Carbonate‐Phyllosilicate Parageneses and Environments of Aqueous Alteration in Nili Fossae and Mars

Abstract

The equilibrium thermodynamics of Fe/Mg-phyllosilicates and carbonates have been investigated using numerical modeling, with emphasis on the effects of evaporation and temperature (hydrothermalism) under conditions related to present-day and primitive Mars, that is, low or high CO 2 partial pressure, under oxidizing or reducing environments. Findings are discussed in relation to alteration and weathering of minerals found in the Nili Fossae region. Results show that nontronite precipitates at very high water-to-precipitate ratio and low temperature, in association with ferrihydrite. During evaporation, this assemblage leads to carbonates: calcite and magnesite, and to low temperature iron-rich serpentine phyllosilicates such as cronstedtite or berthierine under reducing conditions. At high temperature, the initial paragenesis dominated by nontronite leads to hematite and berthierine at low CO 2 partial pressure, and to talc, magnesite and antigorite at high CO 2 partial pressure. Thus, the Fe/Mg-smectites, Mg-carbonate, and Mg-serpentine found near Nili Fossae may result from several alteration events. Weathering of primitive bedrock formed nontronite, with subsequent or concurrent hydrothermalism leading to Mg-serpentine. Further carbonation due to CO 2 -rich fluids led to talc, magnesite and to hydrogen/carbon monoxide/methane in the presence of iron, but at relatively low partial pressures (up to 10 −4 bar). Unless the serpentinization process acted for extended periods of time and over a large portion of the martian surface, it is unlikely it significantly contributed to the atmospheric composition, except maybe for H 2 . However, such water-rich active environments with the presence of redox reactions could have been suitable for the potential emergence of biological activity.Plain Language Summary Multiple locations on Mars exhibit phyllosilicate deposits, which have been used as indicators of temperate paleoenvironments with abundant liquid water. Of these, Nili Fossae is one of the rare occurrences where additional phases such as carbonates or serpentine are found. These minerals also pinpoint to possible hydrothermal activity, which is very important for the putative emergence of biologic activity on the red planet. Our work aims at understanding these various environments using numerical thermodynamic simulations of the mineral assemblages observed in this province. We observe that Mg,Fe-phyllosilicates such as nontronite form in low temperatures environments with very abundant water, and relatively oxidizing conditions (so rather on the surface) while magnesite and serpentine mixtures are characterized by higher temperatures (typically 200-300 °C) in the presence of high CO 2 partial pressures and more reducing conditions. Such hydrothermal reactions should also have led to the formations of hydrogen, carbon monoxide, and perhaps methane, but at concentrations probably too low to allow for global warming on Mars, unless they would have been significantly extended geographically and temporally. Nonethel...

Aqueous alteration processes in Jezero crater, Mars—implications for organic geochemistry

“…Observation of olivine and carbonate mixtures within the Garde target of the Séítah Fm is consistent with orbital infrared observations ( 2 – 4 ) and substantiated by multiple lines of evidence ( 9 – 11 ). Previously proposed hypotheses for the precipitation of these carbonates include low- and high-temperature aqueous alteration of olivine-rich igneous materials, which we will subsequently refer to as ultramafic protolith ( 3 , 15 – 17 ), or precipitation from lake or groundwater ( 4 , 15 – 17 ). Our 10- to 100-μm-scale textural and spectroscopic evidence supports carbonate formation through aqueous alteration of an ultramafic protolith, known as carbonation.…”

“…In ultramafic alteration environments on Earth ( 16 ) and in martian meteorites ( 18 , 19 ), carbonation can be associated with the formation of oxides, hydroxides, and/or Fe- and Mg-rich phyllosilicates, which have not been observed ( 9 ). Carbonation can occur under a wide range of temperatures from low to ambient to hydrothermal and metamorphic ( 15 – 17 ). Other alteration minerals, such as serpentine, have not been definitively observed in the Séitah Fm to date, which could suggest time-limited interactions, low water-to-rock ratios, or ambient fluid temperatures during carbonation ( 3 , 15 – 17 ).…”

“…Carbonation can occur under a wide range of temperatures from low to ambient to hydrothermal and metamorphic ( 15 – 17 ). Other alteration minerals, such as serpentine, have not been definitively observed in the Séitah Fm to date, which could suggest time-limited interactions, low water-to-rock ratios, or ambient fluid temperatures during carbonation ( 3 , 15 – 17 ).…”

“…These observations parallel those made by the Spirit rover in Gusev crater ( 20 ) and within (1.3 billion to 4 billion years ago) martian meteorites ( 18 , 19 ). Modeling has suggested that carbonate deposition could have played a role in the evolution of Mars’ atmosphere ( 3 , 17 , 21 ), but the geological nature of such a depositional mechanism had remained unexplained. Taken together, micron-scale SHERLOC observations of this phenomenon complement previous orbital and meteorite observations and demonstrate the alteration of igneous materials that results in geological deposition of carbonates.…”

Impact Induced Oxidation and Its Implications for Early Mars Climate

“…Other than volcanism, additional sources include serpentinization (Chassefière et al, 2013(Chassefière et al, , 2016Oze & Sharma, 2005;Quesnel et al, 2009;Wordsworth et al, 2015) and UV photolysis (Tarnas et al, 2018). Until present, remote sensing analysis only identified sporadic outcrops of serpentine (e.g., Amador et al, 2018;Ehlmann & Edwards, 2014), casting doubts on the extent of serpentinization in the crust, and uncertainties remain on the reaction rate at low-temperature (Chevrier & Morisson, 2021). Estimated radiolytic hydrogen flux is orders of magnitude smaller than the proposed peak H 2 abundance (Wordsworth et al, 2017), except in the case where all H 2 is locked in a reservoir (e.g., clathrates that may have preserved methane (Chassefière et al, 2013;Chassefière et al, 2016)) and released instantaneously after 1 Gyr (Tarnas et al, 2018).…”