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

• Published in: GEOCHEMICAL JOURNAL Vol. 52; no. 1; pp. 29 - 36
• Main Authors: Li, Xiao-Hu, Wang, Jian-Qiang, Chu, Feng-You, Wang, Hao, Li, Zheng-Gang, Yu, Xing, Wang, Wei
• Format: Journal Article
• Language: English
• Published: GEOCHEMICAL SOCIETY OF JAPAN 2018
• Subjects: Cu isotope; hydrothermal sulfide; isotopic fractionation; oxidation weathering; South Mid-Atlantic Ridge
• ISSN: 0016-7002; 1880-5973
• DOI: 10.2343/geochemj.2.0494

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.