Sm-Nd systematics of Earth are inconsistent with a superchondritic Sm/Nd ratio

The relationship between the compositions of the Earth and chondritic meteorites is at the center of many important debates. A basic assumption in most models for the Earth’s composition is that the refractory elements are present in chondritic proportions relative to each other. This assumption is now challenged by recent Nd/Nd ratio studies suggesting that the bulk silicate Earth (BSE) might have an Sm/Nd ratio 6% higher than chondrites (i.e., the BSE is superchondritic). This has led to the proposal that the present-day Nd/Nd ratio of BSE is similar to that of some deep mantle plumes rather than chondrites. Our reexamination of the long-lived Sm-Nd isotope systematics of the depleted mantle and the continental crust shows that the BSE, reconstructed using the depleted mantle and continental crust, has Nd/Nd and Sm/Nd ratios close to chondritic values. The small difference in the ratio of Nd/Nd between ordinary chondrites and the Earth must be due to a process different from mantle-crust differentiation, such as incomplete mixing of distinct nucleosynthetic components in the solar nebula.