Visiting Accessory Minerals (Apatites, Zircons) in the Martian Regolith Breccia NWA 7533 by Raman and Luminescence Spectroscopy

Abstract:

NWA7533 (and paired NWA7034) is an exceptional martian meteorite as it is a regolith breccia giving unique access to the martian crust (Hewins et al. 2017). Together with in situ observations by MSL Curiosity (Sautter et al. 2015) and reconstruction models of the primitive crust (Bouley et al., 2020), this meteorite has completely changed our view of the martian crust. It provides evidence of complex magma differentiation processes leading to a primitive felsic crust dated at 4.2-4.4 Gy. This regolith breccia actually exhibits a wide diversity of clasts and accessory minerals (present in the matrix and in the clasts), like phosphates and zircons. Dating these minerals has been critical in determining the history of NWA7533. In this study, we use micro-Raman and luminescence spectroscopy, including hyperspectral mapping, to characterize in detail the structure (Raman) and Rare-Earth Element (REE) content (luminescence) of phosphates and zircons at the microscale. Using Raman, apatite and merrillite are detected and appear structurally homogeneous. Luminescence spectra exhibit a more complex pattern with strong REE signatures in merrillite and overgrown apatite mantles, while the REE signature vanishes in independent apatite grains. Together with microtextural observations, this suggest the possible existence of two generations of apatites: one postdating comagmatic merrillite and one formed from REE-depleted liquid(s). Raman investigations show that zircons are generally well crystallized and poorly metamictized by comparison with various terrestrial zircons. Zircons analyzed in the matrix generally show a larger structural heterogeneity compared to those analyzed in a microbasaltic clast. In addition, REE luminescence spectra are different between these two populations of zircons further supporting the possible of two generations of zircons. These data open new questions on the thermal and geochronological history of NWA7533. These observations will have to be completed by in situ geochemical and geochronological investigations to better understand the complex history of NWA7533 and Mars. Last, these data provide a reference spectral dataset for the coming missions going to Mars and carrying Raman spectrometers (NASA Perseverance and ESA/Roscosmos ExoMars rovers).