Mercury (Hg) geochemistry of mid-ocean ridge sediments on the Central Indian Ridge: Chemical forms and isotopic composition

Abstract

To geochemically characterize mercury (Hg) in the deep-sea ridge environment, the total concentration, chemical forms (sequential leaching extraction), and isotopic compositions of Hg in surface sediments from the middle portion of the Central Indian Ridge were investigated. Highly elevated Hg concentrations (up to 13,000 ng/g) in sediments near the hydrothermal vent are associated with intense hydrothermal activity driven by serpentinization. The hydrothermal impact on these sediments is also evident in the REECN fractionation pattern with a remarkably strong positive europium (Eu) anomaly. Most volcanic and hydrothermal Hg in the study area is preferentially precipitated with sulfides; in the hydrothermal vent area, however, scavenging by Fe-Mn hydroxides is another significant removal pathway of Hg. Thus, such precipitation and production of sulfides and hydroxides are a major cause of local enrichment of Hg around the mid-ocean ridge. Most sediments show limited or no mass-independent fractionation (Δ199Hg = +0.02 ± 0.21‰, 2σ, n = 15), indicating that syngenetic magmatic or mantle-derived materials are the dominant Hg source. However, the large variation in mass-dependent fractionation was observed, especially in the vent-distal sediments (δ202Hg = −1.10 ± 0.80‰, 2σ, n = 11), which occurred mainly during the formation of the sulfides and may be associated with preferential precipitation of lighter isotopes. Our study demonstrates that an off-axis high-temperature hydrothermal system driven by exothermic serpentinization of ultramafic mantle rocks may serve as a significant Hg source and provides further insights into grasping the behavior of hydrothermal and volcanogenic Hg in active deep-sea ridge systems.