Decrypting the Formation Conditions of the Basement Carbonate-Bearing Rocks at Nili Fossae

Abstract:

The Nili Fossae region is the site of a number of proposed Landing Sites for the Mars 2020 Rover. A distinguishing feature of many of these sites is the access to large exposures of carbonate (Ehlmann et al. 2008). Serpentinization has been proposed as a formation mechanism of these carbonates, including carbonated (Brown et al. 2010, Viviano, et al. 2013) and low temperature, near surface serpentinization.

The potential for carbonated serpentization at Nili Fossae links the region to Earth analogs in terrestrial greenstone belts such as the Pilbara in Western Australia, where talc-carbonate bearing komatiite cumulate units of the Dresser Formation overlie the siliceous, stromatolite-bearing Strelley Pool Chert unit (Van Kranendonk and Pirajno, 2004). If a similar relationship exists on Mars, investigations of rocks stratigraphically beneath the carbonate-bearing units at Nili Fossae ("the basement rocks") may provide the best chance to examine well preserved organic material from the Noachian. This hypothesis is testable by Mars 2020.

In preparation for the the Mars 2020 landing site, we are examining the thermodynamic relationships that favor formation of serpentine and talc-carbonate and different pressures and temperatures in the crust (Barnes 2007). This will allow us to constrain the low grade metamorphism required to replicate the proposed models of serpentinisation and help us understand the regional metamophic gradient that is critical to furthering our knowledge of the ancient rocks of Nili Fossae.

Refs:
Barnes, S. J. “Komatiites: Petrology, Volcanology, Metamorphism, and Geochemistry.” S.E.G. 13 (2007): 13.

Brown, A. J., et al.. “Hydrothermal Formation of Clay-Carbonate Alteration Assemblages in the Nili Fossae Region of Mars.” EPSL 297 (2010): 174–82.

Ehlmann, B. L. et al. “Orbital Identification of Carbonate-Bearing Rocks on Mars.” Science 322, no. 5909 1828–32.

Van Kranendonk, M.J., and F. Pirajno. “Geochemistry of Metabasalts and Hydrothermal Alteration Zones Associated with Ca. 3.45 Ga Chert+/- Barite Deposits” GEEA 4, no. 3 (2004): 253–78.

Viviano, C. E., et al. “Implications for Early Hydrothermal Environments on Mars through the Spectral Evidence for Carbonation and Chloritization Reactions in the Nili Fossae Region.” JGR 118, no. 9 (2013): 1858–72.