2000 GSA Annual Meeting -- Reno, Nevada
2000 GSA Annual Meeting -- Reno, Nevada
Author(s): CORRIGAN, Catherine M., cmc19@po.cwru.edu, and HARVEY, Ralph P., Dept. of Geological Sciences, Case Western Reserve University, 112 AW Smith, Cleveland, OH 44106-7216; BRADLEY, John, MVA, Inc., 5500 Oakbrook Parkway #200, Norcross, GA, 30093.
Keywords: meteorites Mars ALH84001 alteration phyllosilicates
The absence of macroscopic phyllosilicate minerals in Allan Hills (ALH) 84001 has been cited as an indicator of dry conditions on Mars. Phyllosilicates are, however, present in this meteorite on the nanometer scale. Reports have been made of long, platy and ribbon-like, K-bearing phyllosilicates intergrown with ALH 84001 carbonate and aluminous and MgO-rich but K-poor, very fine-grained phyllosilicates residing in pockets and veins within feldspathic glass. These phyllosilicates are likely phlogopitic micas. Smectite-type clays have been reported, located on the boundaries of carbonate and orthopyroxene and on surfaces of the orthopyroxene not associated with the carbonate. Some of these occurrences are apparently relatively large, up to 500 nm in size. Phyllosilicate minerals have recently been discovered within small (~250 nm) euhedral cavities in orthopyroxene and filling larger (200-800 nm wide) bifurcating veins in the orthopyroxene. The mineralogy of these vein-filling phyllosilicates is possibly smectitic based on analogy to other reports; however, neither can be considered diagnostic as both were based on fairly ubiquitous 10 nm lattice spacing. Given the prevalence of weathering in Antarctic meteorites, the textural context of these phyllosilicates becomes very important. Those phyllosilicates whose origin is most convincingly extraterrestrial are those that are entirely enclosed within feldspathic glass and pyroxene. It has been suggested that the phyllosilicates in feldspar are the result of shock-induced reactions between pre-existing hydrous phases and the parent minerals in the igneous rock. The authors are not speculating here on the origin of the phyllosilicates in orthopyroxene except to note that their textural occurrence is most compatible with an origin as a precipitate. At the current time no convincing structural or compositional distinction between presumed 'martian' and possibly terrestrial phyllosilicates has been put forward. Our current goals are to more clearly identify the type of phyllosilicates present in each of the settings discovered so far to seek petrogenetic clues to their origin.
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