The chondritic neodymium stable isotope composition of the Earth inferred from mid-ocean ridge, ocean island and arc basalts

Accurate knowledge of the composition of Earth's major chemical reservoirs is fundamental for constraining all modern geochemical cycles. Basaltic rocks provide a direct way of sampling the composition of Earth's inaccessible interior. Here, we present the first comprehensive neodymium (Nd) stable isotope analyses for a global compilation of mid-ocean ridge, ocean island, continental intraplate and island arc basalts using a double-spike technique. In these primitive magma compositions magmatic differentiation has no resolvable effect on delta Nd-146/144. Mid-ocean ridge basalts possess an extremely homogenous delta Nd-146/144 with an average composition of delta Nd-146/144=-0.025 +/- 0.013%e (+/- 2 s.d.; n = 33). Ocean island and continental intraplate magmas possess more variable compositions (delta Nd-146/144 = 62 pp m) that are related to the variable incorporation of recycled components in their source regions. Island arc basalts from New Britain (delta Nd-146/144 =61 ppm) reflect the complex interplay between source composition, degree of melting and slab-fluid inputs. Variations are uncorrelated with indicators of magmatic differentiation or slab-fluid addition, rather increasing delta Nd-146/144 with slab depth is attributed to a higher proportion of metasomatized sub-arc mantle in the melting region. A partial melting model for Nd stable isotopes has been constructed using Nd-O force constants calculated using the Born-Lande approximation. Melting of typical mantle peridotite will induce no resolvable fractionations of Nd stable isotopes (Delta Nd-146/144(melt-mantle) < 0.003%e at 1200 degrees C). The lack of fractionation upon partial melting means primitive magmatic rocks can be used to calculate the average composition of the bulk silicate Earth (BSE), which is delta Nd-146/144 = -0.024 +/- 0.031%e (+/- 2 s.d.; n = 80). This BSE composition is indistinguishable at the 95 % confidence level from that of chondritic meteorites, the building blocks of Earth. Therefore, sequestration of significant quantities of Nd into the sulfide matte did not occur, this combined with recent experimental evidence for no Sm-Nd fractionation means the sulfide matte cannot be considered a plausible solution for the delta Nd-146/144 offset between the Earth and chondrites. Despite resolvable variations in delta Nd-146/144 from the canonical value being widespread in terrestrial materials, they are not large enough to generate the difference in radiogenic Nd isotope ratios between the BSE and chondrites. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

The study conducted neodymium stable isotope analyses on various types of basalts globally, revealing that mid-ocean ridge basalts have a very uniform composition, while ocean island and continental intraplate basalts have more variable compositions, related to the varying incorporation of recycled materials in their source regions. Island arc basalts from New Britain reflect the complex interplay between source composition, degree of melting, and slab-fluid inputs.