Highly alkaline,high-temperature hydrothermal fluids in the early Archean ocean

Based on the petrology of hydrothermally altered Archean basaltic greenstones, thermodynamic calculations of phase equilibria were conducted to estimate the composition of a high-temperature (∼350 °C) hydrothermal fluid in an Archean subseafloor basalt-hosted hydrothermal system. The results indicate that the hydrothermal fluid was highly alkaline attributed to the presence of calcite in the alteration minerals under a high-CO₂ condition, and predict a generation of SiO₂-rich, Fe-poor hydrothermal fluids in the Archean subseafloor hydrothermal system. The chemically reactive mixing zones between alkaline hydrothermal fluids and slightly acidic-neutral seawater are characterized by inverse pH and chemical polarity to modern hydrothermal systems, leading to extensive precipitation of silica and iron oxyhydroxides on/under the seafloor. Such processes can be responsible for the abiotic formation of voluminous chert and subseafloor silica dike, the mechanism of silicification, and the pH-controlled generation of banded iron formation that has been arising mainly from the redox chemistry in the Archean ocean. Such high-temperature alkaline fluids could have had a significant role not only in the early ocean geochemical processes but also in the early evolution of life.     

Gradients and phase velocities of ULF geomagnetic disturbances used to determine the source of an impending strong earthquake

Results of studying the behavior of the vectors of gradients and phase velocities of ULF geomagnetic disturbances (F < 1 Hz) in the Japan seismic zone are presented. The gradient and phase velocity vectors along the Earth’s surface were determined using data of the group of three high-sensitivity three-component magnetovariation stations (MVC-3DS) located at triangle vertices at a small (～5 km) distance from one another (magnetic gradiometer). Two such groups of stations were installed in 1999 southwest and southeast of Tokyo at a distance of ～150 km from each other. It has been indicated that, several months before strong earthquakes (M > 5), the values of gradients and phase velocities start anomalously changing, and directions toward sources of impending strong earthquakes appear in the distribution of gradient vector directions. Directions from sources of impending earthquakes appear in the distribution of phase velocity vector directions. It is proposed to use gradients and phase velocities of ULF and ELF geomagnetic disturbances as one of the factors in a short-term prediction of strong earthquakes.     

Nonaqueous titration of functional groups in humic acid

Humic acid was titrated by sodium methoxide in dimethylsulfoxide using platinum-calomel electrode systems. Adding benzoic acid and phenol as internal standards to humic acid yielded two inflections. The titer at the first inflection point was equivalent to the carboxyl groups whose pK_a (H₂O) values were less than 7. The difference between the titers at the two inflection points was equivalent to the phenolic hydroxyl groups whose pK_a (H₂O) values were 7–10. Calculated results for the carboxyl and phenolic hydroxyl groups in humic acid obtained by the nonaqueous titration method agreed closely with those obtained by conventional methods.     

Geochemical characteristics of zircons in the Ashizuri A‐type granitoids: An additional granite topology tool for detrital zircon studies

Zircon is resistant to alteration over a wide range of geological environments, and isotopic ratios within the mineral provide constraints on ages and their parental magmas. Trace element compositions in zircon are also expected to reflect those of their parent magmas, and have a potential as essential indicators for their host rocks. Because most detrital zircons that accumulate at river mouths are derived primarily from granitoids, the classification of zircon within granitoids is potentially meaningful. This study employs the conventional classification scheme of granites (I‐, S‐, M‐, and A‐types). To clarify geochemical characteristics of zircons in A‐type granites, trace element compositions of zircons extracted from the A‐type Ashizuri granitoids were examined. Zircons from the Ashizuri granitoids commonly show enrichments of heavy rare earth elements and positive Ce anomalies, indicating that these zircons were igneous in origin. In addition, zircons in these A‐type granites are characterized by enrichments of Nb, Y, Ta, Th, and U and strong negative Eu anomalies, which exhibit good positive correlations with those in their whole rocks. This fact indicates that these signatures in zircons reflect well those in their parental bodies and are useful in identifying zircons derived from A‐type granite. Based on compilations of available data, zircons from A‐type granites can be clearly discriminated from other‐types of granites within Nb/Sr–Eu anomaly, U/Sr–Eu anomaly, Nb/Sr–U/Sr, and Nb/Sr–Ta/Sr cross‐plots. All indices used in these diagrams were selected based on the geochemical features of both zircon and whole rock of A‐type granites. Application of these discrimination diagrams to detrital zircons will likely provide further insights. For example, some Hadean detrital zircons plot in similar fields to A‐type granites, implying the existence of A‐type magmatism in the Earth's earliest history.     

Preface to the special issue on electromagnetic phenomena related to seismic and volcanic activities from EMSEV in 2016

<正>EMSEV,Electromagnetic Studies of Earthquake and Volcanoes,set up in 2002,is a joint academic organization by multi-associations in IUGG(International Union of Geodesy and Geophysics),including IAGA(International Association of Geomagnetism and Aeronomy),IASPEI(International Association of Seismology and Physics of the Earth’s Interior),IAVCEI(International Association of Volcanology and Chemistry of the Earth’s Interior).On     

Eocene extensional tectonics in the Amakusa region,northern Ryukyu arc

Paleogene surface tectonics in Japan is not well understood because of the paucity of onshore Paleogene stratigraphic records except for those from accretionary complexes. Paralic Paleogene formations remaining in SW Japan are usually so thin that it is difficult to decipher the tectonics from them. However, the Eocene paralic sedimentary package with a thickness of kilometers indicates syn-depositional tectonic subsidence by a few kilometers in the Amakusa archipelago, west of Kyushu Island. Thus, we made a detailed geological map of the Eocene formations in an area of ~50 square kilometers in the northwestern part of the archipelago. We identified NE-SW and NW-SE trending normal faults, most of which were recognized by previous researchers, and also discovered low-angle faults. NW-SE trending ones are known to be of the Miocene. NE-SW trending and low-angle normal faults are the oldest map-scale structures in the Eocene ones. It is not obvious within the above-mentioned area whether those normal faults are accompanied by growth strata. However, the significant southeastward thickening of the Eocene formations across the Amakusa archipelago suggests that they filled a large half graben with the basin margin fault along the eastern side of the archipelago. This basin model is consistent with the N-S to NW-SE transport directions of the low-angle and NE-SW trending normal faults. Since many NE-SW to EW trending Eocene grabens were formed in the offshore regions west of Kyushu Island and in the East China Sea, the Amakusa region was probably a northeastern branch of the rift system. The geologic structures and depositional ages of the Eocene formations indicate that the Eocene extensional tectonics removed the overlying strata to some extent for the high-P/T Takahama Metamorphic Rocks which crops out to the south of our study area.     

Lead isotope study of the late Archean Lac des Iles palladium deposit,Canada: enrichment of platinum group elements by ponded sulfide melt

Lead isotopic compositions of mineral separates are presented from the mineralized zones (Roby, Twilight, and High Grade zones) of the Lac des Iles palladium deposit (approximately 2.69 Ga) and tonalitic country rocks (approximately 2.727 to 2.775 Ga) in the Wabigoon subprovince of the western Superior Province of Canada. Plagioclase separates show Pb isotope ratios similar to those of the late Archean depleted mantle. Sulfide minerals coexisting with plagioclase have a more radiogenic Pb isotope composition, but very low concentrations of U (<0.2 ppb U) and Th (<0.01 ppb) compared to Pb (≫50 ppb). The isotope data are consistent with a model involving the incorporation of radiogenic Pb from country rocks to sulfide melt near the base of the magma chamber or along the conduit of the parental magmas. The proposed interpretation is also supported by decreasing Cu/Pd ratios from earlier barren to late fertile gabbro intrusions in the complex. The ponding sulfide melt became enriched in platinum group elements, as it was interacting with a large magma volume during the evolution of the Lac des Iles igneous complex.     

Three‐dimensional microstructure of samples recovered from asteroid 25143 Itokawa: Comparison with LL5 and LL6 chondrite particles

In this study, the three‐dimensional (3‐D) microstructure of 48 Itokawa regolith particles was examined by synchrotron microtomography at SPring‐8 during the preliminary examination of Hayabusa samples. Moreover, the 3‐D microstructure of particles collected from two LL6 chondrites (Ensisheim and Kilabo meteorites) and an LL5 chondrite (Tuxtuac meteorite) was investigated by the same method for comparison. The modal abundances of minerals, especially olivine, bulk density, porosity, and grain size are similar in all samples, including voids and cracks. These results show that the Itokawa particles, which are surface materials from the S‐type asteroid Itokawa, are consistent with the LL chondrite materials in terms of not only elemental and isotopic composition of the minerals but also 3‐D microstructure. However, we could not determine whether the Itokawa particles are purely LL5, LL6, or a mixture of the two. No difference between the particles collected from Rooms A and B of the sample chamber, corresponding to the sampling sequence of the spacecraft's second and first touchdowns, respectively, was detected because of the statistically small amount of particles from Room B.     

Detection of Pacific tropical water variability by taut-line moorings in the western equatorial Pacific

This study uses temperature and salinity time series acquired with taut-line moorings in the western equatorial Pacific to investigate water mass behavior on the thermocline layer. Basically, it is insufficient to trace water mass variation by the original discrete depth coordinate data because of relatively high variability of density at fixed depth near the thermocline. A reconstruction method based on the density surface motion caused by tidal forcing was used to derive continuous profiles of temperature and salinity from vertically discrete measurements at fixed depths. This method can represent detailed vertical salinity structures and their variation, especially along the potential density surface of 24.8σ_θ, where the salinity maximum of South Pacific tropical water (SPTW) appeared. Variability around the 24.8σ_θ surface at each site was as large as that observed at the surface, which suggests a strong influence of SPTW behavior. High salinity along the 24.8σ_θ surface within the equatorial band of the western Pacific appeared during boreal fall-winter at sites far from New Guinea. In contrast, high salinity appeared near New Guinea during the boreal spring-summer. These features suggest the influence of the New Guinea Coastal Undercurrent. Over longer time scales, several higher salinity events were observed. The most pronounced salinity event occurred during 2007–2008. Interannual variation of the salinity anomaly along the 24.8σ_θ surface was negatively correlated with the Niño 3.4 sea surface temperature anomaly. A long-term salinity anomaly shift from negative to positive occurred around the end of 2002. The relationship with decadal variation in subtropical cell transport is also discussed.