Computer Science
Scientific paper
Jul 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998m%26ps...33..785f&link_type=abstract
Meteoritics & Planetary Science, vol. 33, no. 4, pp. 785-789.
Computer Science
5
Scientific paper
The presence of zoned iron, magnesium, calcium, and manganese in the carbonate phases associated with the cracks and inclusions of the ALH 84001 meteorite provides evidence for constraining the thermal history of the meteorite. Using self- and tracer diffusion coefficients obtained from laboratory experiments on natural calcite, cooling rates are calculated for various temperatures and diffusion distances to assist in the evaluation of the compositional zoning associated with the carbonate phases in ALH 84001. The closure temperature model provides the average temperature below which compositional zoning will be preserved for a given cooling rate, that is, the temperature at which diffusion will be ineffective in homogenizing the phase. The validity of various theories for the formation of the carbonate globules may therefore be examined in view of the diffusion-limited kinetic constraints. Experiments using a thin film-mineral diffusion couple and ion microprobe for depth profiling analysis were performed for the temperature range of 550-800 C to determine self- and tracer diffusion coefficients for calcium and magnesium and in calcite. The resulting activation energies for calcium (Ea(Ca) = 271 ( 80 kJ/mol) and for magnesium (Ea(Mg) = 284 ( 74 kJ/mol) were then used to calculate a series of cooling rate, grain size, and closure temperature curves. The data indicate, for example, that by the diffusion of magnesium in calcite, a 10 mm compositional zone would be completely homogenized at a temperature of 300 C for cooling rates less than 100 K/ma. These data provide no constraint on formation models that propose a low temperature fluid precipitation mechanism, however, they indicate that the carbonate globules were not exposed to a high temperature environment for long time scales following formation.
Cygan Randall T.
Fisler Diana K.
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