Iron, copper, and zinc isotopic fractionation in seafloor basalts and hydrothermal sulfides

Zhigang Zeng,Xiaohui Li,Shuai Chen,Jeroen De Jong,Nadine Mattielli,Haiyan Qi,Christopher Pearce,Bramley J Murton

doi:10.1016/j.margeo.2021.106491

Abstract

Studies of the Fe, Cu, and Zn isotopic compositions of volcanic rocks and sulfides provide an important tool for understanding magmatic, hydrothermal, and alteration processes, thereby enabling the determination of both transition metal sources and the quantification of the petrologic environmental impacts of hydrothermal activities. In this study, the δ56Fe and δ57Fe values of the mid-ocean ridge basalts (MORBs) are higher than those of the seafloor hydrothermal fluids, while the reverse is true for the δ66Zn and δ68Zn values, suggesting that basalt-fluid interactions preferentially incorporate isotopically light Fe and heavy Zn into the fluids, resulting in the relative enrichment of heavier Fe and lighter Zn isotopes in altered basaltic rocks. Most of the δ56Fe values (−1.96 to +0.11‰) of the sulfide minerals are within the range of the vent fluids, but they are significantly lower than those of the MORBs and back-arc basin basalts (BABBs), suggesting that the Fe in the sulfides was mainly derived from the fluids. However, the majority of the chalcopyrite δ56Fe and δ57Fe values are higher than those of the sphalerite and pyrite. This suggests that high-temperature sulfide minerals are enriched in 56Fe and 57Fe, whereas medium- and low-temperature sulfides are depleted in 56Fe and 57Fe. Moreover, the δ65Cu (−0.88 to −0.16‰) and δ66Zn (−0.39 to −0.03‰) values of the sulfide minerals are significantly lower than those of the MORBs, BABBs, and fluids, suggesting that 63Cu and 64Zn were preferentially removed from the fluids and incorporated into the chalcopyrite and sphalerite, respectively. Consequently, vent fluid injection and deposition can cause the heavier Cu and Zn isotopic compositions of hydrothermal plumes, seawater, and sediments.

Highlights

Seafloor hydrothermal fields often contain polymetallic massive sulfides, chimneys, mounds, and their host rocks
Most of the δ56Fe values (− 1.96 to +0.11‰) of the sulfide minerals are within the range of the vent fluids, but they are significantly lower than those of the mid-ocean ridge basalts (MORBs) and back-arc basin basalts (BABBs), suggesting that the Fe in the sulfides was mainly derived from the fluids
This is consistent with the varying fluid temperatures, which ranged from high to low during the pyrite formation (Fig. 7a) (Abraitis et al, 2004; Keith et al, 2016). These values are slightly lower than the δ56Fe values of seawater (− 0.88 to +0.10‰; Rouxel and Auro, 2010) and are similar to the δ56Fe values of hydrothermal fluids (− 1.85 to − 0.14‰; Sharma et al, 2001; Severmann et al, 2004; Rouxel et al, 2008; Moeller et al, 2014) (Fig. 3). All these findings indicate that the fluids may be a source of the light Fe isotopic compositions of the sulfides, and that 54Fe is more likely to be preferentially incorporated into pyrite facies during mixing between seawater and hydrothermal fluids

Summary

Introduction

Seafloor hydrothermal fields often contain polymetallic massive sulfides, chimneys, mounds, and their host rocks (e.g., basalts). Both high (>300 ◦C) and low temperature (

Methods

Results

Discussion

Conclusion