Zircon U–Pb strain chronometry reveals deep impact-triggered flow

Abstract

Large (>100km) meteorite impact cratering events play important roles in surface and biosphere evolution, however, their potential for widespread ductile modification of the lithosphere has been difficult to assess, due partly to our inability to isotopically agecorrelate deep mineral fabrics with surface records. We have integrated benchmark U-Pb zircon dating methods (ID-TIMS, SHRIMP) with new microstructural techniques (EBSD, µXRD) to demonstrate that crystal-plastic deformation can cause rapid out-diffusion of radiogenic Pb and accompanying trace element alteration in crystalline zircon. We have used this phenomenon to directly date fabric in Archean zircons and xenoliths of the lower crust of South Africa at 2023±15 million years, coeval with the 2020±3 million year old Vredefort cratering event at surface, with extent ≥20,000 km 2. Our findings indicate that regional exogenic fabrics, similar to high-temperature tectonic fabrics, exist in ancient crust. Moreover, our results establish that crystal-plastic deformation in the lithosphere can now be directly dated and linked to planetary evolution by zircon U-Pb strain chronometry.

Finite lattice distortion patterns in plastically deformed zircon grains

“…Similar features have been described by Reddy et al (2007), and very likely correspond to type (I) lattice distortions of the classification by Piazolo et al (2012). Moser et al (2009) describe this type of lattice distortion as a result of an impact event. We suggest that this distortion pattern formed due to deformation of the host strong matrix (plagioclase).…”

“…It has been shown repeatedly that ductile deformationrelated microstructures in zircon associated with compositional changes often influence the CL signal (Reddy et al, , 2007Timms et al, 2006). Therefore, a bright CL signal is attributed not only to a high dislocation density but also to a potential trace element enrichment/depletion along the subgrain boundaries, which is supposedly capable of disturbing the isotopic system of the bulk grain (Moser et al, 2009;Timms et al, 2006Timms et al, , 2011.…”

“…In the past, the common explanation for low-temperature diffusion in zircon was the effect of self-irradiation that causes metamictization and thus induces/enhances volume diffusion (Cherniak et al, 1991;Cherniak and Watson, 2003). Recently, a number of authors found evidence for increased trace element diffusion in zircon due to crystal-plastic deformation (Erickson et al, 2013a, b;Flowers et al, 2010;Grange et al, 2013;Kaczmarek et al, 2011;MacDonald el al., 2013;Moser et al, 2009Moser et al, , 2011Nemchin et al, 2009;Piazolo et al, 2012;Reddy et al, 2006Reddy et al, , 2007Reddy et al, , 2009Reddy and Timms, 2010;Schwartz et al, 2010;Timms et al, 2006Timms et al, , 2011Timms et al, , 2012a. These authors have shown that crystalplastic deformation can dramatically affect the content of trace elements in zircon domains and can therefore cause isotopic system resetting.…”

“…Some of those zircons were derived from uncommon occurrences: (1) an impact basin (Moser et al, 2009(Moser et al, , 2011; (2) zircon-rich veins veins within a metaanorthositic body (Piazolo et al, 2012); (3) olivine gabbro derived from the ocean ridge (Reddy et al, , 2007or (4) xenolith from kimberlitic pipe . Other studies were focused on more widespread rock types and smaller zircons grains, less than 100 µm in length (Flowers et al, 2010;Kaczmarek et al, 2011;MacDonald et al, 2013;Reddy and Timms 2010;Schwarzt et al, 2010;Timms et al, 2012b).…”

Mécanismes de solidification des magmas basaltiques :

“…Dans la plupart des études antérieures, l'observation au microscope optique était le seul moyen de distinguer les cristaux déformés des cristaux non déformés. (Flemming, 2007), le zircon (Moser et al, 2009), l'orthopyroxène (Izawa et al, 2009, in review), le diamant (Smith et al, 2010), et l'olivine de météorites (McCausIand et al, 2010) ; aucune étude n'a examiné l'olivine terrestre. Par ailleurs, la caractérisation in situ basée sur la diffraction a été utilisée pour l'étude d'autres types de roches ou minéraux : des roches métamorphiques de ultra haute pression et les minéraux de coésite et quartz associés, par exemple (Ikuta et al, 2007).…”