Experimental study of the copper isotope fractionation between aqueous Cu(II) and covellite, CuS

Abstract

We report the results of a study of the geologically important copper isotope ( 65Cu/ 63Cu) fractionation between aqueous Cu(II) and copper sulphide. The experiments were made in anoxic conditions by precipitating covellite (CuS) from excess Cu as aqueous CuSO 4 through the addition of aqueous Na 2S. Isotopic measurements were collected on a Nu Instruments MC-ICP-MS using sample-standard bracketing and mass bias correction using a nickel internal standard. The results of a series of experiments at 20 °C give a mean fractionation of Δ 65Cu (Cu(II) aq–CuS)=3.06±0.14‰. Additional experiments made at 2°, 10° and 40 °C show that the fractionation factor varies inversely with temperature according to the relation (errors at 1 σ level): Δ Cu( II)– CuS =0.26±0.02×10 6T −2+0.08±0.25. Although the question of whether complete equilibration occurs during the precipitation reaction cannot be resolved, the measured fractionation factors provide a proxy for the natural fractionation processes involving abiogenic covellite formation by low-temperature precipitation. The ∼3‰ Cu(II) aq–CuS fractionation compares with small experimentally measured Cu(II) aq–malachite fractionation (0.20‰ to 0.38±0.04‰ at 30 °C) reported by Maréchal and Sheppard [2002. Isotopic fractionation of Cu and Zn between chloride and nitrate solutions and malachite or smithsonite at 30° and 50 °C. Goldschmidt 2002 Conference, Davos, Geochim. Cosmochim. Acta. 66, (15A), A484.] and a similar fractionation measured in this study for Cu(II) hydroxide precipitation from Cu(II) aq solution. The large Cu(II) aq–CuS isotopic fractionation supports the conclusion that covellite is a Cu(I)S(-I) compound and that redox state is potentially a significant control of abiogenic 65Cu/ 63Cu fractionations in low-temperature geological environments.