Clay mineral formation on Mars: Chemical constraints and possible contribution of basalt out-gassing

• Fe–Mg clay minerals may form during a post-magmatic stage from the degassing volatiles.
• Fe–Mg smectites/chlorites and mafic minerals may crystallize simultaneously at high temperature.
• The equation clays=alteration=liquid water is not as universal as thought.

We focus on processes possibly occurring on Mars that could affect sufficiently large volumes of rocks to be detected by remote sensing techniques.When compared with the chemical modelling of water–rock interactions, the petrographic and mineralogical characteristics of clay deposited in the prismatic joints of a lava flow from the Parana basin (Brazil) suggest that the clay fraction of lava flows may be formed at least partly during an early post-magmatic stage associated with the degassing of acid volatiles.In view of the literature concerning other contexts, such as meteorites or experimental syntheses, we conclude that the crystallization of anhydrous mafic minerals and the formation of clay mineral are not systematically mutually exclusive phenomena. While clay formation is generally related to chemical reaction pathways, it is not necessarily due to the alteration of pre-existing silicates.Such post-magmatic reactions, even if they are likely to represent only a minor contribution to Martian clay formation limited to early Noachian times, require much lower amounts of water compared to conventional hydrothermal alteration or weathering systems. The products of these reactions can be detected over large surface areas, as in the case of the Mars sites, thus allowing us to envisage a greater diversity of paleogeographic scenarios for Early Mars.