Water-Rock Reaction on Mars - as Seen through Earth Analogues

Seidel, Robert (2022). Water-Rock Reaction on Mars - as Seen through Earth Analogues. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.00013eb8


Impact-induced hydrothermal systems are important targets for Mars exploration. Their existence is confirmed by remote observation, in situ investigation by Mars rovers, and the study of Martian meteorites. However, ground truth is lacking for the alteration behaviour of basalt and gabbro in such systems, which dominate the Martian crust. This study uses a detailed petrologic investigation of an alteration event in a terrestrial analogue (Frankenstein Gabbro, Germany), combined with thermochemical modelling, to deduce a reaction path that is guided by ground truth, but independent of the terrestrial composition. Martian host rocks and fluids are substituted for the terrestrial model inputs, to predict the alteration of gabbro in an impact-induced system on Mars.

Alteration in the Frankenstein Gabbro mimics the fracture-controlled alteration expected in Martian impact craters, being focused along tectonically-induced, < 250 μm wide fractures and associated mineral veinlets. Peak conditions are T ≥ 300 °C, at P = 150 bar. The dominant secondary assemblage is Act-Ab-Chl-Ep-Kfs-Prh, formed at pH ~ 6.7 and low water : rock ratio (W/R 10–35). The initial fluid likely resulted from mixing of an autochthonous basement fluid with a limited amount (max. 15 parts per 100) of sediment-derived fluid.

Simulated alteration of Martian gabbro (meteorite NWA 6963) along the analogue-based reaction path predicts a uniform assemblage of amphibole-chlorite at low W/R and variable assemblages at high W/R, for two starting fluids and a control of pure water. Available ground truth suggests the low-W/R assemblage best predicts the alteration in a Martian impact-induced system. The modelled conditions are expected to take place beneath the peak ring of craters ≥ 100 km in diameter. Cooling in situ of the fluids produced at high T, low W/R results in habitable fluids at T = 10–100 °C that are suitable for potential sulfide- and/or sulfate-utilising organisms.

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