ACID-SULFATE WEATHERING OF BASALTIC MATERIALS: A LABORATORY AND FIELD-BASED STUDY WITH IMPLICATIONS FOR MARTIAN WEATHERING ENVIRONMENTS

Previous landed missions have shown acid-sulfate weathering to be a widespread and important process on Mars. While previous studies have documented the general geochemical and mineralogical changes resulting from acid-sulfate weathering of basaltic materials, in this study, we provide detailed information about the microtextural changes that accompany these processes. Our study compared acid-sulfate weathering of natural and simulated basaltic materials under field and laboratory conditions. Laboratory studies utilized both basaltic tephras (JSC Mars-1 and a haloclastite from Iceland) and picritic basalts (1868 and 1859 flows, Hawaii). Experiments were carried out under different acid strengths (pH range ~0-4) and over a range of water-rock ratios (2:1, 1:1, 1:2). Experimental results were compared with naturally weathered samples collected from acid-sulfate environments at Kilauea and Mauna Kea volcanoes, Hawaii. Mineralogical and textural characterization of samples included microscopic (reflected light) and petrographic (transmitted light) thin section studies, as well as X-ray powder diffraction, SEM-EDS and infrared spectral analyses. For the JSC Mars-1 and in line with previous studies, the most pervasive sulfates produced at all acid strengths and rock-water ratios samples were gypsum and alunogen. The Icelandic samples differed in containing alunite in the place of alunogen. Each of the experimental treatments was further distinguished by the relative abundances of accessory and trace minerals produced (e.g. jarosite, sulfides, and elemental sulfur). Textural studies revealed systematic differences in mineral paragenesis, with a progressive increase in the dominance of fibro-radial cements. There was also an observed increase in the thickness of duricrusts, with decreases in pH (acid strength) of solutions and increases in water-rock ratio. Textures observed in natural and experimental samples were compared with microscopic images and inferred compositions of crusted soils, outcrop and basalt rock weathering rinds at the two MER landing sites on Mars.