TRANSPORTATION OF GASEOUS ELEMENTS AND ISOTOPES IN A THERMALLY EVOLVING CHONDRITIC PLANETESIMAL

The behavior of H, C, N and their isotopes in a thermally evolving pla netesimal was evaluated by numerical simulation. Transportation of hea t and gas molecules, and the chemical equilibrium involving these elem ents, were simulated. Our modeled planetesimals initially contain homo geneous amounts of radioactive heat source (Al-26); 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 plan etesimal to its surface, although their transportation efficiencies we re quite different among the three elements, primarily due to differen ces in their affinities to metallic Fe. Significant portions of these elements were redistributed into metallic Fe when the planetesimal was heated at 600 degrees 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 cruciall y depended on O fugacity. To realize effective C transportation, it wa s necessary to assume an oxidizing condition (log fO(2) > log fO(2,(FI F)) + 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 envi ronment sufficient for aqueous alteration of chondritic materials to o ccur. Variations in isotopic ratios of N in taenite observed among equ ilibrated ordinary chondrites can be explained by our gas transportati on model. It is required, however, that carriers of isotopically anoma lous N, perhaps presolar grains, were initially localized on a large s patial scale within a single planetesimal which possibly suggests inco rporation of preaccretionary objects as large as 0.1x of the final mas s of the ordinary chondrite parent body.