Junji Torimoto

Origin of Spinel + Quartz Assemblage in a Si-undersaturated Ultrahigh-temperature Aluminous Granulite and its Implication for the P–T–fluid History of the Phulbani Domain, Eastern Ghats Belt, India

A suite of high-grade rocks including felsic gneiss, aluminous granulite, charnockite and calcsilicate granulite is exposed at Phulbani, which belongs to a petrologically little understood crustal domain (Phulbani domain) of the Eastern Ghats Belt. The aluminous granulite is constituted of corundum þ spinel þ ilmenite þ garnet þ sillimanite þ quartz 6 K-feldspar 6 plagioclase 6 biotite. Textural analysis indicates that corundum, spinel, garnet and/or K-feldspar were formed as a result of biotite dehydration melting of a Si-poor protolith during prograde metamorphism. Although corundum and quartz coexist in micro-scale domains, phase diagram modelling suggests that garnetþ corundum þ spinel þ ilmenite þ sillimanite (up to 800 C at 8 kbar) and garnet þ spinel þ sillimanite þ ilmenite þ quartz (above 950 C at 8 kbar) assemblages were stabilized in two different temperature intervals while attaining the ultrahigh-temperature metamorphic peak. The transformation from corundumto quartz-bearing assemblages was principally governed by chemical reactions. Quartz, formed at the peak stage, produced complex reaction textures involving spinel, corundum, garnet and sillimanite during near-isobaric cooling. Intersection of the same mineral reactions during the prograde and the retrograde paths implies the near-closed-system behaviour of the lower crust, at least at microdomain-scale, possibly achieved after large-scale melt loss along the prograde-to-peak stage of evolution. The pressure–temperature path remained near-isobaric during the prograde and the retrograde evolution of the assemblages. High-density (up to 1 03 g cm) CO2-rich fluid inclusions in aluminous granulite, coarse-grained charnockite and felsic gneiss indicate that peak metamorphism and subsequent evolution occurred under a CO2-dominated fluid regime. The pressure–temperature–fluid evolutionary history of the Phulbani domain shows similarity to that of the adjacent Visakhapatnam domain of the Eastern Ghats Belt and poses questions on the status of the boundary separating these two domains.

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.

Deepest and hottest hydrothermal activity in the Okinawa Trough: the Yokosuka site at Yaeyama Knoll

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°C in temperature, it is the hottest in the OT. Notable Cl depletion (130 mM) and both high H2 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 H2-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.

Supplementary material from "Deepest and hottest hydrothermal activity in the Okinawa Trough: the Yokosuka site at Yaeyama Knoll"

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°C in temperature, it is the hottest in the OT. Notable Cl depletion (130 mM) and both high H 2 and CH 4 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.

A Case Study on Resistivity and Spectral IP properties of modern seafloor hydrothermal deposits in Japan

Summary Resistivity and induced polarization (IP) rock properties are effectively used in the exploration of massive sulphide deposits, formed by ancient seafloor hydrothermal activity. The present study has investigated these properties using sulphide-bearing drill core samples taken from the Ihaya North Knoll and the Iheya-Small Ridge, the location of modern seafloor hydrothermal deposits in Japan. Overall, the results showed that resistivity of sulphide-bearing sediments correlates with porosity, rather than sulphide content. Furthermore, the intensity of spectral IP (phase) was found to depend on the amount of the sulphide minerals, which has a peak at a high frequency range (several kHz). These features are similar to that of typical fine-grained disseminated sulphides. In contrast to the sediments, a massive sulphide sample was found to have unique IP property, possibly due to its large sulphide mineral particles.