Solar System evolution and terrestrial planet accretion determined by Zr isotopic signatures of meteorites

Abstract

Nucleosynthetic isotope signatures in meteorites provide key insights into the structure and dynamics of the solar protoplanetary disk and the accretion history of the planets. We present high-precision Zr isotopic data of a comprehensive suite of non-carbonaceous (NC) and carbonaceous (CC) meteorites, and find that various meteorite groups, including enstatite chondrites, exhibit 96Zr enrichments, whereas there is no resolved 91Zr and 92Zr variability. These new Zr isotope data reveal the same fundamental NC-CC dichotomy observed for several other elements, where CC meteorites are more anomalous compared to NC meteorites and are shifted towards the isotopic composition of Ca-Al-rich inclusions (CAIs). For Zr and other elements, the CC composition is reproduced as a mixture of materials with CAI-like and NC-like isotopic compositions in approximately constant proportions, despite these elements exhibiting disparate nucleosynthetic origins or different cosmo- and geochemical behaviors. These constant mixing proportions are inconsistent with an origin of the dichotomy by thermal processing or selective dust-sorting in the disk but indicate mixing of isotopically distinct materials with broadly solar chemical compositions. This corroborates models in which the NC-CC dichotomy reflects time-varied infall from an isotopically heterogeneous molecular cloud. Among NC meteorites, the isotope anomalies in Zr are linearly correlated with those of other elements, which likewise reflects primordial mixing. Lastly, the new Zr isotope data reinforce the notion that Earth incorporated s-process enriched material from the innermost Solar System, which is not represented by known meteorites. By contrast, contributions to Earth and Mars from outer Solar System CC-like materials were limited, indicating that these planets did not form by pebble accretion, which would have led to high CC fractions.