Abstract
The recently discovered Onnuri hydrothermal vent field (OVF) is a typical off-axis ultramafic-hosted vent system, located on the summit of the dome-like ocean core complex (OCC) at a distance of ∼12 km from the ridge axis along the middle region of the Central Indian Ridge (CIR). The plume chemistry with high methane anomaly was consistent with the precursor of hydrothermal activity; however, the fundamental characteristic of the OVF system, such as the hydrothermal circulation process and source of heat, remains poorly understood. Here, we focus on the geochemical features of surface sediments and minerals collected at and around the OVF region in order to better understand this venting system. The results reveal that the OVF sediments are typified by remarkably high concentrations of Fe, Si, Ba, Cu, and Zn, derived from hydrothermal fluid and S and Mg from seawater; depleted C-S isotope compositions; and abundant hydrothermally precipitated minerals (i.e., Fe–Mn hydroxides, sulfide and sulfate minerals, and opal silica). Notably, the occurrence of pure talc and barite bears witness to strong hydrothermal activity in the OVF, and their sulfur and strontium isotope geochemistry agree with extensive mixing of the unmodified seawater with high-temperature fluid derived from the gabbroic rock within the ultramafic-dominated ridge segment. The findings reveal that the OVF is a representative example of an off-axis, high-temperature hydrothermal circulation system, possibly driven by the exothermic serpentinization of exposed peridotites. Given the widespread distribution of OCC with detachment faults, furthermore, the OVF may be the most common type of hydrothermal activity in the CIR, although the paucity of data precludes generalizing this result. This study provides important information contributing to our understanding of the ultramafic-hosted hydrothermal vent system with a non-magmatic heat source along mid-ocean ridges.
Highlights
Because of the importance of the hydrothermal vent system in ocean chemistry, earth dynamics, and biological communities, as well as the increasing interest in massive metal deposits, the exploration of mid-ocean ridges has recently increased dramatically, revealing considerable diversity in hydrothermal systems
In the Indian Ocean ridges with various speeding rates (
Most surface sediments are composed of yellowish, foraminiferal nanofossiliferous ooze, frequently including angular, dark glossy volcaniclastic particles, except for the sediment samples proximal to the hydrothermal edifices of the Onnuri hydrothermal vent field (OVF) with low CaCO3 contents
Summary
Because of the importance of the hydrothermal vent system in ocean chemistry, earth dynamics, and biological communities, as well as the increasing interest in massive metal deposits, the exploration of mid-ocean ridges has recently increased dramatically, revealing considerable diversity in hydrothermal systems. Systematic deep-sea exploration recently uncovered four new active hydrothermal vent fields in the middle region of the Central Indian Ridge (CIR) between 8 and 14◦S, where ridge morphology and tectonic structure (e.g., detachment faults and ocean core complexes, OCCs) control increased plume incidence at ridge flank and rift wall locations (Kim et al, 2020) These hydrothermal vent fields, located more than 800 km north of previously known vent fields (i.e., the Dodo and Solitaire fields, Nakamura and Takai, 2015) are characterized by particle-poor, diffuse venting with abundant vent fauna and sulfide deposition. Solid evidence for the serpentinization reaction (e.g., heat source) and the fluid temperature of the OVF is still lacking
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