Study of Alterations in the Dar al Gani 735 Martian Meteorite studied by Exomars2022 RLS-like Instrument and HR Raman

1. Introduction

Martian meteorites are, by the moment, the only Martian samples than can be studied in Earth laboratories. The minerals in Martian meteorites contain information about (a)  alteration processes suffered in the Martian surface, (b) due to the shock event that sent Martian materials to the space, (c) originated from pressure and thermal transformation during entry and (d) due to the terrestrial weathering processes after its landing. Thus, any Martian meteorite could have signals of these four alteration processes.

The studies of meteorites make use of microscopic techniques. Among such techniques, Raman micro-spectroscopy is becoming more and more used due to its versatility to detect both crystalline and amorphous minerals at the micrometric level. The RLS (Raman Laser Spectrometer) instrument on board Rosalind Franklin rover of the Exomars 2022 mission has such micrometric capability to study the drilled samples, at 50 microns of spot size.

This work compares the capability of a RLS-like Raman instrument (532 nm continuous excitation laser, focusing at 50 microns), and a High Resolution Raman micro-spectrometer, to detect mineral assemblages in the Dar al Gani 735 Martian meteorite (DaG 735), an olivine-phyric Shergottite with olivine/pyroxene “megacrysts” [1]. The aim is to identify terrestrial and non-terrestrial alterations suffered by the meteorite. Dar al Gani 735 was selected due to the size of individual crystals [1], the presence of large pockets of brownish-colored recrystallized impact glass associated with pyroxene and olivine [3], and the absence of fusion crust but presence of weathering compounds [2].

The Dar al Gani paired Martian Meteorites were ejected from the parent body 1.18 ± 0.17 My ago while the terrestrial age is estimated around 60 ± 20 ky. The isotopic compositions of Sm and Gd suggest a mixing between basaltic lava and regolith to form the bulk of the ejected material [3]. The large amount of literature information about the DaG 735 and paired meteorites [4-10] gave us the basic knowledge to first confirm the positive identification of the referred mineral phases with a HR Raman micro-spectrometer. Then, the RLS-like instrument [11] will tell us which, of the minerals detected with the HR system, could be detected at the 50 microns of spot size

2. Experimental

An InVia micro-Raman instrument, provided with 532 nm excitation lasers and Peltier cooled CCD detector (-70ºC) was used as the HR Raman instrument; details of the working conditions are given elsewhere [12]. The RLS-like instrument used was a BWTek 532 Raman spectrometer, equipped with an objective to focus at 50 microns. Both spectrometers were daily calibrated with the 520.5 cm⁻¹ silicon line. The results were interpreted by comparing the collected Raman spectra with Raman spectra of pure standard compounds.

3. Results and Discussion

The analysis showed the main matrix of pyroxene (clinopyroxenes like diopside-hedenbergite, CaMgSi₂O₆, or augite, CaMgFe)₂Si₂O₆, and ortopyroxene like enstatite, MgSiO₃) with olivine megacrysts (different ratios between forsterite, Mg₂SiO₄, and fayalite, Fe₂SiO₄) together with a remarkable presence of chromite (FeCr₂O₄) were detected as reported in literature [4-6].

 The analysis on cracks/veins shown the largest fractures filled with calcite as a terrestrial weathering compound. Others were filled with hematite (Fe₂O₃) coming from terrestrial weathering of the original Fe-bearing minerals

Apart from the high Ca presence in fractures, the Raman spectrum of the calcite crystals in the bulk shown displaced bands at 1088, 714, 283 and 157 cm^-1, belonging to shocked calcite. Suggesting these calcite crystals as original from Mars altered by the shock effect that formed the meteorite.

Relatively high size crystals of β-anhydrite were detected. This β-anhydrite cannot be assigned to terrestrial weathering because high temperature (>300^oC) is required for its formation. As gypsum changes to β-anhydrite under high pressure (> 2.56 GPa) and temperatures (> 583 K), the source of this β-anhydrite could be shocked original gypsum.

Ilmenite (Fe²⁺Ti⁴⁺O₃) was also detected but at higher wavenumbers, 686, 230 and 375 cm^-1, than terrestrial ilmenite, suggesting again the effect of high pressures to which this ilmenite was subjected (estimation of 18 GPa with the 686 cm^-1 band or 14 GPa with the 230 cm^-1 one) during the shock event.

The identification of anatase is also important to be highlighted. In terrestrial environments, the occurrence of anatase is indicative of low-temperature aqueous alteration, probably from the oxidation of the Fe(II) in ilmenite to form irreversibly hematite and anatase.

All these mineral phases were also detected with the RLS-like Raman spectrometer, suggesting that RLS on board the Rosalind Franklin rover could not only detect mineral but most interestingly all the mineral phases related to aqueous driven alteration processes occurred in Mars.

 4. Conclusions

The characterization of the DaG 735 meteorite with a HR Raman micro-spectrometer was successful because not only the expected mineral phases were detected but also new minerals identified. In particular, Raman spectroscopy was able to detect shocked calcite, anhydrite and ilmenite. Also alterations due to weathering were detected in enough amount as to be also identified by the RLS-like Raman instrument. This provides information about the future data that will be sent by the Raman technique implemented on the Rosalind Franklin rover (Exomars2022 mission from ESA), for the new explorations of Mars materials.

5. Acknowledgements:

This work was financed by the Ministry of Economy and Competitiveness (MINECO, grants ESP2014-56138-C3-2-R and ESP2017-87690-C3-1-R). The authors gratefully acknowledge the support of the SIGUE-Mars consortium (MINECO, grant RDE2018-102600-T).

6. References

[1] Grossman J.N. (2000), Meteorit. & Planet. Sci. 35, A199-A225.

[2] Meyer C., The Martian Meteorite Compendium, 2012, NASA Antarctic Meteorite Program (updated and revised by Righter K., 2017)

[3] Hidaka H. et al. (2009), Earth Planet. Sci. Let., 288, 564-571

[4] Zipfel J. et al. (2000), Meteorit. & Planet. Sci. 35, 95-106.

[5] Mikouchi T.et al. (2001), Meteorit. & Planet. Sci. 36, 531-548.

[6] Folco L. and Franchi I.A. (2000), Meteorit. & Planet. Sci. 35, A54-55.

[7] Wadhwa M.et al. (2001), Meteorit. & Planet. Sci. 36, 195-208.

[8] Borg L.E. et al. (2003), Geochim. Cosmochim. Acta 67, 3519-3536.

[9] Miyaharaa M. et al. (2011), Proc. Nat. Acad. Sci., 108, 5999-6003

[10] Schlüter J. et al. (2002), Meteorit. & Planet. Sci. 37, 1079-1093.

[11] Lopez-Reyes G. (2013), Eur J Mineral. 25, 721-733

[12] Aramendia J. et al. (2018), Trends Anal. Chem., 98, 36-46