Cadmium isotopes in chondrites and acid leachates: Nucleosynthetic homogeneity and a monitor for thermal neutron capture effects
Eniko R. Toth, Manuela A. Fehr, Matthias Friebel, Maria Schönbächler
Geochimica et Cosmochimica Acta
In Press, Journal Pre-proof, Available online 6 February 2020
“Nucleosynthetic isotope variations are well documented for refractory elements in meteorites and the Earth, while moderately volatile elements generally display homogeneous compositions. Cadmium is a moderately volatile element with eight stable isotopes generated by a variety of nucleosynthetic processes. To address the extent of the nucleosynthetic variability in moderately volatile elements, new high precision Cd isotope data are presented for bulk samples of six carbonaceous and one enstatite chondrite. In addition, we report the first Cd isotope results of sequential acid leachates for the CM2 chondrite Jbilet Winselwan. Our new Cd data displays nucleosynthetic homogeneity for bulk chondrites and acid leachates within analytical uncertainties, in agreement with results for other moderately volatile elements. This implies that Cd isotopes were efficiently homogenised prior to incorporation into planetary bodies. We propose that Cd never significantly condensed into dust in stellar environments, or alternatively that such Cd-bearing dust was efficiently destroyed and recycled in the interstellar medium. Our leachate data provides evidence for further homogenisation during thermal processing in the protoplanetary disk including parent body processing. The data shows that Cd in carbonaceous chondrites mainly resides in the more easily dissolved phases, most likely sulphides that were affected by aqueous alteration. Less than 1% of the total Cd was recovered in the final leach fractions that employed HF and mainly dissolve silicates and refractory oxides.
Cadmium is susceptible to thermal neutron-capture effects due to the large neutron capture cross-section of 113Cd (∼20,000 barns). We report variations of up to −0.6 ± 0.3 for ε113Cd (internally normalised to 116Cd/111Cd) in bulk chondrites, which renders Cd a potential thermal neutron-capture monitor. Most neutron dosimeters, such as Pt, Os and Hf, are sensitive to neutron capture in the epithermal energy range and have applications mainly limited to lunar samples or iron and stony-iron meteorites. The additional use of Cd, susceptible to neutron capture in the thermal energy range, therefore provides a new tool to determine the exposure histories of stony meteorites in more detail. Our study demonstrates that thermal neutron-capture effects in carbonaceous and enstatite chondrites can produce resolvable effects and require attention when assessing nucleosynthetic isotope variations.”