The West Pacific Seamount Province (WPSP) represents a series of short-lived Cretaceous hotspot tracks. However, no intraplate volcanoes in advance of petit-spot volcanism erupted near a trench have been identified after the formation of the WPSP on the western Pacific Plate. This study reports new ages for Paleogene volcanic edifices within the northern WPSP, specifically the Ogasawara Plateau and related ridges, and Minamitorishima Island. These Paleogene ages are the first reported for basaltic rocks on western Pacific seamounts, in an area that has previously only yielded Cretaceous ages. The newly found Paleogene volcanisms overprint the Early-middle Cretaceous volcanic edifices, because the seamount or paleo-island material-covered reefal limestone caps on these edifices are uniformly older than the Paleogene volcanism identified in this study. This study outlines several possible causative factors for the Paleogene volcanism overprinting onto existing Cretaceous seamounts, including volcanism related to lithospheric stress, or a younger hotspot track within the northern part of the WPSP that records magmatism from similar to 60 Ma.

In this study, the Sr isotope ratios (IRs; 87Sr/86Sr) of ferromanganese (Fe–Mn) crusts are analyzed through laser ablation inductively coupled plasma multiple-collector mass spectrometry. A sample collected from off Minamitorishima Island showed uniform Sr IRs (0.70906–0.70927) similar to that of present-day seawater with more than 36 mm thickness. Meanwhile, a detritus-rich sample collected from off northeast (NE) Japan showed a wide variation in Sr IRs (0.707761–0.709963). The Sr IR variation in the Fe–Mn crust from off NE Japan suggests detrital influx contributions from both the NE Japan arc (<0.708) and aeolian dust from China (>0.718). Detrital flux from the NE Japan arc increases from the bottom to middle layers, possibly due to the uplift of the Ou backbone range that occurred after ~2 Ma. The increased influx of the aeolian dust in the outer layer is attributable to global cooling in the Quaternary that increased the loess dust transportation from China to the western North Pacific Ocean. Meanwhile, the influence of the detrital influx on the sample from off Minamitorishima Island appeared to be negligible. The Sr IR analysis with high spatial resolution proposed in this study possibly improves the burial history of Fe–Mn nodules.

Recently, small volcanic knolls known as petit-spot are highlighted from seismic viewpoint for a functional necessity to suppress the coseismic displacement during the 2011 Tohoku-Oki earthquake. To better understand physical mechanism of shallow oceanic plate disturbance by the petit-spot magma intrusion, we aim at elucidating the interaction of petit-spot magma and wet sediment in subseafloor volcanic environment. We used petit-spot peperites that are mixtures generated by mingling of magma and wet sediment. The peperites were collected at a petit-spot knoll located east of the outer trench swell in the northwestern Pacific (offshore of northern Japan). The sedimentary portions in the peperites are hardly to poorly consolidated due to heat supply from the magma body, and in places include radiolarian microfossils and zircon grains. Mineral phases of the sedimentary portions were identified to illite, quartz and hydrated sodium aluminum silicate. The radiolarian species and zircon grains were dated at ca. 10.6 to 15.6 Ma and ca. 1 to 108 Ma, respectively. The wide age range can be interpreted that whole sediments on the Early Cretaceous basaltic basement were entirely disturbed and transported to the surface via recent (younger than 1 million years) violate petit-spot magmatism. The subduction of petit-spot with annealed sediment “armor” functioned to prevent coseismic tsunamigenic slip propagation during the 2011 Tohoku-Oki earthquake as the seismic data predicted.

As the alternate survey protocol for AUV-ROV sequential focused geological survey in great water depth, HOV mounted MBES survey followed by the geological observation and sampling was conducted. HOV mounted MBES provided a sub-meter scale high resolution bathymetric map, which efficiently supported the geological observation and resolve down to one meter scale morphological features successfully. Further, a pre-gridded point clouds imagery contains detailed morphological information.

Analyses of elemental abundances by secondary ion mass spectrometry (SIMS) require matrix-matched standard samples to account for the matrix effect on correction factors. This requirement makes it difficult to obtain accurate results for geological samples of variable chemistry. In this study, we prepared 39 volcanic glasses of foiditic, basaltic, basaltic andesitic, rhyolitic and pure SiO2 compositions, including synthetic samples and natural samples collected from the deep seafloor. The measured H2O contents of these samples were in the range 0.02-4.8 wt%. We showed that calibration curves (H2O content vs. (OH-)-O-16/Si-30(SIMS)- ratio) differed according to the composition of the volcanic glasses. Our results demonstrated that for a particular (OH)-O-16/Si-30(SIMS) ratio, water content could differ by up to a factor of five, depending on the composition of the volcanic glass. Although the correction factor (the slope of the calibration curve for water [H2O/((OH-)-O-16/Si-30(-))(SIMS)]) was weakly correlated with SiO2 content, we identified a stronger correlation with the molar weight (g mol(-1), on a one-oxygen mole basis) of the silicate glasses. Our results suggest that modification of the correction factor for the matrix effect on SIMS-based H2O content of volcanic glasses according to their molar weights provides more accurate water contents of silicate glasses, regardless of their chemical composition and water content and without the need for a series of standard glasses of known water contents.

<title>Abstract</title>The Tokoro Belt exposed in NE Hokkaido (Japan) represents part of a Late Cretaceous accretionary complex, which includes variously metamorphosed volcanic rocks that are interbedded with chert, lenticular limestone and some fore-arc sedimentary rocks. The Tokoro Belt is notably different from other Late Cretaceous accretionary complexes around the Pacific Rim because of widespread occurrence of basalts and volcaniclastic rocks in it. The Nikoro Group, characterized by widespread occurrence of volcanic rocks, is divided into western, eastern and southern sections based on the internal structure, geochemical affinities and metamorphic grades of their volcanic lithologies. OIB (ocean island basalt)-type volcanic rocks with low-grade metamorphic overprint predominate in the western and southern sections, whereas MORB (mid-ocean ridge basalt)- and OIA (ocean island alkaline basalt)-type rocks in the eastern section with partly high-pressure metamorphism make up the northern part of the eastern section. Trace element patterns display transitional trends from MORB to OIA geochemical affinities. OIB-type rocks display trace element characteristics similar to those of shield volcano lavas on Hawaii, rather than small and mainly alkaline, Polynesian hotspot lavas; furthermore, they show significant HREE (heavy rare earth element) enrichment probably caused by plume–ridge interaction. Widespread OIBs in the Tokoro Belt represents tectonic slices of a large (&gt;80 km wide) Hawaiian-style, seamount shield volcano on the Izanagi oceanic plate that was accreted into the continental margin of Far East Asia in the Late Cretaceous.

© 2020 We report new noble gas isotopic compositions of submarine basaltic glasses sampled from two seamounts discovered offshore of Chile, and inferred to have erupted as petit-spot volcanoes near the Juan Fernández hotspot. The samples have 3He/4He of 1–15 times atmosphere (Ra). Their neon isotope compositions are similar to those of the Hawaiian Islands and Réunion Island. Their 40Ar/36Ar range from atmospheric to 2300. Although the lavas are likely to be influenced by a hotspot-related component, the cause of the 3He/4He variation must be clarified to ascertain the mantle source. Variations in 3He/4He are not attributable to processes occurring at the Earth's surface such as degassing fractionation, mixing with atmosphere dissolved in seawater, or in-situ post-eruptive addition of 4He. A combination of the Ne-Ar isotope ratios corrected for atmospheric influence and He isotope ratios indicates that the noble gas isotopes of the lavas are a mixture of a hotspot magma, MORB-source, and radiogenic components. The lower 3He/4He are attributed to assimilation with the oceanic lithosphere, suggesting that the pristine 3He/4He of the lavas is hotspot-like. These features can be interpreted as indicating that part of the Juan Fernández plume infiltrated the lithosphere–asthenosphere boundary (LAB), and that the ponding magma has erupted as petit-spot volcanoes because of plate bending. The noble gas data indicate that LAB can be a reservoir for exotic melts, which might have lubricated plate tectonics.

A discontinuity in the seismic velocity associated with the lithosphere-asthenosphere interface, known as the Gutenberg discontinuity, is enigmatic in its origin. While partial mantle melts are frequently suggested to explain this discontinuity, it is not well known which factors critically regulate the melt production. Here, we report geochemical evidence showing that the melt fractions in the lithosphere-asthenosphere boundary were enhanced not only by accumulation of compacted carbonated melts related to recycled ancient marine sediments, but also by partial melting of a pyroxene-rich mantle domain related to the recycled oceanic eclogite/pyroxenites. This conclusion is derived from the first set of Mg isotope data for a suite of young petit-spot basalts erupted on the northwest Pacific plate, where a clearly defined Gutenberg discontinuity exists. Our results reveal a specific linkage between the Gutenberg discontinuity beneath the normal oceanic regions and the recycling of ancient subducted crust and carbonate through the deep Earth.

The Hahajima Seamount is located at the junction between the Izu-Bonin and Mariana forearc slopes in the northwest Pacific Ocean, and between the Philippine Sea Plate and the Ogasawara Plateau on the Pacific Plate. Despite numerous previous studies, the origin of the Hahajima Seamount remains controversial. Here we constrain its origin based on sedimentological and chronological analyses of shallowwater carbonates collected from the eastern slope of the seamount. The carbonates are dominated by floatstones with numerous mollusks. Gravel-sized bioclasts include mollusks (e.g. nerineids and rudists) and local calcareous spines and minor amounts of corals. Sand-sized components include bioclasts of benthic foraminifers and dasycladalean algae, and non-skeletal grains of intraclasts and peloids, with no ooids. Most of the bioclasts are bioeroded and coated with micrite envelopes. The Sr isotope ages of the samples fell into two ranges: Berriasian or Aptian (140.3 Ma or 118.3-113.2 Ma, respectively) and Coniacian to Campanian (86.5-77.2 Ma). Shallow-water carbonates on the Hahajima Seamount are similar in lithology and Sr isotope ages to those on the Ogasawara Plateau located to the east, on the opposite side of the Izu-Bonin Trench (i.e. on the Pacific Plate). This indicates that the shallow-water carbonates on the eastern slope of the Hahajima Seamount were not deposited in situ, but instead originated from the Ogasawara Plateau. The eastern section of this seamount can thus be interpreted as an accretionary wedge.

<jats:p>Abstract. Abstract. We present geochemical and 40Ar∕39Ar age data for a peridotite xenolith-bearing basalt dredged from the Seifu Seamount (SSM basalt) in the northeast Tsushima Basin, southwest Sea of Japan. An 40Ar∕39Ar plateau age of 8.33±0.15 Ma (2σ) was obtained for the SSM basalt, indicating that it erupted shortly after the termination of back-arc spreading in the Sea of Japan. The SSM basalt is a high-K to shoshonitic alkali basalt that is characterized by light rare earth element enrichment. The trace element features of the basalt are similar to those of ocean island basalt, although the Yb content is much higher, indicating formation by the low-degree partial melting of spinel peridotite. The Nd, Sr, and Pb isotopic compositions of the SSM basalt differ from those of back-arc basin basalts in the Sea of Japan. The Sr–Nd isotopic composition of the SSM basalt suggests its source was depleted mid-ocean ridge mantle containing an enriched mantle (EM1) component. The SSM basalt was formed in a post-back-arc extension setting by the low-degree partial melting of an upwelling asthenosphere that had previously been associated with the main phase of back-arc magmatism. </jats:p>

The formation of a petit-spot volcano involves the ascent of an asthenospheric melt to the outboard part of a plate subduction zone where flexure of the plate has taken place in the outer trench swell. On the Pacific Plate, the only previously known examples of such volcanoes were in the vicinity of the Japan Trench. We describe here a newly discovered petit-spot submarine volcano that formed in one of the oldest parts of the Pacific Plate among a cluster of small conical knolls to the southeast of Minamitorishima (Marcus) Island. Geochronological data indicate that this petit-spot volcanic eruption occurred less than 3 million years ago. The volcano erupted on the eastern slope of the outer rise, and it is at an unusually large distance from the Mariana trench axis (similar to 800 km) because this rise, near the older part of the Pacific Plate, is wider than those of other subduction systems.

© 2018 Elsevier B.V. The origins of ferromanganese (Fe–Mn) crusts that form in seawater and elemental partitioning into these crusts have previously been studied using samples from water depths shallower than 4 km. Here we report geochemical analyses of Fe–Mn crusts sampled from petit-spot volcanoes within the Pacific Ocean at water depths of 5.2–6.0 km. The Fe–Mn crusts have low Mn/Fe ratios (0.63–1.5), high contents of Co, Ni, and rare earth elements and Y (REY), and positive Ce anomalies, indicating a hydrogenetic origin. The Fe–Mn crusts are characterized by lower shale-normalized La/Pr ratios (LaSN/PrSN) than typical hydrogenetic Fe–Mn crusts from the Pacific Ocean. Despite negative LaSN/PrSNand NdSN/SmSNand positive EuSN/GdSNand DySN/HoSNcorrelations with water depth, the distribution coefficients of the REY are constant. This indicates that the observed REY variability with water depth in these hydrogenetic Fe–Mn crusts reflects that of seawater. In particular, a strong correlation between LaSN/PrSNand water depth has potential for use as a (paleo)ocean hydro-barometer. The variety of REY compositions of Fe–Mn crusts should be interpreted on the basis of distribution coefficients calculated for each water depth.

The stress field of oceanic lithosphere controls the distribution of submarine petit-spot volcanoes. However, the eruption sites of these petit-spot volcanoes are considered to be limited to concavely flexed regions of lithosphere off the outer rise. Here, we present new data for a recently identified petit-spot lava field on a convexly flexed section of the lithosphere adjacent to the subduction zone offshore of northeast Japan in an area containing more than 80 volcanoes. This area is marked by strongly alkaline lavas that were erupted on the convexly flexed region. As for the concavely flexed region where the petit-spots previously reported, the base of the lithosphere beneath the eruption sites is under extension, whereas the upper part of the lithosphere is under compression. This change in the stress field, from the lower to upper lithosphere, causes ascending dikes to stall in the mid-lithosphere, leading to metasomatic interaction with the surrounding peridotite. The new geochemical data of rocks and xenocrysts presented in this study indicate that strongly alkaline magmas erupted on the convexly flexed region would have ascended more rapidly through the mid-depth of lithosphere because of the extensional regime of the upper lithosphere and decreasing the degree of metasomatic reaction with the surrounding mantle peridotite. The results indicate that the degree of metasomatism and the compositional variations of petit-spot magmas are controlled mainly by the stress field of the lithosphere.

【災害のためプログラム中止】 平成30年北海道胆振東部地震により学術大会のプログラムが大幅に中止となりました．中止となったプログラムの講演要旨については，著者のプライオリティ保護の見地からJ-STAGEに公開し，引用可能とします．ただし，学術大会においては専門家による議論には供されていませんので「災害のためプログラム中止」との文言を付記します．（日本地質学会行事委員会）

Seafloor magnetotelluric (MT) experiments were recently conducted in two areas of the northwestern Pacific to investigate the nature of the old oceanic upper mantle. The areas are far from any tectonic activity, and "normal" mantle structure is therefore expected. The data were carefully analyzed to reduce the effects of coastlines and seafloor topographic changes, which are significant boundaries in electrical conductivity and thus distort seafloor MT data. An isotropic, one-dimensional electrical conductivity profile was estimated for each area. The profiles were compared with those obtained from two previous study areas in the northwestern Pacific. Between the four profiles, significant differences were observed in the thickness of the resistive layer beyond expectations based on cooling of homogeneous oceanic lithosphere over time. This surprising feature is now further clarified from what was suggested in a previous study. To explain the observed spatial variation, dynamic processes must be introduced, such as influence of the plume associated with the formation of the Shatsky Rise, or spatially non-uniform, small-scale convection in the asthenosphere. There is significant room of further investigation to determine a reasonable and comprehensive interpretation of the lithosphere-asthenosphere system beneath the northwestern Pacific. The present results demonstrate that electrical conductivity provides key information for such investigation.

Studies of lithospheric mantle from (ultra) slow spreading ridges have shown that melt extraction at mid-ocean ridges may be incomplete, producing metasomatism/ refertilization of the shallow lithospheric mantle. However, it remains unclear whether similar processes operate off axis and could affect the cooling lithosphere. Here, we report the discovery of a garnet xenocryst in a petit-spot lava sampled on the top of the downgoing Pacific plate in front of Japan. The trace-element composition of this garnet xenocryst, in particular the low chromium, excludes a peridotitic origin, while the flat mid-to heavy rare earth element pattern does not support direct crystallization from melt percolating through the oceanic lithosphere. Garnet formation is therefore interpreted as formed by a subsolidus reaction of a plagioclase-bearing cumulate during the progressive off-axis cooling of the lithosphere. Combining lithosphere cooling models and the specific physical conditions required for subsolidus formation of garnet in tholeiitic systems (0.7-1.2 GPa) indicates that melt percolation to produce plagioclase-bearing cumulate occurs &gt; 150 km off axis. These conditions support that some low-degree melts produced off axis are not collecting to form mid-oceanic ridge basalt (MORB), but percolate and crystallize during the cooling and thickening of the lithospheric mantle. The demonstration of mantle refertilization affecting Pacific lithosphere off axis is critical because such a process could explain the presence of metasomatic domains with distinct physical and chemical properties in the depleted oceanic lithosphere.

Serpentinites (massive and schistose) and listvenite occur as tectonic sheets and lenses within a calcareous metasedimentary melange of the Tulu Dimtu, western Ethiopia. The massive serpentinite contains high-magnesian metamorphic olivine (forsterite [fo] similar to 96mol%) and rare relict primary mantle olivine (Fo(90-93)). Both massive and schistose serpentinites contain zoned chromian spinel; the cores with the ferritchromite rims preserve a pristine Cr/(Cr+Al) atomic ratio (Cr#=0.79-0.87), suggesting a highly depleted residual mantle peridotite, likely formed in a suprasubduction zone setting. Listvenite associated with serpentinites of smaller ultramafic lenses also contain relict chromian spinel having identical Cr# to those observed in serpentinites. However, the relict chromian spinel in listvenite has significantly higher Mg/(Mg+Fe2+) atomic ratios. This suggests that a nearly complete metasomatic replacement of ultramafic rocks by magnesite, talc, and quartz to prevent Mg-Fe2+ redistribution between relict chromian spinel and the host, that is, listvenite formation, took place prior to re-equilibration between chromian spinel and the surrounding mafic minerals in serpentinites. Considering together with the regional geological context, low-temperature CO2-rich hydrothermal fluids would have infiltrated into ultramafic rocks from host calcareous sedimentary rocks at a shallow level of accretionary prism before a continental collision to form the East African Orogen (EAO).

印刷中

The deep carbon cycle plays an important role on the chemical differentiation and physical properties of the Earth's mantle. Especially in the asthenosphere, seismic low-velocity and high electrical conductivity due to carbon dioxide (CO2)-induced partial melting are expected but not directly observed. Here we discuss the experimental results relevant to the genesis of primitive CO2-rich alkali magma forming petit-spot volcanoes at the deformation front of the outer rise of the northwestern Pacific plate. The results suggest that primitive melt last equilibrated with depleted peridotite at 1.8-2.1 GPa and 1,280-1,290 degrees C. Although the equilibration pressure corresponds to the pressure of the lower lithosphere, by considering an equilibration temperature higher than the solidus in the volatile-peridotite system along with the temperature of the lower lithosphere, we conclude that CO2-rich silicate melt is always produced in the asthenosphere. The melt subsequently ascends into and equilibrates with the lower lithosphere before eruption.

A dense field of ferromanganese (Fe-Mn) nodules was discovered on a seamount approximately 300 km east of Minamitorishima Island, in the Japanese Exclusive Economic Zone. To evaluate its potential as a resource for critical metals, we describe results of a geological survey using the SHINKAI 6500 submersible (dive No. 6K 1207) during cruise YK10-05 of R/V Yokosuka in 2010, and geochemical analyses of collected samples. Submersible observations showed that spherical nodules 5-10 cm in diameter almost fully cover the region of high acoustic reflectivity. The large nodules generally consist of three concentric layers: the outermost mottled (sediment-filled) layer L0, the massive black layer L1, and the innermost porous (sediment-filled) layer L2. Elements including Fe, Ti, Co, As, rare earth elements other than Ce, Th, U, and Pb are concentrated in the nodule rim rather than the center. In contrast, Mn, Al, P, Ca, Ni, Zn, Y, Mo, Ce, and W are concentrated in the center, and decrease toward the rim. Geochemical and structural features indicate that the nodules are compositionally and morphologically similar to Fe-Mn crusts, suggesting that they owe their origin solely to prolonged hydrogenetic precipitation of Fe-Mn-(oxyhydr) oxides. As the nodules include metals of economic interest, especially Co, Ni, Mo, and W, this deposit should have high potential for future mining. The changes in nodule composition from the center to the rim may yield information on paleoceanographic events since early Oligocene time.

Shipboard multibeam survey is powerful tool to locate submarine volcanoes especially having small volume. Small submarine volcanoes may represent the initial stages of hotspot activity, but they may also form via litho spheric flexing, regional convection of the mantle, and the presence of fracture zones. Here we describe several volcanoes, flood lavas, and volcanic clusters in French Polynesia using data from archives of multibeam data. The clusters of small volcanoes are similar to petit-spots, and they are not considered to represent the initial stages of a hotspot as they are composed of both young and old edifices, and because the sites are located far from any known hotspot. These newly discovered submarine volcanoes are located in areas with low-velocity seismic shear waves at depths of 60 and 100 km. These lava fields will therefore facilitate geochemical mapping of the mantle in areas unrelated to hotspots, because these lavas may have developed from melts in the shallow mantle beneath French Polynesia. (C) 2016 Elsevier B.V. All rights reserved.

Oceanic lithospheric mantle is generally interpreted as depleted mantle residue after mid-ocean ridge basalt extraction. Several models have suggested that metasomatic processes can refertilize portions of the lithospheric mantle before subduction. Here, we report mantle xenocrysts and xenoliths in petit-spot lavas that provide direct evidence that the lower oceanic lithosphere is affected by metasomatic processes. We find a chemical similarity between clinopyroxene observed in petit-spot mantle xenoliths and clinopyroxene from melt-metasomatized garnet or spinel peridotites, which are sampled by kimberlites and intracontinental basalts respectively. We suggest that extensional stresses in oceanic lithosphere, such as plate bending in front of subduction zones, allowlow-degree melts from the seismic low-velocity zone to percolate, interact and weaken the oceanic lithospheric mantle. Thus, metasomatism is not limited to mantle upwelling zones such as mid-ocean ridges or mantle plumes, but could be initiated by tectonic processes. Since plate flexure is a global mechanism in subduction zones, a significant portion of oceanic lithospheric mantle is likely to be metasomatized. Recycling of metasomatic domains into the convecting mantle is fundamental to understanding the generation of small-scale mantle isotopic and volatile heterogeneities sampled by oceanic island and mid-ocean ridge basalts.

Petit-spot volcanism is a phenomenon ubiquitous on Earth. It originates from melt in the uppermost mantle asthenosphere, occurring where the plate flexes and fractures before subduction. Recent geochemical and petrological studies of petit-spot volcanism lava have shown that understanding this form of volcanism can contribute to the investigation of mantle dynamics and CO2 degassing of Earth. However, geological information constraining the magma source of petit-spot remains limited. Here, we present a comprehensive dataset of geochemistry (major and trace elements, and Sr and Nd isotopic compositions) and Ar-40/Ar-39 ages of alkaline basaltic rocks and glasses to define the geological characteristics of petit-spot volcanoes in the northwestern Pacific. The geochemical and geochronological variations of the basalts indicate that petit-spot volcanism is characterized by a petrogenetically and temporally isolated magma system for each volcano. The basalt geochemistry further indicates that the magmas at the volcanoes were derived from the melting of a heterogeneous regional-scale source under a range of conditions. In addition, slight temporal intra-field migration of petit-spot vent fields against the plate motion was detected. These features indicate that the magma originates from isolated melt ponds at the lithosphere-asthenosphere boundary, and that the speed at which the melt ponds are dragged by the plate is only slightly slower than that of the plate motion. Our results provide detailed insight into eruption processes of asthenosphere melts induced by plate-flexure, and also suggest the complete coupling of the lithosphere to the upper-most asthenosphere in the case of large plate subduction. (C) 2015 Elsevier B.V. All rights reserved.

Petit-spot volcanism is a phenomenon ubiquitous on Earth. It originates from melt in the uppermost mantle asthenosphere, occurring where the plate flexes and fractures before subduction. Recent geochemical and petrological studies of petit-spot volcanism lava have shown that understanding this form of volcanism can contribute to the investigation of mantle dynamics and CO2 degassing of Earth. However, geological information constraining the magma source of petit-spot remains limited. Here, we present a comprehensive dataset of geochemistry (major and trace elements, and Sr and Nd isotopic compositions) and Ar-40/Ar-39 ages of alkaline basaltic rocks and glasses to define the geological characteristics of petit-spot volcanoes in the northwestern Pacific. The geochemical and geochronological variations of the basalts indicate that petit-spot volcanism is characterized by a petrogenetically and temporally isolated magma system for each volcano. The basalt geochemistry further indicates that the magmas at the volcanoes were derived from the melting of a heterogeneous regional-scale source under a range of conditions. In addition, slight temporal intra-field migration of petit-spot vent fields against the plate motion was detected. These features indicate that the magma originates from isolated melt ponds at the lithosphere-asthenosphere boundary, and that the speed at which the melt ponds are dragged by the plate is only slightly slower than that of the plate motion. Our results provide detailed insight into eruption processes of asthenosphere melts induced by plate-flexure, and also suggest the complete coupling of the lithosphere to the upper-most asthenosphere in the case of large plate subduction. (C) 2015 Elsevier B.V. All rights reserved.

Young basaltic knolls have been discovered on the old oceanic lithosphere, namely petit-spot volcanoes. Based on their geochemical signatures, they have presumably originated from partial melts in the asthenosphere. However, there is no direct information on the depth provenance of petit-spot formation. Here we report new geothermobarometric data of rare mantle xenoliths discovered from petit-spot lavas exhibiting a geotherm much hotter than expected for the ca. 140 Ma seafloor on which petit-spots were formed. Such an anomalously hot geotherm indicates that melt porosity around the lithosphere-asthenosphere boundary (LAB) must be as high as a few percent. Such high melt porosity would be possible by continuous melt replenishment. Excess pressure induced by the outer-rise topography enables horizontal melt migration along the LAB and sustains a continuous melt supply to petit-spot magmatism. Given the general age-depth relationship of ocean basins, a melt-rich boundary region could also be a global feature.

北西太平洋のアウターライズ手前（プレート屈曲部）で活動したプチスポット初生マグマが、リソスフェア下部（約1280 ℃、約2.1 GPaの条件）で周囲のカンラン岩と最終平衡にあったことを検証するために、同一海域の別の火山（試料採取地点番号YK06-05 6K#879）と、日本海溝海側斜面に存在する火山（試料採取地点番号KR97-09 10K#56）（噴火当時同様なセッティングにあったと考えられる）に対する複数相飽和実験を行った。その結果、6K#879は約1280 ℃、約1.8 GPaの条件で、かんらん石・斜方輝石・単斜輝石が飽和し、10K#56は約1275 ℃、約1.4 GPaの条件で、かんらん石と斜方輝石が飽和した。プチスポットの初生マグマが、リソスフェア下部（およびそれ以浅）において周囲のカンラン岩と最終平衡にあったことは、マグマ噴出位置や噴火年代の違い関わらず共通する条件である。

Large amounts of CO2 are transferred from Earth's interior to the surface by volcanism. On a geological time scale, the rate of CO2 emission has controlled the evolution of Earth's atmosphere and climate, as well as the dynamic processes that take place in the mantle and core. The total rate of natural CO2 emission from Earth has been estimated on the basis of CO2 flux from arc, mid-ocean-ridge, and hotspot volcanism. However, previous estimates have overlooked the CO2 emitted from a recently discovered type of volcanism-petit-spot volcanism-that occurs on the deep-sea floor. Here, we measure the CO2 and H2O contents of glassy basalts produced by petit-spot volcanism and estimate the initial contents to be &gt; 5 wt% and 1.0-1.1 wt%, respectively. Based on these values and magma flux of petitspot volcanism, we show that the rate of CO2 emission from petitspot volcanoes (2.7-5.4 x 1011 g CO2 yr(-1)) is a few percent of the CO2 emissions from arc and mid-ocean-ridge volcanism, and up to similar to 14% of that from hotspot volcanism. Thus, the contribution to the carbon cycle on Earth of the large amounts of CO2 that have been emitted from the deep-sea floor by petit-spot volcanism has not previously been recognized.

In 2009, petit-spot submarine volcanoes were discovered off the oceanward slope of the central Chile trench, offshore from Valparaiso, Chile, at around 33 degrees S. Ar-Ar dating of mugearite and alkali-basalt from the volcanoes yields ages of 10.11 +/- 0.22 Ma and 6.69 +/- 0.88 Ma, respectively. Back-calculations of plate motion along the present absolute movement direction of the Nazca Plate, conducted using the Ar-Ar age data, indicate that eruption occurred above a zone of plate flexure. The back-calculation results suggest that the mugearite was erupted at a flexural arch prior to arrival at the active site of the Juan Fernandez hotspot. In contrast, the alkali-basalt was erupted on a plate flexure at a site of interaction between a flexural moat and an outer-rise area, where the source material was probably influenced by the Juan Fernandez hotspot. The geochemistry of the lavas supports this interpretation because the concentration ratios of various rare earth and other trace elements in the mugearite are different from those of the alkali-basalt and Juan Fernandez hotspot lavas. Consequently, petit-spot melts could reflect the composition of source materials below the plates at sites of plate flexure.

To assess the ability of densimetry for CO2 fluid in CO2 inclusions, we compare two methods, microthermometry and Raman microspectroscopic densimetry for CO2. The comparative experiment was performed for nine CO2 inclusions in three mantle xenoliths. The results are as follows: (1) microthermometry precisely determines CO2 density with the range of 0.65 to 1.18?g/cm3 compared with Raman microspectroscopic densimetry; (2) CO2 density obtained by Raman microspectroscopic densimetry is fairly consistent with that by microthermometry; (3) it is hard to determine CO2 density in CO2 inclusion with diameter of less than around 3 mu m using microthermometry; and (4) microthermometry can be applied only to the CO2 inclusion whose CO2 density ranges from around 0.65 to 1.18?g/cm3, whereas the Raman microspectroscopic densimetry is applicable to CO2 density ranging from 0.1 to 1.24?g/cm3. The above features carry the potential for estimation of depth origin of mantle-derived rocks. The depth where the rocks were trapped by host magma can be estimated using both geothermometric data and CO2 fluid density in CO2 inclusions in the rocks. Typical precisions of density of CO2 in CO2 inclusions obtained by the Raman microspectroscopic densimetry (similar to 0.01?g/cm3) and by the microthermometry (&lt; 0.001?g/cm3) correspond to uncertainties in the depth origin of 2.4?km and?&lt;?1.7?km, respectively, at 1000?+/-?50?degrees C. In case of the mantle under 7501250?degrees C and 1?GPa, the CO2 fluid has a density ranging from 1.06?g/cm3 to 1.21?g/cm3, which are well measured by the Raman microspectroscopic densimetry. Combination of both densimetries for CO2 in mantle minerals elucidates the deep structure of the Earth. Copyright (c) 2012 John Wiley & Sons, Ltd.

To assess the capacity of fluid inclusions in mantle minerals for CO2 retention, annealing experiments were conducted for two mantle xenoliths with CO2 inclusions for 8 days at 1000 degrees C under atmospheric pressure and fO(2) of 10(-11) MPa. The results show no marked decrease in the CO2 density of the CO2 inclusions for any examined minerals - olivine, orthopyroxene, clinopyroxene, or spinel. The CO2 density of CO2 inclusions in olivine in the present mantle xenoliths is lower than that in pyroxenes or spinel. Results of previous studies indicate that the low CO2 density in olivine is attributable to plastic deformation of olivine around CO2 inclusions during annealing in ascending magma. Results of this study present fundamental implications for deformation mechanisms that arise from internal pressure of fluid inclusions in silicate minerals. We calculated the stress field in minerals having a CO2 inclusion. Results show a steep stress gradient in the host around the inclusion. Such local stress in the mineral induces a local rise in the density of dislocations around the CO2 inclusions. The orthopyroxene used for this study showed a sparse distribution of dislocations around a CO2 inclusion, whereas olivine showed dense dislocations around CO2 inclusions, implying that the low CO2 density of the CO2 inclusions in olivine results from volume expansion of the CO2 inclusions through plastic deformation of the host mineral during annealing of the xenoliths in ascending magma. In this respect, constancy of CO2 density during the annealing experiments for all minerals is an interesting finding. Regarding olivines, the reduction of internal pressure of the CO2 inclusions or interaction of the dense dislocations possibly inhibits decrepitation or further volume expansion of the CO2 inclusions during annealing experiments. However, pyroxenes and spinel show higher and similar CO2 density, which reflects the resistance to plastic deformation and which indicates the effectiveness of CO2 inclusions in these minerals as a depth probe for mantle xenoliths.

Petit-spots, the late Miocene alkali basaltic volcanoes on the Early Cretaceous NW Pacific Plate, originate at the base of the lithosphere. The petit-spot volcanic rocks enclose fragments of tholeiitic basalt, dolerite. gabbro. and mantle peridotite, providing a unique window into the entire section of subducting oceanic lithosphere. We provide here the first direct observations on the deep structure of the Pacific lithosphere using microstructural analyses of a petit-spot peridotite xenolith. The xenolith is a lherzolite that consists mainly of coarse- and medium-grained olivine. orthopyroxene, and clinopyroxene, as well as fine-grained aggregates of spinel and orthopyroxene that probably represent replaced pyrope-rich garnet. A strong deformational fabric is marked by a parallel alignment of millimeter-sized elongate minerals and their crystallographic preferred orientation. The olivine displays a [010] fiber pattern with a girdle of [100] axes and a maximum of [010] perpendicular to the foliation, a pattern which is consistent with a transpressional deformation in high temperature conditions at the base of oceanic lithosphere. Our microstructural observations and seismic data indicate that the lower part of the NW Pacific lithosphere possess an early stage structure of mantle flow at the asthenosphere. This interpretation is compatible with a conventional model in which oceanic lithosphere is thickened during cooling and plate convection. A discrepancy between the weak anisotropy in the petit-spot peridotite and the strong azimuthal anisotropy from the seismic data in the NW Pacific plate implies the existence of a highly anisotropic component in the deep oceanic lithosphere. (C) 2010 Elsevier B.V. All rights reserved.

We compare two intraplate, Pliocene-Pleistocene volcanic fields in different tectonic settings-the central Basin and Range and the northwest Pacific Ocean. Both fields are characterized by widely scattered, small-volume, alkali basaltic volcanoes; within the fields, each volcano apparently originates from a separate, volatile-enriched parental melt from the tipper mantle. There is no evidence at either held for locally anomalous heat flow or ongoing introduction of new fluids into the upper mantle such as might occur above a subducting slab. We conclude that the volcanic fields reflect deformation-driven collection of already existing partial melts in a heterogeneous; upper mantle. Deformation-driven melt collection may be in important mechanism for other diffuse intraplate volcanic fields, and this is consistent with a tectonically controlled, low-flux end member for intraplate fields where magmatism is a passive response to regional deformation. Differences in the degree of fractionation and contamination between the two fields are inferred to be related to flexure-induced vertical variations in the orientation of principal stresses in the northwest Pacific Ocean, which cause stalling of ascending dikes in the lithosphere.

We measured noble gas isotopic compositions of mantle xenoliths and xenocrystic olivines sampled from seamounts-so-called petit-spot volcanoes-on the 135-million-year-old northwestern Pacific Plate. The xenoliths are spinel lherzolites originating from suboceanic lithospheric mantle. The samples' He-3/He-4 ratios are 7.0-8.5 Ra, where Ra signifies atmospheric He-3/He-4. The Ar-40/Ar-36 ratios are as high as 7000. These observations suggest that the noble gas isotopic compositions of suboceanic lithospheric mantle resemble those of mid-ocean ridge basalt (MORB). A mantle source with a He/U ratio as high as an assumed value for MORB source is necessary to maintain the MORB-like He-3/He-4 over 135 million years, implying that melt extraction at mid-ocean ridges only slightly alters the He/U ratio of the oceanic upper mantle. The He-4/Ar-40* ratios of the samples described herein are much lower than the theoretical radiogenic production ratio, where an asterisk denotes correction for atmospheric contribution. The low He-4/Ar-40* is inferred to result from kinetic fractionation in the mantle. When magma migrates through a mantle source, lighter noble gases in the mantle source diffuse selectively into magma channels. The MORB generation does not cause low He-4/Ar-40*, however. If a mantle source is depleted in lighter noble gases during ancient MORB generation, then noble gas isotopic compositions of the mantle source are affected over time by accumulation of radiogenic nuclides. Thereby, the mantle source adopts a radiogenic or nucleogenic noble gas isotopic composition. Recent kinetic fractionation contributes to the low He-4/Ar-40* in the samples. Based on the diffusive fractionation model, more than 100 years are necessary to produce a residual mantle source with He-4/Ar-40*, as observed in the samples. However, petit-spot volcanoes. found as small knolls, seem to erupt within a short period. The low He-4/Ar-40* of the samples implies prior volcanism in this region. No recent volcanism has occurred near this region aside from petit-spot volcanism, indicating that petit-spot volcanoes are polygenetic with a long active period. Petit-spot volcanoes are regarded as common magmatism on the subducting oceanic plate. Consequently, the thermal structure and temperature-dependent physical properties of the oceanic plate are, at least partly, affected by remnant heat of the magmatism. (C) 2009 Elsevier B.V. All rights reserved.

We report Sr, Nd, and Pb isotopic data of young alkaline basalt lava from a new type of volcano (petit-spot) on the northwestern Pacific Plate. Petit-spot lavas show Dupal, or extremely EM-1-like, Si-Nd-Pb isotopic compositions. The data cannot be explained by contamination of pelagic sediment, in spite of the prediction on the basis of geological observation. We thus consider that the geochemistry of petit-spot lava indicates the existence of recycled fertile plate materials, not only the Dupal isotopic signature, in the northern hemisphere Pacific upper mantle unrelated to one or more active plumes. In consideration of published experimental results for fertile plate materials, selective melting of recycled material is a process critical in generating petit-spot lava. Moreover, the small volume of the volcano and low degree of melting in the mantle source needed to form strongly alkalic lavas suggest that petit-spot volcanism is originated front small-scale heterogeneities of recycled material. This idea consistently explains the geochemistry and noble gas isotopic composition of petit-spot lava, and also suggests small-scale heterogeneity widespread in the upper mantle of the Pacific Ocean. Together with a revised view of upper mantle heterogeneity, we propose that gross upper mantle composition is controlled by abundances and scales of regions of recycled material that correspond to differences in the relative position to the Pangea supercontinent. suggesting the link to the tectonic origin of the global scale heterogeneity. (C) 2009 Elsevier Ltd. All rights reserved.

On the western part of the Pacific Plate most seamounts formed during the Cretaceous period in the so-called West Pacific Seamount Province (WPSP). On the northwestern part of the same plate, the Joban and Japanese Seamount Trail (JJST) are also composed of Early Cretaceous seamounts. However, two new groups of knolls were recently discovered during multibeam surveys on the Pacific Plate along the Japan Trench. One group consists of circular knolls that are flat-topped in shape and correspond to eruptive ages of approximately 75 Ma. The other group consists of irregularly shaped knolls, also called petit-spot volcanoes, that are found on the outer-rise systems of the subducting Pacific Plate. These petit-spots seem much younger and available age data suggest that they only formed in the last few million years. Acoustic reflective data, which are simultaneously obtained with bathymetrical data, are a most powerful tool to distinguish the petit-spots from the Cretaceous edifices in the WPSP and JJST. In this paper, we present the results of an exploratory search for these new kind of petit-spot volcanoes along the trenches in the Pacific Ocean, with an emphasis on the Japan and Tonga trenches. The sizes of these irregularly shaped petit-spot volcanoes are several orders of magnitude less than the Cretaceous seamounts and circular knolls, yet they appear to be ubiquitous on the ocean floor, in particular, where incipient melts in the asthenosphere can be squeezed out by tectonic forces.

我々は、平野らが採取した太平洋プレート上に噴出した新鮮なアルカリ玄武岩（ベイサナイトJPT-2）の岩石学的研究を行い、この岩石がマントルの橄欖岩と化学平衡であったときの化学組成と揮発性分量を推定した。高圧融解実験を行い、揮発成分を含むJPT-2CWマグマが、３GPa・1400℃、すなわちザクロ石橄欖岩のソリダス付近で生じたことを明らかにし、さらに橄欖岩にJPT2CWマグマを３wt%加えた試料KLB-1*を用いて、2.2aから3.5GPa, 1200から1500℃の範囲で平均50時間以上の保持実験を行い、アセノスフェアの部分融解条件を再現した。実験結果に基づき、アセノスフェア内部での揮発成分の役割と構造モデルを提唱する。

The volume of magma of the petit spot volcanoes (Hirano et al., 2006) has not been estimated but certainly must be several orders of magnitude less than surrounding Cretaceous volcanoes on the NW Pacific Plate. Note that we found 0-1, 2.5, 4.2, 6.0 and 8.6 Ma volcanoes, implying episodic eruption of magma over a distance of 600 km of plate motion on the flexural Pacific Plate before its subduction but with low volumes of magma production.

Numerous large igneous provinces formed in the Pacific Ocean during Early Cretaceous time, but their origins and relations are poorly understood. We present new geochronological and geochemical data on rocks from the Manihiki Plateau and compare these results to those for other Cretaceous Pacific plateaus. A dredged Manihiki basalt gives an Ar-40-Ar-39 age of 117.9 +/- 3.5 Ma (2 sigma), essentially contemporaneous with the Ontong Java Plateau similar to 2500 km to the west, and the possibly related Hikurangi Plateau similar to 3000 km to the south. Drilled Manihiki lavas are tholefitic with incompatible trace element abundances similar to those of Ontong Java basalts. These lavas may result from high degrees of partial melting during the main eruptive phase of plateau formation. There are two categories of dredged lavas from the Danger Islands Troughs, which bisect the plateau. The first is alkalic lavas having strong enrichments in light rare earth and largeion lithophile elements; these lavas may represent late-stage activity, as one sample yields an Ar-40-Ar-39 age of 99.5 +/- 0.7 Ma. The second category consists of tholefitic basalts with U-shaped incompatible trace element patterns and unusually low abundances of several elements; these basalts record a mantle component not previously observed in Manihiki, Ontong Java, or Hikurangi lavas. Their trace element characteristics may result from extensive melting of depleted mantle wedge material mixed with small amounts of volcaniclastic sediment. We are unaware of comparable basalts elsewhere.

[1] A seismic reflection survey was conducted in the northwestern Pacific to investigate subsurface structure of small volcanoes considered to be formed by the newly-discovered "petit-spot'' intra-plate volcanism. In contrast with an acoustically transparent sedimentary layer in the ambient northwestern Pacific, sedimentary layers are acoustically opaque with interbedded strong reflections beneath the volcanoes. These reflections are possibly caused by inhomogeneous structure due to intrusion of central vent. Subhorizontal strong reflections at base of the sedimentary layer, identified in the vicinity of volcanoes, are probably reflections from sills of dense lavas flowing out within the sedimentary layer. These volcanoes could be monogenetic volcanoes produced by small amount of magma intrusion, &lt; 1 km(3) in volume. Although such small volcanoes have never been discovered unless through a high-resolution bathymetric mapping, the "invisible'' intra-plate volcanism affects successive evolution/modification of the crustal architecture of old-age oceanic plates.

In order to constrain the magma sources and the structural evolution of Hana Ridge, noble gas isotope ratios and Ar-40/Ar-39 ages were determined for this submarine extension of the east rift zone of Haleakala volcano. Ar-40/Ar-39 ages of ten lavas from Hana Ridge are bimodally distributed. Three samples have ages of 1.8-1.9 Ma and seven samples have ages of 1.4-1.5 Ma. The observation of older lavas overlying younger ones in two locations suggests that growth of Hawaiian rift zones occurs endogenously by continuous intrusion of magmas and patchy resurfacing at outbreak points. The 0.5 Ma age range for submarine Hana Ridge lavas, coupled with previously published K/Ar ages of 0.97-1.1 Ma for subaerial Honomanu lavas on the Haleakala Volcano, indicates that Haleakala shield volcanism persisted for nearly 1 My. Furthermore, the new Ar-40/Ar-39 ages, when considered with isotopic compositions of the lavas, suggests that the source for Hana Ridge magmas gradually shifted over a period of 0.5-1.0 My. The majority of samples from the ridge have relatively uniform He-3/He-4 ratios between 18 and 22 Ra, which is higher than He-3/He-4 of subaerial Honomanu tholeiites. Ne-20/Ne-22 and Ne-21/Ne-22 ratios define linearly correlated trend that overlaps with the Loihi-Kilauea trend. He-Ne systematics of the Hana Ridge indicate that the magmas comprising most parts of the ridge were derived from a source with a primordial less-degassed mantle component. Since Hana Ridge lavas predate the subaerial Honomanu lavas, temporal decrease of He-3/He-4 ratios suggests that contribution of this primordial component had decreased in the magma source during establishment of Hana Ridge and Haleakala volcano. Pb-Sr isotopes demonstrate that Hana Ridge magmas are representative of the Kea component. However, such isotopic signatures associated with elevated high 3 He/4 He ratios precludes that the Kea component is a distinct endmember, such as recycled oceanic crust or lithospheric mantle. Alternatively, we propose that it is a common sub-component that is a mixture of Loihi endmember and recycled oceanic crust. Ne isotope ratios of the Hawaiian samples, including the Hana Ridge, show primordial signature irrespective of He-3/He-4 ratios. Such apparent decoupling of He and Ne isotopes may be also attributed to mixing of the Loihi component and recycled component in the mantle plume, either of which components needs to be elementally fractionated prior to mixing. (c) 2006 Elsevier B.V All rights reserved.

The density, and therefore the pressure, Of CO2 fluid inclusions in minerals can be estimated from the Fermi diad splitting of Raman spectra Of CO2. An accurate determination of the pressure Of CO2 fluid inclusions enables the estimation of the depth origin of rocks from the deep Earth. A micro-Raman densimeter was applied to ultramafic-mafic xenoliths sampled along the Ohku coast of Oki-Dogo Island in the Sea of Japan (East Sea). The density Of CO2 fluid inclusions in the mafic granulite was 1.02-1.05 g/cm(3), while those of Iherzolites were 0.98-1.02 g/cm. In contrast, the density Of CO2 fluid inclusions measured in olivine gabbro, clinopyroxenite, and harzburgite were lower ranging from 0.86- to 0.99 g/cm(3). Taking into account the temperature condition estimated using a pyroxene thermometer, the mafic granulite originated from a depth of 27-30 km and the lherzolites from 2529 km. The overlapping depth of 27-29 km can be interpreted as the depth including the Moho discontinuity under Oki-Dogo Island 3.3 Ma. This estimation is consistent with geophysical observations. (c) 2006 Elsevier B.X. All rights reserved.

Volcanism on Earth is known to occur in three tectonic settings: divergent plate boundaries ( such as mid-ocean ridges), convergent plate boundaries ( such as island arcs), and hot spots. We report volcanism on the 135 million-year-old Pacific Plate not belonging to any of these categories. Small alkalic volcanoes form from small percent melts and originate in the asthenosphere, as implied by their trace element geochemistry and noble gas isotopic compositions. We propose that these small volcanoes erupt along lithospheric fractures in response to plate flexure during subduction. Minor extents of asthenospheric melting and the volcanoes' tectonic alignment and age progression in the direction opposite to that of plate motion provide evidence for the presence of a small percent melt in the asthenosphere.

The Hawaiian - Emperor volcanic island and seamount chain is usually attributed to a hot mantle plume, located beneath the Pacific lithosphere, that delivers material sourced from deep in the mantle to the surface(1-5). The shield volcanoes of the Hawaiian islands are distributed in two curvilinear, parallel trends ( termed 'Kea' and 'Loa'), whose rocks are characterized by general geochemical differences(2-5). This has led to the proposition that Hawaiian volcanoes sample compositionally distinct, concentrically zoned, regions of the underlying mantle plume(4,5). Melt inclusions, or samples of local magma 'frozen' in olivine phenocrysts during crystallization, may record complexities of mantle sources(6), thereby providing better insight into the chemical structure of plumes. Here we report the discovery of both Kea- and Loa-like major and trace element compositions in olivine-hosted melt inclusions in individual, shield-stage Hawaiian volcanoes - even within single rock samples. We infer from these data that one mantle source component may dominate a single lava flow, but that the two mantle source components are consistently represented to some extent in all lavas, regardless of the specific geographic location of the volcano. We therefore suggest that the Hawaiian mantle plume is unlikely to be compositionally concentrically zoned. Instead, the observed chemical variation is probably controlled by the thermal structure of the plume.

プレート収斂域で巻き起こる火成活動の物質源として唯一の盲点となっている端成分が存在する．それは沈み込むリソスフェリックマントルである．この成分がプレート収斂域の火成活動に物質的に寄与しているかどうかは定かではないが，島弧マグマの化学組成に見られる様々な不均質に鑑みると，この成分がMORBで代表される典型的なリソスフェリックマントルであるかいずれ知る必要があろう．本研究はその未知の沈み込むリソスフェリックマントルの地球化学的特徴（希ガス同位体組成）を報告するものである．日本海溝海側斜面に極めて新鮮な玄武岩が発見された．この玄武岩には２種類のカンラン石（自形微晶・他形巨晶）が存在し，他形巨晶はその主要元素組成からマントル起源の捕獲結晶であると分かった．それゆえ，もしこの巨晶カンラン石がリソスフェア起源であるならば我々は史上初めて沈み込む直前のリソスフェリックマントルを手に取ったことになる．

Young (6 Ma) alkali-basalts were collected from the toe of the oceanward slope of the northern Japan Trench. Two types of olivine are present in these lavas, xenocrysts with reaction rims and magmatic. The forsterite (Fo) (no. 91-92) values and NiO contents (0.3-0.5 wt%) of the xenocrysts are similar in composition to those of the depleted mantle peridotite. The groundmass olivines have relatively lower Fo values (no. 81-88) and NiO contents (0.1-0.5 wt%). Reaction rims and the vicinity of the silicate inclusion in xenocrysts show the intermediate compositions between the xenocryst and magmatic olivines. Chromian spinel inclusions in the xenocrysts also show the depleted composition in the range of abyssal peridotite. CO2 fluid inclusions in the xenocryst records pressures before entrainment into the host magma up to 0.4 GPa, which corresponds to a depth of up to 14 km of lithospheric mantle. These data indicate that the xenocrysts originate from MORB-depleted mantle.

Improvements in ion collectors of the VG3600 mass spectrometer used for ⁴⁰Ar-³⁹Ar dating in the Radioisotope Center, University of Tokyo, have been performed. We also constructed a new compact vacuum line for extraction and purification of noble gases, and connected it to the mass spectrometer. The new noble gas mass spectrometry system enables us to measure all noble gases (He, Ne, Ar, Kr, and Xe) at low detection limits corresponding to 2×10^-15 cm³ STP or 5×10³ atoms of Kr and Xe. A newly designed W-coil furnace effectively heats small samples (1-100 mg) in a Mo-crucible with condition of low blank levels (10^-10 and 10^-15 cm³ STP for Ar and Xe). Smaller sample sizes required for ⁴⁰Ar-³⁹Ar dating have reduced activity levels. The system was preliminarily applied to I-Xe dating of neutron irradiated meteorites, Bjurböle, Allende and Yamato-74191, and discovered a late formation age of Yamato-74191 (26 Ma after Bjurböle) compared with the Allende meteorite.

The Mineoka ophiolite in the southern Boso Peninsula is situated in a unique tectonic setting in the collisional zone between the Izu and Honshu arcs in Japan. The ophiolitic rocks are composed mainly of tholeiitic pillow basalts and dolerites, alkali-basaltic sheet flows, and calc-alkaline dioritic to gabbroic rocks. The tholeiitic basalts show variable trace element compositions ranging from mid-ocean ridge basalt to island-arc basalt, whereas the alkali-basalts have a within-plate affinity High-Fe and -Ti tholeiitic basalt and within-plate alkali-basalt have Ar/Ar ages of 49 +/- 13 Ma and 19.62 +/- 0.90 Ma, respectively. Three plutonic rocks have K-Ar ages of c. 25, 35 and 40 Ma. These ages are inconsistent with the known ages from the Pacific or Philippine Sea Plate. We infer that the Mineoka ophiolitic assemblage was part of another Tertiary oceanic plate, the &apos;Mineoka Plate&apos;, which underwent island-are volcanism in the Miocene as a result of subduction initiation at a fracture zone or a transform fault system due to a change in the position of the Euler rotation pole of the Pacific Plate at c. 43 Ma. Eruption of within-plate type alkali basalts on the Mineoka Plate took place near the palaeo-Japan continental arc just before the emplacement of the Mineoka ophiolite into the Japanese continental margin.

A belt of disrupted ophiolitic rocks occurs on the Boso Peninsula (Japan), currently located north of the oblique subduction boundary between the Philippine Sea and North American Plates, under which the Pacific Plate has been subducting westwards. This ophiolitic belt (Mineoka Belt) is composed of mafic-ultramafic rocks together with Tertiary chert and limestone and island-arc volcaniclastic rocks. Our detailed structural studies in and around the basaltic rock bodies within the ophiolite reveal three phases of deformation. The first phase is further divided into three stages, all related to oblique normal faulting associated with extensional tectonics at or near a spreading axis. Fluid pressures appear to have fluctuated in association with faulting and veining during this phase. The second phase of deformation is characterized by thrust-related shear zones with a significant strike-slip component and is probably related to the final emplacement of the ophiolite by oblique subduction-obduction processes. The third and final phase of deformation affected not only the ophiolite but also later terrigenous and island-arc pyroclastic rocks. This deformation involved large-scale transpressional dextral slip on forearc sliver faults, which are still active today.

嶺岡帯,鴨川市峠においてアルカリ玄武岩溶岩中に噴出時に取り込まれたと考えられる砂岩を発見した.この砂岩はおもに,斜長石・ダトー石・石英・および珪長質火山岩片から構成され,ダトー石を除けば嶺岡層群神塚層または愛宕山層の砂岩の構成粒子の鉱物組み合わせに似ている.アルカリ玄武岩の活動場は,このような陸源性砂岩が供給されるような場であった事が明らかとなった.アルカリ玄武岩は,その産状と化学組成から,海山を構成していたと考えられるが,アルカリ玄武岩の噴出場と含まれている陸源性砂岩の堆積場は,おそらく海洋プレートと島弧もしくは大陸が会合する収束境界付近であった見込みが高い.アルカリ玄武岩の放射年代は,嶺岡帯のオフィオリティック岩体が本州弧側へ定置したとされる時代の直前であり,本論の結果と調和的である.本研究において嶺岡帯の形成過程を論じるにおいて新たな見地が得られた.

An alkaline volcanic rock sample, peralkaline rhyolite pillow lava, in addition to some radiolarian-bearing pelagic sedimentary rocks, were collected by the Japanese submersible Shinkai 6500 from Quesada Seamount on the oceanward slope of the Mariana Trench. Ar-40-Ar-39 dating resulted in a plateau age of 129.3 +/- 2.6 Ma, which is approximately 10 Myr younger than the radiolarian age of the oldest intercalated tuffaceous radiolarian claystone of early Berriasian age (approximately 140 Ma). Fragments of volcanic glass that are of alkali-basalt (hawaiite) composition may indicate that this tuffaceous claystone was formed during the shield-building volcanic stage. Because the peralkaline rhyolite is a very differentiated volcanic rock and commonly erupts in the post-shield stage, we interpret that the activity lasted for a long period (approximately 10 Myr). This long-lived shield-building stage may be due to the slow moving rate of the Pacific Plate during the Early Cretaceous. (C) 2002 Elsevier Science B.V. All rights reserved.

Alkali pillow basalts were collected from the toe of the oceanward slope of the northern Japan Trench. These alkali-basalts formed as a result of a low degree of partial melting of Pacific Ocean mantle and rapid rise of the magma (no fractionation in shallow magma chambers). Reconstructing Pacific Plate motion based on 40Ar-39Ar age dates of 5.95±0.31 Ma for these basalts indicates that they erupted outboard of outer swell or forebulge of the Japan Trench in the NW Pacific. We suggest that these alkali-basalts represent a new form of intra-plate volcanism, whereby magmatic activity occurs off the forebulge of the downgoing Pacific slab, perhaps using conduits related to fracturing of the slab during bending prior to subduction.