DETERMINATION OF NATURAL CU-ISOTOPE VARIATION BY PLASMA-SOURCE MASS SPECTROMETRY: IMPLICATIONS FOR USE AS GEOCHEMICAL TRACERS

Abstract

Techniques for the high precision measurement of 65Cu/63Cu ratios by multiple-collector plasma-source mass spectrometry has been developed. Two approaches, namely Zn-doping and ''sample-standard bracketing'', have been exploited. By using the ''sample-standard bracketing'' technique, a range of samples including native copper, Cu-carbonate and Cu-sulphides from terrestrial and marine environments have been analysed. An overall variation in 65Cu/63Cu of 22 parts per 104 (22 # units) is observed. This is more than 30 times the 2σ analytical uncertainty of the technique employed, and thus demonstrates the great potential for using stable Cu isotopes as tracers in geological and planetary processes. The variations in #65Cu values observed in this study display some regularity. Those samples involving formation through low temperature aqueous solutions display large differences in #65Cu values even at a single locality, whereas chalcopyrite samples hosted in igneous rocks show similar Cu-isotope compositions worldwide. This indicates that the #65Cu variations arise principally through mass fractionation in low temperature aqueous processes, rather than through source heterogeneity. In contrast to continental sulphides, chalcopyrites from black smoker sulphide chimneys on the ocean floor show large variations in #65Cu values. Relative to active high temperature hydrothermal vents, the old inactive vent deposits are enriched in 63Cu and show smaller variations in #65Cu values. Within in a single active chimney, Cu isotopes become lighter from bottom to top. This variation pattern is explained tentatively by means of a two-stage-process model, which involves: (1) the preferential leaching of 65Cu by hydrothermal processes, and (2) subsequent isotopic exchange between the early formed Cu-sulphides and 65Cu-depleted late-stage hydrothermal fluids. This new capability for Cu-isotope measurement is expected to have a major impact across disciplines ranging from cosmochemistry and geochemistry through biogeochemistry to biochemistry and alimentology.