Dk. Fisler et Rt. Cygan, CATION DIFFUSION IN CALCITE - DETERMINING CLOSURE TEMPERATURES AND THE THERMAL HISTORY FOR THE ALLAN-HILLS-84001 METEORITE, Meteoritics & planetary science, 33(4), 1998, pp. 785-789
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.