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

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 H2. 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 H2-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 H2 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.

[1]  Kentaro Nakamura,et al.  Geochemistry of hydrothermally altered basaltic rocks from the Southwest Indian Ridge near the Rodriguez Triple Junction , 2007 .

[2]  Ko-ichi Nakamura,et al.  Liquid CO2 venting on the seafloor: Yonaguni Knoll IV hydrothermal system, Okinawa Trough , 2006 .

[3]  Ken Takai,et al.  Geochemical and microbiological evidence for a hydrogen-based, hyperthermophilic subsurface lithoautotrophic microbial ecosystem (HyperSLiME) beneath an active deep-sea hydrothermal field , 2004, Extremophiles.

[4]  J. Charlou,et al.  Geochemistry of high H2 and CH4 vent fluids issuing from ultramafic rocks at the Rainbow hydrothermal field (36°14'N, MAR) , 2002 .

[5]  S. Goffredi,et al.  Biogeography and Ecological Setting of Indian Ocean Hydrothermal Vents , 2001, Science.

[6]  E. Oelkers,et al.  The rainbow vent fluids (36°14′N, MAR): the influence of ultramafic rocks and phase separation on trace metal content in Mid-Atlantic Ridge hydrothermal fluids , 2002 .

[7]  Kei Okamura,et al.  Chemical characteristics of newly discovered black smoker fluids and associated hydrothermal plumes at the Rodriguez Triple Junction, Central Indian Ridge , 2001 .

[8]  T. Morishita,et al.  Determination of Multiple Trace Element Compositions in Thin (> 30 μm) Layers of NIST SRM 614 and 616 Using Laser Ablation‐Inductively Coupled Plasma‐Mass Spectrometry (LA‐ICP‐MS) , 2005 .

[9]  D. Günther,et al.  Inter-laboratory note. Laser ablation inductively coupled plasma mass spectrometric transient signal data acquisition and analyte concentration calculation , 1996 .

[10]  Fumiko Nakagawa,et al.  Development of a multibottle gas‐tight fluid sampler WHATS II for Japanese submersibles/ROVs , 2006 .

[11]  W. Seyfried,et al.  Determination of Fe-Cl complexing in the low pressure supercritical region (NaCl fluid): Iron solubility constraints on pH of subseafloor hydrothermal fluids , 1992 .

[12]  Y. Nozaki.  Elemental Distribution: Overview , 2001 .

[13]  W. Seyfried,et al.  Compositional controls on vent fluids from ultramafic-hosted hydrothermal systems at mid-ocean ridges: An experimental study at 400°C, 500 bars , 2003 .

[14]  T. Morishita,et al.  Simultaneous determination of multiple trace element compositions in thin (<30.MU.m) layers of BCR-2G by 193 nm ArF excimer laser ablation-ICP-MS: implications for matrix effect and elemental fractionation on quantitative analysis , 2005 .

[15]  A. Briais.  Structural analysis of the segmentation of the Central Indian Ridge between 20°30′S and 25°30′S (Rodriguez Triple Junction) , 1995 .

[16]  W. Seyfried,et al.  The solubility of chlorite solid solutions in 3.2 wt% NaCl fluids from 300–400°C, 500 bars , 1994 .

[17]  H. Fujimoto,et al.  Three-dimensional magnetic and gravity studies of the Rodriguez Triple Junction in the indian Ocean , 1996 .

[18]  T. Morishita,et al.  Simultaneous in-situ multi-element analysis of minerals on thin section using LA-ICP-MS , 2004 .

[19]  E. Baker,et al.  Variations in hydrothermal methane and hydrogen concentrations following the 1998 eruption at Axial Volcano , 1999 .

[20]  S. D’Hondt,et al.  Radiolytic hydrogen and microbial respiration in subsurface sediments. , 2007, Astrobiology.

[21]  W. Seyfried,et al.  Plagioclase and epidote buffering of cation ratios in mid-ocean ridge hydrothermal fluids: Experimental results in and near the supercritical region , 1989 .

[22]  E. Shock,et al.  Distinguishing ultramafic‐from basalt‐hosted submarine hydrothermal systems by comparing calculated vent fluid compositions , 2000 .

[23]  J. Escartín,et al.  Detachment faults at Mid-Ocean Ridges garner interest , 1998 .

[24]  K. V. Von Damm,et al.  Geochemical controls on hydrothermal fluids from the Kairei and Edmond Vent Fields, 23°–25°S, Central Indian Ridge , 2006 .

[25]  Kentaro Nakamura,et al.  Elemental Mobilizations during Hydrothermal Alteration of Oceanic Lithosphere , 2008 .

[26]  S. Jackson,et al.  A Compilation of New and Published Major and Trace Element Data for NIST SRM 610 and NIST SRM 612 Glass Reference Materials , 1997 .

[27]  G. A. Macdonald,et al.  Chemical Composition of Hawaiian Lavas1 , 1964 .

[28]  P. Hoppe,et al.  Garnet-field melting and late-stage refertilization in "Residual" abyssal peridotites from the Central Indian Ridge , 2002 .

[29]  Dana R. Yoerger,et al.  A Serpentinite-Hosted Ecosystem: The Lost City Hydrothermal Field , 2005, Science.

[30]  W. Seyfried,et al.  Calcium and sodium exchange during hydrothermal alteration of calcic plagioclase at 400°C and 400 bars , 1993 .