Geochemical modeling on Granby Tuffs clays to decipher alteration pathway at Oxia Planum, Mars

Introduction:  ExoMars Rosalind Franklin rover mission will land at Oxia Planum (OP) on Mars. OP is a 3.9Gyr old phyllosilicate-bearing plain located between Mawrth Vallis and Ares Vallis [1]. The Fe,Mg-rich phyllosilicate surfaces detected at Oxia Planum are some of the largest continuous exposures of this type on Mars. Yet understanding of the processes that led to its formation remain elusive.

Orbital NIR spectral features of the phyllosilicates at Oxia suggest Fe-rich vermiculite and/or saponite [3]. Survey of Fe-rich terrestrial vermiculite-bearing rocks [4,5] showed that the best spectral analogy is shown by the basaltic rocks from Granby, Massachusetts, USA. The Granby formation is represented by basaltic flows, dikes and tuffs, all of which altered. Amygdales in vesicular basalts are filled with diverse clay minerals [6]. Among them are micaceous minerals, Al-rich clays and Fe/Mg-rich clays [7]. There formation is investigated here.

Method: EQ3/6 geochemical models are performed using special reactants and solid solutions of olivine, pyroxene, and plagioclase, representing the average Granby tuff composition. The database was enriched with new data on diverse clays, and different scenarios of surface/subsurface alteration pathways are tested.

Relevance for Oxia Planum: Results from these models will be presented in terms of final mineral assemblages and compared with what is known about OP clays. Different scenarios presented in Mandon et al, 2021 will then be challenged regarding the mineral assemblages obtained in our models. Our results demonstrate the value of using analogues studies to decipher past water-rock interactions. By combining mineralogical and chemical analyses on analogues to geochemical models we can obtain a more comprehensive understanding of Mars’s hydrated surface.

Acknowledgments: This project was supported by the EU Horizon 2020 Space program call H2020-COMPET-2015-Grant Agreement no 687302. The study got support from the National Planetology Program (PNP) of the INSU-CNRS and from the CNES Research Proposal Call (APR).

References: [1] P. Fawdon et al., (2024) Journal of Maps 20(1).  [2] J. Vago et al., (2017) Astrobiology 17:6-7. [3] L. Mandon et al, (2021) Astrobiology 21: 464-480. [4] A.M. Krzesińska et al, (2021) Astrobiology 21: 997-1016. [5] H. Dypvik et al, (2021) Planetary and Space Science, 208. [6] R. H. April and D. M. Keller (1992). Clays and Clay Minerals 40: 22-31. [7] Bultel, B., Krzesinska, A., Veneranda, M., Loizeau, D., & Werner, S. (2025, September). EPSC-DPS Joint Meeting 2025 (EPSC-DPS2025 (pp. EPSC-DPS2025).