EBSD and Atom Probe Geochronology of Zircon and Baddeleyite from Early Mars

English Abstract:

A rare record of the earliest history of Mars is accessible in the crystal and lithic clasts of meteorite NWA 7475, one of a group of recently discovered relatively water-rich stones from Rabt Sbayta, Morocco, interpreted to be a suevite-like regolith breccia [1]. SIMS U-Pb dating of the related meteorites by other workers [2,3] has identified zircons as old as 4.44 Ga making them invaluable physical evidence of martian crustal processes. The zircon population experienced a Pb-loss event approximately 3 billion years later, the cause of which is still being investigated. Here we present in-depth examination of the chemical and orientation microstructure of zircon and baddeleyite by EBSD and atom probe tomography (APT) in an effort to gain additional information on Early Mars bombardment history and subsequent metamorphism. FEG-SEM mapping of accessory phases in a thick section identified several hundred small zircon grains in the size range of 4 µm to 25µm in longest dimension (mean= 7µm) and over 100 baddeleyite grains of similar size distribution. A 3D APT map of an approximately 80nm x 80nm x 350nm volume of zircon was homogeneous with respect to trace elements including Li; however, U and Pb were below detection. EBSD mapping of an igneous zircon crystal clast revealed amorphous, U-rich domains that are now metamict, signifying a lengthy upper crustal residence. An APT map from an unfractured, crystalline domain reveals chains of nanoclusters of Fe, Al, Ca. These are interpreted, by analogy with terrestrial observations, to be due to fluid-assisted contamination along fractures during regolith metamorphism. APT geochronology of an anhedral baddeleyite crystal clast yields a Pb isotope ratio consistent with ≥4 Ga formation. A metamorphic nano-scale rim of zircon on this grain appears in a second APT map, and points to martian Si metasomatism of the regolith at some point prior to the meteoroid launch event. These and other nanoscale crystal histories of U-Pb dating phases will be presented to add to our growing picture of martian crustal processes preserved in these rare specimens. [1] Wittman, A., et al., 2015 (in press); MAPS [2] Humayun, M. et al., 2013; Nature [3] Nemchin, A. et al., 2014; Nature Geoscience