Copper isotopic compositions of hydrothermal sulfides from the mid-ocean ridge and implications for later oxidation processes

Considerable Cu isotopic fractionation occurs during the formation of mineral deposits and the oxidative weathering processes of modern seafloor hydrothermal sulfides. This report is the first to describe Cu isotopic compositions of Curich and Zn-rich sulfides, and their oxidation products collected from hydrothermal fields along the South Mid-Atlantic Ridge (13-15 degrees S). Results show that the delta Cu-65 values of these sulfides and their oxidation products are -0.58% oto + 1.36% o, with an average value of Cu-65 = + 0.30 +/- 1.02% o (n = 17, 2 sd). Cu-rich sulfides have slightly lighter Cu isotope compositions, with an average delta Cu-65 = + 0.37 +/- 0.16 parts per thousand (n = 5, 2 sd), whereas Zn-rich sulfides are enriched in heavy Cu isotopes, with an average delta Cu-65 = + 0.77 +/- 0.59 parts per thousand (n = 6, 2 sd). Compared to the Cu-rich and Zn-rich sulfides, their oxidation products are more enriched in light isotopes, with an average delta Cu-65 = -0.24 +/- 0.58% o (n = 6, 2 sd). The marked enrichment of the oxidation products in light Cu isotopes indicates that Cu isotopic fractionation occurs during the subsequent leaching alteration of seawater, whereas high delta S-34 values (up to 14.5 parts per thousand) of oxidation products also reflect large contributions of sulfate reduction S from seawater. The sulfides and oxidation products from the inactive hydrothermal fields reflect that substantial light Cu isotopic compositions can be a light output sink, which might explain the heavy isotopic compositions of the oceans.