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

摘要

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 mu Nd-142, mu Nd-148, and mu Nd-150 values, with data that plot along the mixing line between s-process and terrestrial components in mu Nd-150 versus mu Nd-148 and mu Nd-142 versus mu Nd-148,Nd-150 diagrams. In contrast, the carbonaceous chondrites were characterized by larger anomalies in their mu Nd-142, mu Nd-148, and mu Nd-150 values compared to ordinary chondrites. Importantly, the data for carbonaceous chondrites plot along the s-process and terrestrial mixing line in a mu Nd-150 versus mu Nd-148 diagram, whereas they have systematically lower mu Nd-142 values than the s-process and terrestrial mixing line in mu Nd-142 versus mu Nd-148,Nd-150 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 mu Nd-142 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 Nd-142/Nd-144 ratios between chondrites and terrestrial materials. (C) 2017 Elsevier B.V. All rights reserved.