DETERMINATION OF MINERAL DEFORMATION IN MARTIAN REGOLITH BRECCIAS USING IN SITU MICRO X-RAY DIFFRACTION AND RAMAN SPECTROSCOPY

NWA 8171 and its pairings are pieces of a polymict regolith breccia that represents the very ancient Martian crust. Previous study implies Martian breccias show only weak shock effects. We have investigated five slabs of this Martian breccia using in situ micro-XRD (μXRD) and micro-Raman methods, to quantitatively assess the shock metamorphism exhibited by pyroxene and plagioclase. We have measured the strain-related mosaicity (SRM) in 2D XRD images via full width at half maximum in chi (FWHMχ), and measured peak broadening of the Raman peaks (FWHM_R) related to silicate fundamental stretching modes. μXRD and Raman provide complementary quantitative measurements of the shock effects in the rock-forming minerals which differ in scale due to their different beam diameters (μXRD, 300 µm vs. Raman, 2 µm).

Many lithic and crystal grains in the regolith breccia meteorites show characteristic streaking along χ (the angle subtended along the arc of a Debye ring) in the 2D XRD images due to a mosaic spread of subgrain orientations. Most medium-grained clasts in four slabs have shown streaking (SRM) to varying degrees (FWHMχ), reflecting impacts on Mars. Crystalline clasts in NWA 11220 appear to show greater SRM compared to other regolith breccia samples, suggesting that NWA 11220 is from a part of the regolith breccia that experienced somewhat higher average shock pressure. The Raman spectra of plagioclase clasts in NWA 8171, displaying different shock levels based on 2D XRD images, show quite similar characteristics, with little change in peak position or width. However, one heavily shocked plagioclase clast in NWA 11220, based on the highly extended streaks from the 2D XRD image, shows two exceptionally-broad and medium-intensity bands in the Raman spectrum at around 670 cm^-1 and 1005 cm^-1, which are probably due to the impact-induced formation of amorphous material. For another shocked enstatite clast, all Raman spectra collected from the single grain show increased peak broadening in comparison to an unshocked example from the RRUFF database. Raman spectra of one coarse augite grain in NWA 8171 show peak broadening from the rim to the center, which probably implies heterogeneous shock-metamorphic effects even within a single clast. We will discuss the relationships between SRM in μXRD and the peak broadening in selected Raman peaks for shocked pyroxene and plagioclase, as part of our goal to assess the degree of cumulative shock experienced by the ancient Martian crust, as preserved in its rock-forming minerals.