2000 GSA Annual Meeting -- Reno, Nevada
2000 GSA Annual Meeting -- Reno, Nevada
Author(s): BREARLEY, Adrian J., Dept. of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, brearley@unm.edu
Keywords: ALH84001, clays, TEM, martian, glass
A key component of assessing the hypothesis of fossil life in ALH84001 involves establishing whether water was available to support life during a very early period of martian history. However, mineralogical evidence of water/rock interactions in ALH 84001 is largely absent and where present may be of ambiguous origin because of the possibility of Antarctic weathering. To minimize such ambiguity we have carried out a TEM study of carbonate fragments in ALH 84001 that have been disrupted and entrained within feldspathic shock glass. The fragments occur as 10-50 micron angular grains embedded within the glass. A number of fragments contain variable amounts of phyllosilicates in different occurrences. These include small (<120 nm), irregularly-shaped pockets of crystallites (<10 nm in width), isolated grains within granulated zones and a phyllosilicate vein that crosscuts a carbonate fragment. In all these cases, the phyllosilicates are extremely fine-grained (<20 nm wide parallel to the c-axis) and have typical basal spacings of ~ 1.0 nm consistent with a smectite clay. Several lines of evidence constrain these low temperature clays to be of martian origin. First, the phyllosilicates occur in carbonate fragments that are completely entrained within feldspathic glass and would be isolated from terrestrial fluids. Second, the phyllosilicates occur exclusively in the carbonates and are always truncated abruptly at the interface between the carbonate and the glass. Third, if phyllosilicate formation had occurred within a terrestrial environment then evidence of alteration of the glass would be expected. Finally, dehydration of phyllosilicates at the interface between the glass and the carbonate fragments is locally present. In these regions, the phyllosilicates have a curved, often undulatory morphology and some crystallites appear to be amorphous, evidence that the grains may have decomposed or begun to decompose as a result of heating caused by entrainment within the feldspathic glass. Collectively, these observations provide strong evidence that formation of the phyllosilicates must have occurred before or possibly during the entrainment of the carbonate within the feldspathic glass and hence are almost certainly martian in origin.
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