Physics
Scientific paper
Sep 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998m%26ps...33.1181h&link_type=abstract
Meteoritics & Planetary Science, vol. 33, no. 5, pp. 1181-1195
Physics
3
Scientific paper
Behaviors of H, C, N and its isotopes in a thermally evolving planetesimal were evaluated by numeric 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, though, their transportation efficiencies were quite different among the three elements, primarily due to differences in their affinities to metallic iron. Significant portions of these element were redistributed into metallic iron when the planetesimal was heated at 600oC and above. Nitrogen showed the most prominent siderophile characteristics, resulting in fairly large concentrations of N trapped in metallic iron, which is consistent with observations by Hashizume and Sugiura (1997). Efficiency of C transportation crucially depended on the oxygen fugacity. To realize effective C transportation, it was necessary to assume an oxidizing condition (logfO2 > logfO2,(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 a sufficient environment 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, possibly suggesting incorporation of pre-accretionary objects as large as 0.1=D7 of the final mass of the ordinary chondrite parent body.
Hashizume K.
Sugiura Naoji
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