Transportation of gaseous elements and isotopes in a thermally evolving chondritic planetesimal

Abstract— The behavior of H, C, N and their isotopes in a thermally evolving planetesimal was evaluated by numerical simulation. Transportation of heat and gas molecules, and the chemical equilibrium involving these elements, were simulated. Our modeled planetesimals initially contain homogeneous amounts of radioactive heat source (²⁶Al); and H, C, and N in forms of organic materials, graphite, and in some models, water ice. Vaporized gas molecules were transported from the interior of the planetesimal to its surface, although their transportation efficiencies were quite different among the three elements, primarily due to differences in their affinities to metallic Fe. Significant portions of these elements were redistributed into metallic Fe when the planetesimal was heated at 600 °C and above. Nitrogen showed the most prominent siderophile characteristics, resulting in fairly large concentrations of N trapped in metallic Fe, which is consistent with observations by Hashizume and Sugiura (1997). Efficiency of C transportation crucially depended on O fugacity. To realize effective C transportation, it was necessary to assume an oxidizing condition (log fO₂ > log fO_2,(FiF) + 1) in the initially accreted material. Water vapor, generated at the interior of the planetesimal and transported to its near surface, formed a water-rich layer under certain conditions, providing an environment sufficient for aqueous alteration of chondritic materials to occur. Variations in isotopic ratios of N in taenite observed among equilibrated ordinary chondrites can be explained by our gas transportation model. It is required, however, that carriers of isotopically anomalous N, perhaps presolar grains, were initially localized on a large spatial scale within a single planetesimal, which possibly suggests incorporation of preaccretionary objects as large as 0.1 × of the final mass of the ordinary chondrite parent body.