Zn and Cu isotopic variations in chondrites and iron meteorites:: Early solar nebula reservoirs and parent-body processes

High-precision Zn isotopic variations are reported for carbonaceous chondrites (CC), equilibrated (EOC) and unequilibrated (UOC) ordinary chondrites, iron meteorites from the IAB-IIICD (nonmagmatic) and IIIA (magmatic) groups, and metal from the Brenham pallasite. For irons, delta Cu-65 values are also reported. Data have also been obtained on a coarse-grained type-B calcium-, aluminum-rich refractory inclusion (CAI) from Allende and on acid leaches of Allende (CV3), Krymka (LL3), and Charsonville (H6). Variations expressed as delta Zn-66 (deviation in parts per thousand of Zn-66/Zn-64 in samples relative to a standard) spread over a range of 0.3 parts per thousand for carbonaceous chondrites, 2 parts per thousand for ordinary chondrites, and 4 parts per thousand for irons. The measured Zn-66/Zn-64, Zn-67/Zn-64, and Zn-68/Zn-64 ratios vary linearly with mass difference and define a common isotope fractionation line with terrestrial samples, which demonstrates that Zn was derived from an initially single homogeneous reservoir. The delta Zn-66 values are correlated with meteorite compositions and slightly decrease in the order Cl, CM, CV-CO, and to UOC. The isotopically light Zn of Allende CAI and the acid-resistant residues of Allende and Krymka show that the light component is associated with refractory material, presumably minerals from the spinet-group. This, together with the reverse correlation between relative abundances of light Zn isotopes and volatile element abundances, suggests that Zn depletion in planetary bodies with respect to Cl cannot be ascribed to devolatilization of Cl-like material. These observations rather suggest that refractory material reacted with a gas phase enriched in the lighter Zn isotopes. Alternatively, chondrules with their associated rims should carry a light Zn isotopic signature. The delta Zn-66 values of unequilibrated chondrites are rather uniform, whereas equilibrated chondrites show distinctly more isotopic variability. The values Of delta Cu-65-delta Zn-66 in irons define two trends. The moderate and positively correlated Cu and Zn isotope variations in IIIA and pallasite samples probably reflect crystallization of silicate, sulfide, and solid metal from the liquid metal. The range of delta Zn-66 values of the IAB-IIICD group is large (> 3 parts per thousand) and contrasts with the moderate fractionation of Cu isotopes. We interpret this feature and the negative delta Zn-66-delta Cu-65 correlation as reflecting mixing, possibly achieved by percolation, between metals from a regolith devolatilized at low temperature (enriched in heavy zinc) and metallic liquids formed within the parent body. Copyright (c) 2005 Elsevier Ltd.