Zinc isotopic fractionation between aqueous fluids and silicate magmas: An experimental study

Fluids in magmas play a key role in magma differentiation and transportation of economic metals for ore deposits. As a chalcophile and incompatible element, zinc and its isotopes have been increasingly applied to study the magmatichydrothermal processes. However, zinc isotopic fractionation between aqueous fluids and magmas has not been well constrained. Here we experimentally determined equilibrium fractionation factors of Zn isotopes between aqueous fluids and silicate magmas (Delta Zn-66(fluid-magma) = delta Zn-66(fluid )- delta Zn-66(magma)). The results reveal that aqueous fluids are isotopically heavier than the coexisting silicate magmas. No correlation between Delta Zn-66(fluid-magma) and temperature or chlorine contents in fluids is observed under our experimental conditions. Instead, Delta Zn-66(fluid-magma) is negatively corresponded with NBO/T of the melt (the ratio of non-bridge oxygen and tetrahedron ions), and positively correlated with the molar ratio of Al/(0.5 K + Ca + Fe) in the bulk magmas, suggesting the controlling of silicate composition on Zn isotope fractionation. Our data therefore indicate that the isotopically heavier Zn in the fluids exsolved from magmas may account for the higher delta Zn-66 of pegmatites and high-silica granitic rocks. Moreover, involvement of magmatic fluids can explain the highly variable and remarkably heavier Zn isotopic signatures of fumaroles, thermal spring waters, and seafloor hydrothermal fluids compared to the igneous rocks. This study provides information that can be used to guide research using Zn isotopes to trace fluid activity and magmatism. (C) 2021 Elsevier Ltd. All rights reserved.