Cadmium stable isotope cosmochemistry

Cadmium stable isotope compositions are reported for a comprehensive suite of carbonaceous, ordinary, enstatite, and Rumuruti chondrites as well as achondrites and lunar samples (soils, breccias, pristine anorthosite). The Cd isotope analyses were performed by multiple collector ICP-MS with an external reproducibility of +/- 0.43 parts per thousand (2 sd) for delta Cd-114/110. None of the samples shows evidence of nucleosynthetic anomalies and cosmogenic isotope effects from neutron-capture by Cd-113 were only observed for two lunar samples. The Cd stable isotope compositions of type 1, 2, and some type 3 carbonaceous chondrites, EH4 enstatite chondrites, eucrites, and the Earth are essentially identical at delta Cd-114/110 approximate to 0.0 +/- 0.4. This suggests that the portion of the solar nebula from which the inner solar system bodies accreted was homogeneous with respect to its Cd isotope composition. It also indicates that the primary volatile element depletion of the inner solar system did not involve partial kinetic Rayleigh evaporation or condensation. Furthermore no resolvable Cd isotope effects were,generated during the accretion and initial differentiation of the planetary bodies. In contrast, the analyses reveal large Cd isotope effects for ordinary and some enstatite chondrites, which display delta Cd-114/110 values between about -8 and +16. Smaller fractionations are observed for the Rumuruti and some type 3 to 5 carbonaceous chondrites. These Cd isotope variations are thought to reflect secondary depletion or redistribution of Cd, due to open system thermal metamorphism on the meteorite parent bodies. One CAI and chondrule separates from the Allende meteorite have unexpectedly high Cd concentrations and fractionated light Cd isotope compositions with delta Cd-114/110 approximate to -1 to -4. These characteristics may have been established by the interaction of originally Cd-poor materials with a volatile-rich gas prior to the final accretion of the Allende parent body. The general Cd enrichment of the lunar soil and regolith mainly reflects early volcanic activity. However, decreasing Cd abundances in lunar soils correlate well with an enrichment of the heavy Cd isotopes. This relationship is best explained by suppressed Rayleigh fractionation in response to space weathering. (C) 2007 Elsevier Ltd. All rights reserved.