Mineralogically investigated were massive sulfide samples of chimneys and mounds collected during the "SHINKAI 6500" dives (Dive 562 and Dive 567) from two basalt-hosted hydrothermal ore deposits, being at a short distance of about 1 km, occurring in the eastern end of the Yaeyama Central Graben, the Southern Okinawa Trough. In both sample series (#562- and #567- series), ore minerals are dominated by low-iron sphalerite with small amounts of chalcopyrite and galena. Sphalerite in the #567-series samples shows somewhat wide compositional variation, ranging from 0.4 to 3.8 mole%FeS, with the highest mode of about 1 to 2 mole%FeS. Significant amounts of pyrite and secondary marcasite are present in the #562-series samples with subordinate bornite, while in the #567-series samples, they are lacking, but instead tetrahedrite-tennantite series with atomic ratios of Sb/(Sb+As) being 0.2 to 0.8 is abundant and contains Ag up to 0.7 wt% with mostly less than 0.5 wt%. Neither pyrrhotite nor isocubanite is encountered in the two sample series. Microthermometries of two-phase (liquid + gas) fluid inclusions trapped in calcite (#567-series) have revealed the following results: homogenization temperatures (not corrected for pressure) of 242 to 317 ℃ and salinities of 5.1 to 7.6 wt.%NaCl equiv., being a little more saline than the seawater. The observed mineral assemblages and sphalerite compositions obtained for the Yaeyama Central Graben deposits lead us to indicate that the basalt-hosted mineralizations might have occurred under rather higher sulfidation state. From the mineral associations observed, it is also indicated that both hydrothermal deposits are characterized by a metal association of essentially Zn-Pb-Cu. The mineral assemblages and ore textures observed are similar to those of the Kuroko deposits rather than the Besshi-type deposits, as well as their metal association (Cu-Pb-Zn).

Data pertaining to geophysical exploration have increased substantially and drastically in recent decades due to the acquisition of various global data sets. Both the accuracy of measuring devices and the associated analytical techniques are being constantly improved. We can obtain densely spaced, real-time data from underneath oceanic basements by using oceanographic observation networks. The IFREE (Institute for Research on Earth Evolution) plate dynamics research project has accumulated crustal data by using ocean-bottom seismometers (OBSs), a multichannel seismic (MCS) reflection system, bathymetry data by multi-beam swath system, potential data via a gravity meter and a shipboard towing proton and onboard three component magnetometers. In order to investigate tectonic structures and reveal the causal relationship between the occurrence of interplate earthquakes and the structures, these seismic data and geophysical data are processed and added to an internal database. In this paper, we propose the opening of a publication site by first targeting some of the velocity structural models obtained from OBS data analyses and MCS profiles after time-domain processing. The potential field data of gravity and magnetics along seismic lines are also provided through the user access point. This database can be useful and efficiently contributed for proponents to prepare scientific proposals for the Integrated Ocean Drilling Program (IODP).

We conducted a wide-angle seismic experiment using a large volume (200l) airgun array and 110 ocean bottom seismographs (OBSs) in the central and southern Izu-Ogasawara region. The experiment was KY05-07 cruise using R/V Kaiyo of Japan Agency for Marine-Earth Science and Technology (JAMSTEC) during July 8 and August 1, 2005. Objectives of the experiment are to reveal a velocity structure of the central and southern Izu-Ogasawara region, and to constrain the structural variation along the profile, especially crossing the Sofu-gan tectonic line. The total shot number of the large volume airgun array was 7300, which were recorded by the OBSs and a 12-ch hydrophone streamer towing simultaneously. In this paper, we summarize information of the experiments and introduce OBS and streamer data.

Single-channel seismic reflection surveys were conducted in the northwestern Pacific to investigate subsurface structure and nature of volcanic eruption of small volcanic knolls considered to be formed by "petit-spot" intra-plate volcanism, and also morphology of a sedimentary basin and oceanic basement around the knolls in the area. Seismic profiles reveal the sedimentary layer is acoustically transparent, having some horizontal reflectors within the layer. Oceanic crust of the Pacific Plate lies flatly beneath the sedimentary layer with nearly constant thickness of ~200-300 m. The layer beneath the small knolls is acoustically opaque, possibly caused by inhomogeneous structure due to feeder dykes or cryptodome and volcanic deposits. Strong reflectors at base of the sedimentary layer, identified in the vicinity of small knolls, are probably reflections from volcanic sills. The oceanic crust beneath the small knolls is depressed by weight of the volcanic deposits.

We carried out a deep wide-angle seismic experiment using a large airgun array and 110 ocean-bottom seismographs (OBSs) in the southern Izu-Ogasawara arc area. The experiment was conducted on the R/V Kaiyo of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) from 12 February to 11 March 2005 (KY05-02 cruise). Objectives of this cruise were to determine the velocity structure of the entire arc-backarc system and the difference of crustal structures formed in the Oligocene, Eocene and the present to elucidate crustal growth. An airgun-OBS seismic line was set from the Izu-Ogasawara Trench near Chichijima to the Shikoku Basin across the southern Izu-Ogasawara arc in the arc-backarc system. The OBSs and a 12-channel hydrophone streamer recorded seismic signals, which were shot using a large airgun array with total capacity of 12,000 cubic inches. In this paper, we summarize information of the seismic experiments and introduce OBS data and reflection data.

Seismic reflection characters provide useful information on not only geometry of tectonic structures but also magmatic processes. A multichannel seismic reflection survey has been conducted on November 2000, in order to clarify the subsurface structures around Niijima, Kozushima and Miyakejima of the northern Izu islands, where volcanic activities with earthquake swarm have been repeatedly occurring. As a result, a chain of intermittently continuous reflectors with strong amplitude was observed at 3.5-5 km depth beneath the basement high between Niijima and Kozushima, which is called the Zenisu ridge and an NE-SW oriented echelon ridge across the Northern Izu Arc. The reflective zone has about 10 km length in NW-SE direction whereas it would have extremely thin width in NE-SW direction. Moreover, an isolated reflector is visible below the NW-SE trending basement rise, which is capped by a small seamount chain extending toward SE from Kozushima. Based on the previous study in crustal deformation modeling and seismic characters of the reflections, the reflectors are considered to show the magma intrusions that have been repeatedly occurring there, demonstrating one of the processes of crustal evolution in the study area. Furthermore the seismic attribute analysis shows the remarkable contrast in frequency-dependent attenuation of seismic reflections between the Zenisu ridge and the depression of igneous basement between Miyakejima and the ridge, implying the difference in the extent of destruction of the igneous basement rocks or that in lithology. Finally the AVO and frequency component analyses suggest the existence of volcanic gas at very limited part of the reflectors.