Wishstone to Watchtower: Alteration of Plagioclase-rich Rocks in Gusev Crater, Mars

The Mars Exploration Rover Spirit encountered a remarkable diversity of rocks during its traverse of the Columbia Hills in Gusev crater, manifested both as variations in primary mineralogy and in secondary alteration. The Wishstone and Watchtower Classes represent examples where the less altered (Wishstone) and more altered versions (Watchtower) were recognized as members of an alteration series identified by variations in geochemistry and Fe-bearing mineral phases [1-3]. Work by [1] demonstrated a geochemical relationship consistent with two-component mixing between a high Al₂O₃, TiO₂, CaO, Na₂O, P₂O₅ end-member and a second end-member enriched in MgO, Zn, S, Br, and Cl. The first end-member probably is Wishstone Class, with Watchtower Class intermediate between it and an unrecognized second end-member lithology [1].

New results using mirror-dust corrected spectra from Spirit’s Miniature Thermal Emission Spectrometer [4] affirm the dominant plagioclase component in Wishstone and an amorphous component resembling basaltic glass in Watchtower identified previously [5]. But now we recognize a suite of rocks spanning the full range of alteration, including one dubbed Bruce that appears to be an alteration spectral end-member [4]. The spectra of some rocks with an intermediate level of alteration are well modeled as a simple two-component mixture of Wishstone and Bruce spectra. This is consistent with a style of alteration that progressively obscures spectral contributions of the host rock minerals and is inconsistent with a surface coating. The Bruce spectrum is poorly modeled by primary and secondary phases including phyllosilicates and amorphous silicates. This suggests a style of alteration not recognized in terrestrial settings. Based on the similarity of the Bruce spectrum to TES type 2, this style of alteration may be more widespread on Mars. The conditions that produced this alteration are poorly constrained, hence the relationship to habitability is unknown at this time.

[1] Hurowitz, J. A., et al. (2006), J. Geophys. Res., 111, E12S14. [2] Ming, D. W., et al. (2006), J. Geophys. Res., 111, E02S12. [3] Morris, R. V., et al. (2006), J. Geophys. Res., 111, E02S13. [4] Ruff, S. W., and V. E. Hamilton (submitted). [5] Ruff, S. W., et al. (2006), J. Geophys. Res., 111, E12S18.