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Neodynium isotope data reveal that the Earth is enriched in material from red giant stars relative to its presumed meteoritic building blocks, refuting models of a hidden reservoir of 142 Nd-depleted material or a ‘super-chondritic’ Earth. Chondritic meteorites as proxies for Earth's composition Christoph Burkhardt et al . show that, compared to chondritic meteorites, the Earth's precursor bodies were enriched in neodymium produced by the slow neutron capture 's-process' of nucleosynthesis. This s-process excess leads to a higher 142 Nd/ 144 Nd ratio and, after correction for this effect, the 142 Nd/ 144 Nd ratio of chondritic meteorites and the accessible Earth are indistinguishable within five parts per million. The 142 Nd offset between the accessible silicate Earth and chondritic meteorites therefore reflects a higher proportion of s-process neodymium in the Earth, and not early differentiation processes. The authors conclude that there is no need for hidden-reservoir or 'super-chondritic' Earth models, as previously proposed, and that although chondritic meteorites formed at a greater heliocentric distance and contain a different mix of presolar components than the Earth, they nevertheless may be suitable proxies for the Earth's bulk chemical composition. A long-standing paradigm assumes that the chemical and isotopic compositions of many elements in the bulk silicate Earth are the same as in chondrites 1 , 2 , 3 , 4 . However, the accessible Earth has a greater 142 Nd/ 144 Nd ratio than do chondrites. Because 142 Nd is the decay product of the now-extinct 146 Sm (which has a half-life of 103 million years 5 ), this 142 Nd difference seems to require a higher-than-chondritic Sm/Nd ratio for the accessible Earth. This must have been acquired during global silicate differentiation within the first 30 million years of Solar System formation 6 and implies the formation of a complementary 142 Nd-depleted reservoir that either is hidden in the deep Earth 6 , or lost to space by impact erosion 3 , 7 . Whether this complementary reservoir existed, and whether or not it has been lost from Earth, is a matter of debate 3 , 8 , 9 , and has implications for determining the bulk composition of Earth, its heat content and structure, as well as for constraining the modes and timescales of its geodynamical evolution 3 , 7 , 9 , 10 . Here we show that, compared with chondrites, Earth’s precursor bodies were enriched in neodymium that was produced by the slow neutron capture process (s-process) of nucleosynthesis. This s-process excess leads to higher 142 Nd/ 144 Nd ratios; after correction for this effect, the 142 Nd/ 144 Nd ratios of chondrites and the accessible Earth are indistinguishable within five parts per million. The 142 Nd offset between the accessible silicate Earth and chondrites therefore reflects a higher proportion of s-process neodymium in the Earth, and not early differentiation processes. As such, our results obviate the need for hidden-reservoir or super-chondritic Earth models and imply a chondritic Sm/Nd ratio for the bulk Earth. Although chondrites formed at greater heliocentric distances and contain a different mix of presolar components than Earth, they nevertheless are suitable proxies for Earth’s bulk chemical composition.

The lead-lead isochron age of chondrules in the CR chondrite Acfer 059 is 4564.7 ± 0.6 million years ago (Ma), whereas the lead isotopic age of calcium-aluminum–rich inclusions (CAIs) in the CV chondrite Efremovka is 4567.2 ± 0.6 Ma. This gives an interval of 2.5 ± 1.2 million years (My) between formation of the CV CAIs and the CR chondrules and indicates that CAI- and chondrule-forming events lasted for at least 1.3 My. This time interval is consistent with a 2- to 3-My age difference between CR CAIs and chondrules inferred from the differences in their initial 26 Al/ 27 Al ratios and supports the chronological significance of the 26 Al- 26 Mg systematics.