The mass-independent isotope effects of nucleosynthetic origin that have been identified for numerous refractory elements in meteorite samples are of considerable interest, as they provide constraints on physical conditions in the solar nebula. For volatile elements such investigations are scarce and isotopic effects have yet to be identified for this element group in bulk meteorites.
This study focused on the development of new methods that are able to resolve mass-independent isotope anomalies of volatile Zn and Cd in meteorites at a level of precision that surpasses previous work. The methods employ ion exchange chromatography for sample preparation prior to MC-ICP-MS analyses, which produce Zn and Cd isotope ratio data that have a typical precision of about ±10 to ±50 ppm (2sd).
Application of the techniques revealed small mass-independent Zn isotope anomalies for 11 bulk meteorite samples. Notably, complementary Zn isotope patterns were identified for carbonaceous chondrites on the one hand and ordinary chondrites and IAB iron meteorites on the other. Whilst the available evidence rules out that these anomalies reflect exposure of the meteorites to galactic cosmic rays or heterogeneous distribution of nucleosynthetic r- versus s-process material in the protoplanetary disk, further studies were needed to better constrain the ultimate origin of the isotopic effects.
Sequential acid leaching was carried out for two carbonaceous chondrites. The main leachate fractions revealed Zn isotope patterns that were essentially identical to the bulk meteorites. In contrast, the initial acetic acid leachates of both meteorites had distinct patterns with a small but significant enrichment of s-process isotopes. Based on this result and additional constraints, the Zn isotope anomalies observed for the bulk meteorites most likely reflect mass-independent isotope fractionation from nuclear field shift effects that were produced by evaporation and/or condensation processes in the solar nebula.