Variably weathered ultramafic rocks: Investigation of their mineralogy with SuperCam-like techniques

Abstract

The NASA Perseverance rover is exploring Jezero crater on Mars since February 2021. Orbital data from Nili Fossae and Jezero crater show a wide diversity of mineralogical signatures including primary mineral like olivine as well as diverse secondary minerals like clays and serpentines and one of the strongest spectral signatures of carbonate on Mars from orbit. This olivine and carbonate-bearing unit is regionally extensive. Most of the studies interpreted the relationship between olivine and carbonate-bearing units as a result of weathering processes involving fluid–rock interactions such as serpentinization and carbonation. Since February 2021, Perseverance has confirmed this large diversity of primary and secondary minerals. The detection of serpentine is of particular interest in order to better constrain the past environment at Jezero crater. Among the science payload, SuperCam is combining three spectral techniques (Laser-Induced Breakdown Spectroscopy - LIBS, Raman, and Visible and near InfraRed - VISIR) that give access to the elemental composition of the targets but also to their mineral and molecular structure. Here, we use terrestrial ultramafic rocks with various degrees of alteration that were well characterized from previous studies, and we analyze them with SuperCam replicate laboratory setups to investigate the sensitivity of SuperCam to detect olivine and secondary phases, in particular serpentines. Distinguishing serpentine from olivine by LIBS is challenging since they have similar chemistry. On the other hand, Raman spectroscopy is able to detect all the phases present in the analyzed samples, when their content is high enough. Finally, the VISIR technique detected the secondary phases while it did not observe easily the olivine. We found that the synergy between these three techniques is of particular importance to differentiate such specific phases, which occur as intricate mixtures down to the sub-micrometer scale in natural targets.