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  • 1
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    Missing western half of the Pacific Plate: Geochemical nature of the Izanagi–Pacific Ridge interaction with a stationary boundary between the Indian and Pacific mantles (2015)
    Wiley
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    Publication Date: 2015-09-15
    Description: The source mantle of the basaltic ocean crust on the western half of the Pacific Plate was examined using Pb–Nd–Hf isotopes. The results showed that the subducted Izanagi–Pacific Ridge (IPR) formed from both Pacific (180–∼80 Ma) and Indian (∼80–70 Ma) mantles. The western Pacific Plate becomes younger westward and is thought to have formed from the IPR. The ridge was subducted along the Kurile–Japan–Nankai–Ryukyu (KJNR) Trench at 60–55 Ma and leading edge of the Pacific Plate is currently stagnated in the mantle transition zone. Conversely, the entire eastern half of the Pacific Plate, formed from isotopically distinct Pacific mantle along the East Pacific Rise and the Juan de Fuca Ridge, largely remains on the seafloor. The subducted IPR is inaccessible; therefore, questions regarding which mantle might be responsible for the formation of the western half of the Pacific Plate remain controversial. Knowing the source of the IPR basalts provides insight into the Indian–Pacific mantle boundary before the Cenozoic. Isotopic compositions of the basalts from borehole cores (165–130 Ma) in the western Pacific show that the surface oceanic crust is of Pacific mantle origin. However, the accreted ocean floor basalts (∼80–70 Ma) in the accretionary prism along the KJNR Trench have Indian mantle signatures. This indicates the younger western Pacific Plate of IPR origin formed partly from Indian mantle and that the Indian–Pacific mantle boundary has been stationary in the western Pacific at least since the Cretaceous. This article is protected by copyright. All rights reserved.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Geochemical records from loess deposits in Japan over the last 210 kyr: Lithogenic source changes and paleoclimatic indications (2016)
    Wiley
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    Publication Date: 2016-06-24
    Description: The generation, transport, and accumulation of tropospheric dust have changed with the paleoclimatic changes of the Quaternary Period. Such dust has accumulated in Japan ∼3000 km leeward of the source deserts in China. We analyzed the fractions of windblown fine quartz and bulk major and trace elements, and Sr–Nd–Pb isotopic compositions of loess in SW Japan deposited over the past 210 kyr. The results indicated extensive accumulation of tropospheric dust mixed with tephra fragments derived from the nearby Daisen volcano. The accumulation rate of fine quartz and selected elemental/isotopic compositions can be used as climatic proxies that reflect greater accumulation of dust in times of colder climate. Chemical indices for weathering show enhanced effects of weathering during times of warmer climate. The trace element compositions of the loess deposits are surprisingly similar to those of the Chinese loess, hemipelagic sediments in the Sea of Japan and the western Pacific Ocean, and the distal Chinese dust found in Canada. This similarity indicates that the loess dust shares major fractions of these fine-grained sediments, and that geochemical fractionation during the transport was limited. The Sr–Nd–Pb isotope compositions of the SW Japan loess indicate an origin predominantly in the Gobi Desert. The high-latitude Pacific sediments and high-latitude dust in Canada also show the same signature. However, isotopic compositions of samples from the southern Chinese loess plateau and mid- to low-latitude Pacific sediments are largely derived from the Taklimakan desert, which indicates different delivery pathways of the tropospheric dusts. This article is protected by copyright. All rights reserved.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 3
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    Origin of geochemical mantle components: Role of subduction filter (2016)
    Wiley
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    Publication Date: 2016-07-19
    Description: We quantitatively explore element redistribution at subduction zones using numerical mass balance models to evaluate the roles of the subduction zone filter in the Earth's geochemical cycle. Our models of slab residues after arc magma genesis differ from previous ones by being internally consistent with geodynamic models of modern arcs that successfully explain arc magma genesis, and include element fluxes from the dehydration/melting of each underlying slab component. We assume that the mantle potential temperature ( T p ) was 1400–1650°C at 3.5–1.7 Ga and gradually decreased to 1300–1350°C today. Hot subduction zones with T p ∼1650°C have a thermal structure like modern SW Japan where high-Mg andesite is formed that is chemically like continental crust. After 2.5–1.7 Gyr of storage in the mantle, the residual igneous oceanic crust from hot subduction zones can evolve isotopically to the HIMU mantle component, the residual base of the mantle wedge to EMI, the residual sediment becomes an essential part of EMII, and the residual top of the mantle wedge can become the subcontinental lithosphere component. The Common or Focal Zone component is a stable mixture of the first three residues occasionally mixed with early depleted mantle. Slab residue that recycled earlier (∼2.5 Ga) form the DUPAL anomaly in the southern hemisphere, whereas residues of more recent recycling (∼1.7 Ga) underlie the northern hemisphere. These ages correspond to major continental crust forming events. The east-west heterogeneity of the depleted upper mantle involves sub continental mantle except in the Pacific. This article is protected by copyright. All rights reserved.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 4
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    Geochemical variations in Japan Sea back-arc basin basalts formed by high-temperature adiabatic melting of mantle metasomatized by sediment subduction components (2015)
    Wiley
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    Publication Date: 2015-04-16
    Description: The Yamato Basin in the Japan Sea is a back-arc basin characterized by basaltic oceanic crust that is twice as thick as typical oceanic crust. Two types of ocean floor basalts, formed during the opening of the Japan Sea in the Middle Miocene, were recovered from the Yamato Basin during Ocean Drilling Program Legs 127/128. These can be considered as depleted (D-type) and enriched (E-type) basalts based on their incompatible trace element and Sr–Nd–Pb–Hf isotopic compositions. Both types of basalts plot along a common mixing array drawn between depleted mantle and slab sediment represented by a sand-rich turbidite on the Pacific Plate in the NE Japan forearc. The depleted nature of the D-type basalts suggests that the slab sediment component is nil to minor relative to the dominant mantle component, whereas the enrichment of all incompatible elements in the E-type basalts was likely caused by a large contribution of bulk slab sediment in the source. The results of forward model calculations using adiabatic melting of a hydrous mantle with sediment flux indicate that the melting conditions of the source mantle for the D-type basalts are deeper and hotter than those for the E-type basalts, which appear to have formed under conditions hotter than those of normal mid-oceanic ridge basalts (MORB). These results suggest that the thicker oceanic crust was formed by greater degrees of melting of a hydrous metasomatized mantle source at unusually high mantle potential temperature during the opening of the Japan Sea. This article is protected by copyright. All rights reserved.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 5
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    Reply to comment by Ivan Pineda-Velasco, Tai T. Nguyen, Hiroshi Kitagawa, and Eizo Nakamura on “Diverse magmatic effects of subducting a hot slab in SW Japan: Results from forward modeling” (2015)
    Wiley
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    Publication Date: 2015-07-18
    Description: The Comment by Pineda-Velasco et al . [2015] examined Pb isotope data presented by Kimura et al . [2014]. The authors' points are that (1) there is uncertainty in the analytical results of Kimura et al . [2014] due to the effect of mass fractionation, and therefore, (2) the interpretations of Kimura et al . [2014] based on the extent of crustal assimilation and the estimated Pb isotopic composition of the crustal component are erroneous. In response to the Comment, we report a flaw in the original paper that the samples from the Aono, Daisen, and Kannabe regions were analyzed using conventional TIMS methods and all other samples were analyzed using thallium-spiked multi-collector inductively-coupled-plasma mass spectrometry (TS-MC-ICP-MS). We have re-analyzed the sample powders from Karasugasen, Daisen, and Aono using TS-MC-ICP-MS. Our new results showed considerable overlap with the data in Pineda-Velasco et al . [2015]. Therefore, the isotopic trends shown by the conventional TIMS in Kimura et al . [2014] were analytical artifacts from mass bias. We conclude that the crustal assimilation proposed by Kimura et al . [2014] was erroneous in terms of Pb isotopes, nevertheless some crustal assimilation in the Karasugasen lava is evident from the chemical zoning of hornblende phenocrysts. Although the original Pb isotope argument for crustal contamination was wrong, the ABS4 modeling is unaffected because of no to subtle changes in estimated mantle source compositions in their ABS4 model. This article is protected by copyright. All rights reserved.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 6
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    Slumping and mass transport deposition in the Nankai fore arc: Evidence from IODP drilling and 3-D reflection seismic data (2011)
    Wiley
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    Publication Date: 2011-05-11
    Description: Multiple lines of evidence exist for a range of sediment mass movement processes within the shallow megasplay fault zone (MSFZ) area and the adjacent slope basin in the outer fore arc of the Nankai subduction zone, Japan. Diagnostic features observed in three-dimensional reflection seismic data and in cores of the Integrated Ocean Drilling Program (IODP) document a multifarious mass movement history spanning ∼2.87 million years. Various modes and scales of sediment remobilization can be related to the different morphotectonic settings in which they occurred. From this evidence, we decipher the tectonic control on slumping and mass transport deposition in the Nankai fore arc. Three periods of intensified mass wasting coincided with pulses of enhanced activity on the splay fault: (1) an initial phase of juvenile out-of-sequence thrusting ∼1.95 to 1.7 Ma, (2) a reactivation phase between ∼1.55 and 1.24 Ma, and (3) at about 1 Ma, during a phase of uplift of the fore-arc high and motion along the MSFZ. We suggest that slope oversteepening, extensional stress regimes, and lateral transmission of fluid overpressures may have preconditioned the slope sediments to fail. Individual mass-wasting events may have been triggered by dynamic loading from earthquake waves and/or transient pulses of pore pressure along the splay fault. Overall, our results provide insights into the complicated interplay between tectonic and submarine mass movement processes. We demonstrate that detailed knowledge about the spatial and temporal distribution of submarine mass movements can be integrated into a holistic reconstruction of tectonostratigraphic evolution of accretionary margins.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 7
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    Origins of felsic magmas in Japanese subduction zone: Geochemical characterizations of tephra from caldera-forming eruptions (2015)
    Wiley
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    Publication Date: 2015-06-06
    Description: Dacitic to rhyolitic glass shards from 80 widespread tephras erupted during the past 5 Mys from calderas in Kyushu, and SW, central, and NE Japan were analyzed. Laser ablation inductively coupled plasma mass spectrometry was used to determine 10 major and 33 trace elements and 207 Pb/ 206 Pb– 208 Pb/ 206 Pb isotope ratios. The tephras were classified into three major geochemical types and their source rocks were identified as plutonic, sedimentary, and intermediate amphibolite rocks in the upper crust. A few tephras from SW Japan were identified as adakite and alkali rhyolite and were regarded to have originated from slab melt and mantle melt, respectively. The Pb isotope ratios of the tephras are comparable to those of the intermediate lavas in the source areas but are different from the basalts in these areas. The crustal assimilants for the intermediate lavas were largely from crustal melts and are represented by the rhyolitic tephras. A large heat source is required for forming large volumes of felsic crustal melts and is usually supplied by the mantle via basalt. Hydrous arc basalt formed by cold slab subduction is voluminous, and its heat transfer with high water content may have melted crustal rocks leading to effective felsic magma production. Coincidence of basalt and felsic magma activities shown by this study suggests caldera-forming eruptions are ultimately the effect of a mantle-driven cause. This article is protected by copyright. All rights reserved.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 8
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    Variety and origin of magmas on Shatsky Rise, northwest Pacific Ocean (2012)
    Wiley
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    Publication Date: 2012-08-18
    Description: Shatsky Rise consists of thick (∼30 km maximum) basaltic crust with various geochemical compositions. Geochemistry data indicate that four magma types exist on the plateau; namely normal, low-Ti, high-Nb, and U1349 types. The normal type is the most abundant in volume and appears on all three large edifices of the plateau: Tamu, Ori, and Shirshov massifs. Composition of the normal type is similar to normal mid-ocean ridge basalt (N-MORB) composition, but with slight relative enrichment of the more incompatible elements. The low-Ti type is distinguished from the normal type basalt by slightly lower Ti content at a given MgO. Composition of the high-Nb type is characterized by distinctively high contents of incompatible trace elements. U1349 type basalts are composed of more primitive and depleted compositions compared with the others. The normal type basalts constitute ∼94% of the lava units of the oldest Tamu Massif and non-normal types (i.e., the other three types) basalts comprise ∼57% on the younger Ori Massif, implying that geochemical compositions may have become heterogeneous with time. Petrological examination demonstrates that compositions of the normal-, low-Ti-, and high-Nb-type basalts evolved through fractional crystallization of olivine, plagioclase, and augite in shallow magma chambers (
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 9
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    Sources and physicochemical characteristics of fluids along a subduction-zone megathrust: A geochemical approach using syn-tectonic mineral veins in the Mugi mélange, Shimanto accretionary complex (2012)
    Wiley
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    Publication Date: 2012-07-12
    Description: The Mugi mélange in the Shimanto accretionary complex, southwest Japan, records faulting and fluid flow patterns at the updip limit of the seismogenic region of the Nankai subduction zone. To characterize the origin and behavior of syn-tectonic fluids, we investigated the carbon, oxygen, and strontium isotopic compositions, and rare earth element (REE) patterns of syn-tectonic calcite within veins along fault zones in the mélange, as well as the Sr isotopic compositions and REE patterns of surrounding host rocks. With the exception of intra-basalt veins formed prior to subduction, the δ13C values of veins range from −10‰ to −19‰, suggesting a mixed carbon source (i.e., marine carbonate and organic matter). The vein-forming fluids have positive oxygen isotopic compositions (+2‰ to +9‰ (SMOW)) and high 87Sr/86Sr values (0.70794–0.70850), suggesting that the source was rock-buffered fluids affected both by terrigenous sediments and altered oceanic crust. The veins found in filling the fault zone associated with tectonic underplating have different REE patterns to those of the other veins, implying a difference in physicochemical processes affecting the fault zone near the subduction megathrust.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 10
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    Spatial and temporal evolution of the megasplay fault in the Nankai Trough (2011)
    Wiley
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    Publication Date: 2011-03-23
    Description: The temporal and spatial evolution of a seismogenic megasplay fault in the Kumano area, Nankai Trough (southwest Japan), is revealed by detailed investigation of the three-dimensional structure of the shallow portions of the fault, combined with the results of drilling and dating of cores from Integrated Ocean Drilling Program (IODP) Expedition 316. The ENE striking eastern portion of the splay fault has remained active since the inception of faulting at ∼1.95 Ma. The recent shortening rate is ∼1 m/kyr, which represents ∼1.5%–2.5% of the total plate convergence rate of ∼40–65 m/kyr. The NE striking western portion of the splay fault exhibits a different mode of activity. Early stage activity (before 1.55 Ma) was similar to the eastern portion, but the fault was inactive between 1.55 and 1.24 Ma. The fault was reactivated for a short time at ∼1.24 Ma but again ceased activity after formation of the secondary branch and has been inactive since 1.24 Ma. Cessation of splay fault activity in the western domain after 1.55 Ma may be due to collision with a seamount and resulting bending of the accretionary prism in the splay fault footwall. Continuous activity of the eastern domain of the splay fault after 1.24 Ma may be related to geometrical favorability due to reorientation of the fault after the seamount passed beneath the imbricate thrust zone, leading to initiation of slightly oblique subduction.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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