Spatial distribution of isotopes and compositional mixing in the inner protoplanetary disk

Abstract

The mass-independent isotopic signatures of planetary bodies have been widely used to trace the mixing process that occurred during planet formation. The observed isotopic variations among meteorite parent bodies have been further linked to the modeled mass-weighted mean initial semimajor axes in N-body simulations, assuming a spatial isotopic gradient in the inner protoplanetary disk. However, nucleosynthetic isotopic anomalies of nonvolatile elements and mass-independent oxygen isotopic variation (∆17O) show different relationships with distance from the Sun. Therefore, it is crucial to know whether isotopes were distributed systematically with heliocentric distance in the inner protoplanetary disk. In this study, we performed N-body simulations on compositional mixing during the collisional accretion and migration of planetary bodies to investigate the spatial distributions of Cr and O isotopes in the inner protoplanetary disk. The modeled mass-weighted mean initial semimajor axes of the parent bodies of noncarbonaceous (NC) meteorites and terrestrial planets were used to calculate the isotopic compositions of these bodies. Our simulations successfully reproduced the observed nucleosynthetic Cr isotopic anomaly among Earth, Mars, and the NC meteorite parent bodies, consistent with a spatial gradient of isotopic anomalies in the inner disk. Asteroids originating from different regions in the inner disk were transported to the main belt in our simulations, resulting in the Cr isotopic anomaly variation of the NC meteorite parent bodies. However, the ∆17O distribution among the terrestrial planets and the NC meteorite parent bodies could not be reproduced assuming a ∆17O gradient in the inner protoplanetary disk. The spatial gradient of the nucleosynthetic isotopic anomaly may be a result of changing isotopic compositions in the infalling materials, or reflect the progressive thermal processing of presolar materials. In contrast, the absence of a ∆17O gradient reflects that the oxygen isotopic mass-independent fractionation might have altered the spatial distribution of the nucleosynthetic ∆17O variation in the inner protoplanetary disk before protoplanets formed.