A nucleosynthetic origin for the Earth's anomalous (142)Nd composition.

Abstract

A long-standing paradigm assumes that the chemical and isotopic composition of many elements in the bulk silicate Earth are the same as in chondrites1–4. However, the accessible Earth has a greater 142Nd/144Nd than chondrites. Because 142Nd is the decay product of now-extinct 146Sm (t1/2= 103 million years5), this 142Nd difference seems to require a higher-than-chondritic Sm/Nd of the accessible Earth. This must have been acquired during global silicate differentiation within the first 30 million years of Solar System formation6 and implies the formation of a complementary 142Nd-depleted reservoir that either is hidden in the deep Earth6, or was lost to space by impact erosion3,7. Whether this complementary reservoir existed, and whether or not it has been lost from Earth is a matter of debate3,8,9, but has tremendous implications for determining the bulk composition of Earth, its heat content and structure, and for constraining the modes and timescales of its geodynamical evolution3,7,9,10. Here, we show that compared to chondrites, Earth’s precursor bodies were enriched in Nd produced by the slow neutron capture process (s-process) of nucleosynthesis. This s-process excess leads to higher 142Nd/144Nd, and, after correction for this effect, the 142Nd/144Nd of chondrites and the accessible Earth are indistinguishable within 5 parts per million. The 142Nd offset between the accessible silicate Earth and chondrites, therefore, reflects a higher proportion of s-process Nd 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 for bulk Earth. Thus, although chondrites formed at greater heliocentric distance and contain a different mix of presolar components than Earth, they nevertheless are suitable proxies for Earth’s bulk chemical composition.