Abstract
Isotope anomalies among planetary bodies provide key constraints on planetary genetics and the Solar System's dynamical evolution. However, to unlock the full potential of these anomalies for constraining the processing, mixing, and transport of material in the disk it is essential to identify the main components responsible for producing planetary-scale isotope variations, and to investigate how they relate to the isotopic heterogeneity inherited from the Solar System's parental molecular cloud. To address these issues we measured the Ti and Sr isotopic compositions of Ca,Al-rich inclusions (CAIs) from the Allende CV3 chondrite, as well as acid leachates and an insoluble residue from the Murchison CM2 chondrite, and combine these results with literature data for presolar grains, hibonites, chondrules, and bulk meteorites. Our analysis reveals that the mineral-scale nebular isotopic heterogeneity as sampled by leachates and presolar grains is largely decoupled from the planetary-scale isotope anomalies. Combining isotope anomaly data for a large number of elements reveals that the difference between non-carbonaceous (NC) and carbonaceous (CC) meteorites is the product of mixing between NC material and a nebular reservoir (termed IC) whose isotopic composition is similar to that of CAIs, but whose chemical composition is similar to bulk chondrites. In our preferred model, the distinct isotopic compositions of these two nebular reservoirs reflect an inherited heterogeneity of the solar system's parental molecular cloud core, which therefore has never been fully homogenized during collapse. Planetary-scale isotopic anomalies are thus caused by variable mixing of isotopically distinct primordial disk reservoirs, the selective processing of these reservoirs in different nebular environments, and the heterogeneous distribution of the thereby forming nebular products.
Highlights
The solar system formed $4.567 Ga ago by the gravitational collapse of a cold and dense molecular cloud core whose chemical and isotopic composition reflects $9 Ga of galactic chemical evolution (e.g., Dauphas, 2005)
We investigate how planetary-scale anomalies relate to the mineral-scale isotopic heterogeneity inherited from the solar system’s parental molecular cloud, and whether their formation can be accounted for solely by processes within the accretion disk itself or instead require a change in the isotopic composition of the infalling cloud material
By systematically investigating the isotopic record of planetary materials in multi-element space, we evaluate the main controlling factors for the formation of planetary-scale nucleosynthetic isotope anomalies, and how these anomalies may relate to an isotopic heterogeneity of the solar system’s parental molecular cloud
Summary
The solar system formed $4.567 Ga ago by the gravitational collapse of a cold and dense molecular cloud core whose chemical and isotopic composition reflects $9 Ga of galactic chemical evolution (e.g., Dauphas, 2005). Bulk anomalies are a primordial feature of the solar nebula inherited from heterogeneities in the infalling molecular cloud material, e.g., due to a late injection of freshly synthesized matter (Clayton, 1982; Dauphas et al, 2002, 2004; Bizzarro et al, 2007), or they are caused by fractionation processes of an initially homogeneous cloud, either during infall (Van Kooten et al, 2016), or within the solar nebula itself (Dauphas et al, 2004, 2010; Regelous et al, 2008; Trinquier et al, 2009; Burkhardt et al, 2012b; Paton et al, 2013; Akram et al, 2015; Schiller et al, 2015; Pignatale et al, 2017; Poole et al, 2017) Such fractionation processes may involve the separation of dust grains of different types and sizes, or the selective destruction of thermally labile presolar components. By comparing the results with data for other elements, we develop a quantitative model for the origin of planetary-scale anomalies and the causes for the offset between the non-carbonaceous (NC) and carbonaceous (CC) nebular reservoirs in the context of the evolution of the early solar system
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