Neodymium isotope heterogeneity of ordinary and carbonaceous chondrites and the origin of non-chondritic 142Nd compositions in the Earth

Abstract

We present high-precision Nd isotope compositions for ordinary and carbonaceous chondrites determined using thermal ionization mass spectrometry with dynamic and multistatic methods. The ordinary chondrites had uniform and non-terrestrial μ142Nd, μ148Nd, and μ150Nd values, with data that plot along the mixing line between s-process and terrestrial components in μ150Nd versus μ148Nd and μ142Nd versus μ148,150Nd diagrams. In contrast, the carbonaceous chondrites were characterized by larger anomalies in their μ142Nd, μ148Nd, and μ150Nd values compared to ordinary chondrites. Importantly, the data for carbonaceous chondrites plot along the s-process and terrestrial mixing line in a μ150Nd versus μ148Nd diagram, whereas they have systematically lower μ142Nd values than the s-process and terrestrial mixing line in μ142Nd versus μ148,150Nd diagrams. This shift likely results from the incorporation of calcium- and aluminum-rich inclusions (CAIs), indicating that the Nd isotopic variability in the ordinary chondrites and CAI-free carbonaceous chondrites was caused solely by the heterogeneous distribution of s-process nuclides. The isotopic variation most likely results from nebular thermal processing that caused selective destruction of s-process-depleted (or r-process-enriched) dust grains in the inner Solar System where the parent bodies of ordinary chondrites formed, whereas such grains were preserved in the region of carbonaceous chondrite parent body formation. The Nd isotope dichotomy between ordinary and bulk aliquots of carbonaceous chondrites can be related to the presence of Jupiter, which may have separated two isotopically distinct reservoirs that were present in the solar nebula. After correcting for s-process anomalies and CAI contributions to the Nd isotopes observed in the chondrites, we obtained a μ142Nd value (−2.4±4.8 ppm) that was indistinguishable from the terrestrial value. Our results corroborate the interpretation that a missing reservoir (e.g., a hidden enriched reservoir, erosional loss of crust) is not required to explain the observed differences in 142Nd/144Nd ratios between chondrites and terrestrial materials.