Assessing the shock state of the lunar highlands: Implications for the petrogenesis and chronology of crustal anorthosites.

Our understanding of the formation and evolution of the primary lunar crust is based on geochemical systematics from the lunar ferroan anorthosite (FAN) suite. Recently, much effort has been made to understand this suite's petrologic history to constrain the timing of crystallisation and to interpret FAN chemical diversity. We investigate the shock histories of lunar anorthosites by combining Optical Microscope (OM) 'cold' cathodoluminescence (CL)-imaging and Fourier Transform Infrared (FTIR) spectroscopy analyses. In the first combined study of its kind, this study demonstrates that over ~4.5 Ga of impact processing, plagioclase is on average weakly shocked (<15 GPa) and examples of high shock states (>30 GPa; maskelynite) are uncommon. To investigate how plagioclase trace-element systematics are affected by moderate to weak shock (~5 to 30 GPa) we couple REE+Y abundances with FTIR analyses for FAN clasts from lunar meteorite Northwest Africa (NWA) 2995. We observe weak correlations between plagioclase shock state and some REE+Y systematics (e.g., La/Y and Sm/Nd ratios). This observation could prove significant to our understanding of how crystallisation ages are evaluated (e.g., plagioclase-whole rock Sm-Nd isochrons) and for what trace-elements can be used to differentiate between lunar lithologies and assess magma source compositional differences

Mid-infrared reflectance spectroscopy of aubrite components

Aubrites Peña Blanca Spring and Norton County were studied in the mid-infrared reflectance as part of a database for the MERTIS (Mercury Radiometer and Thermal Infrared Spectrometer) instrument on the ESA/JAXA BepiColombo mission to Mercury. Spectra of bulk powder size fractions from Peña Blanca Spring show enstatite Reststrahlen bands (RB) at 9 µm, 9.3 µm, 9.9 µm, 10.4 µm, and 11.6 µm. The transparency feature (TF) is at 12.7 µm, the Christiansen feature (CF) at 8.1–8.4 µm. Micro-FTIR of spots with enstatite composition in Norton County and Peña Blanca Spring shows four types: Types I and II are similar to the bulk powder spectra but vary in band shape and probably display axis orientation. Type III has characteristic strong RB at 9.2 µm, 10.4 µm, and 10.5 µm, and at 11.3 µm. Type IV is characterized by a strong RB at 10.8−11.1 µm. Types III and IV could show signs of incipient shock metamorphism. Bulk results of this study confirm earlier spectral studies of aubrites that indicate a high degree of homogeneity and probably make the results of this study representative for spectral studies of an aubrite parent body. Spectral types I and II occur in all mineralogical settings (mineral clasts, matrix, melt, fragments in melt vein), while spectral type III was only observed among the clasts, and type IV in the melt. Comparison with surface spectra of Mercury does not obtain a suitable fit, only type IV spectra from quenched impact glass show similarity, in particular the 11 µm feature. Results of this study will be available upon request or via the IRIS database (Münster) and the Berlin Emissivity Database (BED)

Mid-infrared spectroscopy of laser-produced basalt melts for remote sensing application

We obtained mid-infrared spectra and major-element analyses of glasses produced in pulsed laser experiments of basalt. Materials from pits excavated in a basalt slab, as well as of a larger, separated melt droplet were studied. The results of this study show that these glasses exhibits spectral features clearly distinguishable from the unprocessed starting material. Spectra and chemistry show changes, which could be the result of not only melting but also vaporization. Christiansen Features (CF) for the melt glass in the laser-excavated pits are at 8.3–8.5 μm, and a dominating Reststrahlen Band (RB) at 10.1–10.5 μm in wavelength. The spectra of the powdered glass droplet has a CF at 8.8–8.9 μm and a RB at 10.3–10.5 μm. The spectra are clearly different from the spectra of the surrounding starting material, which shows CF between 8.0 and 8.3 μm, and ample RBs between 9.3 μm and 14.7 μm, typical olivine, plagioclase and pyroxene features. The results reflect the chemical composition, which shows significant losses of volatiles like K2O and Na2O, as well as of moderate volatiles like FeO, SiO2, and MgO. Refractories TiO2, Al2O3, and CaO tend to be enriched compared to the bulk starting composition. This indicates loss of material through evaporation. While the spectra of size fractions of the powdered bulk melt glass droplet follow this trend in general, but, because of contamination by the experimental set-up, CaO was found to be strongly enriched in contrast to the other refractories TiO2 and Al2O3. At least the composition of the glasses in the laser-excavated pits could serve as an ‘endmember’ for the sequence of glassy materials expected to be produced in high energy impact processes involving a basaltic target. Correlation of CF with SiO2 contents and the SCFM (SiO2/(SiO2 + CaO + FeO + MgO)) index show similar behaviour of the pit melts like found in earlier studies. However, when the position of the RB in the pit glass is correlated with the SiO2 content, the result shows a different trend compared with earlier studies. Consequently, the data presented in this study could help distinguishing between surface regions formed by volcanic processes and such modified by high-velocity impacts, where evaporation could play a central role. This is of high interest for remote sensing studies of Mercury, which, because of its proximity to the Sun, was probably affected by high-velocity impacts to a very high degree