Nucleosynthetic Samarium, Neodymium, and Barium anomalies in chondritic meteorites and the implications for the chemical evolution of the planetary bodies in the inner Solar System.
Recent studies of variations in the Sm, Nd, & Ba isotopic compositions of bulk meteorites have yielded contradictory results, in terms of early planetary evolution. A key question is whether the observed variations in 142Nd are due to different contributions of nucleosynthetic (p-, s-, & r-process) components, or are caused by differing Sm/Nd ratios generated through planetary differentiation whilst 146Sm was alive. Whereas carbonaceous chondrites are deficient in p-process 144Sm, the 148Sm/154Sm & 150Nd/144Nd ratios of chondrites; eucrites; shergottites; the Moon; and the Earth are constant, indicating that the Solar Nebula possessed a uniform ratio of r/s nuclides. However, carbonaceous chondrites exhibit anomalies in 135Ba and 137Ba consistent with an excess in r-process nuclides. The Ba isotopic compositions of ordinary chondrites and eucrites are indistinguishable from that of the Earth. Thus the Ba, Sm, and Nd isotope ratios that are sensitive to variations in the r/s ratio are consistent, except those of carbonaceous chondrites, suggesting that whereas s- and r-process nuclides were homogeneously distributed in the inner Solar Nebula, radial isotopic heterogeneities did exist within the nebula. The issue of whether different planetary bodies accreted with the same bulk solar Sm/Nd ratio remains unresolved. If Earth is chondritic in its Sm/Nd composition, an early-formed reservoir must exist, but no geochemical evidence of this has yet been found. The eucrite parent body has chondritic Sm/Nd, whereas data from Mars and the Moon are less conclusive.