Grain growth in forsterite-nickel mixtures: Analogues of small parent bodies during early accretion

We report an experimental study on the grain growth kinetics and microstructural evolution of olivine-metal systems. 32 experiments have been performed on four different mixtures of forsterite (Fo) and nickel (Ni) (respectively in vol.%, 95:5, 80:20,30:70 and 10:90). All experiments were carried out at high temperature (1440 degrees C), at oxygen fugacity (fO(2)) similar to 3 log units below the Ni-NiO buffer and for durations between 2 h and 21 days. In aggregates containing isolated forsterite grains in a nickel matrix (Fo + Ni (10:90)) forsterite did not grow, and grain growth of nickel was abnormal. In all other cases, features of normal grain growth were observed and the data were fit to the classic grain growth law d(n) - d(0)(n) = kt. For all these aggregates, grain growth exponents are in the range 4.0 < n < 5.7 and are not correlated with the phase proportion in the mixture. On the other hand, grain growth of forsterite is slower and that of nickel faster as the proportion of nickel increases. Details of the normalised grain size distributions (NGSD) are used to infer that grain growth of forsterite occurred by grain boundary migration rate-limited by nickel grains. For nickel, two types of behaviour were observed. In aggregates containing 5 or 20 vol.% nickel, grain growth occurred by coalescence strongly rate-limited by grain boundary migration of forsterite. In nickel-rich aggregates (Fo + Ni (70:30)), grain growth occurred by grain boundary migration. Comparison with studies from the literature shows that our experimental data are similar to those of Nichols and Mackwell (1991) for porous olivine aggregates, both Ni and pores apparently behaving as mobile minor phases. Extrapolation to time-scales relevant to the formation of ordinary chondrite parent bodies shows that mean grain sizes reached (Fo and Ni) are of the same order of magnitude (100 mu m to 1 mm) as those observed in several types of meteorites. Iron rich phases (kama-cite-taenite and/or troilite) in several H-chondrites are shown to present normal grain growth features and NGSD shapes similar to those in our experimental samples where nickel grains are not interconnected, i.e., Fo:Ni (95:5) and (80:20). The grain growth mechanism for the iron-rich phases in these meteorites may thus also occur by coalescence rate-limited by grain boundary migration of olivine. (C) 2012 Elsevier B.V. All rights reserved.