51st Lunar and Planetary Science Conference (2020) 2302.pdf BADDELEYITE MICROSTRUCTURES IN VARIABLY SHOCKED MARTIAN METEORITES: AN OPPORTUNITY TO LINK SHOCK BAROMETRY AND ROBUST GEOCHRONOLOGY. L. G. Staddon1*, J. R. Darling1, N. R. Stephen2, J. Dunlop1, and K. T. Tait3,4. 1School of Environment, Geography and Geoscience, University of Portsmouth, UK; *
[email protected], 2Plymouth Electron Microscopy Centre, Plymouth University, UK, 3Department of Earth Sciences, University of Toronto, Canada, 4Royal Ontario Museum, Toronto, Canada. Introduction: Baddeleyite (monoclinic ZrO2; m- [6]. Shergottites in which plagioclase has undergone ZrO2) has recently been shown to be capable of provid- complete transformation to maskelynite (S4), suggest- ing robust crystallisation ages for shergottites via in- ing shock pressures of at least ~29 GPa [6], are repre- situ U-Pb isotopic analyses [1,2]. However, in contrast sented by enriched shergottites NWA 8679 and NWA to experimental work [3], these studies also highlight 7257. Both samples are similarly evolved lithologies, that U-Pb isotope systematics of baddeleyite can be abundant in late stage phases such as Fe-Ti oxides, Cl- strongly modified by shock metamorphism. Up to apatite and Si- and K- rich mesostasis, though NWA ~80% radiogenic Pb loss was recorded within North- 7257 is doleritic [9,10]. Enriched basaltic shergottite west Africa (NWA) 5298 [1], with close correspond- NWA 5298 represents the most heavily shocked mar- ence of U-Pb isotopic ratios and baddeleyite micro- tian meteorite studied here (S6), containing both structures [4]. Combined microstructural analysis and maskelynite and vesicular plagioclase melt [4]. U-Pb geochronology of baddeleyite therefore offers Baddeleyite grains >2 µm in length were located tremendous potential to provide robust constraints on via a combination of automated backscattered electron crystallisation and impact ages for martian meteorites.