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Major and minor chemistry, Sr, Nd and Pb isotope ratios, water content and hydrogen isotope ratios of the backarc basin volcanic rocks taken from the active spreading ridge and surrounding area between 12°40′N and 13°15′N in the southern part of the Mariana Trough indicate a complex regional interplay of suprasubduction-zone magmatic sources.

In this area, active backarc spreading occurs along the eastern side of the Mariana Trough, and discrete seamounts align on two parallel chains east of the backarc spreading center. The spreading-center rocks originated from a typical mid-ocean ridge basalt (MORB)-like source, influenced progressively southwestward by proximity to the arc magmatic source and range from basaltic to dacitic in composition. The volcanic rocks from the seamount chains are explained by the mixing of two types of magmatic sources; one is originally a depleted mantle similar to the arc source for magmas farther north and the other is the MORB-like source of the spreading ridge rocks. The influence of the arc source on magma composition of the seamount chains is higher north of 13°N, indicating heterogeneity of the mantle wedge in this region and the merging of the two sources toward the southwest.

Seafloor hydrothermal mineralization of four active hydrothermal fields (Snail, Yamanaka, Archaean and Pika sites) in the Southern Mariana Trough was investigated to clarify the mineralogical and geochemical characteristics of the hydrothermal minerals. The Snail site and the Yamanaka site are located on the crest of the backarc spreading ridge (on-axis). The Pika site sits atop an off-axial volcano, 4.9 km distant from the spreading axis, while the Archaean site sits on the flank of the spreading ridge, about 1.5 km distant from the axis. In the four hydrothermal sites, chimney and mound sulfides are composed mainly of pyrite, marcasite, sphalerite and chalcopyrite. Conduits and interstices of these sulfide minerals are filled with late barite and anhydrite. Mineralizations of off-axial hydrothermal fields have mineralogical and geochemical signatures (Fe-rich and low f condition) similar to those of mineralizations at sediment-free mid-ocean ridge settings. By contrast, mineralizations of on-axial sites show distinctly Zn and Pb-rich (high f condition) signatures similar to those found in arc or rifting settings. Sulfur isotopic compositions of collected sulfide minerals show a similar diversity with a mid-ocean ridge range for the off-axial sites and an island-arc range for the on-axial sites. These isotopic signatures could be explained by varying proportions of magmatic sulfur leached from the underlying volcanic rocks and reduced seawater sulfates.

Ages of sulfide and sulfate mineralized samples collected from active hydrothermal fields in the Southern Mariana Trough were determined. In addition to samples collected from active and inactive chimneys, and sulfide breccia during dive expeditions, massive sulfide ores obtained by shallow drilling were studied. We applied Th/U radioactive disequilibrium dating technique to sulfide minerals, as the collected mineralized samples were dominated by marcasite, pyrite and sphalerite. In addition, electron spin resonance (ESR) dating was applied to a few barite-rich samples, for comparison purpose. A laser step heating Ar-Ar dating of the basement volcanic rock samples was also attempted.

Sulfide chimneys and ores collected from a hydrothermal mound located beside the spreading axis range in age from <100 to 3,520 years old, without notable hiatus. The growth rate of the massive sulfide ore body is calculated to be 0.12–1.5 mm year based on results of the core samples. This age range is comparable for those previously reported for giant hydrothermal mounds of a few 100 m in diameter. These results suggest >1,000 years of continuous hydrothermal activity would be necessary for the formation of a massive sulfide deposit.

Sulfide chimneys and breccia collected from two hydrothermal fields located on an off-axis knoll are up to 9,000 years old. Sulfide breccia collected from an active site on the spreading axis are 2,740 and 7,190 years old. Geophysical studies provided evidence for abundant magma supply in the Southern Mariana Trough, which would have fueled hydrothermal activities in this area for long duration. While geophysical evidence for crustal velocity anomaly below the off-axis knoll suggests mineralization at the off-axis sites is considered to be in the late-stage of the hydrothermal activity, the discrete ages from the on-axis site might reflect episodic hydrothermal activities related to diking events proposed by geophysical and geological studies.

Deep-sea hydrothermal vents harbor diverse prokaryotes. There are a variety of habitat types in a deep-sea hydrothermal field, e.g., active and inactive chimneys, iron-rich mats, venting fluid and hydrothermal plume. Numerous studies have shown the diversity and composition of prokaryotic communities in individual habitats. However, it is still unclear whether and how the characteristics of prokaryotic communities in their respective habitats are different. Previously, we reported 16S rRNA genes in a variety of habitats, i.e., hydrothermally active and inactive chimneys, iron-rich mats, a vent fluid, crustal fluids from boreholes, as well as ambient seawater in a back-arc basin hydrothermal field of the Southern Mariana Trough. Here we summarize the prokaryotic communities in the collected samples at higher taxonomic resolution (up to family level) using the detected 16S rRNA gene sequences and compare them using recently developed bioinformatics tools. The comparative analysis clearly highlights differences in prokaryotic communities among the habitat types on and below the seafloor in the Southern Mariana Trough. Furthermore, descriptions of cultured species and environmental clones close to the detected sequences provide valuable information for understanding of their distribution and potential of ecological roles in deep-sea hydrothermal fields.

The Mariana Trough is a back-arc basin in the Northwestern Pacific. To date, active hydrothermal vent fields associated with the back-arc spreading center have been reported from the central to the southernmost region of the basin. In spite of a large variation of water depth, no clear segregation of vent faunas has been recognized among vent fields in the Mariana Trough and a large snail dominates chemosynthesis-based communities in most fields. Although the Mariana Trough approaches the Mariana Arc at both northern and southern ends, the fauna at back-arc vents within the trough appears to differ from arc vents. In addition, a distinct chemosynthesis-based community was recently discovered in a methane seep site on the landward slope of the Mariana Trench. On the other hand, some hydrothermal vent fields in the Okinawa Trough backarc basin and the Izu-Ogasawara Arc share some faunal groups with the Mariana Trough. The Mariana Trough is a very interesting area from the zoogeographical point of view.

Molecular evolutionary rate of the COI (cytochrome oxidase subunit I) gene in the vent endemic genus (Gastropoda: Provannidae) was estimated to be 0.69 % per million year based on GTR (General time-reversible) + G (Gamma) + I (Proportion Invariant) distances and hypothesized divergence dates of 59–64 Ma (million years ago) between and its sister genus . The population history of , an endemic species to the Mariana Trough, the northwestern Pacific, was reconstructed by analyzing the nucleotide sequences of two fragments of mitochondrial DNA and an intron region of a nuclear gene for ATPS β (ATP Synthetase subunit β) and by extrapolating the estimated COI rate. Two genetically deviated groups with different patterns of geographical distribution were recognized in the analysis of the mitochondrial DNA and their age of divergence was estimated to be 0.91 Ma by the coalescent theory-based analysis of the nuclear gene data. The present geographical distributions of the two groups suggest that their ancestral populations were isolated in the central and southern Mariana Trough, respectively. Rapid expansion (increase of the population size) was suggested to have occurred in both groups at 0.26 and 0.17 Ma, respectively. Periodical changes of hydrothermal activity have apparently controlled the isolation and expansion of the local populations.

The Okinawa Trough is a back-arc basin behind the Ryukyu trench-arc system and located along the eastern margin of the Eurasian continent. Sulfide and sulfate mineralization associated with hydrothermal activity has been recognized in ten hydrothermal fields in the Okinawa Trough. Hydrothermal mineralization recognized in these fields is commonly represented by coexisting occurrence of zinc- and lead-enriched polymetallic sulfides and abundant sulfate minerals. The mineralogy and geochemical signatures present has led researchers to suggest these areas may be a modern analogue for the formation of ancient Kuroko-type volcanogenic massive sulfide (VMS) deposits. Recent seafloor drilling during IODP (Integrated Ocean Drilling Program) Expedition 331 documented the subseafloor structure of a hydrothermal system at the Iheya North Knoll. Mineral textures and hydrothermal assemblages present in the drilled cores obtained from a hydrothermal mound in the proximal area are consistent with Kuroko-type mineralization. Based on geochemical studies, the intra-field diversity of mineralization commonly recognized in the Okinawa Trough can be explained by subseafloor phase separation of the hydrothermal fluid, which reflects shallow water depth (from 700 to 1,600 m). The subseafloor phase separation may. play an important role to accumulate metal elements beneath the seafloor. Based on geophysical and geological studies, the Okinawa Trough is considered a back-arc basin in the rifting stage. Such a tectonic setting is characterized by development of normal faulting in brittle continental crust and frequent intrusion of a magma, which can be expected to provide favorable environment for development of a hydrothermal system

The mid-Okinawa Trough is recognized as an area with extensive volcanism and hydrothermal activity. The Iheya Graben is a depression in the mid-Okinawa Trough, extending approximately 100 km in an ENE-WSW strike. The graben lies 200 m below the surrounding flat-surfaced trough floor (from −1,400 to −1,600 m below sea level). The latest seismic profiles in the western Iheya Graben and adjacent areas reveal numerous normal faults, possibly in association with the rifting activity of the Okinawa Trough. The faulting of the Iheya Graben is non-listric syn-depression faulting, in contrast with dense listric faulting of the adjacent trough floor. The faulting in both areas consists of numerous seafloor-reaching active faults; however, recent activity is concentrated within the Iheya Graben. Non-listric faulting in the Iheya Graben shapes its present structure with large displacements. The displacements and fault propagations indicate the depression of the Iheya Graben was created with an abrupt flexural subsidence followed by extension with normal faulting. The event occurred largely before the formation of the overlying Iheya Ridge, which was reported to be at least 0.2 Ma. Such a significant event may be related to the present extensive volcanism in the region by means of rifting tectonics and magmatism.

ESR (electron spin resonance) ages were determined for barite crystals extracted from hydrothermal sulfide deposits taken at Daiyon-Yonaguni Knoll field, Hatoma Knoll field, Iheya North Knoll field, Hakurei Site of Izena Hole field, Yoron Hole field of the Okinawa Trough. The ages range from 4.1 to 16,000 years, being consistent with detection of Ra in younger samples and radioactive equilibrium/disequilibrium between radium and daughter nuclei. The variation of the ages within each sample is mostly within the statistical error range. The relative order of the ages is consistent with the result of Ra-Pb method, but the determining absolute ages is still an issue. The order of ages of the 5 hydrothermal fields would be arranged, from young to old as follows; Yoron Hole field, Daiyon-Yonaguni Knoll field, Hatoma Knoll field, being nearly equal to Iheya North Knoll field.

This review compiles fluid chemistries of the six known high-temperature hydrothermal fields in the Okinawa Trough (OT) and compares them to global representative fields with various tectonic/geologic backgrounds. The comparisons indicate that the chemical characteristics of the OT hydrothermal fluids are explained by linkages between (1) shallow water depth that constrains the maximum fluid temperature, (2) back-arc tectonic setting that introduces magmatic volatiles into the fluid, (3) probable silicic rock-based fluid-mineral interaction at the hydrothermal reaction zone, and (4) seafloor sediment around the vents that provides both compounds derived from sedimentary organic matter and biogenic compounds, such as methane, produced by microbial ecosystems in the sedimentary environment.

To explain the highly diverse gas compositions and stable isotope ratios of methane among the OT hydrothermal fields, “fluid-sediment interaction” has been further classified into several types with respect to processes (microbial or chemical) and stages of subseafloor fluid circulation (recharge or discharge). This concept, called the Microbial Methanogenesis at Recharge stage (MMR) model, enables us not only to deduce the geochemical origins of the hydrothermal fluid CH in each OT field but also to estimate the geographical distribution of hydrothermal fluid circulation via a two-dimension schematic illustration. The model, which links the fluid geochemistry with the subseafloor fluid migration path, will serve as a base for future studies also for any subseafloor geofluid systems that include hydrothermal systems, subseafloor methane hydrate, and seismogenic fault zone.