Tracing Earth’s Missing Non-Meteoritic Building Materials Through Isotopic Signatures

Planetary bodies exhibit distinct nucleosynthetic isotope signatures that provide insights into the origin of their building blocks [1]. The isotopic composition of Earth and Mars align with nucleosynthetic isotope anomaly trends defined by non-carbonaceous (NC) meteorites, indicating that the terrestrial planets primarily accreted from inner solar system material [2,3]. Furthermore, Earth’s isotopic composition consistently occupies an endmember position within the NC trends. For anomalies in s-process elements this endmember position is absolute, indicating that Earth incorporated a component not represented among known meteorites—an s-process-rich material likely formed in the innermost Solar System and now missing from the meteoritic record.

To investigate the nature of this component, we analyzed fine-grained, matrix-rich fractions from primitive NC chondrites that formed under varying redox conditions and at different heliocentric distances. Our data show that the Ti isotopic composition of these fine fractions deviate from the composition of their bulk host meteorites and are shifted toward—or even beyond—Earth’s isotopic composition. First Zr isotope data from one matrix-enriched fraction indicate that this material is s-process enriched, consistent with an origin in the inner Solar System. This component may represent a missing building block of Earth, heterogeneously distributed within the NC reservoir and selectively preserved in fine-grained material, even if diluted or absent in bulk chondrites.

[1] Dauphas (2017) Nature, 541, 521–524. [2] Burkhardt et al. (2021), Sci. Adv. 7, eabj7601. [3] Mezger et al. (2020), Space. Sci, Rev., 216 :27