Formation of clay-sulfate sedimentary units on Mars via phyllosilicate alteration by acid-sulfate fluids

Phyllosilicate and sulfate-bearing units are ubiquitous on Mars, indicating past water-rich planetary environments that likely transitioned from neutral-to-alkaline pH conditions (phyllosilicate formation) to more acidic pH conditions  (sulfate deposition). Thus, sediments with mixed mineralogy dominated by phyllosilicates and sulfates, informally referred to as the “clay-sulfate units”, may reflect planetary changes in ancient climate with implications on its habitability. Yet, the specific processes and surface conditions that led to the formation of the clay-sulfate units have remained uncertain.

In this study, we investigated the alteration of Mars-analog phyllosilicates (hereafter “clays”) with acidic, sulfate-rich solutions by performing laboratory batch (closed system) and field (open system) experiments to characterize dissolution processes and catalogue diagnostic alteration features produced under a wide range of conditions. Two Fe-rich smectites (nontronites), NAu-1 and NAu-2, and one silicon (IV) oxide (silica), which was used as control, were reacted with two types of acidic,  sulfate-rich solutions that were prepared with (1) natural acid rock drainage labelled ARD, and (2) synthetic sulfuric acid (H₂SO₄) labelled ASf. The initial solutions were adjusted at four pH values (1, 3, 5, and 7) and reacted at 4, 30, and 80°C for up to 6 months. At the end of the experiments, the filtered supernatants were analyzed by ICP-MS while the solids were characterized by X-ray diffraction (XRD), energy-dispersive X-ray fluorescence analyses (ED-XRF), Raman spectroscopy and thermal analysis data, including thermal gravimetry (TG), differential scanning calorimetry (DSC) and evolved gas analysis (EGA).

Results show that structural changes of the acid-treated nontronite clays were detected under all experimental conditions, as evidenced by multiple methods. However, the dissolution of clays was limited even under the most extreme conditions (i.e., NAu-1 reacted with ARD at a pH of 1, at the highest temperature (80°C) and for an extended duration). These results contradict previous studies that suggest that Fe-rich nontronite clays break down easily and dissolve when exposed to highly acidic and high-temperature conditions. Further investigations showed that the dissolution processes were ubiquitous and accompanied by changes in solutions composition and the precipitation of secondary phases, which included Fe(III) oxyhydroxides, a wide range (i.e., Fe(III), Al, Mg, Mn, and Ca) of sulfate minerals, and, in one instance, traces of dioctahedral mica (i.e., illite). These precipitates formed coatings on reacting nontronite clays, thus protecting them from aggressive dissolution. Significantly, the composition of the acid-sulfate solutions plays an essential role in the system evolution, including the geochemical characteristics of the reacting solution and the amount and identity of post-alteration mineralogical assemblages.

Application of our results to Mars reveals that acidic, sulfate-rich fluids were essential for producing clay-sulfate assemblages, such as those found at Gale Crater. Specifically, highly acidic solutions could have induced widespread disintegration of primary clay units and the formation of secondary sulfate deposits. The combined results of our study may allow us to produce a catalogue of alteration features to relate the mineralogical assemblages mapped on Mars to specific solution attributes and environmental conditions in which clays reacted with acidic solutions.