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Petrographic and geochemical characteristics of drill core and surface rock samples from the four hydrothermal vent sites of the Yamanaka, Snail, Archaean, and Pika sites at the Southern Mariana Trough (SMT) are described in order to clarify the geological background of hydrothermal activities. The core samples were drilled by the Benthic Multi-coring System (BMS), penetrating up to ~8 m below seafloor. The recovered samples included both basement rocks and sulfide ores. The basement rocks are further subdivided into basaltic andesite and andesite, most of which are recovered from on-axis and off-axis regions of the SMT, respectively. All of the rocks are characterized by relative enrichment of large-ion lithophile elements with noticeable depletion of Nb and Ta, suggestive of significant influence of subducted slab-derived components into the basement rocks both at the on- and off-axis hydrothermal vent sites. Major and trace element variations of the samples suggest that the Yamanaka, Snail, and Archaean rocks can be explained by a sequence of fractionation of an on-axis magma, whereas only the Pika rocks may be influenced by a different magma component, i.e., off-axis magmatism.

TAIGA11 cruise of R/V Hakurei-maru No.2 was conducted in June, 2011 to study subseafloor geochemical environment below active hydrothermal fields using a shallow drilling system BMS (Benthic Multi-coring System). Three active hydrothermal fields were selected as target fields; the Iheya North Knoll field (27°47′ N, 126°54′ E), the Jade field in the Izena Hole (27°16′ N, 127°05′E), and the Hakurei field in the Izena Hole (27°15′ N, 127°04′ E). In this chapter, we will report chemical composition and isotope ratios of pore fluids extracted from collected sediments. At the Hakurei field in the Izena Hole, BMS drilling attained to 610 cmbsf (cm below the seafloor) in the vicinity of a large massive sulfide mound. The obtained core showed evidence for sulfide and sulfate mineralization below 223 cmbsf. Pore fluid from the corresponding depth showed enrichment in Si, K and Ca, which could be attributed to influence of formation of alteration minerals rather than to involvement of the hydrothermal component. At the Jade field in the Izena Hole, BMS drilling attained to 529 cmbsf at about 300 m apart from the area where high temperature fluid venting congregate. The obtained core comprised grayish white hydrothermal altered mud below 370 cmbsf, although pore fluid showed seawater like composition. At the Iheya North Knoll field, BMS drilling attained to 453 cmbsf at about 200 m apart from the central mound area. The obtained core consisted almost entirely of grayish white hydrothermally altered mud. Pore fluid below 180 cmbsf showed substantial enrichment in major cations (Na, K, Ca and Mg) and Cl, which would be explained as a result of hydration during hydrothermal alteration.

The seafloor massive sulfide (SMS) deposits, which were found in the Izena Hole, the middle Okinawa Trough, show two modes of occurrence, namely, large sulfide mounds on the seafloor and massive sulfide bodies beneath sediments. A 610-cm long sequence of alternating beds of sediment and ore layers was drilled to investigate the lower ore body, at high rates of core recovery (average 64 %), at the northern part of the Hakurei Site in the Izena Hole, the middle Okinawa Trough. Observation on polished sections of the core samples under reflection microscope indicates that the ores (18–166 cmbsf (cm below seafloor)) above the sediment layer (166–221 cmbsf) are porous and contain acicular barite and globular sphalerite with small barite inclusions. These textures are considered to have formed as a result of rapid cooling. On the other hand, ores (221–510 cmbsf) below the sediment layer (166–221 cmbsf) had a massive layer of sphalerite and galena on top (black ore), a middle layer of pyrite and chalcopyrite-rich layer (yellow ore) and a bottom layer of sphalerite and large barite.

Mineralogical and geochemical features of hydrothermal alteration minerals in the sediment cores from the Jade hydrothermal field in the Izena Hole, mid-Okinawa Trough, were studied by XRD, EPMA and TEM-EDS analyses. A core sample 1186MBL collected from the surface sediment near the sulfide chimney venting high temperature fluid up to 320 °C was characterized by occurrence of kaolinite, with sulfide minerals such as sphalerite and galena. The kaolinite would be related to be formed under acidic condition caused by oxidation and dissolution of the sulfide minerals by penetrating seawater. Core samples (1188MB, 1193MB) were collected from the surface sediment in the vicinity of clear hydrothermal fluid venting of ~100 °C, which is located in 400 m distant from the sulfide chimney. In these cores, occurrence of chlorite and smectite was identified. The chlorite in the core 1188MB had chemical composition close to Al-rich chlorite which is classified as sudoite, although chlorite found in other hydrothermal fields in the Okinawa Trough is characterized as significantly Mg-rich chlorite. Core samples of up to 4–6 m length were also collected near the low temperature fluid venting to study alteration in deep layers. One of two long core samples (BMS-J-2) was characterized by chlorite and illite assemblage below 380 cmbsf, while the other (LC-J-2) was characterized by abundant occurrence of K-feldspar below 300 cmbsf. Occurrence of euhedral crystals of K-feldspar in size up to several tens μm suggests the formation by precipitation from high temperature fluid.

Hydrothermal fluids were collected from five hydrothermal fields around the Southern Mariana Trough backarc spreading center for chemical and isotopic analyses. Yamanaka site was interpreted as inactive, so we present results from Snail, Archaean, Pika and Urashima sites. The slightly low pH and negative alkalinity suggests a little bit input of magmatic volatiles, supported by high CO concentrations. Consequently low pH would lead to the hydrothermal fluids rich in Fe compared to the MOR hydrothermal fluids.

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 Ra-Pb and Ra-Th ages were obtained for barite crystals in hydrothermal sulfide deposits taken at the Okinawa Trough and the Southern Mariana Trough. After calibrating the measurement systems with standard samples with pitchblende, it was confirmed that the U and Th concentrations obtained for GSJ samples are consistent with literature values. It was shown that radon does not escape from barite crystals extracted from hydrothermal sulfide deposits, which indicates that Ra-Pb dating method works for these barite crystals. Most of the Ra-Pb and Ra-Th ages are younger than ESR and U-Th ages, where this inconsistency would be explained by the mixture of the barite crystals with younger and older ages, formed by several hydrothermal events.

In estimating the ages of sea floor hydrothermal deposits, the age of the sediments overlying it would give the youngest limit if the sedimentation age is correct. The OSL (optically stimulated luminescence) method was applied to two sediment cores taken by an acrylic corer from the seafloor in the Okinawa Trough. The ages, obtained by the polymineral fine grain method, did not correlate with the stratigraphic sequence within the core, implying either insufficient bleaching of the sediments at the time of the deposition or mixing of sediments of different ages. The polymineral fine grain OSL dating method did not work to date the present sediment samples at the sea floor in the Okinawa Trough.

The radiation from the sulfide deposits taken at Okinawa Trough was measured on-board by a NaI gamma ray spectrometer immediately after the retrieval of the sample onto the research vessel. Gamma ray peaks were identified as those from daughter nuclei of Ra and Ra. The slight change with time of the radiation dose from those deposits is probably due to the geometric arrangement in measurements. As results, no change in radiation dose was observed in the time range up to 90 h after retrieval of the samples from the submarine vehicle.

Time series physical properties of water column were measured at the Hatoma Knoll hydrothermal field on the Okinawa Trough, using a multi-layered Acoustic Doppler Current Profiler (ADCP). We aimed (1) to provide a deep-sea current data within a small and complicated geographical feature of a caldera and (2) to show the possibility of hydrothermal plume observation by ADCP during the measurement in April 2004 and May 2005 cruises. The deep-sea current data were recovered at the depths between seafloor and 40-m or 60-m-layers above the seafloor. Moreover, acoustic reflection signals were sometimes observed up to 120-m layer above the seafloor due to the trail of hydrothermal plumes. A power spectrum density analysis by fast Fourier transform (FFT) showed the dominant time cycle of the horizontal current velocity, which corresponded to tidal periodicity over an approximate 12-hours period, at each layer. Residual currents, which play an important role in material transport, tended to northwest direction and suggested that most of material might keep retaining within the caldera. The causes of the measurement result of the flow which appeared into a pulse form may indicate fluctuation of the blowout of hydrothermal water. The verification of the fluid mechanism by the numerical model in the local field such as the caldera will be necessary in the future.