Investigation of NWA13367 Martian Shergottite by means of VIS-IR imaging spectroscopy

Introduction: 

Studying Martian Meteorites in the laboratory provides fundamental clues about the surface composition as well as the evolution of Mars. Spectroscopic and geochemical investigations furnish valuable insight both regarding the planet composition and information related to secondary processes (i.e. aqueous alteration) [1,2] SNCs meteorites have compositions that are mafic to ultramafic [1,2] essentially basalts or basaltic cumulates. Spectroscopic investigations in the VIS-NIR moreover provide important laboratory data for comparison with rover missions that currently are exploring the Martian surface with in situ spectrometers at even higher spatial resolutions with respect to past instruments.

Here we present reflectance spectroscopic measurements on a large sample of Martian Shergottite, North West Africa (NWA) 13367, by using the VIS-IR hyperspectral imaging technique. The sample has been investigated in the range of 0.35-5.1 micron by means of the SPIM setup at IAPS-CLab [5,6].

Setup and sample description:  the meteorite slab has been investigated with the Spectral Imager (SPIM) instrument in use at C-Lab laboratory at INAF-IAPS. The facility consists in an imaging spectrometer operative in the 0.35-5.1 micron-range [5,6], and contains the laboratory spare of VIR spectrometer onboard the Dawn mission [7]. The setup includes two bidimensional focal planes, a CCD (0.35-1 micron) and an HgCdTe (1-5.1 micron) detectors both hosted, together with the spectrometer, inside a liquid N₂ cooled Thermal Vacuum Chamber. The entry slit is 9x0.038 mm corresponding to a single acquired image on the sample of 256 px, with spatial resolution of 38 micron/px on the target. The sample to analyze is placed outside the TVC on a 3-axis motorized stage: by moving the target at 38 micron-steps and acquiring consecutive frames it is possible to construct a hyperspectral cube of 876 spectral bands and the desired number of lines.

The analyzed sample is a slab of about 5x5 cm. According to the Meteoritical Bulletin, this sample is constituted mainly of olivine (50%), pyroxene (40%), and 5-10% maskelynite, and is characterized by ophitic/poikilitic texture. Pyroxenes range from a low Ca/Fe-rich pigeonite to a Ca-rich augite. Other phases include Fe-Ti oxides, sulfides, and merrillite (Ca-rich phosphate containing Na and Mg).

Measurements and Results: Two scans covering each an area of about 9x6 mm² have been acquired on the same meteorite face. Each image has been constructed by acquiring 150 consecutive frames at 38-micron steps (dimension along the vertical axis). Here we focus on the first of the two scans. To obtain a first preliminary view of the spectral data, several band parameters maps have been retrieved. In particular, here we show the map of (i) band depth at 1 micron [Fig.1], and (ii) band depth at 0.65 micron [Fig.2]. The first map is related to the distribution of iron silicates (pyroxenes and olivine), and as can be seen, these phases cover the almost totality of the distribution map (Fig.1). Nevertheless most of the spectra resemble at first glance those of pyroxenes, characterized by the occurrence of the two Fe²⁺ bands at 1 and 2 micron (fig.3), with the second band varying between 2-2.3 micron.

Fig.1. Map of band depth at 1 mm for NWA 13367 cube, indicative of pyroxene/olivine distribution. This map and the following ones have dimensions of 9 mm x 5.7 mm.

 

In Fig.2 the distribution map of 0.65 micron band depth is shown. Although this absorption band could be consistent with spin-forbidden transition in Ti³⁺ in augite as reported in [8], here it appears, when present, broad and intense. Moreover, it seems to correlate with the 1-micron band more shifted towards longer wavelengths, pointing to a more Fe-rich composition. However it is evident how this phase forms outer rims (yellowish in this map) around core pyroxene grains with different composition (zonation) (Fig.2).

 

Fig.2. Map of band depth at 0.65 mm for NWA 13367 cube. This feature is related to Fe and/or Ti present in clinopyroxene.

 

A first tentative identification of spectral endmembers has been made by using the K-Means classification available on ENVI software. Spectral classes and relative representative endmembers are displayed in fig.3. Here spectra corresponding to pyroxene mineralogy are identified, with different levels of reflectance and also pyroxene composition, as suggested by the changing position of the 1 and 2-mm bands. The other two main classes identified likely represent a hydrated Fe-oxyde and a dark/opaque phase. In some locations spectra characterized by intense organic (C-H) features in the 3-4 micron region are identified (fig.4).

 

Fig.3. Spectral endmembers extracted as a first preliminary analysis through K-Means classification.

Fig.4. Selected spectra showing intense organic absorption features in the 3-4 micron region.

 

Conclusions and Future Work:

We started to investigate by means of VIS-IR imaging spectroscopy a Martian Shergottite (NWA 13367) at the spatial resolution of 38 mm. As preliminary spectral analysis, we retrieved several maps, for example at 1 micron (Fe silicates) and 0.65 micron (Ti or Fe-rich clinopyroxene). In particular, in some zones, the pyroxenes show zonation, with a likely Fe-rich outer rim. The correlation of 0.65-micron band depth with 1-micron band position will be further investigated. The 38-micron spatial resolution will allow to investigate in detail both the mineralogy of this sample and also at least some aspects related to the texture at the sub-mm scale. Moreover, additional analyses will be performed on a portion of this sample by means of coupled FTIR spectroscopy and mass spectrometry.

 

References: [1] Udry A. et al. JGR Planets 125 2020. [2] Treiman A H et al PSS, 48 1213 1230 2000 [3] Hicks L.J. et al. Geoc. Et Coscmoc. Acta, 136 194 210 2014. [4] Bishop J. L. et al. 80th Met. Soc. 6115 2017. [5] Coradini A. et al., Vol. 6, EPSC-DPS2011-1043, 2011. [6] De Angelis S. et al., Rev.Sci.Instr. 86, 093101, 2015. [7] De Sanctis M.C. et al., Space Sc. Rev., 163:329–369, 2011. [8] Klima R.L., et al., M&PS, 42, n.2, 235-253, 2007.

 

Acknowledgements: Scientific activities with SPIM are funded within the ExoMars program by the Italian Space Agency (ASI).