147Sm-143Nd systematics of Earth are inconsistent with a superchondritic Sm/Nd ratio

摘要

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 ¹⁴²Nd/¹⁴⁴Nd 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 ¹⁴³Nd/¹⁴⁴Nd ratio of BSE is similar to that of some deep mantle plumes rather than chondrites. Our reexamination of the long-lived ¹⁴⁷Sm-¹⁴³Nd isotope systematics of the depleted mantle and the continental crust shows that the BSE, reconstructed using the depleted mantle and continental crust, has ¹⁴³Nd/¹⁴⁴Nd and Sm/Nd ratios close to chondritic values. The small difference in the ratio of ¹⁴²Nd/¹⁴⁴Nd 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.