Abstract:
The high-temperature (T = 1623 K) transformations of olivine single crystals in contact with graphite powder have been studied on time scales of hours to days by scanning electron microscopy, transmission electron microscopy, and electron microprobe. Microstructures have been characterized in a reaction layer composed of Fe-Ni precipitates and iron-depleted olivine, which developed between the surface and an inner reaction front. Composition profiles of Mg, Si, Fe, and O (direct measurement, no assumption for stoichiometry) have been determined in the reaction layer, which is characterized by oxygen and silicon losses, constant Si/O ratio, and constant total Fe content. A detailed study of the reaction layer has shown that it contains indeed two reaction fronts (inner and outer) corresponding, respectively, to the beginning of metal precipitation and to the total depletion of Fe2+ from olivine.The propagation of the two reaction fronts located at position l as a function of run duration, t, follows a parabolic law: l = k.t. The two rate constants, k, are equal to 5.0 +/- 0.5 x 10-15 m2 s-1 and 3.1 +/- 0.1 x 10-14 m2 s-1 for the inner and outer fronts, respectively. A numerical modelling in finite differences is proposed for testing the parameters involved in the propagation of the reaction fronts. Propagation of chemical fronts appears mainly rate limited by the interdiffusion of iron/nickel and magnesium in the olivine lattice. Transport and volatilization of oxygen and silicon from the interior to the surface are not rate limiting and probably involve defects allowing fast effective diffusion for these elements.This coupled reduction/volatilization reaction has the potential of fractionating (Fe/Si) and (Mg/Si) ratios in olivines processed at high temperatures under reducing conditions. We have evidenced that chemical and microstructural observations made in ''dusty olivines'' from chondrules in unequilibrated type 3 chondrites could be explained by the reactions described in this study. Coupling this result with the kinetic data suggests that ''dusty olivines'' could have been generated by transitory thermal events with cumulative durations of a few hours.