Iron-Copper-Zinc Isotopic Compositions of Andesites from the Kueishantao Hydrothermal Field off Northeastern Taiwan

Abstract

The studies of iron (Fe), copper (Cu), and zinc (Zn) isotopic compositions in seafloor andesites are helpful in understanding the metal stable isotope fractionation during magma evolution. Here, the Fe, Cu, and Zn isotopic compositions of andesites from the Kueishantao hydrothermal field (KHF) off northeastern Taiwan, west Pacific, have been studied. The majority of δ56Fe values (+0.02‰ to +0.11‰) in the KHF andesites are consistent with those of MORBs (mid-ocean ridge basalts). This suggests that the Fe in the KHF andesites is mainly from a MORB-type mantle. The Fe-Cu-Zn isotopic compositions (δ56Fe +0.22‰, δ65Cu +0.16‰ to +0.64‰, and δ66Zn +0.29‰ to +0.71‰) of the KHF andesites, which are significantly different from those of the MORBs and the continental crust (CC), have a relatively wide range of Cu and Zn isotopic compositions. This is most likely to be a result of the entrainment of the sedimentary carbonate-derived components into an andesitic magma. The recycled altered rocks (higher δ56Fe, lower δ66Zn) could preferentially incorporate isotopically light Fe and heavy Zn into the magma, resulting in relative enrichment of the lighter Fe and heavier Zn isotopes in the andesites. The majority of the δ56Fe values in the KHF andesites are higher than those of the sediments and the local CC and lower than those of the subducted altered rocks, while the reverse is true for δ66Zn, suggesting that the subseafloor sediments and CC materials (lower δ56Fe, higher δ66Zn) contaminating the rising andesitic magma could preferentially incorporate isotopically heavy Fe and light Zn into the magma, resulting in relative enrichment of the heavier Fe and lighter Zn isotopes in the andesites. Thus, the characteristics of the Fe and Zn isotopes in back-arc and island-arc volcanic rocks may also be influenced by the CC and plate subduction components.