Abstract
The ion microprobe has been used to analyse olivine in seven pallasites for Ni. The results are in the range 22–41 μg/g and are significantly lower than published electron probe analyses. The olivine-metal equilibrium temperatures deduced from thermodynamic calculations are correspondingly lower, and it is likely that equilibrium between olivine and metal was maintained to somewhat below the temperature of the γ-α transformation in metal (∼700°C). The results appear to be consistent with cooling rates of ∼1K/Ma derived from metal phase compositions, and certainly more so than the previous electron probe data. A Ni profile across an olivine grain in Eagle Station shows a decrease from 40 μg/g at the centre to 25 μg/g at the edge, illustrating the effect of limited diffusion of Ni in olivine during cooling. A correlation between grain size and central Ni content was also observed in Eagle Station, the smallest grains having the lowest Ni content.
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