Growth mechanism and additional constraints on FeNi metal condensation in the solar nebula
Chemically zoned FeNi metal grains in the metal-rich chondrites QUE 94411 and Hammadah al Hamra 237 formed by gas-solid condensation in the temperature range from similar to1500 to 1400 K during highly energetic thermal events in the solar nebula. We observe a linear correlation between the apparent diameter of the zoned FeNi metal grains and their inferred condensation temperature interval, which indicates that the grain growth rate was essentially constant. This lends strong support for a kinetic "hit-and-stick" growth model that yields growth timescales of similar to20-85 hours and gas cooling rates of similar to1-2 K h(-1) for six representative zoned metal grains studied in QUE 94411. In the core regions of the zoned metal grains the Ni concentration is systematically lower than the thermodynamically predicted values, suggesting that solid-state diffusion played an important role in shaping the zoning profiles. Combined with existing data, our observations provide a set of constraints on the physics and chemistry of large-scale, high-temperature processes in the earliest solar nebula, which present astrophysicists with profound challenges.