Isotopic Evolution of the Inner Solar System Inferred from Molybdenum Isotopes in Meteorites

Gerrit Budde,Christoph Burkhardt,Fridolin Spitzer,Alessandro Morbidelli,Thorsten Kleine,Thomas S Kruijer

doi:10.3847/2041-8213/ab9e6a

Abstract

Abstract The fundamentally different isotopic compositions of non-carbonaceous (NC) and carbonaceous (CC) meteorites reveal the presence of two distinct reservoirs in the solar protoplanetary disk that were likely separated by Jupiter. However, the extent of material exchange between these reservoirs, and how this affected the composition of the inner disk, are not known. Here we show that NC meteorites display broadly correlated isotopic variations for Mo, Ti, Cr, and Ni, indicating the addition of isotopically distinct material to the inner disk. The added material resembles bulk CC meteorites and Ca–Al-rich inclusions in terms of its enrichment in neutron-rich isotopes, but unlike the latter materials is also enriched in s-process nuclides. The comparison of the isotopic composition of NC meteorites with the accretion ages of their parent bodies reveals that the isotopic variations within the inner disk do not reflect a continuous compositional change through the addition of CC dust, indicating an efficient separation of the NC and CC reservoirs and limited exchange of material between the inner and outer disk. Instead, the isotopic variations among NC meteorites more likely record a rapidly changing composition of the disk during infall from the Sun’s parental molecular cloud, where each planetesimal locks the instant composition of the disk when it forms. A corollary of this model is that late-formed planetesimals in the inner disk predominantly accreted from secondary dust that was produced by collisions among pre-existing NC planetesimals.

Highlights

Nucleosynthetic isotope anomalies reveal a fundamental dichotomy between non-carbonaceous (NC) and carbonaceous (CC) meteorites (Warren 2011; Budde et al 2016), which sample two spatially distinct reservoirs that coexisted in the early solar system for several million years (Ma; Kruijer et al 2017)
The prolonged spatial separation of the NC and CC reservoirs most likely reflects the formation of Jupiter, which acted as an efficient barrier against material exchange either by its growth itself (Morbidelli et al 2016; Kruijer et al 2017) or through a pressure maximum in the disk near the location where Jupiter later formed (Brasser & Mojzsis 2020)
While there are large s-process Mo isotope variations among meteorites within both the NC and CC groups, all CC meteorites are characterized by an approximately constant r-process excess over NC meteorites (Budde et al 2016; Kruijer et al 2017; Poole et al 2017; Worsham et al 2017). This difference makes Mo isotopes ideally suited to identify any compositional change of the NC reservoir, because the continuous addition of CC dust to the NC reservoir would result in a characteristic isotopic shift of the NC composition toward an enrichment in r-process Mo isotopes over time

Summary

Introduction

Nucleosynthetic isotope anomalies reveal a fundamental dichotomy between non-carbonaceous (NC) and carbonaceous (CC) meteorites (Warren 2011; Budde et al 2016), which sample two spatially distinct reservoirs that coexisted in the early solar system for several million years (Ma; Kruijer et al 2017). While there are large s-process Mo isotope variations among meteorites within both the NC and CC groups, all CC meteorites are characterized by an approximately constant r-process excess over NC meteorites (Budde et al 2016; Kruijer et al 2017; Poole et al 2017; Worsham et al 2017) This difference makes Mo isotopes ideally suited to identify any compositional change of the NC reservoir, because the continuous addition of CC dust to the NC reservoir would result in a characteristic isotopic shift of the NC composition toward an enrichment in r-process Mo isotopes over time. Mo isotopes with unprecedented precision and use these data, combined with published data for other meteorite groups, to assess any compositional heterogeneity within the inner disk that may have arisen through material exchange between the NC and CC reservoirs

Correction of CRE Effects

Mo Isotope Variations among NC Meteorites

Mixing Trends in the NC Reservoir

Spatial and Temporal Variations in the NC Reservoir

Implications for Planetesimal Formation in the Inner Disk