ABSTRACT

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 Cu-rich and Zn-rich sulfides, and their oxidation products collected from hydrothermal fields along the South Mid-Atlantic Ridge (13–15°S). Results show that the δ⁶⁵Cu values of these sulfides and their oxidation products are –0.58‰ to +1.36‰, with an average value of ⁶⁵Cu = +0.30 ± 1.02‰ (n = 17, 2 sd). Cu-rich sulfides have slightly lighter Cu isotope compositions, with an average δ⁶⁵Cu = +0.37 ± 0.16‰ (n = 5, 2 sd), whereas Zn-rich sulfides are enriched in heavy Cu isotopes, with an average δ⁶⁵Cu = +0.77 ± 0.59‰ (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 δ⁶⁵Cu = –0.24 ± 0.58‰ (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 δ³⁴S values (up to 14.5‰) 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.