Mixing and Homogenization in the Early Solar System: Clues from Sr, Ba, Nd, and Sm Isotopes in Meteorites

Abstract

High-precision barium isotopic compositions of large samples of an ordinary chondrite and a eucrite are identical to the terrestrial values. In contrast, the carbonaceous chondrites reveal excesses in (135)Ba and (137)Ba of around + 39 and + 22 parts per million (ppm), respectively; no anomalies are resolvable in (130,132,138)Ba. High- precision Sr isotopic compositions of all meteorites are identical within error. The data are consistent with the carbonaceous chondrites having an excess in the r-process (135,137)Ba with respect to Earth, eucrite parent bodies, and ordinary chondrites. The carbonaceous chondrites, however, display no variation in the r- and s- process Sm and Nd isotopes, suggesting that the r- process sources of Ba and the lanthanides were decoupled. The homogeneity of Ba and Sm isotopes in the Earth, eucrite parent body, and ordinary chondrite indicates that the solar nebula that fed planetesimals between similar to 1 and similar to 2.4 AU was well mixed with respect to these isotopes. It was heterogeneous beyond similar to 2.7 AU where carbonaceous chondrite parent bodies formed. These observations also indicate that the best estimate of the Nd isotopic composition of the Earth is obtained from ordinary chondrites and not from carbonaceous chondrites, as is normally assumed. Since the terrestrial upper mantle shows a (142)Nd anomaly of +18 +/- 8 ppm with respect to the ordinary chondrites, this is further evidence that the upper mantle retains a memory of early Earth differentiation and sequestration of a reservoir with an average Sm/Nd ratio lower than that of chondrites.