Dioctahedral Phyllosilicates Versus Zeolites and Carbonates Versus Zeolites Competitions as Constraints to Understanding Early Mars Alteration Conditions

Abstract

Widespread occurrence of Fe,Mg‐phyllosilicates has been observed on Noachian Martian terrains. Therefore, the study of Fe,Mg‐phyllosilicate formation, in order to characterize early Martian environmental conditions, is of particular interest to the Martian community. Previous studies have shown that the investigation of Fe,Mg‐smectite formation alone helps to describe early Mars environmental conditions, but there are still large uncertainties in terms of pH range, oxic/anoxic conditions, etc. Interestingly, carbonates and/or zeolites have also been observed on Noachian surfaces in association with the Fe,Mg‐phyllosilicates. Consequently, the present study focuses on the dioctahedral/trioctahedral phyllosilicate/carbonate/zeolite formation as a function of various CO2 contents (100% N2, 10% CO2/90% N2, and 100% CO2), from a combined approach including closed system laboratory experiments for 3 weeks at 120°C and geochemical simulations. The experimental results show that as the CO2 content decreases, the amount of dioctahedral clay minerals decreases in favor of trioctahedral minerals. Carbonates and dioctahedral clay minerals are formed during the experiments with CO2. When Ca‐zeolites are formed, no carbonates and dioctahedral minerals are observed. Geochemical simulation aided in establishing pH as a key parameter in determining mineral formation patterns. Indeed, under acidic conditions dioctahedral clay minerals and carbonate minerals are formed, while trioctahedral clay minerals are formed in basic conditions with a neutral pH value of 5.98 at 120°C. Zeolites are favored from pH ≳ 7.2. The results obtained shed new light on the importance of dioctahedral clay minerals versus zeolites and carbonates versus zeolites competitions to better define the aqueous alteration processes throughout early Mars history.

Experimental hydrothermal alteration of basaltic glass with relevance to Mars

“…A similar observation was inferred by Viennet et al. (), and it may indicate that NIR is more sensitive for discriminating such minerals.…”

“…; Viennet et al. ), but before only reacted at fully saturated conditions. Major element composition of the original basalt is presented in Table .…”

“…Hydrothermal experiments and reaction path modeling performed by Viennet et al. () showed a correlation between di‐ and trioctahedral smectites and dissolved CO 2 . The trioctahedral varieties were favored at low levels of dissolved CO 2 and alkaline conditions.…”

Detection of Carbonates in Martian Weathering Profiles

“…To better characterize the environment of weathering, we use here the mineral associations as a proxy and discuss the conditions of the system drawn from a terrestrial analog study (Gaudin et al, ), geochemical modeling (Zolotov & Mironenko, ), and laboratory experiments (Gaudin et al, ; Viennet et al, ).…”

“…If Mars experienced a humid climate for a long time, a dense atmosphere mostly composed of CO 2 and lower amounts (few percent) of other greenhouse gasses is required (Wordsworth et al, 2017). Under such climatic conditions, both geochemical models (Schaefer, 1990;Zolotov & Mironenko, 2016) and laboratory experiments (Gaudin et al, 2018;Viennet et al, 2017) have shown the coprecipitation of hydrated minerals and small amounts of carbonates. Therefore, the apparent absence of carbonates in these weathering profiles remains enigmatic and debates on inhibited formation or subsequent dissolution of the carbonates are yet inconclusive (Edwards & Ehlmann, 2015).…”