Tuesday

The nakhlite martian meteorites contain hydrothermal veins, the result of H2O-CO2-rich fluids originating from shallow subsurface ice-bearing rocks and the dissolution of olivines and feldspathic mesostasis. Carbonates crystallised at up to 150-200 °C, followed by crystalline saponite and Al-rich ferric serpentine, and then a rapidly cooled metastable assemblage with poorly crystalline gel at ~50 °C [Bridges and Schwenzer, 2012].

Our experiment tests this model of alteration, including the temperature, pH and composition of the fluids. The samples include nakhlite analogue mixtures of olivines, pyroxenes, and/or plagioclase, immersed in purified water at 35 °C under a synthetic martian atmosphere (98 % CO2 and 2 % Ar, 30 ppm Kr and 8 ppm Xe) for 1, 3, and 9 month experiments, using the Mars Brines experimental apparatus at NASA Ames [Schwenzer et al. 2017].

The resulting samples revealed veins of clays in the olivine. The secondary minerals examined under HR-TEM showed crystalline material with T-O lattice d-spacings of 7.0 Å. Synchrotron Fe-K XAS measurements show positive shifts of ~1 eV in the 1s→3d pre-edge peak centroid position compared to the surrounding olivine, as a result of an increase in the ferric content (Fe3+/ΣFe ≤0.5) associated with the alteration. The average chemical composition of the clay material is MgO 44.8, Al2O3 0.9, SiO2 51.8, FeO 1.1, Fe2O3 1.3 wt%. These Fe-K XAS and HR-TEM results are similar to the ferric alteration material observed in the nakhlites [Hicks et al., 2014].