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Since the discovery of the Kairei hydrothermal field at the southernmost end of the Central Indian Ridge (CIR) in 2000, only four active hydrothermal vent fields have subsequently been discovered. These four hydrothermal fields show remarkable diversity in the chemical compositions of fluids and associated ecosystems. Focused geophysical mapping and rock sampling indicate that different tectonic setting constrains the different hydrothermal activity for each field. Two hydrothermal fields in the southern CIR are located on the axial rift-valley wall. The hydrogen-rich Kairei hydrothermal field at 25°19′S is constrained by both basaltic magmatism and detachment faulting that exhume ultramafic rocks on a shallow subsurface, whereas no evidence of ultramafic exposure is recognized in the typical mid-ocean ridge type Edmond hydrothermal field at 23°52′S. Two other hydrothermal fields have been newly discovered in the central CIR. The Solitaire field at 19°33′S is located about 2.6 km away from the neo-volcanic zone and is likely influenced by intra-plate volcanism. The Dodo field at 18°20′S is located at the center of the axial valley floor, where a basaltic sheet-lava flow buries the seafloor. The lava morphology and the existence of an adjacent large off-axis seamount support the idea that excess melt is supplied in this segment. The anomalous magmatism is likely influenced by mantle plume components or by a large-offset transform fault just north of the segment. The large diversity found in the four hydrothermal fields along the CIR provides important insights on the tectonic control of global hydrothermal systems.

In the nearly 40 years since the discovery of the deep-sea hydrothermal vent site at the Galápagos spreading center, more than 300 sites of high-temperature hydrothermal venting have been discovered and investigated around the world. Surprisingly, however, most of these sites are located in the Pacific and Atlantic Oceans, whereas only five hydrothermal vent sites have been discovered in the Indian Ocean. During the TAIGA project, we conducted four research cruises to investigate four of the five Indian Ocean hydrothermal vent sites (two of which were newly discovered during one of the TAIGA cruises) located along the Central Indian Ridge (CIR). The results of geological and geochemical analyses demonstrate wide variation in fluid chemistry, reflecting the diverse geological background of the CIR hydrothermal vent fields. Although the CIR is an intermediate-spreading ridge, the geological and geochemical features of the Kairei hydrothermal field appear to be similar to ultramafic rock-hosted hydrothermal fields found along the slow-spreading Mid-Atlantic Ridge (MAR). By contrast, the Dodo hydrothermal field shares similarities with the hydrothermal vent sites found along the fast-spreading East Pacific Rise. The Solitaire and Edmond hydrothermal fields are characterized by high pH and Cl levels, respectively, although the geological background underlying the unusual chemistry of their hydrothermal fluids is still uncertain. Additionally, extensive microbiological analyses of the Kairei hydrothermal vent site revealed that its microbial communities are affected by the chemical characteristics of the hydrothermal vent fluids. Macrofaunal analyses also revealed new faunal data for the Indian Ocean hydrothermal vents, including novel genera and families that are potentially indigenous to the Indian Ocean hydrothermal systems. In particular, the discovery and characterization of a new morphotype of “scaly-foot” gastropod raises the question of the mechanism and physiological role of iron sulfide mineralization. The results of our investigations extended knowledge of the Indian Ocean hydrothermal systems including geochemical variations of hydrothermal fluids, their relationships to the geological background, the biodiversity and biogeography of the hydrothermal vent-associated microbial and faunal communities. This, in turn, will provide important insight into the relationships among geological backgrounds, hydrothermal processes, and biological activities, not only at the Indian Ocean hydrothermal vents but also in global mid-ocean ridge hydrothermal systems.

Two hydrothermal fields, the Kairei and Edmond hydrothermal fields, are known in the southern Central Indian Ridge (CIR). The Kairei hydrothermal field at 25°19′S is associated with hydrogen-rich hydrothermal activity, whereas the Edmond hydrothermal field at 23°52′S is recognized in the typical mid-ocean ridge type hydrothermal activity. Differences of lithology and geological background between two hydrothermal fields are reflected in the different type of hydrothermal activity. We recovered more than 870 kg of rock samples by dredging from the southern part of the CIR adjacent to the Kairei and Edmond hydrothermal fields during the KH-10-6 cruise. Here, we present new petrological and geochemical data for MORB samples taken between the CIR-S1 and CIR-S4 segments with the aim of constraining distributions of lithology at the southern CIR, and discuss the petrogenesis and the mantle source for these basalts. The MORB melts that formed rocks within the CIR-S1, CIR-S2, and CIR-S4 segments equilibrated with mantle olivine at approximately 10 kbar, and were erupted after undergoing only minor fractionation. MORB samples from the CIR-S4 segment have slightly depleted trace element compositions, whereas MORB samples from the off-ridge part of the CIR-S1 segment are highly depleted. MORB samples from the Knorr seamount have enriched compositions involved a minor amount of hotspot-derived material, as indicated by previous isotope analyses. The presence of a depleted MORB source beneath the off-ridge section of the CIR-S1 segment indicates that the older mantle material at the boundary between the CIR-S1 and CIR-S2 segments was highly depleted. In turn, this suggests that the source mantle beneath the southern CIR is heterogeneous both along and across the present spreading axis, and that the composition of the mantle in this area is a function of the degree of mixing between depleted and enriched sources.

Peridotites and related gabbroic rocks are widely exposed in the Central Indian Ridge, where the H-rich-fluid-bearing Kairei hydrothermal field exists. We report on petrological and mineralogical characteristics of peridotites and gabbroic rocks recovered from an oceanic core complex at a latitude of 25° South (25°S OCC) and the Yokoniwa Rise around the Kairei hydrothermal field. Gabbros recovered from the 25°S OCC show a wide range of variations in terms of mineral chemistry and mineral assemblages (olivine-gabbro, gabbronorite to highly evolved oxide gabbro) and are similar to those from the Atlantis Bank of the Southwest Indian Ridge, an ultraslow-spreading ocean ridge. Peridotites recovered from 25°S OCC and the Yokoniwa Rise are generally characterized by moderately to highly depleted melt components. The partial melting of these peridotites is followed by chemical modification through interaction with a wide range of melts from relatively less evolved to highly evolved characteristics. Moderately to highly depleted melt components in the studied peridotites can be explained as being either residue after a relatively high-melt productivity period in intermediate-spreading ridges or a geochemically distinctive domain which has suffered from partial melting in the past rather than partial melting beneath the present mid-ocean ridge systems.

In 2010, we conducted seven surveys for the deep-sea hydrothermal plume through conductivity-temperature-depth profiler (CTD) “tow-yo” cast in the area of the Kairei field. We observed a turbidity anomaly with a maximum thickness of 120 m, the upper limit of which was at 2,150 m water depth, approximately 300 m above the Kairei field hydrothermal vents (~2,440 m). The depth of upper limit of turbidity anomaly around Kairei field was the same height as in previous reports. Because the maximum height of hydrothermal plumes are regulated by the density (temperature and salinity) of the end-member hydrothermal fluid and dilution by the ambient seawater, the height of the plume suggested that the hydrothermal activity of the Kairei field was also the same as 17 years ago. Deep sequencing of microbial 16S rRNA genes showed that the SUP05 phylotypes and , which are known as the potential sulfur oxidizer and/or possibly hydrogen oxidizer, were propagated in the early stage of the hydrothermal plume and in the hydrothermal fluid–seawater mixing zone near the Kairei hydrothermal vents. Our exploration found a hydrothermal plume at 14 km north of the Kairei field, which had different H/CH ratio expected from the end-member hydrothermal fluid of Kairei field and the ambient seawater mixing. The north plume had a lower H, higher CH concentration, and higher microbial cell density than those in the hydrothermal plume around Kairei field. The north hydrothermal plume represented too oxic condition to harbor methane production by anaerobic methanogens. In addition, our microbial community structure analysis based on deep sequencing of 16S rRNA genes more than 10,000-amplicon reads per one sample showed no signal of methanogenic archaea. This suggests little in situ methanogenesis from H in the plume. It seems likely that high concentration of methane in the north plume is derived from another hydrothermal plume source rather than the Kairei hydrothermal fluids. Further studies will be needed to understand the cause of high methane concentration in the north plume.

In deep-sea hydrothermal vent fields, faunal distribution is associated with the geochemical environments generated by hydrothermal vent activity. Hydrothermal vent fields on the Central Indian Ridge (CIR) are associated with vent fauna which is thought as a mixture of Atlantic and Pacific and are discretely distributed along the ridge axis of more than 1,000 km apart. In this chapter, faunal distribution in hydrothermal vent fields on the CIR is summarized at the intra- and inter-field levels. The species composition of the vent fauna in the four vent fields hitherto known is reviewed and updated, and faunal resemblance among the four vent fields of the CIR appears to reflect the number of species recorded, indicating that faunal surveys are not sufficient in describing the whole vent fauna on the CIR. All the genetic studies of the CIR vent fauna have indicated a high genetic connectivity among the local populations, despite the many potential dispersal barriers existing between the vent fields. On the basis of the spatial distribution of vent species in a vent field, typical vent fields on the CIR were classified into six zones, of which the central two zones are often covered by swarms in the Kairei and Edmond fields. The close relationship between vent fauna from the CIR and the western Pacific, compared to those from other regions, is highlighted. Knowledge of the Indian Ocean vent fauna is limited, and further quantitative information on the biodiversity of vent fauna will provide clues to the formation of biogeographical regions and the dispersal of vent fauna among deep-sea hydrothermal vent fields.

The Southern Mariana Trough back-arc basin is a currently active back arc basin, and it has fast spreading morphologic and geophysical characteristics, suggesting an additional magma supply, even though the full spreading rate is categorized as slow spreading. Five hydrothermal vent sites have been found within 5 km around the spreading axis at 13°N. The Japanese TAIGA Project selected this area as one of three integrated target sites, and TAIGA Project members conducted series of JAMSTEC research cruises for different types of geophysical surveys, together with dive observation and samplings by the submersible Shinkai 6500. We reviewed the results from these geophysical surveys and the volcanic rock samples to summarize the products from the TAIGA Project. The results provide strong constraints on the mantle dynamics and the crustal formation at the Southern Mariana Trough back-arc basin; all the results support that they are influenced by hydration derived from the subducting slab with accompanying the additional magma supply. Furthermore, the results from the geophysical and geological surveys for the five hydrothermal vent sites provide characteristic features on the hydrothermal activity and the features are different between on-axis and off-axis hydrothermal sites. The on-axis hydrothermal site is associated with an episodic diking event followed by fissures in a fourth order ridge segment, and its duration and size vary depending on the episodic diking event and on the fissures following. In contrast, the formation of the off-axis hydrothermal sites is closely related to the residual heat from the volcanism rather than tectonic stresses accompanied by faults, and the off-axis hydrothermal activity is for a long period and in a large scale. We summarized all the evidence to propose our scenario of the mantle dynamics, the crustal formation, and the hydrothermal activity of the Southern Mariana Trough back-arc basin.

The Southern Mariana Trough is an active back-arc basin with hydrothermal activity. We investigated relations between the back-arc spreading system and the hydrothermal system in this area by conducting a seismic reflection/refraction survey and a three-month campaign of seismic observations using ocean bottom seismometers. From a 3D seismic velocity structure analysis, we mapped a low-velocity structure just beneath the spreading axis, a high-velocity structure with convex upward beneath an off-axis knoll, and a thickening of layer 2 (to about 3 km) over the refraction survey area compared with normal mid-ocean ridges. We found very low seismicity in the hydrothermal area and high seismicity in areas of high topographic relief that probably represent arc volcanoes. The low-velocity structure at the axis suggests that there is some magmatic activity beneath the axis in the form of sheetlike mantle upwellings. These may constitute the hydrothermal heat source at this site. The high-velocity structure with convex upward at the off-axis knoll suggests the presence of off-axis volcanism there. The very low seismicity suggests that this volcanism may have ceased, thus residual heat of this off-axis volcanism may contribute the heat for hydrothermal activity at this site. A comparison of the velocity structure with other back-arc spreading zones and mid-ocean ridges shows that the Southern Mariana Trough has a relatively thick layer 2 with lower seismic velocities, suggesting that the crust was formed by magmas with high volatile contents, consistent with upwelling mantle influenced by subduction. The very low seismicity at the hydrothermal sites indicates that there are no faults or fractures related to the hydrothermal activity. This suggests that the activity is not related to tectonic stresses there.

The electrical resistivity of the oceanic crust is sensitive to the porosity of the crust and the fluid temperature within crustal fractures and pores. The spatial variation of the crustal porosity and the fluid temperature that is related to a hydrothermal circulation can be deduced by revealing an electrical resistivity structure of the oceanic crust involving a hydrothermal site. We carried out a magnetometric resistivity experiment using an active source to reveal an electrical resistivity structure of the oceanic crust at the Snail site on the ridge crest of the Southern Mariana Trough. Active source electric currents were transmitted along and across the ridge axis in a 4,000 m area including the Snail site. Five ocean bottom magnetometers were deployed around the Snail site as receivers to measure the magnetic field induced by the transmission of the active source electric currents. The amplitude of the induced magnetic field was calculated by maximizing data density and the signal to error ratio in the data, and locations of the transmissions were determined using several types of calibration data. An optimal 1-D resistivity structure of the oceanic crust, averaged over the experimental area, was deduced by least squares from the data of the amplitude of the magnetic field and the location of the transmission. After calculating magnetic field anomalies, which are deviations of the observed amplitude from the prediction of the optimal 1-D resistivity model, an optimal 3-D resistivity structure was deduced from the magnetic field anomalies through trial and error 3-D forward modeling. The optimal 1-D resistivity structure is a two-layer model, which consists of a 5.6 Ω-m upper layer having a 1,500 m thickness and a 0.1 Ω-m underlying half-space. Using Archie’s law and porosity profiles of the oceanic crust, the resistivity of 5.6 Ω-m at depths ranging from 800 to 1,500 m suggests the presence of high-temperature fluid related to the hydrothermal circulation. The resistivity of 0.1 Ω-m below 1,500 m depth may represent a magma mush that is a heat source for the hydrothermal circulation. The optimal 3-D resistivity structure includes a conductive anomaly (0.56 Ω-m in approximately 300 m area down to 400 m depth) immediately below the Snail site, two resistive anomalies (56 Ω-m with slightly larger volumes than the conductive anomaly) adjacent to the conductive anomaly on the across-ridge side, and three conductive anomalies away from the Snail site. The conductive anomaly immediately below the Snail site suggests hydrothermal fluid, and the adjacent resistive anomalies suggest areas of low porosity. The size and distribution of the conductive and resistive anomalies near the Snail site constrains the size and style of the hydrothermal circulation.

We compiled multi-narrow beam bathymetric data and geomagnetic field data obtained by a series of JAMSTEC research cruises in the Southern Mariana Trough back-arc basin, where is selected as one of three integrated target sites for the Japanese TAIGA Project. The bathymetric data are used to trace the non-transform offsets that define the ridge segments at the off-axis, and to characterize the seafloor morphology signatures from the bathymetry profiles across the spreading axes of two ridge segments. The geomagnetic field data are used to derive the crustal magnetization distribution and to estimate the spreading rate of the southern segment. Both of the spreading rate and the seafloor deepening rate of the southern segment support highly asymmetric seafloor spreading; much faster spreading in the west side of the spreading axis compared to the east side (trench side). We estimated the full spreading rate as 46 km/Myr with its half rate of 33 km/Myr for the west side and 13 km/Myr for the east side. In contrast to the southern segment, our results indicate that the northern segment has a different style of the asymmetric seafloor spreading; that is accompanied by an obvious trace of a ridge jump to the trench side. The local symmetry axis in the bathymetry profiles locates at a distance of 18 km to the west from the spreading axis, suggesting that it is the failed spreading axis due to the ridge jump. The location of this failed spreading axis coincides with the center of the bull’s eye feature in the Mantle Bouguer anomaly, suggesting that the ridge jump to the trench side with an increase in the magma supply. We propose that the influence of the low viscosity region in the mantle wedge due to hydration driven by water release from the subducting slab leads to the highly asymmetric seafloor spreading; the low viscosity mantle would preferentially captures the mantle upwelling zone beneath the spreading axis as the spreading axis has been kept in the area closed to the low viscosity region in the mantle wedge, resulting in the highly asymmetric seafloor spreading. Further, the different styles of the asymmetric seafloor spreading between the northern segment and the southern segment probably show evidence that the influence varies with the distance from the low viscosity region in the mantle wedge.