Hidenori Kumagai

Abstract The Martian atmospheric Ne may reflect recent gas supply from its mantle via volcanic degassing, due to its short (∼100 Myr) escape timescale. The isotopic ratio of the Martian atmospheric Ne would therefore provide insights into that of the Martian mantle, further suggesting the origin of Mars volatiles during planetary formation. Mass spectrometric analysis of the Martian atmospheric Ne, however, has faced challenges from interference between ²⁰Ne⁺ and ⁴⁰Ar⁺⁺. Previous studies using a polyimide membrane for ²⁰Ne/⁴⁰Ar separation were limited by the drawbacks of elastomeric O-rings to support the membrane, such as low-temperature intolerance, outgassing, and the need to endure environmental conditions during the launch and before/after landing on Mars. This study proposes a new method employing a metal C-ring to secure a 100 μm polyimide sheet within vacuum flanges. Environmental tests, including vibration, shock, extreme temperatures, and radiation exposure, were conducted on the gas separation flanges. Pre- and post-test analyses for He, Ne, and Ar demonstrated the membrane-flange system’s resilience. Gas permeation measurements using terrestrial air effectively permeated ⁴He and ²⁰Ne, while reducing ⁴⁰Ar by more than six orders of magnitude. This study achieved a <3% accuracy in determining the ²⁰Ne/²²Ne ratio, sufficient for assessing the origins of Ne in the Martian mantle. Furthermore, experiments with a 590 Pa gas mixture simulating the Martian atmosphere achieved a 10% accuracy for the ²⁰Ne/²²Ne isotope ratio, with gas abundances consistent with numerical predictions based on individual partial pressures. These results validate the suitability of the developed polyimide membrane assembly for in situ Martian Ne analyses.

We determined the K-Ar ages of phengite occurring as a gangue silicate mineral in the massive sulfide ores from the Kotsu and Iyo Besshi-type volcanogenic massive sulfide (VMS) deposits in the Sanbagawa high-P/T metamorphic belt, southwest Japan. The K-Ar ages of phengite are dated to be ca. 45 or 85 Ma and these ages correspond to the timing of initial or terminal stages of regional retrograde metamorphism. Contrastingly, the Re-Os isochron ages of sulfide mineral aggregates from the Kotsu and Iyo Besshi-type VMS deposits are ca. 145 Ma, which is much older than those of phengite K-Ar ages. Since exactly the same ore samples were used for both K-Ar and Re-Os geochronology, it was confirmed that the K-Ar isotope system was reset due to the regional metamorphism up to lower epidote-amphibolite facies, although the Re-Os isotope system retained the original geochemical and isotopic composition even though the regional (Sanbagawa high-P/T) metamorphism.

地質試料に対する希ガス同位体分析は、大気混染との戦いである。一方、今世紀に入っての二次イオン質量分析計(SIMS)やレーザーアブレーション誘導結合プラズマ質量分析計(LA-ICP-MS)などの局所同位体分析の発展は地球史初期の様相をはじめ地質試料から多様な情報を引き出しつつある。熱年代学にも活用されている希ガス同位体分析がこの流れにどの程度追随できるかは予断を許さない。この小論では、一世代前の希ガス質量分析装置を用いた筆者らの局所微量分析の試みを報告する。

As the alternate survey protocol for AUV-ROV sequential focused geological survey in great water depth, HOV mounted MBES survey followed by the geological observation and sampling was conducted. HOV mounted MBES provided a sub-meter scale high resolution bathymetric map, which efficiently supported the geological observation and resolve down to one meter scale morphological features successfully. Further, a pre-gridded point clouds imagery contains detailed morphological information.

<title>Abstract</title>Seafloor massive sulphide (SMS) deposits, modern analogues of volcanogenic massive sulphide (VMS) deposits on land, represent future resources of base and precious metals. Studies of VMS deposits have proposed two emplacement mechanisms for SMS deposits: exhalative deposition on the seafloor and mineral and void space replacement beneath the seafloor. The details of the latter mechanism are poorly characterised in detail, despite its potentially significant role in global metal cycling throughout Earth’s history, because in-situ studies require costly drilling campaigns to sample SMS deposits. Here, we interpret petrographic, geochemical and geophysical data from drill holes in a modern SMS deposit and demonstrate that it formed via subseafloor replacement of pumice. Samples from the sulphide body and overlying sediment at the Hakurei Site, Izena Hole, middle Okinawa Trough indicate that sulphides initially formed as aggregates of framboidal pyrite and matured into colloform and euhedral pyrite, which were replaced by chalcopyrite, sphalerite and galena. The initial framboidal pyrite is closely associated with altered material derived from pumice, and alternating layers of pumiceous and hemipelagic sediments functioned as a factory of sulphide mineralisation. We infer that anhydrite-rich layers within the hemipelagic sediment forced hydrothermal fluids to flow laterally, controlling precipitation of a sulphide body extending hundreds of meters.

We recovered sedimentary and plutonic rocks from the off-ridge portion of the Southwest Indian Ridge (37 degrees 00.02' E, 44 degrees 49.73' 5, 2165 m deep). The petrography, geochemistry, and geochronology of these plutonic rocks were analyzed. Ar-Ar dating of biotite in the plutonic rocks yielded early Paleozoic ages of approximately 475 and 490 Ma, indicating that these rocks are likely related to Antarctic orogenies during the late Neo-proterozoic to early Paleozoic. In addition, the samples exhibited trace element compositions and low initial epsilon Nd values (-8 and -12) similar to those of rocks from western Dronning Maud Land and the Transantarctic Mountains, Antarctica. Such similarities suggest that these plutonic rocks from the off-ridge portion of the Southwest Indian ridge were derived from western Dronning Maud Land or the Transantarctic Mountains and deposited by melting icebergs as dropstones.

Although seafloor massive sulfide (SMS) deposits are crucially important metal resources that contain high-grade metals such as copper, lead, and zinc, their internal structures and generation mechanisms remain unclear. This study obtained detailed near-seafloor images of electrical resistivity in a hydrothermal field off Okinawa, southwestern Japan, using deep-towed marine electrical resistivity tomography. The image clarified a semi-layered resistivity structure, interpreted as SMS deposits exposed on the seafloor, and another deep-seated SMS layer at about 40-m depth below the seafloor. The images reinforce our inference of a new mechanism of SMS evolution: Upwelling hydrothermal fluid is trapped under less-permeable cap rock. The deeper embedded SMS accumulates there. Then hydrothermal fluids expelled on the seafloor form exposed SMS deposits.

We observed the initial release rate of metals from four fresh (i.e., without long time exposure to the atmosphere) hydrothermal sulfide cores into artificial seawater. The sulfide samples were collected by seafloor drilling from the Okinawa Trough by D/V Chikyu, powdered under inert gas, and immediately subjected to onboard metal-leaching experiments at different temperatures (5°C and 20°C), and under different redox conditions (oxic and anoxic), for 1-30h. Zinc and Pb were preferentially released from sulfide samples containing various metals (i.e., Mn, Fe, Cu, Zn, Cd, and Pb) into seawater. Under oxic experimental conditions, Zn and Pb dissolution rates from two sulfide samples composed mainly of iron disulfide minerals (pyrite and marcasite) were higher than those from two other sulfide samples with abundant sphalerite, galena, and/or silicate minerals. Scanning electron microscopy confirmed that the high metal-releasing sample contained several galvanic couples of iron disulfide with other sulfide minerals, whereas the low metal-releasing sample contained fewer galvanic couples or were coated by a silicate mineral. The experiments overall confirmed that the galvanic effects with iron disulfide minerals greatly induce the initial release of Zn and Pb from hydrothermal sulfides into seawater, especially under warm oxic conditions.

The commercial use of metals such as copper, lead, and zinc has markedly increased in recent years, resulting in increased interest in deep-sea mining of seafloor hydrothermal sulfide deposits. However, the full extent of the impact of deep-sea mining at hydrothermal field deposits on the environment remains unclear. In addition to impacting the deep sea, the leaching of heavy metals from extracted sulfide mineral may also affect the upper ocean zones as the sulfide rock is retrieved from the seafloor. Here, we used a delayed fluorescence-based bioassay using the marine cyanobacterium Cyanobium sp. NIES-981 to evaluate the toxicity of three sulfide core samples obtained from three drill holes at the Izena Hole, middle Okinawa Trough, East China Sea. Leaches from two of the cores contained high concentrations of zinc and lead, and they markedly inhibited delayed fluorescence in Cyanobium sp. NIES-981 compared with control. By examining the toxicity of artificial mixed-metal solutions with metal compositions similar to those of the leaches, we confirmed that this inhibition was a result of high zinc and lead concentrations into the leaches. In addition, we conclude that this delayed fluorescence-based bioassay is a viable method for use by deep-sea mining operations because it is quicker and requires less laboratory space and equipment than the standard assay.

Microbial life inhabiting subseafloor sediments plays an important role in Earth's carbon cycle. However, the impact of geodynamic processes on the distributions and carbon-cycling activities of subseafloor life remains poorly constrained. We explore a submarine mud volcano of the Nankai accretionary complex by drilling down to 200 m below the summit. Stable isotopic compositions of water and carbon compounds, including clumped methane isotopologues, suggest that ~90% of methane is microbially produced at 16° to 30°C and 300 to 900 m below seafloor, corresponding to the basin bottom, where fluids in the accretionary prism are supplied via megasplay faults. Radiotracer experiments showed that relatively small microbial populations in deep mud volcano sediments (102 to 103 cells cm-3) include highly active hydrogenotrophic methanogens and acetogens. Our findings indicate that subduction-associated fluid migration has stimulated microbial activity in the mud reservoir and that mud volcanoes may contribute more substantially to the methane budget than previously estimated.

Copyright © 2018 by the International Society of Offshore and Polar Engineers (ISOPE) A series of scientific drilling at the submarine hydrothermal fields were conducted to decipher mineralization process of the hydrothermal deposits in the middle Okinawa Trough. By using LWD technique, in-situ measurements of temperature, flow-in of the fluid, electromagnetic nature in the boreholes were successfully achieved. Further, the physical properties of the drilled formations showed good agreement with those of cored samples analyzed on-board and shore-based.

Since the initial discovery of hydrothermal vents in 1977, these 'extreme' chemosynthetic systems have been a focus of interdisciplinary research. The Okinawa Trough (OT), located in the semi-enclosed East China Sea between the Eurasian continent and the Ryukyu arc, hosts more than 20 known vent sites but all within a relatively narrow depth range (600-1880 m). Depth is a significant factor in determining fluid temperature and chemistry, as well as biological composition. However, due to the narrow depth range of known sites, the actual influence of depth here has been poorly resolved. Here, the Yokosuka site (2190 m), the first OT vent exceeding 2000 m depth is reported. A highly active hydrothermal vent site centred around four active vent chimneys reaching 364 degrees C in temperature, it is the hottest in the OT. Notable Cl depletion (130 mM) and both high H-2 and CH4 concentrations (approx. 10 mM) probably result from subcritical phase separation and thermal decomposition of sedimentary organic matter. Microbiota and fauna were generally similar to other sites in the OT, although with some different characteristics. In terms of microbiota, the H-2-rich vent fluids in Neuschwanstein chimney resulted in the dominance of hydrogenotrophic chemolithoautotrophs such as Thioreductor and Desulfobacterium. For fauna, the dominance of the deep-sea mussel Bathymodiolus aduloides is surprising given other nearby vent sites are usually dominated by B. platifrons and/or B. japonicus, and a sponge field in the periphery dominated by Poecilosclerida is unusual for OT vents. Our insights from the Yokosuka site implies that although the distribution of animal species may be linked to depth, the constraint is perhaps not water pressure and resulting chemical properties of the vent fluid but instead physical properties of the surrounding seawater. The potential significance of these preliminary results and prospect for future research on this unique site are discussed.

Submarine hydrothermal deposits are one of the promising seafloor mineral resources, because they can store a large amount of metallic minerals as sulfides. The present study focuses on the electrical properties of active modern submarine hydrothermal deposits, in order to provide constraints on the interpretation of electrical structures obtained from marine electromagnetic surveys. Measurements of resistivity and spectral induced polarization (IP) were made using drillcore samples taken from the Iheya North Knoll and the Iheya Minor Ridge in Okinawa Trough, Japan. These hydrothermal sediments are dominantly composed of disseminated sulfides, with minor amounts of massive sulfide rocks. The depth profiles of resistivity and spectral IP properties were successfully revealed to correspond well to layer-by-layer lithological features. Comparison with other physical properties and occurrence of constituent minerals showed that resistivity is essentially sensitive to the connectivity of interstitial fluids, rather than by sulfide and clay content. This suggests that, in active modern submarine hydrothermal systems, not only typical massive sulfide rocks but also high-temperature hydrothermal fluids could be imaged as low-resistivity anomalies in seabed surveys. The spectral IP signature was shown to be sensitive to the presence or absence of sulfide minerals, and total chargeability is positively correlated with sulfide mineral abundance. In addition, the massive sulfide rock exhibits the distinctive IP feature that the phase steadily increases with a decrease of frequency. These results show the effective usage of IP for developing and improving marine IP exploration techniques.

Deep-sea hydrothermal vents discharge abundant reductive energy into oxidative seawater. Herein, we demonstrated that in situ measurements of redox potentials on the surfaces of active hydrothermal mineral deposits were more negative than the surrounding seawater potential, driving electrical current generation. We also demonstrated that negative potentials in the surface of minerals were widespread in the hydrothermal fields, regardless of the proximity to hydrothermal fluid discharges. Lab experiments verified that the negative potential of the mineral surface was induced by a distant electron transfer from the hydrothermal fluid through the metallic and catalytic properties of minerals. These results indicate that electric current is spontaneously and widely generated in natural mineral deposits in deep-sea hydrothermal fields. Our discovery provides important insights into the microbial communities that are supported by extracellular electron transfer and the prebiotic chemical and metabolic evolution of the ocean hydrothermal systems.

Halogens and noble gases within submarine basaltic glasses are critical tracers of interactions between the surface volatile reservoirs and the mantle. However, as the halogens and noble gases are concentrated within seawater, sediments, and the oceanic crust this makes the original volatile signature of submarine basaltic lavas susceptible to geochemical overprinting. This study combines halogen (Cl, Br, and I), noble gas, and K concentrations within a single submarine basaltic quenched margin to quantify the amount of seawater assimilation during eruption, and to further elucidate the mechanisms of overprinting. The outer sections of the glass rim are enriched in Cl compared to the interior of the margin, which maintains mantle-like Br/Cl, I/Cl, and K/Cl ratios. Low Br/Cl and K/Cl in the outer sections of the basaltic glass margin indicate that the Cl enrichment in the outer glass is derived from the assimilation of a saline brine component with up to 70% of the Cl within the glass being derived from brine assimilation. Atmospheric noble gas contamination is decoupled from halogen contamination with contaminated outer sections maintaining MORB-like Ar-40/Ar-36, suggesting seawater-derived brine assimilation during eruption is not the dominant source of atmospheric noble gases in submarine basalts. Volatile heterogeneities in submarine basalts introduced during and after eruption, as we have shown in this study, have the potential to expand the range of mantle halogen compositions and only by better understanding these heterogeneities can the Br/Cl and I/Cl variance in mantle derived samples are determined accurately.

Natural gamma ray logging was carried out at the five wells in Izena Hole off-shore Okinawa by D/V Chikyu on November to December in 2016. This is one of the research cruise under an umbrella of Cross-ministerial Strategic Innovation Promotion Program (SIP), "Next-generation technology for ocean resources exploration (Zipangu in the Ocean program)". The purpose of this survey is to understand geological profile and distribution of the subsurface hydrothermal deposit. It is difficult to take good quality and recovery cores in hydrothermal brittle formation. On the other hand, the continuous natural gamma ray logging data allows us to describe continuous vertical formation profile. The memory-type logging tool was stuck out just below the drill bit at the bottom of borehole after cutting cores. The tool measures logging data during pulling out of the hole. Natural gamma ray, temperature and pressure were measured in the boreholes. The gamma ray data by logging tool has good correlation with that in the core data by laboratory measurement until ~30-80m below sea floor at the good core recovery intervals. The logging data are only continuous profile below the depth till maximum 180m below sea floor. The gamma ray data indicate classification of sediment layer, ore body, and alternate layer in this region. The temperature logging indicates potential of hydrothermal deposit activity. The pressure data shows static water column.

During the Integrated Ocean Drilling Program (IODP), a total of 41.54 m of basement rock, consolidated volcaniclastic sediment, was recovered beneath a thin sediment cover. The drilled site is at the eastern end of the crestal area of Bowers Ridge, a north and westward sweeping offshoot of the Aleutian Arc into the Bering Sea. The volcanic sequence recovered from Holes U1342A and U1342D was divided into six major lithologic units. We used the single grain Ar-40-Ar-39 dating method performed by step-wise heated laser fusion technique to date andesites of Unit 1. Thereby two ages of Oligocene volcanism (34-32 Ma, 28-26 Ma) were distinguished each other according to our Ar-40-Ar-39 data. These ages refute a hypothesized Cretaceous origin in the North Pacific as an exotic arc massif or sector of the Hawaiian Emperor chain and indicate that the Bowers Ridge is a Bering-Sea formed arc or remnant arc that ceased forming in the latest Oligocene to the earliest Miocene time. (C) 2015 Elsevier Ltd. All rights reserved.

© 2016 JFES. All rights reserved. The CK16-01/Expedition 908 was conducted at the active hydrothermal field in the Iheya-North Knoll and Noho in Okinawa trough Japan by D/V Chikyu in Feb-Mar 2016. This expedition drilled hydrothermal fields recently discovered in the Iheya Small Ridge (Noho site: as reported in the JOGMEC news release on December 4, 2014) using LWD and geothermal tools. The purpose of this study in 2016 are to establish the origin and development model of deep water hydrothermal deposit, and to examine continuity of reservoir and mineral deposits in this area. The project was a part of “Scientific Research on Genesis of Marine Resources” for “Next-generation Technology for Ocean Resources Exploration,” of the Cross-ministerial Strategic Innovation Promotion Program (SIP). LWD was conducted at 3 sites in Iheya-North Knoll and at 4 site in Noho. The gamma ray, various resistivities, and high resolution resistivity borehole image allows us to understand the futures of hydrothermal sediments. Real-time annular pressure and temperature allow us to analyze dynamic thermal formation fluid flow. The cores were acquired and quick measurement and analysis were carried out by onboard scientists. Monitoring devices were successfully installed at active hydrothermal field in Iheya and Noho to record temperature, pressure and flow rate of the hot fluid, and to observe its process of deposits.

Studies on olivine-rich troctolites from oceanic ridges propose that hybridized mantle rocks may locally constitute small portions of the lower oceanic crust. The exact reaction process by which they originate is still debated and their hybrid nature is controversial. We show that textural and chemical inheritances of the pre-existing mantle are preserved in olivine-rich troctolites recently sampled at the Central Indian Ridge. The occurrence of a large orthopyroxene of a probable mantle origin suggests that these rocks formed through the reactive overprint of a mantle peridotite. Combining our data with those of olivine-rich troctolites worldwide, we show that the clinopyroxenes from these rocks follow chemical trends slightly distinct to those of the oceanic gabbros. These chemical trends can be ascribed to crystallization from melts assimilating mantle peridotites, suggesting that a "mantle flavor" can be locally retained in these hybrid rocks. The present distribution of Ol-rich troctolites suggests that melt-mantle reaction processes by which these rocks originate is likely to be more diffuse at slower spreading environments, where extensive melt-rock reactions within a thick thermal boundary layer enhances the conversion of the shallow oceanic mantle into hybrid crustal rocks. (C) 2015 Elsevier B.V. All rights reserved.

Peridotites and related gabbroic rocks are widely exposed in the Central Indian Ridge, where the H2-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.

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.

© 2015 JFES. All rights reserved. The CK14-04/Expedition 907 was conducted at the active hydrothermal field on the Iheya-North Knoll at Okinawa trough Japan by D/V Chikyu in July 2014. High temperature hydrothermal fluid flow exceeding 300degC was expected at the active hydrothermal deposits from the previous surveys. To achieve successful survey and to protect LWD tools, 1) pre-survey dynamic temperature modeling was carried out, 2) continuous circulation system "Non Stop Driller", which makes constant drilling fluid circulation even during pipe connection, was used, 3) LWD tools were upgraded up to 175degC, was used, and 4) real-time annular temperature and pressure were monitored. Flow rate was carefully controlled with the Non Stop Driller System after the pre-survey temperature modeling. The six wells were drilled with LWD successfully. The water depths are 886m to 1132m. High quality natural gamma-ray and resistivity, annular temperature and annular pressure mounted on arcVISION and TeleScope throw a light to active hydrothermal deposits first ever. The maximum annular temperate anomaly at 84degC was observed. The combination of natural gamma-ray and resistivity logs, and time series of annular temperature and pressure allows us innovative interpretation for deep water hydrothermal deposits. These results suggest that a LWD survey can be a new method for investigating sea-floor hydrothermal deposits.

To elucidate the evolution of hydrothermal activities, we conducted an interdisciplinary study including geochemistry and biology to develop a method of obtaining reliable age information. As geochemical dating techniques, two methods applicable for hydrothermal ore minerals were developed and improved: electron spin resonance method and uranium–thorium disequilibrium method. Cross checks between the two methods generally showed good agreement for the range of hundreds to thousands of years. As biological analysis, the biodiversity among faunal communities in the targeted areas was analyzed at the species and DNA levels. Species and genetic diversity of the local fauna were not always correlated to geochemical dating, either in the southern Mariana Trough region or in the Okinawa Trough region. Although the results are not simple, comparison of age information obtained from analyses of these two disciplines potentially provides important constraints for discussion of the history and evolution of hydrothermal activities.

The dose rates in sea water in the Southern Mariana Trough sea floor hydrothermal area were investigated. The dose rate in sea water was less than detection limit (about 0.1 mGy/year) near the sea surface while it was around 2 mGy/year at the sea floor of hydrothermal area, where those levels of dose rates will be negligible for ESR (electron spin resonance) dating of barite of sulfide deposits. During the dive by a submarine vehicle, there were three locations at which the level of dose rate increases more than ten times as much as the usual level. One was very close to a hydrothermal vent, but no hydrothermal activities were found near the other locations.

© The Author(s) 2015. 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. Keywords.

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 230Th/234U 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 39Ar-40Ar 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 &lt 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-1 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 &gt 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 offaxis 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.

Polymetallic sulfides deposited in seafloor hydrothermal vents have recently attracted attention as potential deep-sea mineral resources for base, rare, and precious metals such as Cu, Zn, Pb, In, Ga, Ge, Au, and Ag. For future exploitation of this type of deep-sea mineral resources, development of effective methods for exploring seafloor hydrothermal activity is a key to provide the most promising list of fields. However, conventional exploration methods are likely laborious and time-consuming, and a more efficient methods for exploration of seafloor hydrothermal vents are to be further developed. In the last decade, water column observation using multibeam echo souder (MBES) systems have become successfully applied to exploration of seafloor hydrothermal vents. In 2013 and 2014, we conducted extensive water column surveys using MBES systems in the mid-Okinawa Trough. During the surveys, we detected 10 hydrothermal vent sites, including previously known sites, belonging to four relatively large hydrothermal vent fields, located at the Izena Hole, Iheya North Knoll, Iheya Small Ridge, and a seamount 15 km northwest of the Izena Hole. All of the hydrothermal vent sites are in groups of 2-3 vent sites belonging to a hydrothermal field. Morphological features of the acoustic water column anomalies (rising vertically up to similar to 1000 m from the seafloor without a significant change of width) implied that the acoustic water column anomalies were not caused directly by hydrothermal vent fluid flows. The depth of the top of the acoustic water column anomalies (similar to 500 m) corresponded rather well to the depth of the CO2 phase transition from liquid/clathratehydrate to vapor. This suggests that the acoustic water column anomalies are attributed to water mass including dense liquid CO2 droplets with clathrate-hydrate crusts, which are originally derived from the seafloor hydrothermal fluid discharges.

Kumagai, H, Adrovic, F, Iwase, R, Kinoshita, M, Machiyama, H, Hattori, M &amp; Okano, M, 2012, , InTech.

Seismic reflection data around the Iheya North Knoll hydrothermal field provide insights into geological structures that control subseafloor hydrothermal fluid flow in the sediment-covered continental backarc basin of the mid-Okinawa Trough. We identified the seismic expression of widespread porous volcaniclastic pumiceous deposits and intrusions as a result of silicic arc volcanism. The porous and permeable volcanic deposits are distributed in an area extending updip from the thick succession of the deep trough to the seafloor at the hydrothermal field. Their regional structure focuses the flow of hydrothermal fluids derived from the surrounding trough-fill sediments and directs them to the vents of the hydrothermal field. The high concentrations of CH4 and NH4 in the fluids of the hydrothermal field are likely derived from the interaction of migrating fluids with trough-fill sediments. (C) 2011 Elsevier B.V. All rights reserved.

We measured helium-3 (He-3), sulfur (S), fluorine (F), chlorine (Cl) and bromine (Br) concentrations in vesicles of Mid Ocean Ridge Basalts (MORBs) and Back-Arc Basin Basalts (BABBs). The aim of this study was to characterize the MORB-source composition and to estimate the fluxes of S, F, Cl and Br from the mantle. A newly developed gas extraction method called the "Frozen Crushing Method" was applied. Measured concentrations in vesicles were (4-31) x 10(-15) mol/g of He-3, (20-420) x 10(-9) mol/g of S, (60-5000) x 10(-9) mol/g of F, (160-450) x 10(-9) mol/g of Cl and (10-1300) x 10(-9) mol/g of Br. We estimated global fluxes of S, F, Cl and Br at Mid Ocean Ridges using the molar ratio of each element on the He-3 in the samples and the known mantle He-3 flux of 527 mol/yr. Estimated fluxes using element concentrations measured in vesicles are (1.2 +/- 0.7) x 10(10) mol/yr of S, (1.4 +/- 1.0) x 10(11) mol/yr of F, (1.4 +/- 0.5) x 10(10) mol/yr of Cl and (3.1 +/- 2.8) x 10(10) mol/yr of Br. In addition, fluxes calculated on the basis of estimated concentrations in the bulk basalts are (1-26) x 10(10) mol/yr of S, (2-120) x 10(11) mol/yr of F, less than (2-120) x 10(10) mol/yr of Cl and (4-240) x 10(10) mol/yr of Br.

Magmatic processes during the earliest stage of subduction initiation are still not well understood. We examined peridotites recovered from an exhumed crust-mantle section exposed along the landward slopes of the northern Izu-Bonin Trench using the Japan Agency for Marine-Earth Science and Technology's remotely operated vehicle KAIKO7000II. Based on the Cr# [Cr/(Cr + Al) atomic ratio] of spinel, two distinctive groups, (1) high-Cr# (&gt;0.8) dunite and (2) medium-Cr# (0.4-0.6) dunite, occur close to each other and are associated with refractory harzburgite. Two distinctive melts were in equilibrium with these dunites: a boninitic melt for the high-Cr# dunite and a mid-oceanic ridge basalt (MORB)-like melt for the medium-Cr# dunite. The TiO2 content of the latter melt is lower than typical MORB compositions. We suggest that the medium-Cr# dunite was a melt conduit for a basalt recently reported from the Mariana forearc that was erupted at the inception of subduction. The wide range of variation in the Cr#s of spinels in dunites from the Izu-Bonin-Mariana forearc probably reflects changing melt compositions from MORB-like melts to boninitic melts in the forearc setting due to an increase of slab-derived hydrous fluids and/or melts during subduction initiation.

Kumagai, H, Tsukioka, S, Yamamoto, H, Tsuji, T, Shitashima, K, Asada, M, Yamamoto, F &amp;amp; Kinoshita, M, 2010, &#039;Hydrothermal plumes imaged by high-resolution side-scan sonar on a cruising AUV,Urashima&#039;, vol. 11, no. 12.

&amp;emsp;As our understanding of seafloor hydrothermal systems grows, we recognize they are not always stable and sometimes show dramatic changes. In this review, the authors present a compilation of geochemical and geochronological studies that are helpful when investigating the evolving processes of submarine hydrothermal systems.&lt;br&gt;&amp;emsp;Chapter II describes the systematics and methodology of three dating techniques with discussions on their application to minerals formed by seafloor hydrothermal activities. The K-Ar (Ar-Ar) technique is popular for dating igneous rocks, but it is not appropriate for dating hydrothermal minerals because potassium is a trace component of sulfide/sulfate minerals. Following recent progress, micro-analytical techniques applying laser fusion are applicable for dating fluid inclusions and/or hydrothermal alteration minerals, which could provide important geochronological information. Uranium and thorium series disequilibrium dating have been employed for previous geochronological studies of hydrothermal minerals obtained from submarine ore deposits. To cover a wide time range, it is necessary to use various combinations of parent and daughter nuclides. Applying ESR dating to hydrothermal minerals is a rather new challenge. Although it needs several investigations to establish the methodology, it could be a useful rapid dating technique for a time range of less than one thousand years.&lt;br&gt;&amp;emsp;Chapter III introduces studies focusing on the evolution of seafloor hydrothermal activities over a short time scale (one week to a few years). Detection of event plumes associated with seafloor lava eruption brought an awareness of episodic hydrothermal activity triggered by magmatic perturbation. Subsequent dive studies revealed evolving geochemical processes, such as major changes of volatiles and elemental species concentrations of venting fluid. With remote real-time monitoring of acoustic T-waves generated by seafloor seismic activities, event detection and response cruises have been conducted successfully to investigate various evolving processes in more detail.&lt;br&gt;&amp;emsp;Chapter IV introduces studies focusing on the evolution of seafloor hydrothermal activities over a long time scale (tens of thousands of years). Radiometric dating studies of hydrothermal minerals such as sulfide and manganese oxide collected from the TAG mound, which is one of the largest hydrothermal mound structures, reveal an age distribution over at least 15000 years separated by quiescent intervals lasting up to 2000 years. On slow spreading ridges such as the Mid-Atlantic ridge, major fracture systems focus the hydrothermal discharge at one place for more than one thousand years with repeated reactivation.&lt;br&gt;&amp;emsp;In Chapter V, the authors discuss the direction of future studies. Although hydrothermal systems on mid-oceanic ridges have been well studied, those related to arc-backarc magmatic activities could provide more appropriate fields for studying the evolutionary process of submarine hydrothermal systems. Combining geochronological studies with geochemical and mineralogical studies would be important for reconstructing the evolution process in more detail.

Frictional melting experiments were performed on fine grain homogeneous gabbroid with high temperatures induced by frictional heating using a high-velocity apparatus. We examined whether rapid fault movement can equilibrate fault rock gas with atmospheric components by measuring volatile gas and noble gas isotopes from a gabbroid sample using a quadrupole mass spectrometer to detect released gas from the simulated fault rock.The anticipated rapid equilibration of volatiles during the frictional melting of rocks implies that the noble gas and volatile were released and mixed with the atmosphere during this experiment. Gases released from the sample were collected in a small aluminum tube in nitrogen atmosphere before and after the frictional melting experiment. The gas comprised carbon dioxide, water vapor, hydrogen, helium, and other noble gases. The He/Ar ratio and H2 concentration are higher than the pre-analysis of N2 atmosphere. This release of volatiles is consistent with the pseudotachylyte-like post experimental texture of specimen. It is also consistent with the co-seismic geochemical anomaly observed along a natural fault system.

Isotope ratios of noble gases in certain minerals are believed to be modified easily by their ambient reservoirs when the minerals reach temperatures that are higher than their closure temperatures. The thermal history of a rock or a geologic event, even a fault movement, can therefore be determined quantitatively by application of radiometric dating methods to different minerals. However, neither traditional K-Ar nor Ar-Ar dating methods are readily applicable to faulted rocks because, in the case of faulting, it is difficult to prove that the temperature of an event was greater than the closure temperatures of minerals. Frictional melting experiments performed on. ne grain homogeneous gabbroid with high temperatures induced by frictional heating using a high-velocity friction apparatus were performed to test whether rapid fault movement can reset the noble gas isotope ratio of fault rocks. The temperature on the artificial fault plane is much higher than the closure temperature of the K-Ar system, as inferred from the calculated cooling age. The anticipated rapid equilibration of volatiles during the frictional melting of rocks implies that the noble gas isotope composition/ratio was reset to an atmospheric value during this experiment. We measured noble gas isotopes in a gabbroid sample using laser fusion analysis. The resetting of noble gases is only observed in glass that had melted completely. Rejuvenation and/or apparent increases in the K-Ar ages were only observed in a narrow, 3 mm-wide zone around the fault plane, which appears to be thermally altered and mechanically fractured in thin sections. Because of frictional heating, such glassy materials, called pseudotachylytes, sometimes form along the fault plane in natural faults as a result of complete melting of rock induced by frictional heating. Thus these results suggest that K-Ar ages of fault activity can be quantitatively obtained from the analysis of pseudotachylytes. (C) 2008 Elsevier B. V. All rights reserved.

This paper presents the first detailed studies on the petrology of abyssal peridotites and related fault rocks recovered from an oceanic core complex (OCC) in the southern part of the Central Indian Ridge using the submersible SHINKAI 6500 of the Japan Agency for Marine-Earth Science and Technology. Less deformed, statically serpentinized peridotites were recovered from the ridge-facing slope, whereas highly deformed rocks were recovered from sheet-like structures on the top surface of the OCC. The top surface of the OCC is interpreted to be the main detachment fault. The serpentinized peridotites are consistent with an origin as residues after moderate degrees (13-15%) of partial melting; these were later chemically modified as a result of the infiltration of evolved melts of probable granitic composition resulting in the formation of leucocratic veins. The deformed rocks from the detachment fault are divided into talc-rich and chlorite-rich parts, probably formed as a result of interaction of hydrothermal fluids with peridotite and gabbro precursors along the detachment fault, respectively. Deformation and alteration were locally concentrated along the detachment fault, resulting in mechanical mixing of both altered gabbros and serpentinized peridotites in the deformed rocks during the exhumation of the OCC associated with long-lived fault activity. Our results reveal that gabbros and peridotites are tectonically exposed in oceanic core complexes on the seafloor along the intermediate-spreading CIR, as well as in slow-spreading regions. Fluid-mobile elements such as Li, Rb, Ba, Pb, Sr and U are higher in serpentines than their precursor mantle minerals. The uranium content in serpentine is variable but is abundant in the outermost margin of the precursor minerals. The trace element compositions of serpentine appear to have been continuously changed along with changes in the chemistry of the hydrothermal fluids as temperature decreased. Fluid-mobile elements were thus added and/or leached out during the serpentinization of the peridotite combined with later seafloor weathering.