CATION DIFFUSION IN CALCITE - DETERMINING CLOSURE TEMPERATURES AND THE THERMAL HISTORY FOR THE ALLAN-HILLS-84001 METEORITE

The presence of zoned Fe, Mg, Ca, and Mn in the carbonate phases assoc iated with the cracks and inclusions of the Allan Hills (ALH) 84001 me teorite 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 calculat ed for various temperatures and diffusion distances to assist in the e valuation of the compositional zoning associated with the carbonate ph ases in ALH 84001. The closure temperature model provides the average temperature below which compositional zoning will be preserved for a g iven cooling rate, that is, the temperature at which diffusion will be ineffective in homogenizing the phase. The validity of various theori es for the formation of the carbonate globules may be examined, theref ore, in view of the diffusion-limited kinetic constraints. Experiments using a thin film-mineral diffusion couple and ion microprobe for dep th profiling analysis were performed for the temperature range of 550- 800 degrees C to determine self- and tracer-diffusion coefficients for Ca and Mg and in calcite. The resulting activation energies for Ca (E -a(Ca) = 271 +/- 80 kJ/mol) and for Mg (E-a(Mg) = 284 +/- 74 kJ/mol) w ere used then to calculate a series of cooling rate, grain size, and c losure temperature curves. The data indicate, for example, that by the diffusion of Mg in calcite, a 10 mu m compositional zone would be com pletely homogenized at a temperature of 300 degrees C for cooling rate s <100 K/Ma. These data provide no constraint on formation models that propose a low-temperature fluid precipitation mechanism; however, the y indicate that the carbonate globules were not exposed to a high-temp erature environment for long time scales following formation.