Trace Element Mobility During Acid Aqueous Alteration of Martian Basalt

Abstract

It is widely believed that aqueous alteration on Mars has been strongly influenced by low pH conditions over a considerable fraction of Martian geological history. Previous experimental work demonstrated that these low pH environments result in the mobility of elements, such as Fe(III) and Al, that are relatively insoluble under most near surface conditions on Earth. Although these studies have increased our understanding of major element behavior, little is known about the mobility of trace elements under Martian conditions. A series of aqueous alteration experiments on synthesized Martian basalt, reported in this thesis, allows better understanding of the potential mobility of Ni, Zn, and Cr on the Martian surface. These elements were chosen because of their variability in rocks and soils analyzed on the Martian surface by the Mars Exploration Rovers. Experiments were performed on a starting basalt composition similar to the average composition of the Martian upper crust over a range of pH (0-4) and under varying degrees of crystallinity (glass and crystalline basalt). Similar experiments were also performed on natural chromites to assess their reactivity and the role this mineral plays in the mobility of Cr. In the basaltic glass experiments Ni was found to be the most mobile element, followed by Cr and then Zn. The mobility of cations in crystalline basalt is dependent on their mineralogical setting. Consequently, Ni was also found to be the most mobile element in the crystalline basalt due to its abundance in olivine, the most susceptible mineral present. During the chromite alteration experiments, chromite was found to dissolve non-stoichiometrically. The reactivity of basalt and chromite with various acidic solutions is such that they could make an important contribution of Ni, Zn and Cr ions to aqueous solutions on the Martian surface. However, the liberation of these elements appears to depend on their coordination within the structure of the material. A cation in a network modifying site will be released into solution more readily than one in a network forming site. This may be influenced by the presence of other cations and their availability to charge balance or compete for that site. Once mobile in Martian solutions, Ni, Zn and Cr may have been available for incorporation into secondary mineral phases, such as sulfates, oxides, chlorides and clays, or adsorption onto their surfaces.