Laboratory Hydrothermal Alteration of Basaltic Tephra by Acid Sulfate Solutions: an Analog Process for Martian Weathering

Introduction: Sulfur is a major chemical component of the Martian surface materials and is thought to have a volcanic origin. Pathfinder and Viking chemical analyses revealed SO3 compositions of “soils” and dust from about 4 to over 7 wt. % [1,2]. Although we know that S is chemically important, we do not know its speciation. Small amounts of sulfates (e.g., Ca and Mg sulfates) and sulfides (e.g., pyrrhotite, pyrite) have been identified in SNC meteorites [e.g., 4]; however, sulfates have not been detected from remote spectral measurements, which are sensitive to levels of 10-15 wt. % abundance [5,6]. It is likely that the high sulfur content in the Martian soil is secondary (i.e., S from volcanic gases), because it is unlikely that basalt magmas can contain these high S compositions [7]. Because of the oxidizing nature and high Fe and S contents of the surface “soils”, Fe-bearing sulfates (e.g., jarosite, natrojarosite, schwartmannite) have been suggested as possible phases in the Mars “soil” and dust [e.g., 8-11]. Other candidate sulfate-bearing phases are Al, Ca and Mg sulfates [e.g., 2,12]. The objective of this study is to conduct simulated Mars-like weathering experiments in the laboratory to determine the weathering products that might form during oxidative, acidic weathering of Mars analog materials. Materials and Methods: Hydrothermal sulfuricacid weathering experiments were performed using either a single hydrothermal vessel to simulate a closed hydrologic system or a series of three hydrothermal vessels to simulate the reactions of leachates moving through fresh basalt in an open hydrologic system [13]. For closed hydrologic system experiments, 10 mL of 0.5 N H2SO4 and 1 mL of 38% H2O2 were added to 0.1 g of <150 μm HWPC100, which is an olivine-rich basalt from Hawaii with a composition of glass>olivine>plagioclase [14]. Samples and solutions were heated at 150°C for 216 hours in a 20 mL Teflonlined hydrothermal vessel. For open system experiments, 10-mL of 0.5 N H2SO4 and 1 mL of 38% H2O2 were added to 0.1 g of <150 μm HWPC100 in 20-mL Teflon-lined hydrothermal vessels and heated at 150°C for 72 hours. Upon completion of the experiment, the leachate (L-1) from the hydrothermal vessel was transferred to a second Teflon-lined hydrothermal vessel along with 1-mL of fresh H2O2 and 0.1 g of fresh HWPC100. The second hydrothermal vessel was heated at 150°C for 72 h and, again, the leachate (L-2) was transferred to a third Teflon-lined hydrothermal vessel along with 1-mL of fresh H2O2 and 0.1 g of fresh HWPC100. A small amount of leachate (≈ 1 mL) from each run was saved for solution chemical analysis by atomic absorption spectroscopy. Residues (i.e., run product solids) in all experiments were washed in 95% ethanol and freeze-dried. Run products were analyzed by x-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy and wet chemical procedures. Results and Discussion: In closed hydrologic system experiments, olivine was converted to amorphous silica and soluble Mgand Fe-sulfates, which remain highly soluble in high acidity and low oxidant (H2O2) solutions. However, in the open hydrologic system, amorphous silica, MgSO4nH2O, gypsum, and jarosite formed at the end of the experimental run (Fig. 1). Dissolution and formation processes may be described by the following reactions in both the closed and open hydrologic systems: