Possible formation pathways for zeolites in closed-basin lakes on noachian Mars: Insights from geochemical modeling

Abstract

Zeolites are among the most common and widespread authigenic silicate minerals found in saline-alkaline paleolacustrine environments on Earth. Analcime, a Na-zeolite, has been detected on crustal outcrops on Mars using orbital spectral image data. Identifying other zeolite species is complicated by the lack of diagnostic absorption features in spectral data and spectral similarity to some polyhydrated sulfates. Therefore, analcime is the only zeolite species so far unambiguously identified as present on Mars. The presence of certain zeolite mineral species or assemblages in a closed basin paleolake can be used to track hydrological changes in that basin. This paper discusses the application of geochemical modeling to identify zeolite phases that could have formed in closed lake basins of late Noachian Mars. The composition of the high-silica Buckskin sample from Gale crater is used as a starting material because 1) most zeolite-rich assemblages in closed hydrologic systems on Earth originate from the decomposition of high-silica volcanic glass, and 2) geochemical modeling studies are already available for the more common lower silica basalts on Mars. This study, along with previous studies involving basaltic starting materials, allow a comparison between terrestrial and Martian data and an exploration of the differences in secondary mineral precipitation with substrates of different silica contents. The EQ3/6 geochemical modeling code was adopted, and Late Noachian Mars was assumed to be warm and semi-arid, with an atmosphere with 1.3 bars of PCO2 and current Mars PO2 (14.7 μbar). The Buckskin sample was reacted with three different starting solutions including two different rainfall chemistries and an acidic H2SO4 − HCl solution, which could have formed through volcanic degassing. The mineral formation and re-dissolution patterns under different Water/Rock (W/R) ratios are also explored.The results show that near neutral and acidic solutions in closed-basin lakes filled with high-silica volcaniclastic materials could eventually produce zeolite minerals like those in saline-alkaline lakes on Earth. Analcime forms only in water-limited environments (low W/R ratio), while chabazite is present even at relatively high W/R ratio. Clinoptilolite transforms to analcime with an increase in pH, with decreased W/R ratio, and with time. Analcime was found to be the most stable zeolite mineral over the greatest range of conditions in this environment, associated with increased pH, consistent with its presence as a very common constituent of saline-alkaline lake deposits on Earth. The study shows that the mineral precipitation patterns associated with the dissolution of high-silica materials through acid weathering are comparable to the mineral precipitation patterns following the dissolution of basaltic rocks by acid weathering under 0 − 25oC conditions or hydrothermal conditions as described in previous laboratory, field, and geochemical modeling studies.