Isotopic fractionation of Cu and Zn between chloride and nitrate solutions and malachite or smithsonite at 30 degrees and 50 degrees

Abstract

Interpretation of the isotopic variations in natural Cu systems (~9 permil) and, to a much lesser extent, Zn systems (~1 ‰) requires experimental fractionation data. Malachite (CuCO3.Cu(OH)2) and smithsonite (ZnCO3) were synthetised using a slightly modified experimental protocol of the slow precipitation method of Melchiorre et al. [1]. Powdered calcite was replaced by malachite or smithsonite by reaction with a copper or zinc salt solution. Solutions of 0.05 M, 0.1 M and/or 0.5 M CuCl2, Cu(NO3)2, ZnCl2 and Zn(NO3)2 were prepared from reagent-grade solids and distilled water, and isotopically (18O/16O, 65Cu/63Cu, 66Zn/64Zn) characterised. After reacting in a closed tube for 3 to 4 weeks in a constant temperature bath at 30°C and 50°C, the solid products were separated from the solution. Before isotopic analysis, the presence of Ca in the solid phases was measured by ICP-MS and found to be negligible. In the absence of possible interference from Ca, Cu and Zn were analysed without purification by MC-ICP-MS. All experiments were run in duplicate. The analytical precision of δ65Cu and δ66Zn is ± 0.04 ‰ (2 σ level). The delta values are expressed with respect to Cu NIST 976 and to an in-house solution of Zn JMC 30749L. [Note: αsolution-mineral = (65Cu/63Cu)solution/(65Cu/63Cu)mineral]. Measured chloride and nitrate solution-mineral fractionations factors at 30°C are 1.00038 and 1.00020 for malachite and 1.00004 and 1.00011 for smithsonite respectively. Minerals are thus depleted in 65Cu and 66Zn relative to the solutions. Compared to the 30°C data, fractionation factors at 50°C are slightly smaller for malachite (chloride: 1.00031 and nitrate: 1.00017) but indistinguishable for smithsonite. For Cu, fractionation factors are the same for 0.05 M, 0.1 M and 0.5 M solutions but those for chloride solutions are systematically larger than those for nitrate solutions, as observed for chromatographic experiments [2]. Similar to observations on natural systems [3, 4] and ion exchange experiments [2], the Cu-isotope fractionations are substantially larger than those for Zn. The experimental fractionations are small and rather insensitive to temperature variations and cannot simply account for the 6 ‰ range of variations in natural malachites [5]. Changes in solute chemistry and/or distillation type processes are probably important during malachite formation. References [1] Melchior et al. (1999) Econ. Geol., 94, 245. [2] Maréchal and Albarède (2002) GCA, 66, 1499. [3] Maréchal et al. (1999) Chem. Geol., 156, 251. [4] Zhu et al. (2000) Chem. Geol., 163, 139. [5] Maréchal (1998) thesis.