Quantitative Raman calibration of sulfate-bearing polymineralic mixtures: a S quantification in sedimentary rocks on Mars

The NASA 2020 Mars mission is a Curiosity-type rover whose objective is to improve the knowledge of the geological and climatic evolution of Mars and to collect rock samples for return to Earth. The new rover has a payload of seven instruments including the SuperCam instrument which consists of four tools including a Raman spectrometer. This Raman device will be non-destructive and will analyse the surface remotely in order to determine the mineralogy of rocks and, by extent, to detect and quantify major elements such as sulfur. Sulfur has been detected as sulfate (Ca,Mg,Fe-sulfates) in sedimentary rocks. This element is difficult to quantify using the laser ablation tool of the ChemCam instrument on-board the Curiosity rover.We propose a Raman calibration to constrain the sulfur abundance in polymineralic mixtures. We acquired Raman signatures on binary and ternary mechanical mixtures containing Ca and Mg sulfates, mixed with natural silicate minerals supposed to be relevant to basaltic-sedimentary rocks at the surface of Mars: olivine, clinopyroxene, orthopyroxene and plagioclase. Using the Voigt function to process the Raman spectra from samples extracted from our mixtures allows us to calculate the initial proportions of our preparations of Ca and Mg sulfates. From these simulations, calibration equations have been provided allowing us to determine sulfate proportions (CaSO4and MgSO4) in a mixture with basaltic minerals. With the presented calibration, S can be quantified at a lower limit of 0.7 wt.% in Martian soil.