The Southwest Indian Ridge (SWIR) is an ultraslow-spreading ridge where large hydrothermal fields (HFs) are widely distributed. The HFs differ in geological settings, basement rock compositions and mineral associations, but are commonly associated with massive sulfides and sulfide-rich hydrothermal vents that are rich in mercury (Hg). However, the source of Hg remains not well understood. This is a first report on the concentration and isotopic composition of Hg in sulfides from two large HFs, named Duanqiao and Yuhuang, in the SWIR. Sulfides from Duanqiao and Yuhuang showed elevated Hg concentrations, ranging from 3.5 x 10(2) to 8.1 x 10(3) ng/g and 4.4 x 10(2) to 4.4 x 10(4) ng/g, respectively, which suggest that seafloor mass sulfide deposits can be an important sink of Hg to the deep marine environment. In both HFs, pyrite (4.4 x 10(2)-4.4 x 10(4) ng/g) and sphalerite (8.3 x 10(2)-6.0 x 10(3) ng/g) show higher Hg concentrations than chalcopyrite (3.5 x 10(2) ng/g), suggesting that the replacement of Fe(II) and Zn(II) by Hg(II) is a major form of Hg incorporation in sulfides. Sulfides from Yuhuang show relatively larger delta Hg-202 and Delta Hg-199 ranges of -1.23 to -0.05 parts per thousand and -0.10 to 0.20 parts per thousand, respectively, compared with those from Duanqiao (delta(202) Hg: -0.63 to -0.12 parts per thousand; Delta(199) Hg: 0.02-0.10 parts per thousand), suggesting that the Hg sources are different in the two HFs. The differences in Delta(199) Hg suggest the dominance of magmatic/mantle Hg in the Duanqiao HF, but a mixture of magmatic/mantle and seawater Hg to the Yuhuang HF. We thus propose Hg isotopes as a potential source tracer of Hg in HFs and infer that magmatism may not only serve as a direct Hg source in HFs, but may also drive seawater circulation and cause the precipitation of Hg from hydrothermally circulated seawater. (C) 2020 Elsevier Ltd. All rights reserved.