Geological background of the Kairei and Edmond hydrothermal fields along the Central Indian Ridge: Implications of their vent fluids’ distinct chemistry

Abstract

Hydrogen-rich hydrothermal areas, such as those in the Indian Ocean, may have had an influence on early evolution of life on Earth and thus have attracted interest because they may be a proxy for ancient ecosystems. The Kairei and Edmond hydrothermal fields in the Indian Ocean are separated by 160 km, but exhibit distinct fluid chemistry: Kairei fluids are hydrogen-rich; Edmond fluids are hydrogen-poor. At this region, the Central Indian Ridge shows an intermediate spreading rate, 48 mm year )1 as full rate, where the hydrothemal fields occur. Kairei field vent fluids show persistently high concentrations of H 2 . The Kairei field seems to be unique among hydrogen-enriched hydrothermal regions: most similar hydrogen-rich hydrothermal activity occurs along slowly spreading ridge, <40 mm year )1 . The geological and tectonic aspects of the Kairei and Edmond hydrothermal fields were studied to try to elucidate geological constraints on hydrogen production. Visual observations of the seafloor near Kairei from a submersible revealed olivine-rich plutonic rocks with olivine gabbro-troctolite-dunite assemblages exposed within 15 km of the vent field, with serpentinized ultramafic mantle rocks on the Oceanic Core Complex (OCC). The OCC area might be a recharge zone of Kairei hydrothermal activity producing H 2 -rich vent fluids. The chaotic seafloor within 30 km of the Kairei field reflects a magma-starved condition persisting there for 1 Myr. Asymmetric geomagnetic and gravity anomalies near the Kairei field can be used to infer that patchy olivine-rich intrusions are scattered within mantle ultramafics, where infiltrated seawater reacts with magma and ultramafic rocks or olivine-rich rocks. The heterogeneous uppermost lithosphere containing shallow olivine-rich rock facies surrounding the Kairei field provides abundant H 2 into the vent fluid through serpentinization. The hydrogen-rich Kairei field is hosted by basalt, with mafic-ultramafic olivine-rich lithology; the ordinary, hydrogen-poor Edmond field is hosted by a normal basaltic lithology. The contrasting geochemical signatures of the two fields reported here can also be found in ancient rocks from a juvenile Earth. This suggests that lithology-controlled generation of hydrogen may have operated for a long time and be relevant to the origin of life on Earth.

Tectonic Background of Four Hydrothermal Fields Along the Central Indian Ridge

“…11.3b). However, the fluid shows a typical sulfur-rich content, and there is no evidence of ultramafic exposures around the Edmond field (Van Dover et al 2001;Gallant and Von Damm 2006;Kumagai et al 2008). Unlike the other hydrothermal fields on fast-spreading ridge, the Edmond filed is not directly linked with the neo-volcanic zone, and the upward fluid path may be controlled by the normal faults of the riftvalley.…”

“…The hydrothermal fluid of the KHF is unique in its high H 2 and low CH 4 contents (Van Dover et al 2001;Gallant and Von Damm 2006;Kumagai et al 2008). In general, the high content of H 2 in the hydrothermal fluid is explained by the generation of H 2 through the serpentinization of ultramafic rocks.…”

“…The H 2 concentration in the Edmond fluid is markedly low, and in the range of typical basalt-hosted hydrothermal fluids (Gallant and Von Damm 2006;Kumagai et al 2008).…”

“…11.3a). Known as Uraniwa Hills, this OCC consists of two domed highs (Kumagai et al 2008): the northern hill is a rectangular-shaped dome, whereas the southern hill is a more elongated massif with corrugations on the surface. A prominent breakaway ridge bounds the eastern end of this massif.…”

“…The KHF is a blacksmoker-type high temperature field, but fluids have an unusually high concentration of H 2 (2.5-8.5 mmol/kg), despite similarities to typical basalt-hosted mid-ocean ridge hydrothermal fluids in its other mineral and gas element compositions (Van Dover et al 2001;Gallant and Von Damm 2006;Kumagai et al 2008). Takai et al (2004) proposed that the Kairei ecosystem is sustained by the primary production of hydrogenotrophic, hyperthermophilic methanogens, utilizing H 2 and CO 2 as primary energy and carbon sources.…”