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Volume 46 (2012)
- Issue 4 Pages 189-
- Issue 3 Pages 137-
- Issue 2 Pages 79-
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Volume 46, Issue 4
Displaying 1-7 of 7 articles from this issue
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Special section “Geofluids”
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Tetsu KOGISO, Hikaru IWAMORI, Tetsuya YOKOYAMA2012 Volume 46 Issue 4 Pages 189-190
Published: December 31, 2012
Released on J-STAGE: February 20, 2017
JOURNAL FREE ACCESSGeofluids, especially aqueous and carbonate ones have great influences on physicochemical properties of Earth materials, which in turn play significant roles in physical and chemical processes in the Earth's interior. Recent findings of close relationships between the loci of fluid liberation and those of seismicity in subduction zones have led many researchers to a speculation that just geofluids are the key player in seismic activities in subduction zones as well as in igneous activities. Geofluids in subduction zones also play critical roles in the global carbon cycle, which is one of the most important factors that control climate changes of the Earth's surface. This special issue "Geofluids" reports various aspects of aqueous, carbonate, and hydrocarbon fluids that reside in the shallow and deep interiors of subduction zones. Many of the six papers in this special issue were written by presenters in the session "Geofluids: the role of fluids in the Earth's interior" in the annual meeting of the Geochemical Society of Japan 2011. We hope that the papers in this special issue would provide readers with up-to-date understandings of the roles of geofluids in various activities and phenomena in this restless planet.View full abstractDownload PDF (299K)
Reviews
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Aitaro KATO2012 Volume 46 Issue 4 Pages 191-203
Published: December 31, 2012
Released on J-STAGE: February 20, 2017
JOURNAL FREE ACCESSThe very dense and well-covered ray-paths from the many earthquake clusters recorded by portable seismic stations provide us excellent opportunities to image detailed crustal structures and to determine hypocenters and stress field with high-accuracy within the seismogenic zone. Based on these high-resolution images, I here review roles of crustal fluids on earthquake generations, in terms of, 1) inland large earthquakes, 2) slow slips along the subduction zone boundary, 3) non-volcanic swarms. The presence of crustal fluids in source regions of the inland large earthquakes is expected to weaken the crustal material, causing local contractive deformation of the crust. Stress loading through the weak crust reactivates pre-existing weak faults within ancient rift systems, leading to devastating intraplate earthquakes in northeast Japan. Recent studies of slow earthquakes point to the involvement of high pore fluid pressures near the plate boundary in the occurrence of slow earthquakes. In Tokai region, it has been revealed that combination of dehydration fluids with heterogeneous fluid transport properties in the overlying fore-arc plate generates variations of fluid pressures along the downgoing plate boundary, which in turn controls the occurrence of slow earthquakes along the plate boundary. Beneath the non-volcanic seismic swarm region in Wakayama district, presence of crustal fluids has been demonstrated based on slow velocity and high conductive anomalies. Crustal fluids dehydrated from the subducting oceanic crust could infiltrate into the mantle wedge and crust, leading up to the intensive non-volcanic seismic swarm.View full abstractDownload PDF (3647K) -
Kazuaki OKAMOTO, Masayo KOBAYASHI, Miyuki ARAKAWA, Seiya FUKUMURA2012 Volume 46 Issue 4 Pages 205-215
Published: December 31, 2012
Released on J-STAGE: February 20, 2017
JOURNAL FREE ACCESSDehydrated fluid in deep subduction zone causes various geological phenomena such as earthquake, and arc volcanism. It has been considered that there is a correlation between the double seismic zone and metamorphic dehydration reaction in deep slab. The location of the upper limits of the upper seismic plane correspond to metamorphic facies boundary where H2O contents change in subducting crust; numerous earthquakes from 60 to 110 km depths in the lawsonite-blueschist facies, many earthquakes in the lower crust of the slab from 110 to 150 km depths in the lawsonite-amphibole eclogite facies and few earthquakes in the lawsonite eclogite facies. Recent petrological researches have revealed that both blue schist and lawsonite eclogite are stable in the same pressure and temperature condition because chemical variation including water content creates both lawsonite-amphibole eclogite and lawsonite eclogite in different portion of subducted crust. Partial melting would occurred in eclogite in deep subduction zone if warm slab is subducted. In descending slab, the eclogite would reach wet solidus defined as phengite-, through zoisite-, and amphibole-decomposition reactions with increasing temperatures. The lower plane of the double seismic zone, is considered to be related to dehydration reaction in the slab. Metamorphic olivine has been described in vein from serpentinite mylonite. The vein was created by dehydration reaction to decompose antigorite under shear deformation. In the cold slab beneath Tohoku arc, the reaction has a negative slope in P-T space and forms olivine + orthopyroxene + fluid. In the warm slab beneath SW Japan, the reaction has a positive slope in P-T space and forms olivine + talc + fluid. The above these dehydration reactions are well-described in the serpentinite from high P/T metamorphic belt from Spain, and Italy, respectively.View full abstractDownload PDF (1793K) -
Tsuyoshi ISHIKAWA, Tetsuro HIRONO2012 Volume 46 Issue 4 Pages 217-230
Published: December 31, 2012
Released on J-STAGE: February 20, 2017
JOURNAL FREE ACCESSIn this paper, we review recent progress on an attempt at evaluating coseismic fluid-rock interactions in fault zones on the basis of trace element and isotope analyses of fault rocks. The slip zone rocks from Taiwan Chelungpu fault at 1 km depth exhibit marked decreases of lithium, rubidium, cesium and 87Sr/86Sr and an increase of strontium relative to adjacent host sedimentary rocks. Model calculations reveal that these trace element and isotope spectra were produced by coseismic fluid-rock interactions at >350℃, which may have caused a dynamic decrease of friction along the fault through thermal pressurization. The slip zone rocks from a major reverse fault in the Boso Emi accretionary complex at 1-2 km depth also show similar evidence for coseismic fluid-rock interactions at high temperatures. For the slip zone rocks from the Shimanto accretionary complex in Kure area, which represent rocks of ancient megaspray fault at 2.5-5.5 km depth, the signals derived from high-temperature fluids overlap with those from melting, indicating coseismic fluid-rock interactions followed by frictional melting. These results demonstrate that high-temperature fluid-rock interactions widely occur during seismic slip and geochemical characteristics of the fault rocks are useful indicators of such coseismic events.View full abstractDownload PDF (1578K)
Original paper
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Takehiko HIRAGA, Yasufumi WATANABE, Tomonori MIYAZAKI2012 Volume 46 Issue 4 Pages 231-242
Published: December 31, 2012
Released on J-STAGE: February 20, 2017
JOURNAL FREE ACCESSInvestigation of intergranular regions of fine-grained metamorphic rocks using various electron microscopy reveals the presence of intergranular pores previously filled with an aqueous fluid. Such intergranular fluid inclusions exhibit a characteristic shape which minimizes interfacial energy at intergranular regions. The distribution of the inclusions indicates that they were formed from fluid filled intergranular microcracks that ovulated into inclusions due to the initially unstable form of a fluid film at grain boundaries. A simple calculation of interfacial stresses produced by anisotropic thermal contraction of quartz grains during cooling of the quartz aggregate demonstrates that most grain boundaries in crustal rocks experience intense intergranular cracking accompanied by infiltration of fluids. Presence of the fluid filled cracks might explain the observations of low electrical resistivity and seismic wave speeds at middle to shallow crustal depths. The inhibition of the crack formation due to the interfacial stress relaxation in ductile crustal regions results in higher electrical resistivity and seismic wave speeds at greater deep. Intergranular chemical components in mantle rocks are ubiquitously found in mantle xenoliths. We can attribute these components to intergranular melts which were present in the mantle.View full abstractDownload PDF (3527K)
Reviews
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Junji YAMAMOTO, Hidemi ISHIBASHI2012 Volume 46 Issue 4 Pages 243-255
Published: December 31, 2012
Released on J-STAGE: February 20, 2017
JOURNAL FREE ACCESSThe occurrence of subduction-related fluid in mantle wedge is responsible for various geological phenomena in subduction zone. Identification of the fluid will especially constitute a valuable contribution to the interpretation of geophysical observations regarding subduction-related events. As a way to identify the fluid, it is effective to analyze fluid within peridotite xenoliths collected at subduction zone. We frequently observe fluid in the peridotite xenoliths as tiny fluid inclusions, which are composed of various volatiles. Though water or brine has been supposed as a dominant fluid composition in mantle wedge, the occurrence of CO2 fluid is prominent above all. Density of CO2 fluid in shallower mantle wedge is around 1.1 g/cm3, which is far lower than that of surrounding rocks, resulting in upward flow of the CO2 fluid. That is, the CO2 fluid within mantle wedge is an allochthonous component. Further studies on the distribution and origin of the CO2 fluid would clarify circulation system of the fluid in mantle wedge. Here, we review studies examining geochemical aspects and the origin of the CO2 fluid in peridotite xenoliths. Assuming an oxygen fugacity and temperature, carbon in shallower mantle wedge should exist as CO2 fluid, which is fairly consistent with the observation of the peridotite xenoliths. Carbon isotopic composition of the CO2 fluid suggests the possible occurrence of subduction-related carbonic fluid in mantle wedge. To make better elucidation of the origin of the fluid inclusions, it is necessary to combine it with any other isotopic indices such as nitrogen isotopes. Determination of the origin of the fluid will enhance our understanding on the mechanism of geological events in subduction zone.View full abstractDownload PDF (2481K)
Original paper
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Tomohiro TOKI, Ryutaro HONDA, Kazuki OOMINE, Urumu TSUNOGAI, Daisuke D ...2012 Volume 46 Issue 4 Pages 257-274
Published: December 31, 2012
Released on J-STAGE: February 20, 2017
JOURNAL FREE ACCESSChemical and isotopic compositions of seeping gas from the coastal area of Yagaji Island were measured. The gases composed mainly of methane. The δ13C and δD of methane suggest that the origin of the methane is thermal decomposition of organic matter. The δ13C of methane and CH4/(C2H6+C3H8) ratio in the gases suggest long distance migration from the sources. The 3He/4He ratios in the Yagaji gases suggest thermal decomposition of organic matter occurs in deeply-buried sediments. The Yagaji gases would be generated from marine organic matter in the Motobu group around 4 km below the ground.View full abstractDownload PDF (2260K)
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