Experimental epithermal alteration of synthetic Los Angeles meteorite: Implications for the origin of Martian soils and identification of hydrothermal sites on Mars

摘要

The dissolution behavior and secondary minerals generated by experimental alteration of a synthetic Los Angeles basaltic shergottite (LA) were evaluated at epithermal (75°C) conditions. Starting pH values of 1.1 and 3.6 and water-rock ratios ranging from 100 to 2000 were studied using flow-through and batch reactor experimental apparatus. Dissolution of plagioclase dominates effluent solution chemistry under all experimental conditions. Little Mg is released to solution because nearly 100% of the Mg in unaltered LA is contained in pyroxene M1 sites, which are relatively resistant to dissolution. Secondary minerals generated include siliceous residue, amorphous SiO2, lepidocrocite and ferrihydrite, anhydrite, alunogen, rhomboclase, ferrinatrite, halite, and unidentified Ca and Na sulfates. The dominance of alteration products of plagioclase suggests that LA-type basalts have not contributed significantly to the Martian soils. The search for hydrothermal sites by remote sensing may be hampered by the general similarity of the alteration products found in the near-surface epithermal environment to those thought to exist in Martian soils (sulfates, iron oxyhydroxides, amorphous silica). However, the unique chemical composition of secondary products formed by LA alteration may make detection of recent hydrothermal alteration of evolved basaltic rocks more straightforward. In addition, alteration in terrestrial epithermal environments associated with mafic to intermediate volcanic rocks generally produces abundant sulfides and kaolinite, two potentially unique mineral phases which might also indicate the presence of active or extinct hydrothermal systems.