1999 GSA Annual Meeting -- Denver, Colorado
1999 GSA Annual Meeting -- Denver, Colorado
Author(s): BARKER, William W., barker@geology.wisc.edu; and BANFIELD, Jillian F., Dept. of Geology and Geophysics, UW-Madison, Weeks Hall, 1215 West Dayton St., Madison, WI 53706
Keywords: pyroxene, weathering, smectite, lithobiontic, Mars
We examined Ar ion milled thin sections of ALH84001 for evidence of biogeochemical weathering of pyroxene using analytical HRTEM. Specifically, we sought evidence for low temperature, hydrous alteration of the silicates and for relationships between primary and secondary minerals that would be consistent with a set of criteria for microbial-mineral interactions developed based on observations of terrestrial samples. For example, lithobiontic communities impact silicate mineral weathering in two ways; either directly by close approach and attachment of cells via extracellular polymers where space permits, or indirectly through acceleration of reactions by soluble, low molecular weight organic compounds. We have shown previously that the mineralogy and textures developed during low temperature reactions of chain silicate weathering differ based on the proximity of cells to a reacting mineral surface. In cases of cell attachment, the reactant phase dissolves and secondary minerals nucleate within extracellular polymers, resulting in complex mixtures of organic polymers, clay minerals, and secondary oxyhydroxides that coat all mineral and cell surfaces. In areas where insufficient space precludes direct colonization, topotactic smectite forms as the primary mineral reacts. We characterized three distinct areas within ALH 84001. Cracks within pyroxene and pyroxene surfaces at the carbonate globule/pyroxene interface and the oxide/sulfide assemblage were examined for evidence of clay mineral formation and direct or indirect microbial activity. To date, we have found no clay minerals within cracks in pyroxene at distance from carbonate globules, either in crushed zones or single crystals of pyroxene. Likewise, no clays have been detected within the oxide/sulfide mineral assemblage. Very minor amounts of topotactic nontronitic smectite occurs within ALH84001 at the carbonate/pyroxene interface, often spatially associated with late-stage carbonate dissolution textures and fractures filled with an as yet unidentified nanocrystalline Mg mineral assemblage. While this clay does prove low temperature hydration reactions within the meteorite, the textural relationships are analogous to inorganic reactions or indirect biological effects. The limited clay development does not support a protracted mineral-water interaction history on Mars, and may be consistent with very slight terrestrial weathering of pyroxene.
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