MICROTEXTURES FORMED DURING ACID-SULFATE WEATHERING OF BASALTIC TEPHRA: RESULTS OF A FIELD AND LABORATORY-BASED STUDY

The Mars Exploration Rover mission has provided a wealth of geochemical and petrographic data from the surface of Mars. The data collected suggests that acid-sulfate weathering may be a globally important process. While previously published work has documented the geochemical and mineralogical characteristics of an acid-sulfate environment in basaltic materials, in this study, we provide in-depth descriptions of the micro-textural changes that accompany these processes. This study compared acid-sulfate weathering of basaltic materials under field and laboratory conditions. Experimental results were compared with natural, acid sulfate weathered basalts collected from active fumaroles at Kilauea and extinct fumaroles on Mauna Kea, Hawaii. Results from the study showed that discernible differences exist between experimental treatments and natural samples, both in mineralogy and the micro-textures observed. Petrographic work allowed the differentiation between a vapor-dominated (fumarolic, phreatic-zone) alteration and alteration in a saturated, vadose-zone environment. For example, samples collected from altered tephra beds near the summit of Mauna Kea contained isopachous, platy alunite crystals with fine-grained coatings of jarosite and hematite, surrounding a clay-rich, altered tephra. Thin section studies to determine mineral and fabric paragenesis suggested that initial acidic alteration took place within a vadose (saturated) zone environment that precipitated isopachous alunite cement. This was followed by alteration within a phreatic (capillary fringe) environment that deposited jarosite and hematite as fine-grained coatings on alunite. These observations may be explained by alteration following an eruption of hot basaltic ash onto snow cover under previous glacial conditions on Mauna Kea, followed by downslope draining of a perched aquifer. Knowledge gained from our analog studies was applied to a preliminary analysis of MER-A Microscopic Imaging data for presumed acid-sulfate weathered outcrops in the Columbia Hills (Gusev Crater). Results suggest that these rocks represent alteration under both phreatic and vadose zone conditions.