Ultraviolet Photooxidation and O2 Chemical Oxidation of Fe2+ -Smectites and Implications for Mars

Abstract

Clay minerals detected in ancient Martian terrains help constrain Mars’ climate and aqueous alteration history. Since Mars’ primordial origin, atmospheric redox conditions have evolved from reducing to oxidizing and clay minerals may record the effects of that transition. Ferrous trioctahedral smectites of varying iron content were synthesized and subjected to oxidation by O2 and by UV irradiation to address these as potential oxidizing agents. UV irradiation (112.77 hours with 450 W Hg lamp) of smectites equivalent to approximately four years of flux on the Martian surface caused incomplete oxidation (Fe3+/ΣFe = 16-18%). O2 experiments (two hour, twelve hour, two day, and five day) produced more oxidation in smectites with higher Fe content at the same exposure times. Photooxidation caused octahedral sheet contraction; however, chemical oxidation allowed more contraction to occur in the high Fe smectites. The mid and high Fe smectites had observable changes in their visible-near infrared (VNIR) reflectance spectra with the formation of a nontronite (Fe3+, Mg)2-OH feature at 2.3 µm, even with partial oxidation. With both oxidation experiments, the reflectance spectra lost its initial MMM-OH features (AlAl(Fe2+,Mg) and Fe2+MgMg-OH) and produced a single nontronite-like MM-OH feature. UV irradiation produced a secondary nontronite phase, possibly on the surface of the higher Fe content smectites; however there was no evidence for iron ejection. Ferrous smectites are capable of undergoing UV photooxidation under aqueous conditions and this process could have occurred during early Martian history. Distinguishing between UV and O2 oxidation in smectites cannot be completed exclusively with Martian spectra; however, the lack of secondary oxides may hint at alteration history based on the nature of mineral assemblages detected on Mars.