Journal of Mineralogical and Petrological Sciences
Albite porphyroblasts occur in schists in higher-grade zones of the Sanbagawa metamorphic belt, J... more Albite porphyroblasts occur in schists in higher-grade zones of the Sanbagawa metamorphic belt, Japan. In this study of pelitic schists from the Nagatoro area of the belt, various microstructures in albite porphyroblasts were identified and indicate that grain size reduction of the albite porphyroblasts occurred after their formation. Schists with recognizable porphyroblasts contain fine grains of albite aligned in the matrix. In comparison, schists without recognizable albite porphyroblasts contain lenticular aggregations of albite grains in quartz-rich layers that suggest grain size reduction of the original porphyroblasts. These observations suggest that the absence of albite porphyroblasts does not necessarily indicate different pressure, temperature, or chemical conditions but can instead be explained by grain size reduction of albite porphyroblasts during deformation.
The beginning of the recrystallization of minerals within a subducting oceanic plate provides a v... more The beginning of the recrystallization of minerals within a subducting oceanic plate provides a valuable record of dehydration within the subduction zone. Pelitic schists of the Nagatoro area, Kanto Mountains, Japan, record the initial stages of garnet growth. Consequently, these rocks were studied to analyze garnet nucleation and growth during metamorphism of the Sanbagawa metamorphic belt, one of the world's most comprehensively studied subduction complexes. The garnet grains are small, euhedral, and occur only within micaceous lamellae that define the schistosity. Crystal size distribution analyses revealed most of the garnet grains follow the log-normal size distribution, indicating that they formed in the same event. A few exceptionally large garnet grains exist that do not seem to follow the log-normal distribution. The latter garnet grains contain a rounded fragmental area with a different chemical composition inside the core. It is possible that detrital fragments of garnet contribute to the irregular crystal size distribution of garnet in the studied area. Many of the smaller (log-normal) garnet grains have relatively large, homogeneous Mn-rich cores. The lack of chemical zoning within the garnet cores suggests that they grew under constant pressure and temperature in response to overstepping of the garnet-in reaction. The chemical composition changes very sharply at the boundary between the core and the surrounding mantle. The size of the Mn-rich core is different from sample to sample, suggesting that the nucleation was controlled by the local chemical condition of each sample.
Kusatsu-Shirane Volcano is an active Quaternary andesitic-to-dacitic volcano located in the Centr... more Kusatsu-Shirane Volcano is an active Quaternary andesitic-to-dacitic volcano located in the Central Japan Arc. We conducted a detailed petrological investigation of orthopyroxene (opx)-magnetite (mt) symplectites associated with olivine in the Sessho lava, an andesitic lava flow from Kusatsu-Shirane. We concluded that the symplectites are pseudomorphs after olivine and were formed through the breakdown of olivine in a mafic magma as a result of oxidation during mixing with a felsic magma. Various olivines and opx-mt symplectites that show different stages of the progressive breakdown reaction of olivine coexist in a single lava flow. We suggest that basaltic recharge into the magma reservoir beneath Kusatsu-Shirane occurred repeatedly, leading to a hybrid andesite magma with different types of olivine and symplectite being erupted at Kusatsu-Shirane Volcano.
In central Shikoku, SW Japan, eclogitic rock masses expose themselves among the sedimentary schis... more In central Shikoku, SW Japan, eclogitic rock masses expose themselves among the sedimentary schists of the Sambagawa metamorphic belt. When and how they were juxtaposed might reveal the nature of subduction zone tectonics. Garnet is known to have conspicuous zoning in the Sambagawa metamorphic belt. Apart from grains having a normal zoning with a bell-shaped Mn profile, grains with a composite chemical zoning pattern are found exclusively in the schists close to the eastern Iratsu metagabbro mass in the Besshi district. It is reasonable to consider that composite zoning formed as a result of an event related to the juxtaposition. P-T paths of the characteristic zoning in garnet were inversely calculated using differential thermodynamic method (Gibbs' method) , to extract the influence of the emplacement of the tectonic block. The derived P-T paths of composite zoning, compared to the prograde P-T path of the Sambagawa metamorphism, showed temperature decreases during their growth interval. It is suggested that the growth interval of garnet corresponds to the juxtaposition of the eclogitic rock masses and the pelitic Sambagawa schists, and that the juxtaposition occurred during subduction.
Japanese stone industry has been thought to have developed to supply materials for the western ar... more Japanese stone industry has been thought to have developed to supply materials for the western architectures that were introduced in Japan in the late 19th century. It is also thought that it simply reduced because the imported stone materials became inexpensive. They are true, in a way, but the fact is more intricate, according to the interview survey carried out in several granite production areas. It is important to learn and record the complexity in the industrial history of one of the Japanese undergroung resources. Characteristic Japanese manner in the stone industry is described in this article. Difference in the industrial structure and history between several production areas are then documented. Modern quarrying and stone manufacturing industry in Japan was established shortly after the introduction of western architecture, which used stones. Before that, stone was not popular in architectures. Therefore, stone was accepted in Japanese construction as decorating or finishi...
Geoscience Studies: historical, philosophical and STS studies Convener:*Michiko Yajima(Tokyo Medi... more Geoscience Studies: historical, philosophical and STS studies Convener:*Michiko Yajima(Tokyo Medical and Dentel University), Shigeyuki Aoki(Center for Cultural Research and Studies, University of Aizu), Toshihiro Yamada(Chiba Prefectural Funabashi High School), Shigeo Yoshida(Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University), Chair:Shigeyuki Aoki(Center for Cultural Research and Studies, University of Aizu), Shigeo Yoshida(Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University) Tue. Apr 29, 2014 4:15 PM 5:45 PM 422 (4F) Topics on history, philosophy and STS (science, technology and society)studies of geoscience will be discussed. Any related subject is welcome, such as history of mining, methodology of geoscience, sociology of natural hazards, and politics of environmental issues.
The empirical relationship between the degree of compactional thinning and geochemical compositio... more The empirical relationship between the degree of compactional thinning and geochemical composition for fully consolidated mudstones was proposed for the first time. New data were provided by sedimentological, ichnological, and geochemical analyses of the mudstone samples from the Eocene Naharigawa Formation, Kochi Prefecture, Japan. Based on these analyses, the degree of compaction of the Naharigawa mudstones as a percentage of shortening (Cms) was calculated as 83.01 %. To quantitatively estimate the empirical relationship, the Cms values for various mudstones, whose geochemical compositions are available by previous studies, were further calculated and compiled. As a result of the compilation, a significant and strong negative correlation between the Cms values and CaO abundances (r =−0.75;p<0.05) was recognized. Furthermore, the Cms values show negative correlation between Ca concentrations normalized to terrigenous material (CaO/Al2O3, CaO/TiO2) that are considered as proxies...
Nodules are known to occur within the alternate layers of sand and mud stones in Muroto Pninsula,... more Nodules are known to occur within the alternate layers of sand and mud stones in Muroto Pninsula, Shikoku. Those nodules apparently have been formed within the deep-sea sediment of the trench, the timing and the details of their formation, however, is not known. This report describes the occurrence of the nodules within the alternate layers of sand and mud. The occuuence and the cross-sectional observation of the nodules both support that the nodules are trace fossils and they formed within compacted layers of mud underneath the surface of the sea floor. Spatial distribuion and occurrence of the nodules were observed within an outcrop of alternate layers of sand and mud. The alternate layers were straight, with no evidence of large deformation associating the accretion. The thickness of the mud stone layer and the sand stone layer was approximately 10 cm and 2 cm, respectively. The size of the nodules was around 3 to 5 cm (radius). Nodules occurred in many different layers. Most nod...
Volcanic and igneous activities, and these long-term forecasting Convener:*Teruki Oikawa(Institut... more Volcanic and igneous activities, and these long-term forecasting Convener:*Teruki Oikawa(Institute of Geology and Geoinformation, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology), Daisuke MIURA(Geosphere Sciences, Civil Engineering Research Laboratory, Central Research Institute of Electric Power Industry), Takeshi Hasegawa(Department of Earth Sciences, College of Science, Ibaraki University), Nobuo Geshi(Geological Survey of Japan, The National Institute of Advanced Industrial Science and Technology), Yoshihiro Ishizuka(Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology), Chair:Maya Yasui(College of Humanities and Sciences, Nihon University), Mitsuhiro Yoshimoto(Faculty of Science, Hokkaido University) Thu. May 1, 2014 9:00 AM 10:45 AM 411 (4F) This session focuses on generation and accumulation processes of magmas, magma-crust interaction and degassing, and modes of eruption, long-term forecast ...
The early Toarcian (Early Jurassic) oceanic anoxic event (T-OAE) was a significant palaeoenvironm... more The early Toarcian (Early Jurassic) oceanic anoxic event (T-OAE) was a significant palaeoenvironmental perturbation that led to marked changes in ocean redox condition and ecosystem. This event is characterized by the widespread occurrence of a ~3–7‰ negative excursion in the carbon-isotope (δC) composition of marine organic and inorganic matter and terrestrial plant material. In addition, one of the distinct phenomena during the early Toarcian is the abrupt rise of pCO2 and consequent global warming, which led to enhanced hydrological cycles and ocean anoxia. Despite such global impacts of the event, the precise palaeoenvironmental and palaeoecological changes during the event from sections outside of the Boreal and Tethys realms are uncertain. To address this issue and further expand our understanding of the nature of the event, we investigated the Nishinakayama Formation of the Toyora area, southwest Japan, which represents an organic-rich silty mudstone-dominated succession depo...
Reconstruction of the local thermal structure within metamorphic rocks are important since the th... more Reconstruction of the local thermal structure within metamorphic rocks are important since the thermal structure shows the deformation regime during the exhumation of the terrain. The deformation regime of the Sanbagawa metamorphic rocks of the Kanto Mountains have long been controversial, whether it is one plastic continuous body or it is composed of several parts separated by faults or thrusts. One of the key must lie in the area where apparent isograd is outcropped. Most of the Nagatoro area is categorized as the zone I by Hashimoto et al. (1992), which is the lowest metamorphic grade zone and is equivalent to the Chlorite Zone defined in the Shikoku Area of the Sanbagawa Metamorphic Belt. Zone I is defined by the mineral assemblage without garnet. Appearance of garnet is the index of zone II, which is the second lowest metamorphic grade zone defined in this area. Several outcrops in this area are known as being the zone II rocks since they show the mineral assemblage including g...
Kusatsu–Shirane Volcano is an active Quaternary andesitic-to-dacitic volcano located in the Centr... more Kusatsu–Shirane Volcano is an active Quaternary andesitic-to-dacitic volcano located in the Central Japan Arc. We conducted a detailed petrological investigation of orthopyroxene (opx)–magnetite (mt) symplectites associated with olivine in the Sessho lava, an andesitic lava flow from Kusatsu–Shirane. We concluded that the symplectites are pseudomorphs after olivine and were formed through the breakdown of olivine in a mafic magma as a result of oxidation during mixing with a felsic magma. Various olivines and opx–mt symplectites that show different stages of the progressive breakdown reaction of olivine coexist in a single lava flow. We suggest that basaltic recharge into the magma reservoir beneath Kusatsu–Shirane occurred repeatedly, leading to a hybrid andesite magma with different types of olivine and symplectite being erupted at Kusatsu–Shirane Volcano.
The beginning of the recrystallization of minerals within a subducting oceanic plate provides a v... more The beginning of the recrystallization of minerals within a subducting oceanic plate provides a valuable record of dehydration within the subduction zone. Pelitic schists of the Nagatoro area, Kanto Mountains, Japan, record the initial stages of garnet growth. Consequently, these rocks were studied to analyze garnet nucleation and growth during metamorphism of the Sanbagawa metamorphic belt, one of the world’s most comprehensively studied subduction complexes. The garnet grains are small, euhedral, and occur only within micaceous lamellae that define the schistosity. Crystal size distribution analyses revealed most of the garnet grains follow the log-normal size distribution, indicating that they formed in the same event. A few exceptionally large garnet grains exist that do not seem to follow the log-normal distribution. The latter garnet grains contain a rounded fragmental area with a different chemical composition inside the core. It is possible that detrital fragments of garnet ...
Journal of Mineralogical and Petrological Sciences, 2018
Traditional metamorphic geology admitted that the appearance of specific index mineral, such as g... more Traditional metamorphic geology admitted that the appearance of specific index mineral, such as garnet, indicated the rocks experienced higher peak metamorphic condition. It is largely true, but it is still unclear what is going on in the rocks at the very start of the garnet formation. One of the key must lie in the area where apparent isograd is outcropped. Nagatoro area is located in the Kanto Mountains, and it is where the low grade metamorphic rocks of the Sanbagawa metamorphic belt are exposed. Appearance of garnet is the index of the higher metamorphic grade in this area. Several outcrops in Nagatoro area are known to contain garnet, thus being good samples of the outcropped isograd. In this study, spatial distribution of garnet within such an outcrop several tens of meters long was investigated. The samples are pelitic schists, and the mineral assemblage was basically quartz, plagioclase, muscovite, and chlorite. 36 of the 55 samples contained garnet. Garnet grains were small, most of them with diameters less than 100 µm. Most of the garnet grains were euhedral to subhedral and were found within micaceous lamellae which form the foliations of the pelitic metamorphic rocks. The micaceous lamellae were constituted mainly by muscovite with lesser amount of chlorite. Occurrence of garnet-bearing rocks within the outcrop seemed to be restricted in certain structural layers. The structural layers are known to be nearly parallel to the lithologic boundary in this area. Mapped chemical profiles of garnet revealed that the garnet grains exhibited euhedral growth of the crystal. The core part was relatively large and homogeneous, with quite a high Mn end-member (spessartine) content (X Sps > 50%). Irregular shaped inner core was preserved. These features indicate that the texture preserves the beginning stage of garnet growth. The trend of chemical composition of garnet rim and adjacent chlorite is consistent with the bulk rock chemistry control. Spatial distribution of garnet, at the start of its growth, was probably controlled by the bulk rock chemistry.
The early Toarcian oceanic anoxic event (T-OAE; ~183 Ma) was a significant palaeoenvironmental pe... more The early Toarcian oceanic anoxic event (T-OAE; ~183 Ma) was a significant palaeoenvironmental perturbation associated with marked changes in oceanic redox conditions. However, the precise redox conditions and redox history of various water masses during the T-OAE, especially those from outside the Boreal and Tethyan realms, are unclear. To address this issue, we present pyrite framboid data from an upper Pliensbachian to lower Toarcian succession deposited on the NW Panthalassic margin in a shallow-water setting (Sakuraguchi-dani section, Toyora area, SW Japan). 21/12/2017 Available data on redox-sensitive trace elements from the same succession suggest that dysoxic bottom-water conditions generally prevailed, with intermittent short-term oxygenation events. Size-distribution analysis of pyrite framboids reveals that framboid size populations from the silty mudstones during the OAE were characterized by small mean diameters and standard deviations. This suggests that euxinic conditions at least intermittently occurred during the T-OAE interval. Most likely, this water-column euxinia was associated with the expansion of an oxygen minimum zone linked to increased primary productivity. This interpretation is consistent with a previously reported increase in fluvial discharge and thus nutrient flux caused by a strengthening of the hydrological cycle.
Please find accompanying this letter a revision of the above manuscript that was recently reviewe... more Please find accompanying this letter a revision of the above manuscript that was recently reviewed for Earth and Planetary Science Letters. We have detailed below the changes made to our manuscript in response to the comments raised by the Editor. We provide two copies of the manuscript: one with track changes with our revisions highlighted, and one clean copy. We are delighted that the submission has been judged of sufficient quality for publication in Earth and Planetary Science Letters after minor revision. In our revision, we have addressed all the issues raised by the Editor, and we have outlined below detailed responses to all comments. Thank you for the revision and for attending to the reviewer comments. I have read the revision. The only things that now need looking at, I think, are some very minor suggestions I have made for better phrasing -which you are welcome to take or leave. I have marked these on a hard copy and will send a scan of this to you by separate e-mail. Please could you attend to these final points, and then we should be able to accept it for publication. => Thank you very much for your thoughtful editing. During revision, we changed the manuscript as suggested by your comments.
Journal of Mineralogical and Petrological Sciences
Albite porphyroblasts occur in schists in higher-grade zones of the Sanbagawa metamorphic belt, J... more Albite porphyroblasts occur in schists in higher-grade zones of the Sanbagawa metamorphic belt, Japan. In this study of pelitic schists from the Nagatoro area of the belt, various microstructures in albite porphyroblasts were identified and indicate that grain size reduction of the albite porphyroblasts occurred after their formation. Schists with recognizable porphyroblasts contain fine grains of albite aligned in the matrix. In comparison, schists without recognizable albite porphyroblasts contain lenticular aggregations of albite grains in quartz-rich layers that suggest grain size reduction of the original porphyroblasts. These observations suggest that the absence of albite porphyroblasts does not necessarily indicate different pressure, temperature, or chemical conditions but can instead be explained by grain size reduction of albite porphyroblasts during deformation.
The beginning of the recrystallization of minerals within a subducting oceanic plate provides a v... more The beginning of the recrystallization of minerals within a subducting oceanic plate provides a valuable record of dehydration within the subduction zone. Pelitic schists of the Nagatoro area, Kanto Mountains, Japan, record the initial stages of garnet growth. Consequently, these rocks were studied to analyze garnet nucleation and growth during metamorphism of the Sanbagawa metamorphic belt, one of the world's most comprehensively studied subduction complexes. The garnet grains are small, euhedral, and occur only within micaceous lamellae that define the schistosity. Crystal size distribution analyses revealed most of the garnet grains follow the log-normal size distribution, indicating that they formed in the same event. A few exceptionally large garnet grains exist that do not seem to follow the log-normal distribution. The latter garnet grains contain a rounded fragmental area with a different chemical composition inside the core. It is possible that detrital fragments of garnet contribute to the irregular crystal size distribution of garnet in the studied area. Many of the smaller (log-normal) garnet grains have relatively large, homogeneous Mn-rich cores. The lack of chemical zoning within the garnet cores suggests that they grew under constant pressure and temperature in response to overstepping of the garnet-in reaction. The chemical composition changes very sharply at the boundary between the core and the surrounding mantle. The size of the Mn-rich core is different from sample to sample, suggesting that the nucleation was controlled by the local chemical condition of each sample.
Kusatsu-Shirane Volcano is an active Quaternary andesitic-to-dacitic volcano located in the Centr... more Kusatsu-Shirane Volcano is an active Quaternary andesitic-to-dacitic volcano located in the Central Japan Arc. We conducted a detailed petrological investigation of orthopyroxene (opx)-magnetite (mt) symplectites associated with olivine in the Sessho lava, an andesitic lava flow from Kusatsu-Shirane. We concluded that the symplectites are pseudomorphs after olivine and were formed through the breakdown of olivine in a mafic magma as a result of oxidation during mixing with a felsic magma. Various olivines and opx-mt symplectites that show different stages of the progressive breakdown reaction of olivine coexist in a single lava flow. We suggest that basaltic recharge into the magma reservoir beneath Kusatsu-Shirane occurred repeatedly, leading to a hybrid andesite magma with different types of olivine and symplectite being erupted at Kusatsu-Shirane Volcano.
In central Shikoku, SW Japan, eclogitic rock masses expose themselves among the sedimentary schis... more In central Shikoku, SW Japan, eclogitic rock masses expose themselves among the sedimentary schists of the Sambagawa metamorphic belt. When and how they were juxtaposed might reveal the nature of subduction zone tectonics. Garnet is known to have conspicuous zoning in the Sambagawa metamorphic belt. Apart from grains having a normal zoning with a bell-shaped Mn profile, grains with a composite chemical zoning pattern are found exclusively in the schists close to the eastern Iratsu metagabbro mass in the Besshi district. It is reasonable to consider that composite zoning formed as a result of an event related to the juxtaposition. P-T paths of the characteristic zoning in garnet were inversely calculated using differential thermodynamic method (Gibbs' method) , to extract the influence of the emplacement of the tectonic block. The derived P-T paths of composite zoning, compared to the prograde P-T path of the Sambagawa metamorphism, showed temperature decreases during their growth interval. It is suggested that the growth interval of garnet corresponds to the juxtaposition of the eclogitic rock masses and the pelitic Sambagawa schists, and that the juxtaposition occurred during subduction.
Japanese stone industry has been thought to have developed to supply materials for the western ar... more Japanese stone industry has been thought to have developed to supply materials for the western architectures that were introduced in Japan in the late 19th century. It is also thought that it simply reduced because the imported stone materials became inexpensive. They are true, in a way, but the fact is more intricate, according to the interview survey carried out in several granite production areas. It is important to learn and record the complexity in the industrial history of one of the Japanese undergroung resources. Characteristic Japanese manner in the stone industry is described in this article. Difference in the industrial structure and history between several production areas are then documented. Modern quarrying and stone manufacturing industry in Japan was established shortly after the introduction of western architecture, which used stones. Before that, stone was not popular in architectures. Therefore, stone was accepted in Japanese construction as decorating or finishi...
Geoscience Studies: historical, philosophical and STS studies Convener:*Michiko Yajima(Tokyo Medi... more Geoscience Studies: historical, philosophical and STS studies Convener:*Michiko Yajima(Tokyo Medical and Dentel University), Shigeyuki Aoki(Center for Cultural Research and Studies, University of Aizu), Toshihiro Yamada(Chiba Prefectural Funabashi High School), Shigeo Yoshida(Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University), Chair:Shigeyuki Aoki(Center for Cultural Research and Studies, University of Aizu), Shigeo Yoshida(Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University) Tue. Apr 29, 2014 4:15 PM 5:45 PM 422 (4F) Topics on history, philosophy and STS (science, technology and society)studies of geoscience will be discussed. Any related subject is welcome, such as history of mining, methodology of geoscience, sociology of natural hazards, and politics of environmental issues.
The empirical relationship between the degree of compactional thinning and geochemical compositio... more The empirical relationship between the degree of compactional thinning and geochemical composition for fully consolidated mudstones was proposed for the first time. New data were provided by sedimentological, ichnological, and geochemical analyses of the mudstone samples from the Eocene Naharigawa Formation, Kochi Prefecture, Japan. Based on these analyses, the degree of compaction of the Naharigawa mudstones as a percentage of shortening (Cms) was calculated as 83.01 %. To quantitatively estimate the empirical relationship, the Cms values for various mudstones, whose geochemical compositions are available by previous studies, were further calculated and compiled. As a result of the compilation, a significant and strong negative correlation between the Cms values and CaO abundances (r =−0.75;p<0.05) was recognized. Furthermore, the Cms values show negative correlation between Ca concentrations normalized to terrigenous material (CaO/Al2O3, CaO/TiO2) that are considered as proxies...
Nodules are known to occur within the alternate layers of sand and mud stones in Muroto Pninsula,... more Nodules are known to occur within the alternate layers of sand and mud stones in Muroto Pninsula, Shikoku. Those nodules apparently have been formed within the deep-sea sediment of the trench, the timing and the details of their formation, however, is not known. This report describes the occurrence of the nodules within the alternate layers of sand and mud. The occuuence and the cross-sectional observation of the nodules both support that the nodules are trace fossils and they formed within compacted layers of mud underneath the surface of the sea floor. Spatial distribuion and occurrence of the nodules were observed within an outcrop of alternate layers of sand and mud. The alternate layers were straight, with no evidence of large deformation associating the accretion. The thickness of the mud stone layer and the sand stone layer was approximately 10 cm and 2 cm, respectively. The size of the nodules was around 3 to 5 cm (radius). Nodules occurred in many different layers. Most nod...
Volcanic and igneous activities, and these long-term forecasting Convener:*Teruki Oikawa(Institut... more Volcanic and igneous activities, and these long-term forecasting Convener:*Teruki Oikawa(Institute of Geology and Geoinformation, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology), Daisuke MIURA(Geosphere Sciences, Civil Engineering Research Laboratory, Central Research Institute of Electric Power Industry), Takeshi Hasegawa(Department of Earth Sciences, College of Science, Ibaraki University), Nobuo Geshi(Geological Survey of Japan, The National Institute of Advanced Industrial Science and Technology), Yoshihiro Ishizuka(Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology), Chair:Maya Yasui(College of Humanities and Sciences, Nihon University), Mitsuhiro Yoshimoto(Faculty of Science, Hokkaido University) Thu. May 1, 2014 9:00 AM 10:45 AM 411 (4F) This session focuses on generation and accumulation processes of magmas, magma-crust interaction and degassing, and modes of eruption, long-term forecast ...
The early Toarcian (Early Jurassic) oceanic anoxic event (T-OAE) was a significant palaeoenvironm... more The early Toarcian (Early Jurassic) oceanic anoxic event (T-OAE) was a significant palaeoenvironmental perturbation that led to marked changes in ocean redox condition and ecosystem. This event is characterized by the widespread occurrence of a ~3–7‰ negative excursion in the carbon-isotope (δC) composition of marine organic and inorganic matter and terrestrial plant material. In addition, one of the distinct phenomena during the early Toarcian is the abrupt rise of pCO2 and consequent global warming, which led to enhanced hydrological cycles and ocean anoxia. Despite such global impacts of the event, the precise palaeoenvironmental and palaeoecological changes during the event from sections outside of the Boreal and Tethys realms are uncertain. To address this issue and further expand our understanding of the nature of the event, we investigated the Nishinakayama Formation of the Toyora area, southwest Japan, which represents an organic-rich silty mudstone-dominated succession depo...
Reconstruction of the local thermal structure within metamorphic rocks are important since the th... more Reconstruction of the local thermal structure within metamorphic rocks are important since the thermal structure shows the deformation regime during the exhumation of the terrain. The deformation regime of the Sanbagawa metamorphic rocks of the Kanto Mountains have long been controversial, whether it is one plastic continuous body or it is composed of several parts separated by faults or thrusts. One of the key must lie in the area where apparent isograd is outcropped. Most of the Nagatoro area is categorized as the zone I by Hashimoto et al. (1992), which is the lowest metamorphic grade zone and is equivalent to the Chlorite Zone defined in the Shikoku Area of the Sanbagawa Metamorphic Belt. Zone I is defined by the mineral assemblage without garnet. Appearance of garnet is the index of zone II, which is the second lowest metamorphic grade zone defined in this area. Several outcrops in this area are known as being the zone II rocks since they show the mineral assemblage including g...
Kusatsu–Shirane Volcano is an active Quaternary andesitic-to-dacitic volcano located in the Centr... more Kusatsu–Shirane Volcano is an active Quaternary andesitic-to-dacitic volcano located in the Central Japan Arc. We conducted a detailed petrological investigation of orthopyroxene (opx)–magnetite (mt) symplectites associated with olivine in the Sessho lava, an andesitic lava flow from Kusatsu–Shirane. We concluded that the symplectites are pseudomorphs after olivine and were formed through the breakdown of olivine in a mafic magma as a result of oxidation during mixing with a felsic magma. Various olivines and opx–mt symplectites that show different stages of the progressive breakdown reaction of olivine coexist in a single lava flow. We suggest that basaltic recharge into the magma reservoir beneath Kusatsu–Shirane occurred repeatedly, leading to a hybrid andesite magma with different types of olivine and symplectite being erupted at Kusatsu–Shirane Volcano.
The beginning of the recrystallization of minerals within a subducting oceanic plate provides a v... more The beginning of the recrystallization of minerals within a subducting oceanic plate provides a valuable record of dehydration within the subduction zone. Pelitic schists of the Nagatoro area, Kanto Mountains, Japan, record the initial stages of garnet growth. Consequently, these rocks were studied to analyze garnet nucleation and growth during metamorphism of the Sanbagawa metamorphic belt, one of the world’s most comprehensively studied subduction complexes. The garnet grains are small, euhedral, and occur only within micaceous lamellae that define the schistosity. Crystal size distribution analyses revealed most of the garnet grains follow the log-normal size distribution, indicating that they formed in the same event. A few exceptionally large garnet grains exist that do not seem to follow the log-normal distribution. The latter garnet grains contain a rounded fragmental area with a different chemical composition inside the core. It is possible that detrital fragments of garnet ...
Journal of Mineralogical and Petrological Sciences, 2018
Traditional metamorphic geology admitted that the appearance of specific index mineral, such as g... more Traditional metamorphic geology admitted that the appearance of specific index mineral, such as garnet, indicated the rocks experienced higher peak metamorphic condition. It is largely true, but it is still unclear what is going on in the rocks at the very start of the garnet formation. One of the key must lie in the area where apparent isograd is outcropped. Nagatoro area is located in the Kanto Mountains, and it is where the low grade metamorphic rocks of the Sanbagawa metamorphic belt are exposed. Appearance of garnet is the index of the higher metamorphic grade in this area. Several outcrops in Nagatoro area are known to contain garnet, thus being good samples of the outcropped isograd. In this study, spatial distribution of garnet within such an outcrop several tens of meters long was investigated. The samples are pelitic schists, and the mineral assemblage was basically quartz, plagioclase, muscovite, and chlorite. 36 of the 55 samples contained garnet. Garnet grains were small, most of them with diameters less than 100 µm. Most of the garnet grains were euhedral to subhedral and were found within micaceous lamellae which form the foliations of the pelitic metamorphic rocks. The micaceous lamellae were constituted mainly by muscovite with lesser amount of chlorite. Occurrence of garnet-bearing rocks within the outcrop seemed to be restricted in certain structural layers. The structural layers are known to be nearly parallel to the lithologic boundary in this area. Mapped chemical profiles of garnet revealed that the garnet grains exhibited euhedral growth of the crystal. The core part was relatively large and homogeneous, with quite a high Mn end-member (spessartine) content (X Sps > 50%). Irregular shaped inner core was preserved. These features indicate that the texture preserves the beginning stage of garnet growth. The trend of chemical composition of garnet rim and adjacent chlorite is consistent with the bulk rock chemistry control. Spatial distribution of garnet, at the start of its growth, was probably controlled by the bulk rock chemistry.
The early Toarcian oceanic anoxic event (T-OAE; ~183 Ma) was a significant palaeoenvironmental pe... more The early Toarcian oceanic anoxic event (T-OAE; ~183 Ma) was a significant palaeoenvironmental perturbation associated with marked changes in oceanic redox conditions. However, the precise redox conditions and redox history of various water masses during the T-OAE, especially those from outside the Boreal and Tethyan realms, are unclear. To address this issue, we present pyrite framboid data from an upper Pliensbachian to lower Toarcian succession deposited on the NW Panthalassic margin in a shallow-water setting (Sakuraguchi-dani section, Toyora area, SW Japan). 21/12/2017 Available data on redox-sensitive trace elements from the same succession suggest that dysoxic bottom-water conditions generally prevailed, with intermittent short-term oxygenation events. Size-distribution analysis of pyrite framboids reveals that framboid size populations from the silty mudstones during the OAE were characterized by small mean diameters and standard deviations. This suggests that euxinic conditions at least intermittently occurred during the T-OAE interval. Most likely, this water-column euxinia was associated with the expansion of an oxygen minimum zone linked to increased primary productivity. This interpretation is consistent with a previously reported increase in fluvial discharge and thus nutrient flux caused by a strengthening of the hydrological cycle.
Please find accompanying this letter a revision of the above manuscript that was recently reviewe... more Please find accompanying this letter a revision of the above manuscript that was recently reviewed for Earth and Planetary Science Letters. We have detailed below the changes made to our manuscript in response to the comments raised by the Editor. We provide two copies of the manuscript: one with track changes with our revisions highlighted, and one clean copy. We are delighted that the submission has been judged of sufficient quality for publication in Earth and Planetary Science Letters after minor revision. In our revision, we have addressed all the issues raised by the Editor, and we have outlined below detailed responses to all comments. Thank you for the revision and for attending to the reviewer comments. I have read the revision. The only things that now need looking at, I think, are some very minor suggestions I have made for better phrasing -which you are welcome to take or leave. I have marked these on a hard copy and will send a scan of this to you by separate e-mail. Please could you attend to these final points, and then we should be able to accept it for publication. => Thank you very much for your thoughtful editing. During revision, we changed the manuscript as suggested by your comments.
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