Effect of LIBS-Induced Alteration on Subsequent Raman Analysis of Iron Sulfides

Mineral alteration is a possible side effect of spectroscopic techniques involving laser ablation, such as laser-induced breakdown spectroscopy (LIBS), and is related to the interaction of the generated plasma and ablated material with samples, dust, or ambient atmosphere. Therefore, it is essential to understand these interactions for analytical techniques involving laser ablation, especially for space research. In this combined LIBS–Raman analytical study, pyrite (FeS_2) and pyrrhotite (Fe_{1–x}S) samples have been consecutively measured with LIBS and Raman spectroscopy, under three different atmospheric conditions: ∼10^{–4} mbar (atmosphereless body), ∼7 mbar, and Martian atmospheric composition (Martian surface conditions), and 1 bar and Martian atmospheric composition. Furthermore, a dust layer was simulated using ZnO powder in a separate test and applied to pyrite under Martian atmospheric conditions. In all cases, Raman spectra were obscured after the use of LIBS in the area of and around the formed crater. Additional Raman transitions were detected, associated with sulfur (pyrite, 7.0 mbar and 1.0 bar), polysulfides (all conditions), and magnetite (both minerals, 1.0 bar). Magnetite and polysulfides formed a thin film of up to 350–420 and 70–400 nm in the outer part of the LIBS crater, respectively. The ZnO-dust test led to the removal of the dust layer, with a similar alteration to the nondust pyrite test at 7.0 mbar. The tests indicate that recombination with the CO_2-rich atmosphere is significant at least for pressures from 1.0 bar and that plasma–dust interaction is insignificant. The formation of sulfur and polysulfides indicates fractionation and possible loss of volatile elements caused by the heat of the LIBS laser. This should be taken into account when interpreting combined LIBS–Raman analyses of minerals containing volatile elements on planetary surfaces

Effect of initial water composition on thermodynamic modeling of hydrothermal alteration in basalt—A case study of the Vargeão Dome impact structure

The impact-generated hydrothermal system at Vargeão Dome, Brazil, is a unique potential analogue for impact-generated hydrothermal systems on Mars. Its evolution can be understood through thermodynamic modeling, for which one of the necessary parameters is the composition of the involved water. The exact water composition for Vargeão at the time of the impact is unknown, and, moreover, the effect of this uncertainty is often underestimated in thermodynamic modeling. Here, the effect of initial water composition was tested by using a randomized set of initial solutions for thermodynamic modeling of the evolution of the Vargeão Dome impact-generated hydrothermal system. It was found that even small changes in composition could affect the precipitation of common minerals like calcite and quartz. Therefore, it is necessary to perform a sensitivity analysis for any thermodynamic model in which the initial solution is poorly constrained. Subsequently, the found effects were used to constrain water compositions for the Vargeão Dome system at the time of the impact, by eliminating randomized solutions of models precipitating different minerals from those observed in reality. Using a simple set of rules, it was possible to constrain the total amount of dissolved solids between 6 and 2000 mg/L, as well as provide approximate boundaries for all individual elements present in the solution

Hydrothermal alteration at the basalt-hosted Vista Alegre impact structure, Brazil

Hydrothermal systems provide a possible habitat for early life and are key targets in the quest for life outside Earth. In impact craters on Mars, hydrous minerals can represent products of impact-generated hydrothermal systems (IGHS) or minerals already present in the crust and exposed during impact-caused excavation. Because of its basaltic target rock, similar in composition to Martian crust, the Vista Alegre impact structure in Brazil is one of the very few analog structures that may reveal the origin of these minerals, if evidence of hydrothermal alteration is established. This work presents the results of a systematic search for evidence of hydrothermal alteration at the Vista Alegre impact structure. Four types of alteration were identified, all within a 2.5–3.0 km radius from the crater center: a zircon-bearing melt veinlet, two sets of hydrothermal veins consisting predominantly of calcite and chabazite, and local alteration comprising saponite. Thermodynamic modeling suggests subsequent heating and cooling for each of the hydrothermal vein sets. Combined thermodynamic and spectrometric evidence indicates that development of a vigorous IGHS is unlikely. If similar processes occur on Mars, hydrous minerals are more likely preimpact phases exposed by excavation, rather than being formed through an IGHS