Nucleosynthetic components of the early solar system inferred from Ba isotopic compositions in carbonaceous chondrites

The Ba isotopic composition of primitive meteorites is one of the more useful tracers to study nucleosynthetic processes in the early solar system, because the seven stable Ba isotopes consist of p-, r-, and s-process nucleosynthetic components and possible decay products from presently extinct Cs-135. Ba isotopic analyses were performed on acid leachates from five carbonaceous chondrites, Murchison (CM2), Sayama (CM2), Allende (CV3), Isuna (CO3) and Maralinga (CK4), and one basaltic achondrite, Juvinas (Eucrite). The Ba isotopic data of acid treatment fractions in carbonaceous chondrites suggest the presence of independent nucleosynthetic components for s- and r-processes in the solar system. The isotopic patterns from the acid residual fractions of two CM2 meteorites, Murchison and Sayama, show an enrichment of s-process isotopes, which is similar to that of presolar SiC grains. On the other hand, their acid leachates have an excess of r-process components among the Ba isotopes. In many cases of carbonaceous chondrites other than CM meteorites, the excess of r-process isotopes is too small for a detailed discussion about the nucleosynthetic contribution. This may be interpreted as metamorphic destruction of presolar grains in meteorite matrices associated with petrographic-type meteorites. Juvinas shows no isotopic anomalies of Ba, because the isotopic record in the early solar system was reset possibly by magmatic processes in the parent body. (C) 2003 Elsevier B.V. All rights reserved.