Raman spectroscopy of shocked enstatite-rich meteorites

We present Raman patterns of enstatite in different classes of enstatite-rich chondrites and achondrites of various shock levels as previously reported from petrographic observations and X-ray diffraction analyses. Thin sections or mineral separates of four enstatite chondrites (LaPaz Icefield [LAP] 02225, MacAlpine Hills [MAC] 02837, Pecora Escarpment [PCA] 91020, and Itqiy), three aubrites (Larkman Nunatak [LAR] 04316, Khor Temiki, and Allan Hills [ALH] 84008), and a ureilite (Sayh al Uhaymir [SaU] 559) were examined by laser Raman spectroscopy. We find that the frequencies of fundamental Raman peaks of enstatites from the chondrites and aubrites deviate by 2cm(-1) from the values for unshocked enstatite. This small difference implies a negligible effect of shock metamorphism on peak positions. Significant differences (<6cm(-1)) for peak positions are found for the pyroxenes of SaU 559 and may be attributed to minor substitution of Fe and Ca for Mg. Linear regressions of peak widths of enstatite chondrites against their established shock stages show a strong positive correlation for each mode (r(2)>0.94). From this linear relationship, the 343 and 1014cm(-1) peaks of the aubrites coincide with S4 determined from petrography. For Itqiy, we find S4-5, while the shock levels of SaU 559 exceed the petrologic scheme (S1-6), suggesting that the ureilite might have sustained multiple shock events or have been deformed in a high-pressure environment. Alternatively, for Itqiy (peak 343cm(-1)) and SaU 559 (all peaks) enstatites, minor substitutions of Fe and Ca for Mg may have further broadened their peak widths.