Geochemical characterization of carbonates using RLS-like Raman data and Raman-LIBS technique combination: applications for Mars exploration

Raman Spectroscopy is an analytical technique that will be deployed on Mars in the following years and could be part of other payloads for planetary exploration missions in the future. Its ability for identification of mineral phases and its interest in Mars has been deeply discussed in bibliography [1]. Perseverance rover, to be launched in 2020, and ExoMars rover, to be launched in 2022, will carry three Raman instruments, different in concept and capabilities. SHERLOC (mounted on Perseverance’s arm) is a UV Raman instrument mainly focused in the direct detection of biomarkers, SuperCam (mounted on Perseverance’s mast) is a standoff, multi-technique, instrument that performs Raman and LIBS at distances of several meters from the rover. Finally, RLS, mounted in Rosalind Franklin Rover, in the Pasteur analytical laboratory, is a continuous wave, 532 nm excitation source Raman instrument. While the first one is focused in detection limits of organics, RLS is intended to investigate mineralogy and possible biomarkers, while SuperCam, due to its standoff and time resolved design, is a different concept to que other two Raman instruments, as it is also capable of fusing data from different techniques. Carbonates are minerals of great interest for astrobiology, and, as suggested by CRISM data, the landing site selected for the NASA/Mars 2020 rover mission (Jezero crater) presents a variety of Fe-Ca-Mg carbonate units [2]. For Oxia Planum, Rosalind Franklin’s landing site, although no carbonates have been detected in that area by orbiter data, Earth analogues suggest that small amounts of carbonates might be found in the clay rich area. On Earth, top bench Raman spectrometers can be effectively used to discriminate carbonates and to determine the Mg/Fe concentration ratio of mineral species from dolomite (CaMg(CO3)2) - ankerite (CaFe(CO3)2) and magnesite (MgCO3) - siderite (FeCO3) solid solutions series [3]. The previously mentioned instruments might present limitations derived from the design constrains of space exploration. Resolution, far from ideal, and low intensity of the signal, are two of the main factors that could affect the possible calculations done with data from the three Raman instruments. SuperCam is a special case, as it is able to obtain data from several techniques from the same spot of the sample, and that might help to overcome those difficulties. In this work a complete set of Ca-Mg-Fe carbonates is analysed by different Raman instruments, including automated contact instruments and combined standoff developments. The initial characterization of the samples is done with XRD, as gold standard. Then, a characterization of all those carbonates based only on Raman data sets was done, aiming to evaluate the impact of resolution in the classification power of Raman-based calculations. A detailed vibrational mode analysis was carried out for interpreting the structural modifications induced by cationic substitution. Here, after a detailed interpretation it was found that Raman active internal modes are less sensitive to the carbonate chemistry than the external modes (i.e. the 155cm-1 and 286cm-1 respectively). Same collection of carbonates is analysed using standoff Raman-LIBS combination. In this case we will evaluate how having the complementary information of the elemental composition improves the results obtained by standoff Raman spectroscopy [4], as LIBS is more sensitive to the possible changes in the cations in the samples. Using these data sets, a combination of univariate and multivariate calculations are done to evaluate their classification capacity. As commented before, LIBS can classify better these minerals thanks to its lower detection limit and a better functionality in standoff configuration. However, the effect from other phases, different from carbonates, might disturb the LIBS calculations, reason why having an assessment of all the phases in play by Raman spectroscopy is of great interest, supporting the idea of the power of technique combination. 1 F. Rull, S. Maurice, I. Hutchinson et al. Astrobiology, Vol. 17 (2017), No. 6-7 2 B.H.N. Horgan, R.B. Anderson, G. Dromart, E.S. Amador, M.S. Rice Icarus, 339 (2020) 113526. 3 P. Kristova, L. Hopkinson, K. Rutt, H. Hunter, G. Cressey, American Mineralogist, 98 (2013) 401-409. 4 J.A. Manrique-Martinez et al. Journal of Raman Spectroscopy (2020) 1-16.