evolution of animal and plant life. The bed has not yet been radioisotopically dated. Here, we report 207 secondary ion mass spectrometry (SIMS) ages from 102 zircon (ZrSiO4) grains from the Ludlow (n = 2) and stratigraphically higher Downton (n = 1) bone beds. SIMS ages are middle Ordovician (471.6 ± 20.7 Ma) to late Devonian (375.7 ± 14.6 Ma, 238U–206Pb, ±1σ analytical uncertainty). Cathodoluminescence images show that the youngest ages appear affected by alteration. Chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) U–Pb geochronology was utilized to improve precision. Detrital zircon grains from Downton yield 424.91 ± 0.34/0.42/0.63 Ma and from Ludlow 424.85 ± 0.32/0.41/0.62 Ma (n = 5 each, 238U–206Pb, ±2σ analytical, tracer or systematic uncertainty). These ages provide a maximum deposition age. Results overlap the basal Přídolí age (423.0 ± 2.3 Ma) in its stratotype (Požáry Section, Reporyje, Prague, Czech Republic). The Ludlow Bone Bed marks the base of the local Downton Group, which has previously been correlated with the base of the Přídolí Series. The CA-ID-TIMS ages are older than those for other land arthropod-bearing sediments, such as the Cowie Harbour Fish Bed and Rhynie Chert.© 2020 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License (http://creativecommons.org/ licenses/by/4.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethic

Shocked monazite chronometry: integrating microstructural and in situ isotopic age data for determining precise impact ages

Monazite is a robust geochronometer and occurs in a wide range of rock types. Monazite also records shock deformation from meteorite impact but the effects of impact-related microstructures on the U–Th–Pb systematics remain poorly constrained. We have, therefore, analyzed shock-deformed monazite grains from the central uplift of the Vredefort impact structure, South Africa, and impact melt from the Araguainha impact structure, Brazil, using electron backscatter diffraction, electron microprobe elemental mapping, and secondary ion mass spectrometry (SIMS). Crystallographic orientation mapping of monazite grains from both impact structures reveals a similar combination of crystal-plastic deformation features, including shock twins, planar deformation bands and neoblasts. Shock twins were documented in up to four different orientations within individual monazite grains, occurring as compound and/or type one twins in (001), (100), (10 1 ¯) , {110}, { 212 } , and type two (irrational) twin planes with rational shear directions in [ 0 1 ¯ 1 ¯ ] and [ 1 ¯ 1 ¯ 0 ]. SIMS U–Th–Pb analyses of the plastically deformed parent domains reveal discordant age arrays, where discordance scales with increasing plastic strain. The correlation between discordance and strain is likely a result of the formation of fast diffusion pathways during the shock event. Neoblasts in granular monazite domains are strain-free, having grown during the impact events via consumption of strained parent grains. Neoblastic monazite from the Inlandsee leucogranofels at Vredefort records a 207Pb/206Pb age of 2010 ± 15 Ma (2σ, n = 9), consistent with previous impact age estimates of 2020 Ma. Neoblastic monazite from Araguainha impact melt yield a Concordia age of 259 ± 5 Ma (2σ, n = 7), which is consistent with previous impact age estimates of 255 ± 3 Ma. Our results demonstrate that targeting discrete microstructural domains in shocked monazite, as identified through orientation mapping, for in situ U–Th–Pb analysis can date impact-related deformation. Monazite is, therefore, one of the few high-temperature geochronometers that can be used for accurate and precise dating of meteorite impacts

Durendal, translated: Islamic object genealogies in the chansons de geste

The transfer of Saracen arms into Frankish ownership is a leitmotif of many chansons de geste, but one whose significance for translatio imperii has yet to be elucidated. In this essay, I focus on the Chanson d’Aspremont, a twelfth-century epic set in Calabria that narrates the pre-history of Durendal, Roland’s sword of Song of Roland fame, as an object inherited by Roland from its former royal Muslim owner. Drawing on cultural history and a number of object-translation models derived from material and spolia studies, I read the sword’s symbolic transfer as evidence of Norman desire for and appropriation of former Fatimid imperium in Sicily