Hidenori Kumagai

Diversity of melt conduits in the Izu-Bonin-Mariana forearc mantle: Implications for the earliest stage of arc magmatism

ABSTRACT 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 Technology9s remotely operated vehicle KAIKO7000II . Based on the Cr# [Cr/(Cr + Al) atomic ratio] of spinel, two distinctive groups, (1) high-Cr# (>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 TiO 2 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.

Petrology of local concentration of chromian spinel in dunite from the slow-spreading Southwest Indian Ridge

This is the first detailed report on local concentration of chromian spinel in a dunite from an ultraslow spreading ridge, the Southwest Indian Ridge (SWIR). The sample was collected from an outcrop with detailed observations using submersible SHINKAI 6500 of the Japanese Marine Science Technology Center. The dunite occurs as a tabular-shaped layer in a lherzolite host outcrop. Spinel is found as a string of small micropods 2–3 centimeters in size. These spinel micropods make a layer in the middle part of a spinel-poor dunite ( 0.4) compared with other peridotite samples in the studied area (lherzolites to harzburgite with low-Cr# spinel, typically Cr# ≤ 0.3). The occurrence and chemical compositions of clinopyroxene in the enstatite-poor harzburgite suggest that some clinopyroxenes crystallized from infiltrated interstitial melts. The host peridotites are interpreted as a residue of relatively low degrees of partial melting consistent with a location along the SWIR far from a mantle hot spot. This was then followed by crystallization of clinopyroxene from interstitial melt in the dunite. Irrespective of their small size, the lithological relationships between the spinel micropods and the host peridotites are the same as those for podiform chromitite in ophiolites and orogenic peridotites. The spinel Cr# in the micropods (0.3) is compatible with the lower range of those in basalts from SWIR far from hot spot. The spinel micropods were mainly formed by interaction between relatively depleted peridotite and a locally significant volume of basaltic melt traversing the upper mantle. This study coupled with the previous works on chromitites suggest that podiform chromites occur in every geodynamic setting, though economic concentrations of chromite (Cr-rich spinel) are unlikely to occur in the mantle at ultraslow spreading ridges.

Hydrothermal plumes imaged by high‐resolution side‐scan sonar on a cruising AUV, Urashima

An autonomous underwater vehicle (AUV) is an ideal platform that enables a closer approach to the seafloor than other platforms. The AUV provides remarkable advancement not only in imaging small-scale bathymetry with high resolution but also in imaging the spatial distribution of scattering signals within a water column. Acoustic echoes can be recorded continuously by AUVs, potentially providing information related to hydrothermal plumes within a water column. In May 2007, such an AUV, Urashima, of the Japan Agency for Marine–Earth Science and Technology, captured fantastic acoustic images within the water column above an active hydrothermal field, the Iheya-North field, off Okinawa. Numerous filamentous and hyperbolic echoes were recorded on the side-scan sonar images. According to the temperature and geochemical anomaly, the scattering signals within the water column were hydrothermal plumes upwelling from the vent area. This proof of concept has implications for long-range exploration for vent field with AUVs.

Spontaneous and Widespread Electricity Generation in Natural Deep‐Sea Hydrothermal Fields

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.

Transition from seamount chain to intraplate volcanic ridge at the East Pacific Rise

A number of large submarine intraplate volcanic ridges have been discovered throughout the South Pacific Basin, but their origins are enigmatic. Recent shipboard geophysical surveys reveal that the Sojourn Ridge, one of these large intraplate ridges, becomes a chain of discrete seamount volcanoes that intersects the ridge axis. This seamount chain exhibits several features that suggest that it is directly related to the Sojourn Ridge. The Sojourn Seamount Chain grows continuously in both volume and number of seamounts with distance from the spreading axis; several loci of recent volcanic activity along the chain are evident in the side-scan imagery, and a mantle Bouguer anomaly low underlies the entire length of the chain. This evidence provides new constraints on the origin of intraplate volcanic ridges. The continuation of the Sojourn Ridge system as a volcano chain that extends to within 5 km of the spreading axis implies active generation of magma and a focusing mechanism, such as flexural stresses induced by the mass of the volcanic pile, as the probable mechanism for developing volcanic ridges and long seamount chains.

Petrology of Peridotites and Related Gabbroic Rocks Around the Kairei Hydrothermal Field in the Central Indian Ridge

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.

Dating of Hydrothermal Mineralization in Active Hydrothermal Fields in the Southern Mariana Trough

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.

Deep-biosphere methane production stimulated by geofluids in the Nankai accretionary complex

Scientific drilling at a submarine mud volcano shows that geofluid migration stimulates methanogenesis in the deep biosphere. 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.

Depth profiles of resistivity and spectral IP for active modern submarine hydrothermal deposits: a case study from the Iheya North Knoll and the Iheya Minor Ridge in Okinawa Trough, Japan

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.Graphical abstract.

Evaluating Hydrothermal System Evolution Using Geochronological Dating and Biological Diversity Analyses

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.