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• First comprehensive study of Nd stable isotopes in wide range of terrestrial basalts. • Partial melting modelling demonstrates that δ146Nd is unfractionated upon melting. • Confirmation the bulk silicate Earth and chondrites have the indistinguishable δ146Nd. • Variations in δ146Nd induce no analytical artefacts when calculating radiogenic isotope compositions. 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 δ146/144Nd. Mid-ocean ridge basalts possess an extremely homogenous δ146/144Nd with an average composition of δ146/144Nd = −0.025 ± 0.013‰ (±2 s.d.; n = 33). Ocean island and continental intraplate magmas possess more variable compositions (δ146/144Nd = 62 ppm) that are related to the variable incorporation of recycled components in their source regions. Island arc basalts from New Britain (δ146/144Nd = 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 δ146/144Nd 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 (Δ146/144Nd melt-mantle < 0.003‰ at 1200 °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 δ146/144Nd = −0.024 ± 0.031‰ (±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 142Nd/144Nd offset between the Earth and chondrites. Despite resolvable variations in δ146/144Nd 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. [ABSTRACT FROM AUTHOR]

Neodymium (Nd) stable isotopes have the potential to provide new constraints on a diverse range of geological processes from planetary formation and magmatic differentiation to weathering and ocean circulation. In this contribution, we present a technique for the high-precision measurement of Nd stable isotope ratios by thermal ionisation mass spectrometry (TIMS). We use a 145Nd–150Nd double spike (DS), composed of 28% 145Nd and 67% 150Nd, to correct for mass dependent fractionation resulting from sample preparation and mass spectrometry. Isotope ratios are expressed as δ146Nd which is the per mil deviation in the measured 146Nd/144Nd ratio relative to reference material JNdi-1. Repeated analyses show that an internal precision (n = 400 cycles; 2 se) of ≤0.005‰ on δ146Nd can be achieved in agreement with theoretical predictions. An interlaboratory comparison of the primary standard JNdi-1 over a three-year period shows that resolvable offsets in δ146Nd are induced within and between mass spectrometers as the result of systematic biases in the efficiency of the Faraday collectors. Following normalisation of the data to JNdi-1 = 0‰ the long-term reproducibility of δ146Nd on a range of international reference materials processed through chemical separation is better than ±0.015‰ (2 sd). In addition to stable isotope compositions this method allows for the simultaneous measurement of 143Nd/144Nd with a precision of ≤11 ppm. Over a wide range of 143Nd/144Nd (0.5118–0.5132) the DS analyses here agree within analytical uncertainty with published values using conventional techniques. To allow the accurate age correction of these radiogenic isotope compositions in ancient geological samples (>2 Ga) here for the first time we have combined a DS and an elemental tracer spike. A 149Sm tracer spike was calibrated and used to obtain isotope dilution Sm concentrations and ultimately parent-daughter ratios. Measured Sm–Nd ratios are within uncertainty of previous estimates for a range of reference materials, with the addition of the Sm spike having no resolvable effect on δ146Nd values. This technique allows δ146Nd, 143Nd/144Nd and Sm/Nd to be obtained simultaneously, therefore allowing constrains to be placed on both the source or age of a material and the processes involved in its formation. [ABSTRACT FROM AUTHOR]

The non-chondritic neodymium (Nd) 142 Nd/ 144 Nd ratio of the silicate Earth potentially provides a key constraint on the accretion and early evolution of the Earth. Yet, it is debated whether this offset is due to the Earth being formed from material enriched in s -process Nd isotopes or results from an early differentiation process such as the segregation of a late sulfide matte during core formation, collisional erosion or a some combination of these processes. Neodymium stable isotopes are potentially sensitive to early sulfide segregation into Earth's core, a process that cannot be resolved using their radiogenic counterparts. This study presents the first comprehensive Nd stable isotope data for chondritic meteorites and terrestrial rocks. Stable Nd measurements were made using a double spike technique coupled with thermal ionisation mass spectrometry. All three of the major classes of chondritic meteorites, carbonaceous, enstatite and ordinary chondrites have broadly similar isotopic compositions allowing calculation of a chondritic mean of δ 146/144 Nd = −0.025 ± 0.025‰ (±2 s.d.; n = 39 ). Enstatite chondrites yield the most uniform stable isotope composition (Δ 146/144 Nd = 26 ppm), with considerably more variability observed within ordinary (Δ 146/144 Nd = 72 ppm) and carbonaceous meteorites (Δ 146/144 Nd = 143 ppm). Terrestrial weathering, nucleosynthetic variations and parent body thermal metamorphism appear to have little measurable effect on δ 146/144 Nd in chondrites. The small variations observed between ordinary chondrite groups most likely reflect inherited compositional differences between parent bodies, with the larger variations observed in carbonaceous chondrites being linked to varying modal proportions of calcium–aluminium rich inclusions. The terrestrial samples analysed here include rocks ranging from basaltic to rhyolitic in composition, MORB glasses and residual mantle lithologies. All of these terrestrial rocks possess a broadly similar Nd isotope composition giving an average composition for the bulk silicate Earth of δ 146/144 Nd = −0.022 ± 0.034‰ ( n = 30 ) . In the samples here magmatic differentiation appears to only have an effect on stable Nd in highly evolved magmas with heavier δ 146/144 Nd values observed in samples with >70 wt% SiO 2 . The average stable Nd isotope composition of chondrites and the bulk silicate Earth are indistinguishable at the 95% confidence level. However, mantle samples do possess variable stable Nd isotope compositions (Δ 146/144 Nd = 75 ppm) with an average δ 146 / 144 Nd value of −0.008‰. If these heavier values represent the true composition of pristine mantle then it is not possible to completely rule out some role for core formation in accounting for some of the offset between the mantle and chondrites. Overall, these results indicate that the mismatch of 142 Nd between the Earth and chondrites is best explained by a higher proportion of s -process Nd in the Earth, rather than partitioning into sulfide or S-rich metal in the core. [ABSTRACT FROM AUTHOR]