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排序方式: 共有449条查询结果,搜索用时 31 毫秒
101.
Wataru Kanda Mitsuru Utsugi Yoshikazu Tanaka Takeshi Hashimoto Ikuko Fujii Toshiaki Hasenaka Nobuaki Shigeno 《Journal of Volcanology and Geothermal Research》2010,189(1-2):158-171
Since August 2000, we have recorded the total intensity of the geomagnetic field at the summit area of Kuchi-erabu-jima volcano, where phreatic eruptions have repeatedly occurred. A time series analysis has shown that the variations in the geomagnetic field since 2001 have a strong relationship to an increase in volcanic activity. These variations indicate thermal demagnetization of the subsurface around the presently active crater. The demagnetization source for the early variations, until summer 2002, was estimated at about 200 m below sea level. For the variations since 2003, the source was modeled on the basis of the expansion of a uniformly magnetized ellipsoid. The modeling result showed that the source is located at 300 m above sea level beneath the crater. We carried out an audio-frequency magnetotelluric survey with the aim of obtaining a relation between the demagnetization source and the shallow structure of the volcano. A two-dimensional inversion applied to the data detected two good conductors, a shallow thin one which is restricted to a region around the summit area, while the other extends over the edifice at depths between 200 and 800 m. These conductors are regarded as clay-rich layers with low permeability, which were assumed to be generated through hydrothermal alteration. The demagnetization source for the early variations was possibly located at the lower part of the deep conductor and the source after 2003 lies between the two conductors, where groundwater is considered to be abundant. Based on these results, as well as on seismological, geodetic, and geochemical information, we propose a heating process of the Kuchi-erabu-jima volcano. In the initial stage, high-temperature volcanic gases supplied from the deep-seated magma remained temporarily at the level around the lower part of the less permeable deep conductor since the ascent path had not yet been established. Then, when the pathway developed as a result of repeated earthquakes, it became possible for a massive flux of volcanic gases to ascend through the conductor. The high temperature gases reached the aquifer located above the conductor and the heat was efficiently transported to the surrounding rocks through the groundwater. As a consequence, an abrupt increase of the gas flux and diffusion of the heat through the aquifer occurred and the high-temperature zone expanded. Since the high-temperature zone is located beneath another conductor, which acts as caprock, we assume that the energy of the phreatic explosion is accumulated there. 相似文献
102.
The effect of microorganisms on Fe precipitation rates at neutral pH in the field was examined. The studied area was a cave having Fe-stalactites composed mainly of ferrihydrite and associated microorganisms. The microorganisms were covered with ferrihydrite. Water associated with stalactites was slightly supersaturated with respect to ferrihydrite, and had a dissolved oxygen concentration of 2 ppm, a pH of 6, and an Fe concentration of approximately 14 ppm. Fe precipitation rates were estimated from decreases in Fe concentrations in water during flowing through the Fe-stalactites. The estimated Fe precipitation rate in the field ranges from 6.8×10−8 to 4.0×10−7 mol/l/s. These values are in good agreement with bulk estimates of Fe-stalactite growth rates derived from the length increase (1.3 cm) of one formation over 30 days. The estimated Fe precipitation rates are faster by about four orders of magnitude than expected inorganic precipitation rates. On-site Fe precipitation experiments with sterilized and unsterilized Fe-stalactites and without Fe-stalactites indicate that microorganisms are the controlling factor accelerating Fe precipitation rates at neutral pH. These results suggest that microbially accelerated Fe precipitation rates are more likely related to exopolysaccharides and microbial surface properties than metabolic precipitation mechanisms. 相似文献
103.
Geology and Mineralization at the Dajing Tin-polymetallic Ore Deposit, Inner Mongolia, China 总被引:6,自引:1,他引:6
Abstract: Dajing is a large-scale tin–polymetallic ore deposit in north China with Sn, Cu, Pb, Zn, Ag, and minor elements such as Co and In. The deposit is controlled by a passage-host structure system. Two groups of NE direction faults such as F2 and F1 are characterized by sinistral and multiphase activities, and the syn-ore NE faults with their derivative faults nearly in N-S direction constitute an important structure system as channelway for ore fluid. Around F2 fault (mineralization center), metallic elements display horizontal zoning outwards from Sn, Cu to Cu, Sn, Pb, Zn, Ag and to Pb, Zn, Ag. Controlled by sinistral pull-apart vein system, the WNW direction host structure yielded the ore vein to "W" shape on horizontal plan while to staircase shape on vertical profile. Multiperiodic activities of the host structures system lead to multistage of ore mineralization. Four mineralization stages are recognized in the main mineralization, i.e. 1) cassiterite–arsenopyrite–quartz stage, 2) cassiterite-sulfide (or Cu–poly–metallic mineralization) stage, 3) massive pyrite stage, and 4) galena-sphalerite stage. Besides, fluid activities are identified here at the pre-mineral stage of the formation of quartz veins and sheared deformation, and at the post-mineral stage of quartz–calcite–fluorite deposition.
The earlier tin-rich ore solution raised from NE faults and its nearly N-S derivatives filled the NW-WNW direction fractures, and formed tin ore veins. Subsequent mineralization of Cu-polymetallic stage, pyrite stage and galena-sphalerite stage, overlapping on the same fracture system, sequentially expanded outwards from the fractures, and produced different kinds of ore types and mineralization zoning. 相似文献
The earlier tin-rich ore solution raised from NE faults and its nearly N-S derivatives filled the NW-WNW direction fractures, and formed tin ore veins. Subsequent mineralization of Cu-polymetallic stage, pyrite stage and galena-sphalerite stage, overlapping on the same fracture system, sequentially expanded outwards from the fractures, and produced different kinds of ore types and mineralization zoning. 相似文献
104.
Electrokinetic phenomena associated with a water injection experiment at the Nojima fault on Awaji Island, Japan 总被引:2,自引:0,他引:2
Hideki Murakami Takeshi Hashimoto Naoto Oshiman Satoru Yamaguchi Yoshimori Honkura Norihiko Sumitomo 《Island Arc》2001,10(3-4):244-251
Abstract Self-potential variations were measured to estimate the magnitude of electrokinetic and hydrological parameters (zeta potential and permeability) of the Nojima Fault zone in Awaji, Japan. The study observed self-potential variations that seemed to be associated with water flow from the injection well to the fracture zone, which were induced by turning the injection on and off. Amplitudes of the variations were a few to 0.03 V across 320–450 m dipoles. These variations can be explained well with an electrokinetic model. The quantity k/ζ (permeability/zeta potential) is in the range 1.6 × 10−13 − 5.4 × 10−13 m2 /V. Permeability of the Nojima fault zone can be estimated as approximately 10−16 –10−15 m2 on the assumption that the zeta potential is in the range –0.01 to –0.001 V. 相似文献
105.
Takeshi Hashimoto 《Island Arc》2001,10(3-4):306-317
Abstract A resistivity survey method using artificial telluric noise was examined and applied to a field of a fault zone. The electric earth current was measured at 50 sites in the Nojima Fault zone, which is in the northwestern part of Awaji Island, southwestern Japan. The dominant component of the observed electric field is supposed to be leakage currents from DC electric railways running outside the island. Amplitude and polarization of the stray current were systematically investigated and were revealed to represent the subsurface electrical structure of the study area. Some features on the fault zone's electrical structure have been pointed out, including: (i) an electrical boundary that corresponds to a geological one between granite (resistive) and sediments (conductive); and (ii) a low resistivity spot on the surface rupture of the earthquake fault. The structure estimated in the present study is both qualitatively and quantitatively consistent with previous resistivity surveys done using other methods pursued in the same area. It shows the validity of the 'stray current method' as one that is easy and uses low-cost resistivity exploration tools in a region where the effect of artificial noise caused mainly by leakage currents from electrical railways cannot be ignored. 相似文献
106.
Helium and carbon isotopes in fluorites: implications for mantle carbon contribution in an ancient subduction zone 总被引:1,自引:0,他引:1
Junji Yamamoto Makoto Watanabe Yoshiyuki Nozaki Yuji Sano 《Journal of Volcanology and Geothermal Research》2001,107(1-3)
The concentrations of helium and carbon in fluorite associated with Cretaceous to Neogene (90–13 Ma) granitic magmatism in the Japanese arc have been measured. Concentrations of Li, U, Th and Gd were measured to correct for secondary generated 3He. The CO2/3He of fluorites are almost uniform (1.5×1010–4×1010) and in fair agreement with the range of present island arc volcanic gases. The calculated mantle C contribution in the Mesozoic subduction zone appear to have been identical to the present one (7–19%) indicating that the C flux from the mantle in supra-subduction zone environments has remained fairly constant during the past 70 million years. 相似文献
107.
Yuji Sano Toshitaka Gamo Kenji Notsu Hiroshi Wakita 《Journal of Volcanology and Geothermal Research》1995,64(1-2)
Secular variations in 13C/12C ratios and chemical compositions of gas samples from October 1986 to July 1992 are reported from a 92–95 °C steam well located about 3 km north of Mt. Mihara, an active volcano on Izu-Oshima Island, Japan. The δ13C value steeply increased from −2.97‰ (relative to PDB carbonate) in December 1986 to −1.15‰ in March 1988 and then gradually decreased to −1.75‰ in July 1992. Over the same period, the CO2 content changed similarly with time, even though the experimental error is relatively large. These variations are consistent with helium isotope changes. Initially rapid and then slow enhancements of 3He/4He ratio, δ13C value and CO2 content are invoked by violent eruptions of Izu-Oshima volcano from 15 November to 18 December 1986. After the eruptive activity, depletion of magmatic gas emission and subsequent mixing with crustal fluids in the hydrothermal system may produce the gradual decreases of 3He/4He ratio, δ13C value and CO2 content. Taking into account the rates of these decreases, we suggest that helium and carbon isotope ratios will return to the situation of before the magmatic eruption within 15 years. 相似文献
108.
Dario Tedesco Gerarda Miele Yuji Sano Jean Paul Toutain 《Journal of Volcanology and Geothermal Research》1995,64(1-2)
Intensive gas emanations occur throughout the island of Vulcano, Italy. Sharp fluctuations recorded in the crater gas composition suggest the presence of two separate volcanic reservoirs and continuous mixing with another source, “crustal” waters. This mixing differs between the beach and crater fumaroles. Gas samples from three crater fumaroles with temperatures ranging from 200 to 550 ° C were sampled repeatedly over a one year period. During the same interval of time, six samples from submarine and subaerial beach fumaroles and water well gases were also sampled. Gases from one crater fumarole (F5) showed variations of (3He/4He)fumarole to (3He/4He)air between 5 and 6 correlated with variations of several chemical species. High 3He/4He ratios for the beach fluids, similar to those of crater fluids, suggest the existence of a unique large magmatic reservoir at depth feeding both the crater and beach intermediate reservoirs. However, temporal changes clearly indicate variable degrees of fluids mixing, and the geographic distribution of the 3He/4He ratios as well as the chemical composition of the fluids suggest the existence between the magma reservoir and the surface of two intermediate different reservoirs, independently related to crater and to beach fumaroles. 相似文献
109.
Takeshi Nishimura 《Geophysical Journal International》1996,127(3):773-782
We investigate the particle orbits of long-period (about 20 s) P waves observed with the global seismic network. By analysing 84 three-component seismograms recorded at 25 stations from 60 earthquakes occurring beneath 300 km, we quantitatively evaluate the orbits by three sets of eigenvalues and eigenvectors, using a covariance matrix method. The eigenvalues for P waves recorded at stations located on continents are explained by the standard horizontal layered structure model (iasp91). On the other hand, the orbits observed at stations close to island arcs are affected not only by the horizontal layered structure but also by heterogeneity due to subducting plates, mantle diapirs and so on. On the basis of a single-scattering model for a plane P wave, we quantify the heterogeneities by an isotropic scattering coefficient g0. Fitting the theoretical eigenvalues to the observed ones, we estimate g0 for the crust and upper mantle beneath continents to be less than 0.0005 km-1 , and the mean g0 for the structure beneath island arcs to be about 0.0015 to 0.003 km-1 . 相似文献
110.