Okinawa

According to Haneda (1955), the centipede Orphaneus brevilabiatus is found widely in Micronesia, the East Indies, the Malay Peninsula, Indochina, Formosa and Okinawa. The centipedes (about 6 cm long) are often found in native houses, on the walls and inside furniture, and they secrete a greenish luminous slime when irritated. When exposed to a chloroform vapor, they discharge strikingly luminous slime from every segment of the body. 

Distribution. The first indication of the occurrence of palytoxin in fish was presented in 1969 (16). The filefish Altera scripta belonging to family Monacathidae was traditionally known in Okinawa, Japan, to contain a toxic substance in the gut and, thus, to kill pigs when fed to them. The presence of fragments of Palythoa sp. in the guts and the resemblance in solubility between the fish toxin and palytoxin led the authors to a conclusion that the toxic principle in the filefish viscera was palytoxin. Incidence of human intoxication due to eating the filefish was not confirm. ... 

Fig. 4.19 Map showing occurrence of Heterotropa in Japan. Compounds bearing the methy-lenedioxy group occur on Taiwan, Okinawa, and associated islands. Compounds that lack the methylenedioxy group occur in Japan. Arrows show possible routes of colonization of the two types...

Reef (Coll and Wright, 1987). Several related compounds have also been identified from plants collected on Amami Island (north of Okinawa) by Ichikawa et al. (1974). 

Two hypotheses of seafloor depth at the time of mineralization have been proposed based on foraminiferal data, ca. 3500 m (Guber and Ohmoto, 1978 Guber and Merrill, 1983) and 1500 m (Kitazato, 1979). Considering seafloor depth of present-day ore formation at back-arc basins and fluid inclusion data mentioned above, shallow seafloor depth hypothesis (Kitazato, 1979) seems more likely. If the pressm e-temperature condition of Kuroko ore fluids was close to the boiling curve, the depth could be estimated to be 1,000-1,500 m, which is similar to that for present-day back-arc mineralization such as Okinawa Trough. 

Precipitation of barite and quartz. Barite and quartz are the most common gangue minerals in the submarine hydrothermal ore deposits such as Kuroko deposits and back-arc basin deposits (e.g., Okinawa, Mariana deposits) (Halbach et al., 1989 Shikazono, 1994 Shikazono and Kusakabe, 1999). These minerals are also common in midoceanic ridge deposits. 

Barite is abundant in back-arc basin hydrothermal system such as Okinawa, Manus and Mariana (Shikazono and Kusakabe, 1999). 

The active back-arc extension in the Okinawa Trough and the Taupo Depression in New Zealand can be explained by the injection model. 

Figure 1.164. Distribution of Cenozoic basalts and active rift systems in the northeast China region. Arrows indicate the horizontal convective current in the upper mantle associated with the upwelling of the asthenosphere beneath the region. A Baikal Rift B Shanxi Graben C Tancheng-Lujiang Fault D Okinawa Trough (Tatsumi et al., 1990).

Halbach, P., Nakamura, K., Wahsner, M., Lange, J., Sakai, H., Kaselitz, L., Hansen, R.-D., Yamano, M., Post, J., Prause, B., Seifent, R., Michaelis, W., Teichmann, R, Kinoshita, M., Marten, A., Ishibashi, J., Czerwinski, S. and Blum, N. (1989) Probable modern analogue of Kuroko type massive sulfide deposits in the Okinawa Trough back-arc basin. Nature, 333, 496-499. 

Miyashita (1995) proposed that the epithermal mineralizations in Hokusatsu district (Kyushu) are related to strata volcanoes in the volcanic depression and not to caldera formation, hy compiling the data on the ages of mineralization and volcanic activities, gravity and geology. He considered that the Hokusatsu volcanic depression zone is an extension of the Okinawa Trough. This volcanic depression started from 5-6 Ma (Kamata and Watanabe, 1985), where bimodal volcanism is found. 

Recently, several submarine hydrothermal sites have been discovered from the seafloor of back-arc depression zones and volcanic fronts near the Japanese Islands (Okinawa Trough and Izu-Bonin) (Fig. 2.29). The studies on these areas are described below. 

Honma et al. (1991) have shown that the Okinawa Trough basalts have significantly high K, Rb and Sr contents and D/H, 0/ 0 and Sr/ Sr ratios than N-Morb have and these are due to generation of magma from normal-type mantle peridotite modified by component from the subducted slab and crustal contamination. 

Halbach et al. (1997) reported lead isotope data on volcanic rocks, sediments and ores from the hydrothermal JADE field in the Okinawa Trough and pointed out that lead isotopic compositions of Okinawa JADE ores are very similar to Kuroko ores (Fig. 2.31) and both sediments and volcanic rocks contributed comparable amounts of lead to the deposit. 

Fig. 2.30. Location and tectonic setting of Okinawa Trough. The solid circles mark the hydrothermal fields listed in Table 2.12. The broken lines show contours of 1000 m water depth. The triangles indicate Quaternary volcanoes (Ishibashi and Urabe, 1995).

Mica, kaolins (kaolinite and halloysite), Mg-rich chlorite, talc, montmorillonite and chlorite/montmorillonite are abundant in the host epiclastic sediments and pumiceous tuffs at Jade in the Okinawa Trough (Marumo and Hattori, 1997). 

No. Host Axial Seamount CASM 14 B North Fiji Basin 24 B Mariana Trough at 18 N 11 A Valu Fa, Lou Basin 47 B, A, D Eastern Manus Basin 26 D Jade Okinawa Trough 17 R... 

The H2S concentration of hydrothermal solution is plotted in Fig. 2.33. Based on these data, we can estimate the temperature of hydrothermal solution buffered by alteration mineral assemblages such as anhydrite-pyrite-calcite-magnetite and pyrite-pyrrhotite-magnetite for Okinawa fluids. 

Location No. 1 N. Fiji White Lady 7 Lau Vai Lili 13 Mariana Alice Spring 16 Okinawa Minami-Ensei 18 Okinawa Izena 21 Izu-Bonin Suiyo OBS EPR... 

Site No. Name of site 1 Okinawa Trough JADE 2 Okinawa Trough CLAM 3 Okinawa Trough South Ensei 4 Izu-Bonin Suiyo SM. 5 Mid-Mariana Trough Alice Springs 6 South Mariana Trough Forecast vent 7 Manus Basin Vienna Woods 8 North Fiji Basin 9 Lau Basin Vai Lili... 

Fig. 2.33. H2Saq concentration.s as a function of temperature for hot spring fluids at midocean ridges as a function of redox. Assuming AMPC (anhydrite-magnetite-pyrite-calcite) and PPM (pyrite-pyrrhotite) buffers redox in sub-seafloor reaction zones and a pressure of 500 bars, dissolved H2Saq concentrations indicate temperatures of approximately 370-385°C. Solid star Okinawa. (Modified after Seyfried and Ding, 1995.)...

Among the back-arc deposits, the features of the Okinawa Trough deposits are similar to those of Kuroko deposits, but North Fiji and Mariana deposits are different. For instance, gold, silver, zinc, lead and arsenic are rich in the Okinawa Myojin-Knoll Caldera and some Kuroko deposits but poor in the North Fiji and Mariana deposits. 

The difference in mineralogy of the Kuroko and present-day back-arc deposits are (1) metastable phases such as native sulfur, wurtzite, and amorphous silica are poor in the Kuroko deposits (2) arsenic minerals such as jordanite, tetrahedrite-tennantite, native arsenic, and realgar are common in the present day back-arc deposits (Okinawa Myojinsho Knoll Caldera), but rare in Kuroko deposits except tetrahedrite-tennantite (3) secondary minerals such as cerussite and covellite are common in present day back-arc deposits (e.g., Okinawa, Myojinsho Knoll Caldera) (4) Dendritic texture is common in the present day back-arc deposits. 

These differences are considered to be attributed to the dilferences in compositions of rocks and alteration minerals interacted with circulating seawater or modified seawater at elevated temperatures. For example, high K and Li concentrations in the hydrothermal solution in the Mid-Okinawa Trough baek-arc basin (Jade site) are due to the interaction of hydrothermal solution with acidic volcanic rocks (Sakai et al., 1990). It is evident that the chemical compositions of hydrothermal solution are largely alfected by water-rock interaction at elevated temperatures. 

Fig. 2.39. Na /K+ atomic ratios of well discharges plotted at measured downhole temperatures. Curve A is the least squares fit of the data points above 80°C. Curve B is another emperical curve (from Truesdell, 1976). Curves C and D show the approximate locations of the low albite-microcline and high albite-sanidine lines derived from thermodynamic data (from Fournier, 1981). Small solid subaerial geothermal water Solid square Okinawa Jade Open square South Mariana Through Solid circle East Pacific Rise 11°N Open circle Mid Atlantic Ridge, TAG.

Okinawa Trough and at Escanaba Trough are even higher than that of seawater (0.7092), strongly suggesting the contribution of sedimentary material with high Sr/ Sr ratios (Gamo, 1995). 

Although the data are not plentiful, it is clear that the hydrothermal solutions of sediment-hosted ridges and back-arc basin covered by sediment (Okinawa Trough) contain high amounts of ammonium (2.8-13.6 p.molal) (Scott, 1997). This means that ammonium was derived by thermal maturation reaction of organic matter in sediments by the following reactions (Gamo et al., 1991),... 

S S data on H2S and sulfides from Okinawa Trough (Okinawa Izena) show a high S S value (- -8%o) (Sakai, H. pers. conun cited in Ishibashi andUrabe, 1995). 8 " S values from other districts are similar to those of midoceanic ridges -i-2.1%o to 4-3. l%o, Mariana Trough (Kusakabe et al., 1990) -f0.3%o to -t-2.2%o, Minami Ensei Knoll (Nedachi et al., 1992 4-2.l%o to 4-2.8%o, Kita-Bayonnaise caldera (lizasa et al., 1992) 4-0.9% to 4-1.2% (Kaikita caldera) (vein part) (Ishibashi and Urabe, 1995). S " S values from the Okinawa (Izena) sulfides are higher than any of S S data from midoceanic ridge sulfides and H2S of hydrothermal solutions. 

B values of hydrothermal solution from back-arc basin are lower than those from midoceanic ridge hydrothermal solution (Gamo, 1995). 8"B values of Okinawa Trough hydrothermal solution are particularly low (—5%o to — 10%c) (Ishikawa and Nakamura, 1993), suggesting a contribution of sedimentary boron to hydrothermal solution. 

The iron content of sphalerite from back-arc deposits is lower than midoceanic ridge deposits. Pyrrhotite and wurtzite are not common in back-arc deposits, although they were identified from Iheya Ridge, Middle Okinawa Trough, and Mariana. 

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