Experimental study of chemical coupling between reduction and volatilization in olivine single crystals

The high-temperature (T = 1623 K) transformations of olivine single crystal s 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 reac tion layer composed of Fe-Ni precipitates and iron-depleted olivine, which developed between the surface and an inner reaction front. Composition prof iles of Mg, Si, Fe, and O (direct measurement, no assumption for stoichiome try) have been determined in the reaction layer, which is characterized by oxygen and silicon losses, constant Si/O ratio, and constant total Fe conte nt. A detailed study of the reaction layer has shown that it contains indee d two reaction fronts (inner and outer) corresponding, respectively, to the beginning of metal precipitation and to the total depletion of Fe2+ from o livine. The propagation of the two reaction fronts located at position I as a funct ion of run duration, t, follows a parabolic law: l = root k . t The two rat e constants, k, are equal to 5.0 +/- 0.5 x 10(-15) m(2) s(-1) and 3.1 +/- 0 .1 x 10(-14) m(2) s(-1) for the inner and outer fronts, respectively. A num erical modelling in finite differences is proposed for testing the paramete rs involved in the propagation of the reaction fronts. Propagation of chemi cal fronts appears mainly rate limited by the interdiffusion of iron/nickel and magnesium in the olivine lattice. Transport and volatilization of oxyg en and silicon from the interior to the surface are not rate limiting and p robably involve defects allowing fast effective diffusion for these element s. This coupled reduction/volatilization reaction has the potential of fractio nating (Fe/Si) and (Mg/Si) ratios in olivines processed at high temperature s under reducing conditions. We have evidenced that chemical and microstruc tural observations made in "dusty olivines" from chondrules in unequilibrat ed type 3 chondrites could be explained by the reactions described in this study. Coupling this result with the kinetic data suggests that "dusty oliv ines" could have been generated by transitory thermal events with cumulativ e durations of a few hours. Copyright (C) 2000 Elsevier Science Ltd.