Rates and mechanisms of chemical weathering of ferromagnesian silicate minerals on Mars

Abstract

Ferric-bearing assemblages on Mars indicate that oxidative weathering of surface basalts has occurred during the evolution of the Red Planet. In aqueous environments chemical weathering proceeded through stages of dissolution of Fe 2+-bearing silicate and sulfide minerals, oxidation of dissolved Fe 2+ to Fe 3+ ions, and hydrolysis of dissolved Fe 3+ to insoluble ferric-bearing oxide, oxyhydroxide, and hydroxysulphate phases. To determine when these ferrolysis reactions occurred on Mars and to estimate rates of chemical weathering of minerals in Martian surface rocks, experimental data for terrestrial olivines and pyroxenes with compositions resembling assemblages in SNC meteorites are reviewed. Dissolution of ferromagnesian silicates is generally stoichiometric under anoxic conditions. Nonstoichiometric dissolution of pyroxenes may occur initially, both as a result of preferential release of cations in the M2 sites and armoring of crystal surfaces by protective layers of insoluble ferric oxides when dissolution occurs under oxidizing conditions. Dissolution rates of Fe 2+ ions decrease in the order: olivine ≥ pyroxene M2 > pyroxene M1. Dissolution rates of these minerals are highest in acidic solutions, decrease with rising pH and at low temperatures, are least at near-neutral pH, and rise again when pH is in the alkaline range. Since low temperatures currently exist on Mars, dissolution rates of basaltic minerals are probably stoichiometric and extremely slow on the present-day Martian surface, but may have been much faster in the past, especially if acidic groundwater and a more temperate climate prevailed. Ferrous iron released during chemical weathering was metastable with respect to ferric iron when oxygenated groundwater on Mars exceeded pH 3.5. Rates of oxidation of dissolved Fe 2+ ions are strongly pH dependent; they are very slow in acidic solutions, in contrast to mineral dissolution rates. Rates of oxidation also decrease with increasing ionic strength and at low temperatures. However, in saline solutions, rates of oxidation eventually increase with rising ionic strength, including brines near eutectic temperatures that have been proposed on Mars. Rates of oxidation of Fe 2+ dissolved in slightly acidic brines at −25°C, for example, are estimated to be about 10 5 times slower than those of Fe 2+ occurring in terrestrial river water and bottom ocean water. Ferroan saponites precipitated from cold acidic saline solutions containing dissolved Mg 2+, Fe 2+, and silica are unstable when exposed to oxygen as a result of intracrystalline redox reactions, such as (Fe 2+ + OH −) clay + 1 4 O 2 = (Fe 3+ + O −2) clay + 1 2 H 2O . The partially dehydroxylated mixed-smectite plus interlayer ferric oxide assemblages that formed during exposure to the Martian atmosphere may account for difficulties in identifying clay silicates in remote-sensed reflectance spectral measurements of Mars due to loss of identifiable hydroxyl groups. Ferrolysis of dissolved Fe 2+ ultimately yielded hydrous ferric oxide deposits on Mars, the formation of which was influenced by pH, temperature, ionic strength, ion-pair formation, and the rate of oxidation in Martian groundwater. Deposition rates of hydrous ferric oxides into episodic ocean basins that occurred in the past on Mars were comparable to those that produced terrestrial Precambrian iron-formations. On present-day Mars, dissolved Fe 2+ ions may persist indefinitely, particularly in frozen permafrost. However, sublimation and evaporation of day-time equatorial melt-waters may cause localized oxidation of dissolved Fe 2+ ions, leading to the precipitation of nanophase ferric-bearing oxides, hydroxysulfate and partially dehydroxylated clay silicate assemblages that litter bright regions on the surface of Mars.