Growth kinetics of FeS melt in partially molten peridotite: An analog for core-forming processes

The growth kinetics of molten FeS pools in partially molten peridotite were investigated by time-series experiments in a piston-cylinder apparatus. The starting materials were mixed powders of peridotite + FeS, with FeS = 6%, 12% and 18% by volume in order to characterize the effect of volume fraction on the growth laws of FeS. The initial particle size of FeS was about 3.5 μm. The samples were annealed at temperatures between 1573 and 1723 K at 1.5 GPa for durations ranging from a few seconds to 100 h. The size of FeS pools was determined by analysis of backscattered electron images. The increase of pool size (G) of FeS with time (t) follows a growth law: Gn − G0n = k · t (k = k0 exp(− Q / RT)). Samples with higher FeS volume fraction have larger pool size at the same conditions. The growth exponent (n) at 1573 K strongly depends on initial volume fraction of FeS and varies between ∼2.6 and ∼6.4, whereas those at 1723 K are almost constant (∼2.3) irrespective of the initial volume fraction. The growth exponent (n) tends to decrease with increasing temperature and volume fraction of silicate melt for each run series of different initial volume fraction of FeS. Low volume fractions of FeS and silicate melt leads to sluggish growth of the pools due to pinning of the silicate mineral phases. The activation enthalpy for pool growth is 331 ± 40 kJ/mol based on the results from samples with 18 vol.% FeS, which show the smallest variation of growth exponent over a range of temperature. These FeS coarsening experiments may serve as tentative analogs for the behavior of a liquid metal phase in hot proto-planetary objects. Assuming exponential heating of such bodies in the early solar system—and allowing for a significant Zener pinning effect of Fe pools—the time spent above the silicate solidus may be insufficient to grow the pools beyond the size where diffusive equilibration with the silicate surroundings can be maintained: in other words, diffusive equilibration may assured because of slow coarsening kinetics.