Biology
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p31g..05m&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P31G-05
Biology
[3617] Mineralogy And Petrology / Alteration And Weathering Processes, [5220] Planetary Sciences: Astrobiology / Hydrothermal Systems And Weathering On Other Planets, [6225] Planetary Sciences: Solar System Objects / Mars, [8424] Volcanology / Hydrothermal Systems
Scientific paper
Recent observations made at Mars by orbiting spacecraft and landers have documented the widespread occurrence of sulfate-rich rocks across the planet. Although the settings and origins of these rocks are likely to be variable, at least some appear to have originated from hydrothermal alteration of volcanic rocks. In order to place constraints on the chemical and mineralogical processes that might give rise to sulfate-rich rocks in hydrothermal settings on Mars, we are studying of acid-sulfate alteration of basalt in fumarolic environments using a combination of field work, experiments, and numerical modeling. Examination of acid-sulfate altered basaltic cinders from active fumaroles at Cerro Negro volcano in Nicaragua indicates that the initial stage of alteration results predominantly in the formation of amorphous silica, gypsum, and natroalunite/natrojarosite, with minor amounts of iron oxides/oxyhydroxides. Laboratory experimental alteration of basalt cinders by sulfuric acid at 145 °C generated a similar suite of minerals, with amorphous Si-rich gel, anhydrite, and Fe-bearing natroalunite as the primary products, along with minor amounts of Fe oxides/oxyhydroxides and magnesium sulfates. Crystalline silicates, including clay minerals, are not observed in either the field samples or experimental products. During the initial stage of alteration in both field and laboratory samples, igneous phenocrysts decompose rapidly while the glass remains intact. Over time, cations inlcuding Ca, Mg, Fe, Al, and Na are progressively leached from the glass in the field samples, while the glass largely maintains its original morphology despite being composed predominantly of SiO2. Evaluation of the laboratory and field results with numerical geochemical models indicates that formation of the observed alteration products requires that (a) igneous silicate phenocrysts (plagioclase, augite, olivine) react much faster than basaltic glass and (b) there are kinetic inhibitions to the precipitation of Si- and Al- bearing phyllosilicates, as well as hematite and goethite. Using observations from the terrestrial system as constraints, numerical models based on martian basalt compositions indicate that the initial stages of acid-sulfate alteration of martian pyroclastic deposits in hydrothermal environments should result in a suite of alteration minerals dominated by amorphous silica, anhydrite or gypsum (depending on temperature), Fe-bearing natroalunite and kieserite, along with minor amounts of iron oxides/oxyhydroxides and relict glass.
Hynek Brian Michael
McCollom Thomas M.
Rogers Karyn L.
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