Thermodynamic and mass balance constraints on iron-bearing phyllosilicate formation and alteration pathways on early Mars

Widespread occurrences of phyllosilicates, predominantly nontronite and Mg saponite, in the Noachian‐aged crust of Mars place constraints on water availability and surface conditions. However, the Noachian atmosphere likely lacked oxidants capable of forming nontronite from the basaltic crust and it...

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Bibliographic Details
Published in:Journal of geophysical research. Planets 2013-10, Vol.118 (10), p.2124-2136
Main Author: Catalano, Jeffrey G.
Format: Article
Language:English
Subjects:
Atmosphere
Cosmochemistry. Extraterrestrial geology
Earth sciences
Earth, ocean, space
Exact sciences and technology
Extraterrestrial geology
Formations
Geochemistry
Iron
Magnesium
Mars
Mars surface
nontronite
Oxidation
phyllosilicates
Saponite
Thermodynamics
Weathering
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Summary:Widespread occurrences of phyllosilicates, predominantly nontronite and Mg saponite, in the Noachian‐aged crust of Mars place constraints on water availability and surface conditions. However, the Noachian atmosphere likely lacked oxidants capable of forming nontronite from the basaltic crust and its origins are thus uncertain. In this study, scenarios for the formation of iron‐bearing phyllosilicates from the weathering and hydrothermal alteration of Martian basalt were investigated using geochemical modeling to establish plausible pathways of nontronite generation and the effect of redox conditions on associated secondary phases. Hydrothermal alteration produced FeII and Mg saponite, serpentine, and zeolites at 100°C and chlorite, prehnite, and talc at 250°C, consistent with phases observed in crater deposits on Mars. Anoxic weathering produced primarily FeII and Mg saponite. High water‐rock ratios produced Al‐clays and amorphous silica and high pCO2 conditions inhibited smectite formation because of carbonate precipitation. Oxic weathering produced primarily nontronite and Mg saponite. Later oxidation of anoxic weathering products also formed nontronite‐Mg saponite assemblages; oxidation products of hydrothermal assemblages included nontronite, Mg saponite, and either Al smectites or residual zeolites or prehnite. These calculations demonstrate that there are multiple plausible routes to form Fe/Mg smectites on early Mars and that an oxidizing atmosphere need not have existed in the Noachian. Decoupling iron oxidation from initial clay formation raises the possibilities that Mars transitioned to conditions favoring widespread iron oxidation after phyllosilicate formation ceased and that residual, unoxidized ferrous iron phyllosilicates still exist today in the Martian subsurface. Key Points Multiple geochemically plausible routes exist for forming nontronite on Mars Iron oxidation on Mars may have occurred later than phyllosilicate formation Early Mars need not have had an oxidizing atmosphere
ISSN:2169-9097
2169-9100
DOI:10.1002/jgre.20161