Kinetic condensation of metals in the early solar system: Unveiling the cooling history of solar nebula by refractory metal nuggets

Abstract Refractory Metal Nuggets (RMNs; submicrometer highly siderophile element rich metal alloys) are observed in Ca, Al-rich inclusions (CAIs) and other components of primitive meteorites, and some RMNs could have condensed from the Solar Nebula. In order to study the condensation of RMNs in the Solar Nebula, NUCON – a kinetic condensation model – has been developed to simulate the nucleation and condensation of refractory siderophile metal phases. NUCON treats RMNs as solid solutions where multiple elements can accrete onto one RMN. To achieve this goal, the homogeneous nucleation theory is modified to compute the nucleation of solid solutions. Also, a numerical method is developed to compute the integration of condensation and evaporation rates of an RMN. Equilibrium among gaseous phases is also considered, including monatomic gases and oxides. The oxygen fugacity of the simulated Solar Nebula can also be modified by adjusting carbon abundance. NUCON shows that the nucleation of RMNs was inhibited even when the cooling rate of the Solar Nebula was below 0.1 K/year, and RMNs experienced kinetic condensation largely deviated from the equilibrium condensation. This study modeled the condensation of RMNs in the RMN-forming regions with different cooling rates, total pressures, and oxygen fugacities, and explored how these parameters affect the radii and Ni/Fe ratios of RMNs. To reproduce the RMNs reported in literature, most of which have radii from 100 to 1000 nm, the cooling rate during the accretion of refractory siderophile metals in RMN-forming regions should be in the order of 1 K/year. The timescale of refractory-metal condensation is in the order of 102 years. In addition, RMNs have been measured to have Ni/Fe ratios from almost zero to over unity, and NUCON shows that the cooling rate during Fe Ni accretion in RMN-forming regions should be in the order of 10 K/h so that the observed Ni/Fe ratios of RMNs can be reproduced. The timescale of Fe Ni condensation is in the order of 10 h. Thus, NUCON predicts a transition from slow cooling to rapid cooling that is likely to have occurred during RMN condensation.