Abstract
Baddeleyite (monoclinic; m-ZrO2) is a widespread accessory phase within shergottites. However, the effects of shock loading on baddeleyite U-Pb isotopic systematics, and therefore its reliability as a geochronometer within highly shocked lithologies, are less well constrained. To investigate the effects of shock metamorphism on baddeleyite U-Pb chronology, we have conducted high-resolution microstructural analysis and in-situ U-Pb isotopic measurements for baddeleyite within enriched basaltic shergottites Northwest Africa (NWA) 7257, NWA 8679 and Zagami. Electron backscatter diffraction (EBSD) analyses of baddeleyite reveal significant microstructural heterogeneity within individual thin sections, recording widespread partial to complete reversion from high-pressure (≥3.3 GPa) orthorhombic zirconia polymorphs. We define a continuum of baddeleyite microstructures into four groupings on the basis of microstructural characteristics, including rare grains that retain magmatic twin relationships. Uncorrected U-Pb isotopic measurements form Tera-Wasserburg discordia, yielding new 238U-206Pb discordia ages of 195 ± 15 Ma (n = 17) for NWA 7257 and 220 ± 23 Ma (n = 10) for NWA 8679. Critically, there is no resolvable link between baddeleyite microstructure and U-Pb isotope systematics, indicating negligible open-system behaviour of U-Pb during zirconia phase transformations. Instead, we confirm that high post-shock temperatures exert the greatest control on Pb mobility within shocked baddeleyite; in the absence of high post-shock temperatures, baddeleyite yield robust U-Pb isotope systematics and date the age of magmatic crystallization. Low bulk post-shock temperatures recorded within Zagami (≤220 °C), and suggested within NWA 7257 and NWA 8679 by baddeleyite microstructure and other petrological constraints, confirm that the previously derived baddeleyite age of Zagami records magmatic crystallization, and provide greater age diversity to 225 Ma to 160 Ma enriched shergottites. While our data yield no resolvable link between microstructure and U-Pb isotopic composition, we strongly recommend that microstructural analyses should represent an essential step of baddeleyite U-Pb chronology within planetary (e.g., martian, lunar, asteroidal) and shocked terrestrial samples, allowing full contextualisation prior to destructive isotopic techniques. Microstructurally constrained in-situ U-Pb analyses of baddeleyite thus define new opportunities for the absolute chronology of martian meteorites and, more broadly, shocked planetary materials.
Highlights
Our ability to place absolute constraints on the timing of martian crustal processes is critical to our understanding of the evolution of the planet’s surface and interior
This apparent dichotomy led to the shergottite age paradox (Nyquist et al, 2001; Head et al, 2002; Bouvier et al, 2005, 2008, 2009; El Goresy et al, 2013; Moser et al, 2013; Zhou et al, 2013; Bellucci et al, 2016; Darling et al, 2016), with debate strongly centred on the true crystallization age of shergottites and, due to the !20 GPa shock pressures typically experienced by shergottites during ejection from the martian surface (Fritz et al, 2005), the role of shock resetting in calculated ages (Bouvier et al, 2008; Gaffney et al, 2011; El Goresy et al, 2013; Vaci and Agee, 2020)
The low peak- and post-shock temperatures experienced by Zagami and shergottites with comparable bulk shock characteristics ( 220 °C; Malavergne et al, 2001; Fritz et al, 2005; Stoffler et al, 2018) strongly argue that baddeleyite microstructures document reversion from orthorhombic zirconia (o-ZrO2); except in localised areas of shock melting, it is highly unlikely that post-shock temperatures reached the $1170 °C required to induce transformation to tetragonal zirconia (t-ZrO2)
Summary
Our ability to place absolute constraints on the timing of martian crustal processes is critical to our understanding of the evolution of the planet’s surface and interior. While crater chronology indicates the martian surface is dominated by ancient, pre-Amazonian crust (i.e., 4.5–3.0 Ga; Hartmann and Berman, 2000), Rb-Sr, Sm-Nd, Lu-Hf and U-Pb isochrons of shergottites dominantly yield late Amazonian ages of 600 Ma (Nyquist et al, 2001; Udry et al, 2020; Vaci and Agee, 2020). In-situ U-Pb dating of baddeleyite (monoclinic zirconia; m-ZrO2), a widespread accessory phase within enriched shergottites (Herd et al, 2017), has been key in resolving this conundrum (Moser et al, 2013; Zhou et al, 2013; Darling et al, 2016) Such in-situ approaches allow characterization of petrographic context prior to isotopic analyses and are considered less sensitive to perturbation than traditional dating techniques, where preferential leaching, terrestrial or martian contamination, and the inclusion of trace phases may modify the isotopic compositions of mineral separates. Five U-Pb baddeleyite ages have attested to the late Amazonian age of shergottites: 235 ± 37 Ma for Roberts Massif (RBT) 04261 (Niihara, 2011), 192 ± 10 Ma for Grove Mountains (GRV) 020090 (Jiang and Hsu, 2012), 185 ± 9 Ma for Zagami (Zhou et al, 2013), 187 ± 33 Ma for Northwest Africa (NWA) 5298 (Moser et al, 2013), and 188 ± 8 Ma for NWA 8653 (Wu et al, 2021)
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