Geochemistry of Copper and Zinc Isotopes: method, natural variability and oceanographic application

The natural variations of the stable isotopes of copper (masses 63 and 65) and zinc (masses 64, 66, 67, 68 and 70) are likely to provide valuable information on the internal and external cycles of these metals and on the processes of associated exchanges. However, the isotopic geochemistry of transition elements has been little studied because of the lack of an adequate analytical technique. A procedure for analyzing the isotopic abundances of copper and zinc by plasma source mass spectrometry and multi-collection (plasma 54) has been developed. The isotopic compositions are expressed relative to a standard (NIST 976 copper or JMC zinc). For isotope pairs ⁶⁵Cu/⁶³Cu and ⁶⁶Zn/⁶⁴Zn, the analytical precision is 0.04 pm in delta units. Significant isotopic variations (9 pm) are measured between copper minerals formed at low temperatures, without these variations being clearly related to the nature of the mineral. The isotopic compositions of zinc also show natural variations (0.9 pm). On the other hand, the isotopic analysis of the two trace metals in marine particles sedimenting through the water column of the central Atlantic Ocean (Eumeli sites) reveals the existence of isotopic variations greater than the error. analytical: 0.12 o/oo for ⁶⁶Zn, and 0.25 o/oo for ⁶⁵Cu. The elemental and isotopic balances indicate an enrichment in heavy isotopes within the organic matter during the remineralisation of the particles. Rather than source effects, exchange processes between the particulate phase and the dissolved phase therefore seem to be responsible for the isotopic variations observed over time. This work shows that the isotope ratios of copper and zinc measured with great precision can be used as geochemical and biogeochemical tracers. (author)