Geochemical Journal, Vol. 52, 2018
XIAO-HU LI*, JIAN-QIANG WANG, FENG-YOU CHU, HAO WANG, ZHENG-GANG LI, XING YU, WEI WANG
Key Laboratory of Submarine Geosciences, Second Institute of Oceanography, State Oceanic Administration, 36 Baochubei Road, Hangzhou 310012, P.R. China
(Received December 18, 2016; Accepted July 10, 2017)
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.
Key words: Cu isotope, hydrothermal sulfide, oxidation weathering, isotopic fractionation, South Mid-Atlantic Ridge