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

We report the results of a study of the geologically important copper isotope ((CU)-C-65/(CU)-C-63) fractionation between aqueous Cu(II) and copper sulphide. The experiments were made in anoxic conditions by precipitating covellite (CuS) from excess Cu as aqueous CuSO4 through the addition of aqueous Na2S. 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 degreesC give a mean fractionation of Delta(65)Cu (Cu(II)(aq) - CuS) = 3.06 + 0.14parts per thousand. Additional experiments made at 2degrees, 10degrees and 40 degreesC show that the fractionation factor varies inversely with temperature according to the relation (errors at I a level): Delta(Cu(II)-Cus) = 0.26 +/- 0.02 x 10(6) T-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 similar to 3parts per thousand Cu(II)(aq) -CuS fractionation compares with small experimentally measured Cu(II)(aq) -malachite fractionation (0.20parts per thousand to 0.38 + 0.04parts per thousand at 30degreesC reported by Marechal and Sheppard [2002. Isotopic fractionation of Cu and Zn between chloride and nitrate solutions and malachite or smithsonite at 30degrees and 50 degreesC. 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 Cu-66/Cu-63 fractionations in low-temperature geological environments. (C) 2004 Published by Elsevier B.V.