The neodymium stable isotope composition of the silicate Earth and chondrites

The non-chondritic neodymium (Nd) Nd-142/Nd-144\ 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 delta Nd-146/144 = -0.025 +/- 0.025% (+/- 2 s.d.; n = 39). Enstatite chondrites yield the most uniform stable isotope composition (Delta Nd-146/144 = 26 ppm), with considerably more variability observed within ordinary (Delta Nd-146/144 = 72 ppm) and carbonaceous meteorites (Delta Nd-146/144 = 143 ppm). Terrestrial weathering, nucleosynthetic variations and parent body thermal metamorphism appear to have little measurable effect on delta Nd-146/144 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 delta Nd-146/144 = 0.022 0.03470) (n = 30). In the samples here magmatic differentiation appears to only have an effect on stable Nd in highly evolved magmas with heavier delta Nd-146/144 values observed in samples with >70 wt% SiO2. 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 (Delta Nd-146/144 = 75 ppm) with an average delta Nd-146/144 value of 0.008%0. 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 Nd-142 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. (C) 2017 The Authors. Published by Elsevier B.V.