Molybdenum Isotope Dichotomy in Meteorites Caused by s-Process Variability

Abstract

Molybdenum isotopes measured in most individual presolar silicon carbide grains are dominated by s -process contributions from the helium intershells of asymptotic giant branch (AGB) stars. The much smaller isotopic variations in molybdenum in meteorites and their components are largely controlled by s -process enrichments or depletions relative to terrestrial composition but lie along two parallel s -process mixing lines separated by what has been suggested to be an r -process contribution. The two mixing lines are populated by carbonaceous-chondrite- and noncarbonaceous-chondrite-related meteorites (CC and NC groups, respectively). We have compared molybdenum isotopic data for presolar grains with those from meteorites and renormalized the meteorite data in a way that is consistent with s -, r -, and p -process contributions observed in presolar SiC grains. The results indicate that (1) there seems to be a fixed ratio between p - and r -process contributions in all data, (2) the dichotomy in molybdenum isotopes between the CC and NC groups can be explained by variations in the isotope makeup of the s -process contribution to the meteoritic samples, (3) this variability is similar to the variations in s -process molybdenum from different AGB stars deduced from presolar grain analyses, and (4) the larger range of isotopic compositions found in refractory inclusions is also consistent with s -process isotope variability.