Multiscale spectroscopic characterization of ungrouped achondrites

Reflectance spectroscopy remains the most important tool to remotely infer mineralogical and chemical information of Earth and planetary surfaces (Bishop at al. 2019). Meteorites represent a sampling of planetary bodies with variable differentiation degree, mineralogy, and chemistry. For this reason, an accurate laboratory characterization of meteorites is of crucial importance for the correct understanding of spectral features of planetary bodies. This is especially true for the VNIR spectral range (~ 0.3-2.5 µm) where most of the spacecraft instruments operate and earth-based telescopes work. On the other hand, IR spectroscopy is of paramount importance to detect subtle chemical and mineralogical variations in minerals and rocks. Here we report a spectroscopic and mineralogical characterization of a suite of ungrouped achondrites which do not fit any classification scheme and their origin, in terms of planetary bodies, is either unknown or highly uncertain. We acquired reflectance spectra in both VNIR (0.35-2.5 µm and IR (~600 to ~7000 cm-1 or ~2 - ~17 µm) spectral ranges. The spectroscopic characterization has been performed with a spatial resolution spanning two orders of magnitude (from ~ 5·10-5 to ~ 5·10-3 m) allowing us to relate the bulk spectroscopic response of the rocks with the spectroscopic features of their constituent minerals. Bidirectional reflectance (VNIR and bulk FTIR) measurements were performed at IAPS-INAF, µ-FTIR measurements were executed at DAFNE-Light synchrotron radiation facility at LNF - INFN, while EPMA and SEM analysis were performed at MEMA - Department of Earth Sciences of Florence. Our results show a wide chemical and mineralogical variation between the samples. The mineralogical and chemical variation is clearly reflected on spectroscopic features. In Fig 1 we report the MidIR reflectance spectra and SEM-EDS images for sample NWA 5400. Fig. 1a shows the bidirectional reflectance spectrum for the powders (spot size: 6 mm). Fig. 1b we report the SEM-EDS image of a small area of the sample with the detected phases indicated by different colors. In Fig. 1c we show the µ-FTIR results on the same area in (b). It is interesting to note how the µ-FTIR spectra of olivine are slightly different in term of band position and area likely indicating small chemical heterogeneities within the sample.Figure 1. FTIR/SEM-EDS results for sample NWA 5400. (a) Bidirectional FTIR reflectance spectrum on powdered sample [spot size: 6 mm]. (b) SEM-EDS (false colors) of a small area (1 x 0.6 mm) of the sample. The different colors correspond to the detected mineral phases. (c) µ-FTIR spectra collected in the same area showed in (b). Location of the spot analysis is marked by colored dots. Yellow area corresponds to the spatial resolution of the µ-FTIR analysis (150 x 150 µm) in this sample.Fig. 2(a) shows representative VNIR spectrum for high-pyroxene content meteorites like NWA 6704. Figure 2(b) shows a representative VNIR spectrum for medium-high content olivine samples like NWA 10503. The spectral features are highly dependent from modal abundances of mafic minerals (mainly olivine and pyroxene) so that it is possible to identify two main groups: olivine-dominant achondrites and pyroxene-dominant achondrites. Cloutis et al. (1986) proposed a plot to show the relationship between spectral features and olivine/pyroxenes content. The same plot was later considered by Gaffey et al. (1993) to discriminate the S-type asteroids on the basis of the band I center and the ratio between the area of band I and band II (BAR). Recently this relationship has been also used to relate spectral variation of achondrites and possible parental bodies (e.g. Carli et al. 2018; Lucas et al 2019) and is reported in Fig 3 for our samples.Figure 2. Typical VNIR reflectance spectra for pyroxene-dominant achondrites (a) and olivine-dominant achondrites (b)Reflectance spectroscopy coupled with a thorough mineralogical and geochemical investigation of meteorites proved to be a powerful technique to infer quantitative conclusions about their origin, history and associate them with their provenance. Using the plot originally proposed by Gaffey et al. (1993) we clustered our samples in four distinct regions which likely correspond to potentially four different family of small objects. µ-FTIR was instead successfully applied to retrieve small-scale information including mapping the chemical and mineralogical heterogeneities.