Timing of acid weathering on Mars: A kinetic‐thermodynamic assessment

Abstract

[1] Weathering of olivine basalt by H 2 SO 4 -HCl aqueous solutions at the conditions of early Mars was investigated through numerical modeling in a system open with respect to CO 2 and O 2 only. The model includes dissolution rates of primary and secondary minerals and oxidation rate of aqueous Fe 2+ , as well as chemical equilibration among solutes, dissolved gases, and precipitates. The results reveal fast dissolution of Fe-Mg minerals at low pH, followed by preferential dissolution of plagioclase at higher pH. Correspondingly, solutions evolve from acidic, Mg-Ca-Fe 2+ -Fe 3+ -Al 3+ compositions toward Na-rich alkaline fluids. The period over which neutralization and mineral precipitation may occur is shorter at higher initial pH, lower water to rock ratios, and larger mineral surface areas. Early stages of weathering are characterized by the formation of amorphous silica, goethite, and kaolinite, while zeolites and carbonates form considerably later at higher pH, where silica dissolves. Slow oxidation of Fe 2+ causes precipitation of ferrous phyllosilicates. Comparison with Martian observations indicate that amorphous silica, Fe 3+ oxyhydroxides, and Mg-, Ca-, and Fe-sulfates could have formed during multiple short-term episodes of acid weathering that were terminated by freezing and/or evaporation. Throughout history, impact generation of oxidants (e.g., O 2 , SO 3 , NO 2 ) caused formation of strong acids and incremental Fe 2+ oxidation, the processes that are not efficient during O 2 -deficient periods of volcanic degassing. Although impact-generated acid rainfalls could have caused intense weathering and erosion in Noachian time, dilution of acids and a prolonged existence of surface solutions favored local neutralization of solution, and formation, transport and deposition of clays.

Source-to-Sink

“…Chemical weathering has occurred overall within a cold, waterlimited, low-pH S-cycle (Hurowitz andMcLennan 2007, McLennan andGrotzinger 2008), during which chemical reactions were incomplete (Madden et al 2004, Tosca andKnoll 2009) short-lived episodes terminated by freezing and/or evaporation (Zolotov and Mironenko 2007). The most comprehensive interpretation of the general evolution of chemical weathering on Mars comes from global mineralogical mapping in which (1) a Noachian ''phyllosian era'' characterized by aqueous alteration and clay-mineral generation, yields to (2) a Hesperian ''theiikian era'' characterized by sulfate generation and (3) a late Hesperian-Amazonian ''siderikian era'' characterized by the formation of anhydrous ferric oxides in slow, superficial weathering without liquid water ( Fig.…”

Mars Sedimentary Geology: Key Concepts and Outstanding Questions

“…The overall nature of chemical weathering departs significantly from Earth's carbon cycle, instead occurring within a sulfur cycle dominated by low pH, water limitation, and cold environments (Hurowitz and McLennan, 2007;McLennan and Grotzinger, 2008), where chemical reactions are incomplete (Madden et al, 2004;Tosca and Knoll, 2009) and terminated by freezing and evaporation (Zolotov and Mironenko, 2007). Another fundamental point of departure is that there is fairly broad consensus that plate tectonics do not, and have not, operated on Mars.…”

Volcanic hydrothermal systems as potential analogues of Martian sulphate-rich terrains

“…High silica abundances in soils from Meridiani Planum, Gusev crater and in some chasmata of Valles Marineris are most likely a product of intensive acid rock leaching processes and silica precipitation from saturated thermal fluids (Glotch & Bandfield, 2006;Milliken et al, 2008;Weitz et al, 2010;Wendt et al, 2011). These observations suggest that low-pH acid-sulphate weathering was a major alteration pathway at some point during Mars' geological history (Bibring et al, 2006;Ming et al, 2006;Zolotov & Mironenko, 2007). In contrast to Earth, where the carbon cycle is a major controlling factor in surface geochemistry, it seems that sulphur cycling processes dominated on Mars at least during part of its evolution (Gaillard et al, 2013).…”

“…The presence of sulphates at Meridiani Planum, Valles Marineris and Gusev crater suggest that aqueous alteration by low pH sulphate-rich fluids was involved Zolotov & Mironenko, 2007;Ming et al, 2008). Although none of the SNC (shergottite, nakhlite and chassigny) Martian meteorites are pervasively altered, minor amounts of sulphates and halogens like Cl and Br suggest that hydrothermal briny aqueous fluids have interacted with the host rocks and secondary phases formed afterwards upon the evaporation of these solutions (Bridges et al, 2001;Sutton et al, 2001;Greenwood, 2005).…”