記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1029/2020GC009114

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The lithophile elements and isotopic compositions of oceanic basalts suggest they derive from different mantle components. We present new analytical results of un-degassed deep-marine basaltic glasses from various regions, and we find strong linear correlations of R² ≥ 0.997 between H₂O and F as well as H₂O and Ce, for which we propose mantle trends. The mantle trends represent global variations of mantle components, ranging from a depleted dry peridotite of mid-ocean ridge basalt (MORB) (DMM, depleted MORB source mantle: H₂O = 100 ppm; H₂O/Ce = 200; H₂O/F = 10) to a hydrous peridotite of Hawaiian ocean island basalt (OIB) of deep mantle origin (FOZO, focal zone: H₂O = 750 ppm; H₂O/Ce = 200; H₂O/F = 18.5). Accordingly, we defined the correlation as a volatile DMM–FOZO trend. Based on our findings, we report novel H₂O–F–Ce systematics to discriminate the degree of water depletion in the source mantle and the rehydration of the mantle with recycled surface water through oceanic plate subduction. Using this method, most OIBs are distinguished clearly from the DMM–FOZO trend, and we find that the water in their sources originates from recycled water derived from the hydrated oceanic crust and sediment after various degrees of dehydration (75–95%) in subduction zones.

DOI： 10.1016/j.chemgeo.2019.06.014

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

DOI： 10.5575/geosoc.2018.0056

記述言語：英語   掲載種別：研究論文（学術雑誌）  

A new olivine reference material – MongOL Sh11-2 – for in situ analysis has been prepared from the central portion of a large (20 × 20 × 10 cm) mantle peridotite xenolith from a ~ 0.5 My old basaltic breccia at Shavaryn-Tsaram, Tariat region, central Mongolia. The xenolith is a fertile mantle lherzolite with minimal signs of alteration. Approximately 10 g of 0.5–2 mm gem quality olivine fragments were separated under binocular microscope and analysed by EPMA, LA-ICP-MS, SIMS and bulk analytical methods (ID-ICP-MS for Mg and Fe, XRF, ICP-MS) for major, minor and trace elements at six institutions world-wide. The results show that the olivine fragments are sufficiently homogeneous with respect to major (Mg, Fe, Si), minor and trace elements. Significant inhomogeneity was revealed only for phosphorus (homogeneity index of 12.4), whereas Li, Na, Al, Sc, Ti and Cr show minor inhomogeneity (homogeneity index of 1–2). The presence of some mineral and fluid-melt micro-inclusions may be responsible for the inconsistency in mass fractions obtained by in situ and bulk analytical methods for Al, Cu, Sr, Zr, Ga, Dy and Ho. Here we report reference and information values for twenty-seven major, minor and trace elements.

DOI： 10.1111/ggr.12266

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Unusual mafic rock fragments deposited in Plio-Pleistocene-aged marine sediments were recorded at Integrated Ocean Drilling Program (IODP) Site U1359, in Wilkes Land, East Antarctica. These fragments were identified from sediment layers deposited between c. 3 and 1.2 Ma, indicating a sustained supply during this time interval. Clinopyroxenes in these basalts are Al-Ti diopside-hedenbergite, uncommon in terrestrial magmatic rocks. A single strong peak in the Raman spectra of a phosphate-bearing mineral at 963 cm
^-1
supports the presence of merrillite. Although not conclusive, petrological traits and oxygen isotopic compositions also suggest that the fragments may be extra-terrestrial fragments affected by shock metamorphism. Nevertheless, it is concluded that the basaltic fragments incorporated in marine sediments at Site U1359 represent ice-rafted material supplied to the continental rise of East Antarctica, probably from the bedrocks near the proximal Ninnis Glacier. Further studies on Plio-Pleistocene sediments near Site U1359 are required to characterize the unusual mafic rocks described.

DOI： 10.1017/S0954102018000299

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The ca 3.4 Ga Strelley Pool Formation (SPF) of the Pilbara Craton, Australia, represents a Paleoarchean sedimentary succession preserving well-described and morphologically diverse biosignatures such as stromatolites and cellularly preserved microfossils. The SPF microfossil assemblage identified from three greenstone belts includes relatively large (20-80 μm in width), acid-resistant, organic-walled lenticular microfossils, which can be extracted using a palynological technique. In this study, we present results of measurements of over 800 palynomorphic specimens of SPF lenticular microfossils from 2 remote (∼80 km apart) localities that represent different depositional environments and thus different habitats, as evidenced by their distinct lithostratigraphic association and trace element geochemistry. We demonstrate statistically that the two populations are distinct in oblateness from a polar view and furthermore that each population comprises subpopulations defined by different areas and oblateness. This study may provide the earliest morphological evidence for speciation of unicellular organisms, which could have been allopatric (geographic) and adaptive. It can also be suggested that SPF lenticular microbes had highly organized cytoskeleton indispensable for strict control of the cell morphology of large and robust microbes, which in turn were likely advantageous to their prosperity and diversification.

DOI： 10.1089/ast.2017.1799

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We investigated the Sr–Nd–Hf–Pb isotope compositions of clinopyroxene separates and strongly leached bulk rock powders from the basalt lavas on Raivavae Island in the South Pacific. These data allowed us to explore the magma chamber processes of the two basalt groups: the high-μ-type (μ = ²³⁸ U/²⁰⁴Pb, HIMU) Rairua basalts erupted at ~ 9 Ma, and the depleted mantle (DM)-type Anatonu basalts erupted at ~ 6 Ma. We assessed different analytical techniques for clinopyroxene sample treatment, including leaching, sample digestion, chemical separation, and determination of parent/daughter ratios for use in age corrections. The age-corrected initial Sr–Nd–Hf isotope compositions of clinopyroxene samples solely overlap on the bulk rock compositions of each basalt type. The initial Pb isotope compositions show significant differences between the clinopyroxenes and bulk rock samples in both the Rairua and Anatonu basalts. The clinopyroxenes have less radiogenic Pb than their hosts, reflecting the magma chamber processes. The overall variations in the bulk rock Pb isotopes are explained by mixing of melts from the HIMU- and DM-type sources in the plume. In detail, early stage Rairua magma was more influenced by DM melts from which Rairua clinopyroxene was crystallized. Subsequent supply of dominant HIMU melts drove the bulk rock magma chemistry more HIMU-like. The Anatonu DM magma was largely from depleted mantle that differs from the Pacific MORB source. Subordinate contribution of a Pacific alkali-basalt-like component is suggested from the clinopyroxene chemistry of Anatonu perhaps from impregnated pyroxenite in the lithospheric mantle.

DOI： 10.1016/j.chemgeo.2017.12.015

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The extreme Sr, Nd, Hf, and Pb isotopic compositions found in Pitcairn Island basalts have been labeled enriched mantle 1 (EM1), characterizing them as one of the isotopic mantle end members. The EM1 origin has been vigorously debated for over 25 years, with interpretations ranging from delaminated subcontinental lithosphere, to recycled lower continental crust, to recycled oceanic crust carrying ancient pelagic sediments, all of which may potentially generate the requisite radiogenic isotopic composition. Here we find that δ²⁶Mg ratios in Pitcairn EM1 basalts are significantly lower than in normal mantle and are the lowest values so far recorded in oceanic basalts. A global survey of Mg isotopic compositions of potentially recycled components shows that marine carbonates constitute the most common and typical reservoir invariably characterized by extremely low δ²⁶Mg values. We therefore infer that the subnormal δ²⁶Mg of the Pitcairn EM1 component originates from subducted marine carbonates. This, combined with previously published evidence showing exceptionally unradiogenic Pb as well as sulfur isotopes affected by mass-independent fractionation, suggests that the Pitcairn EM1 component is most likely derived from late Archean subducted carbonate-bearing sediments. However, the low Ca/Al ratios of Pitcairn lavas are inconsistent with experimental evidence showing high Ca/Al ratios in melts derived from carbonate-bearing mantle sources. We suggest that carbonate-silicate reactions in the late Archean subducted sediments exhausted the carbonates, but the isotopically light magnesium of the carbonate was incorporated in the silicates, which then entered the lower mantle and ultimately became the Pitcairn plume source.

DOI： 10.1073/pnas.1719570115

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Boninites are widely distributed along the western margin of the Pacific Plate extruded during the incipient stage of the subduction zone development in the early Paleogene period. This paper discusses the genetic relationships of boninite and antecedent protoarc basalt magmas and demonstrates their recycled ancient slab origin based on the T–P conditions and Pb–Hf–Nd–Os isotopic modeling. Primitive melt inclusions in chrome spinel from Ogasawara and Guam islands show severely depleted high-SiO₂, MgO (high-silica) and less depleted low-SiO₂, MgO (low-silica and ultralow-silica) boninitic compositions. The genetic conditions of 1 346 °C at 0.58 GPa and 1 292 °C at 0.69 GPa for the low- and ultralow-silica boninite magmas lie on adiabatic melting paths of depleted mid-ocean ridge basalt mantle with a potential temperature of 1 430 °C in Ogasawara and of 1 370 °C in Guam, respectively. This is consistent with the model that the low- and ultralow-silica boninites were produced by remelting of the residue of the protoarc basalt during the forearc spreading immediately following the subduction initiation. In contrast, the genetic conditions of 1 428 °C and 0.96 GPa for the high-silica boninite magma is reconciled with the ascent of more depleted harzburgitic source which pre-existed below the Izu–Ogasawara–Mariana forearc region before the subduction started. Mixing calculations based on the Pb–Nd–Hf isotopic data for the Mariana protoarc basalt and boninites support the above remelting model for the (ultra)low-silica boninite and the discrete harzburgite source for the high-silica boninite. Yb–Os isotopic modeling of the high-Si boninite source indicates 18–30 wt% melting of the primitive upper mantle at 1.5–1.7 Ga, whereas the source mantle of the protoarc basalt, the residue of which became the source of the (ultra)low-Si boninite, experienced only 3.5–4.0 wt% melt depletion at 3.6–3.1 Ga, much earlier than the average depleted mid-ocean ridge basalt mantle with similar degrees of melt depletion at 2.6–2.2 Ga.

DOI： 10.1111/iar.12221

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Many seamounts on the Ontong Java Plateau (OJP) occur near the Stewart Arch, a topographic high that extends parallel to the North Solomon Trench along the southern margins of the plateau. Despite the thick sediment cover, several volcanic cones with strong acoustic reflection were discovered on the submarine flank of the Nuugurigia Seamount. From such volcanic cones, basalts were successfully sampled by dredging. Radiometric dating of basalts and ferromanganese encrustation indicate eruption age of 20–25 Ma, significantly younger than the ~ 122 Ma main OJP plateau and post-plateau basalts. The age range coincides with the collision of the OJP with the Solomon Arc. The Nuugurigia basalts geochemically differ from any other rocks sampled on the OJP so far. They are alkali basalts with elevated Sr, low Zr and Hf, and Enriched Mantle-I (EMI)-like isotopic composition. Parental magmas of these alkali basalts may have formed by small-degree melting of peridotitic mantle impregnated with recycled pyroxenite material having enriched geochemical composition in the OJP's mantle root. We conclude that small-volume alkali basalts from the enriched mantle root migrated through faults or fractures caused by the collision along the Stewart Arch to form the seamount. Our results suggest that the collision of the OJP with the Solomon arc played an important role in the origin of similar post-plateau seamounts along the Stewart Arch.

DOI： 10.1016/j.lithos.2017.09.029

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The South China Sea is the largest marginal basin in the world, and is located in the confluence of three major tectonic plates (Pacific-Philippine Sea, Indo-Australia, Eurasia). Its tectonic history and evolution throughout the Cenozoic has been the subject of much discussion with regards to its mechanism, timing, and relationship to neighboring geological features. We conducted the first Re-Os isotope study of the South China Sea cores recovered from ocean drilling (ODP-Ocean Drilling Program and IODP-International Ocean Discovery Program) and from Philippine oil exploration wells in order to investigate whether or not the basin's tectonic evolution will be reflected in the seawater Os isotope record. These cores represent the northern and southern conjugate margins of the South China Sea, and should have complementary records of the tectonic events in the basin. We found three important negative anomalies in the South China Sea's seawater Os isotope record at 35–33 Ma, 26–23 Ma, and ~ 13–6 Ma, which are temporally related to tectono-magmatic events. At the northern margin, the record starts with the lowest (¹⁸⁷Os/¹⁸⁸Os)_i values of 0.37 and 0.32 for the oldest samples, which then progresses to more radiogenic values for younger sediments. The trend is consistent with the start of seafloor spreading at ~ 33 Ma that saw the peak in the flux of mantle-derived material, which later slowed down until the second peak between 26 and 23 Ma. However, these unradiogenic values also mark the Eocene-Oligocene Transition and more likely implies South China Sea connectivity to global ocean circulation since pre-Oligocene. In both margins, slight to marked decrease in (¹⁸⁷Os/¹⁸⁸Os)_i values deviate from the global trend and coincides with the ridge jump at ~ 23 Ma. In the southern conjugate margin, (¹⁸⁷Os/¹⁸⁸Os)_i show a general decreasing tendency opposite that of the northern margin and the global trend starting from Mid- to Late Miocene (~ 13–6 Ma). Mass balance modeling suggests that the two younger negative excursions may be accounted for by mantle and hydrothermal Os inputs of ~ 372 mol/my at ~ 23 Ma and post ~ 13 Ma. We suggest that the post ~ 13 Ma protracted decrease in (¹⁸⁷Os/¹⁸⁸Os)_i values for the southern margin reflects localized input of mantle-derived lithogenic Os from the Luzon Arc combined with post-spreading magmatic intrusions, and possibly gradual changes in paleoceanographic condition at the southern East Sub-basin.

DOI： 10.1016/j.margeo.2017.07.018

記述言語：英語   掲載種別：研究論文（学術雑誌）  

An intra-arc rift (IAR) is developed behind the volcanic front in the Izu arc, Japan. Bimodal volcanism, represented by basalt and rhyolite lavas and hydrothermal activity, is active in the IAR. The constituent minerals in the rhyolite lavas are mainly plagioclase and quartz, whereas mafic minerals are rare and are mainly orthopyroxene without any hydrous minerals such as amphibole and biotite. Both the phenocryst and groundmass minerals have felsic affinities with a narrow compositional range. The petrological and bulk chemical characteristics are similar to those of melts from some partial melting experiments that also yield dry rhyolite melts. The hydrous mineral-free narrow mineral compositions and low-Al₂O₃ affinities of the IAR rhyolites are produced from basaltic middle crust under anhydrous low-temperature melting conditions. The IAR basalt lavas display prominent across-arc variation, with depleted elemental compositions in the volcanic front side and enriched compositions in the rear-arc side. The across-arc variation reflects gradual change in the slab-derived components, as demonstrated by decreasing Ba/Zr and Th/Zr values to the rear-arc side. Rhyolite lavas exhibit different across-arc variations in either the fluid-mobile elements or the immobile elements, such as Nb/Zr, La/Yb, and chondrite-normalized rare earth element patterns, reflecting that the felsic magmas had different source. The preexisting arc crust formed during an earlier stage of arc evolution, most probably during the Oligocene prior to spreading of the Shikoku back-arc basin. The lack of systematic across-arc variation in the IAR rhyolites and their dry/shallow crustal melting origin combines to suggest re-melting of preexisting Oligocene middle crust by heat from the young basaltic magmatism.

DOI： 10.1007/s00410-017-1345-1

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The similar, but not identical chemical properties of W compared with Mo suggest that the stable isotope system of W could be a novel proxy to explore the modern and ancient ocean as is the case in the well-established utility of Mo isotopes. We experimentally investigated the isotopic fractionation of W during adsorption on Fe and Mn (oxyhydr)oxides (ferrihydrite and δ-MnO₂), a key process in the global ocean budget of this element. Our adsorption experiments confirmed that W isotopes fractionate substantially on both ferrihydrite and δ-MnO₂: lighter W isotopes are preferentially adsorbed on both oxides as a result of equilibrium isotopic exchange between dissolved and adsorbed species, and the obtained values of Δ^186/183W_{liquid–solid} ( = δ¹⁸⁶W_dissolved − δ¹⁸⁶W_adsorbed) are 0.76 ± 0.09‰ for ferrihydrite and 0.88 ± 0.21‰ for δ-MnO₂ (2σ, n = 6). Compared with the case of Mo isotopes, fractionation of W isotopes is (i) of comparable magnitude between ferrihydrite and δ-MnO₂, and (ii) much smaller than that of Mo on δ-MnO₂. Our previous XAFS observations and newly-performed DFT calculations both indicate that the observed W isotopic fractionations are caused by the symmetry change from Td (tetrahedral) WO₄ ²⁻ to distorted Oh (octahedral) monomeric W species via formation of inner-sphere complexes on both ferrihydrite and δ-MnO₂. The similar isotopic fractionations between the two oxides relate to the strong tendency for W to form inner-sphere complexes, which causes the symmetry change, in contrast to the outer-sphere complex of Mo on ferrihydrite. The smaller isotopic fractionation of W compared with Mo on δ-MnO₂ despite their similar molecular symmetry seems to be due to their different degrees of distortion of Oh species. Our findings imply that the isotopic composition of W in modern oxic seawater is likely to become heavier relative to the input by removal of lighter W isotopes via adsorption on ferromanganese oxides in analogy with the Mo isotope budget. In contrast, the isotopic composition of W in ancient seawater should have evolved in response to the extent of deposition of both Fe and Mn oxides; this is likely to be different compared with that of the Mo isotopes, which is strongly associated with the occurrence of Mn oxides relative to Fe oxides.

DOI： 10.1016/j.gca.2017.01.031

記述言語：英語   掲載種別：研究論文（学術雑誌）  

It has been proposed that the oceanic crust-mantle boundary may partly form through interactions between melts and the shallow mantle. Although this process may have a key role in defining the chemical signature of the oceanic crust, the effect of such melt-mantle interactions on the isotopic composition of the reacting melts has rarely been investigated. Here we report Re-Os isotopes and platinum group element (PGE) compositions of residualmantle harzburgites (olivine forsterite, Fo_91-90) and troctolites (olivine Fo_89-87) likely produced by melt-rock reactions at the Central Indian Ridge crust-mantle boundary. Although the harzburgites have high Os (4000-2500 pg/g) and unradiogenic isotopic signatures (¹⁸⁷Os/¹⁸⁸Os = 0.128-0.124), the troctolites show variable Os (383-16 pg/g) and highly fractionated ¹⁸⁷Os/¹⁸⁸Os ratios (0.129-0.140), which correlate with mineral (e.g., olivine Fo) and bulk-rock (e.g., CaO, Ni, and Cu) compositions. We relate this isotopic variability to the melt-mantle interaction process by which the troctolites generated. In particular, we suggest that the radiogenic Os signature was produced by selective assimilation of radiogenic interstitial sulfides from the reacted mantle. This mechanism is able to mimic the isotope variability of erupted melts, sustaining the idea that the crust-mantle boundary can act as a reactive filter for the melts extracted from the source region.

DOI： 10.1130/G37428.1

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The 3.4-Ga Strelley Pool Formation (SPF) at the informally named 'Waterfall Locality' in the Goldsworthy greenstone belt of the Pilbara Craton, Western Australia, provides deeper insights into ancient, shallow subaqueous to possibly subaerial ecosystems. Outcrops at this locality contain a thin (<3 m) unit of carbonaceous and non-carbonaceous cherts and silicified sandstones that were deposited in a shallow-water coastal environment, with hydrothermal activities, consistent with the previous studies. Carbonaceous, sulfide-rich massive black cherts with coniform structures up to 3 cm high are characterized by diverse rare earth elements (REE) signatures including enrichment of light [light rare earth elements (LREE)] or middle rare earth elements and by enrichment of heavy metals represented by Zn. The massive black cherts were likely deposited by mixing of hydrothermal and non-hydrothermal fluids. Coniform structures in the cherts are characterized by diffuse laminae composed of sulfide particles, suggesting that unlike stromatolites, they were formed dominantly through physico-chemical processes related to hydrothermal activity. The cherts yield microfossils identical to previously described carbonaceous films, small and large spheres, and lenticular microfossils. In addition, new morphological types such as clusters composed of large carbonaceous spheroids (20-40 μm across each) with fluffy or foam-like envelope are identified. Finely laminated carbonaceous cherts are devoid of heavy metals and characterized by the enrichment of LREE. This chert locally contains conical to domal structures characterized by truncation of laminae and trapping of detrital grains and is interpreted as siliceous stromatolite formed by very early or contemporaneous silicification of biomats with the contribution of silica-rich hydrothermal fluids. Biological affinities of described microfossils and microbes constructing siliceous stromatolites are under investigation. However, this study emphasizes how diverse the microbial community in Paleoarchean coastal hydrothermal environment was. We propose the diversity is at least partially due to the availability of various energy sources in this depositional environment including reducing chemicals and sunlight.

DOI： 10.1111/gbi.12150

記述言語：英語   掲載種別：研究論文（学術雑誌）  

A high concentration of molybdenum (Mo) is a striking feature of modern marine hydrothermal manganese (Mn) crusts from both low- and high-temperature hydrothermal systems; however, the origin of that Mo is poorly constrained. In this study, we analyzed Mo isotopic composition (δ^98/95Mo) in a Mn crust collected from the Ryukyu arc system, and assessed the possible use of δ^98/95Mo to constrain the origin of Mo in hydrothermal Mn crusts. Along with Mo isotope analysis, we also measured the abundance of major and trace elements and the Re-Os isotopic composition. The Mn crust sample had a high Mn content (42%-47%) and was low in Fe (0.2%-0.5%). Among the manganophile elements (Co, Ni, Cu, Zn, and Mo), only Mo showed high concentrations (>550ppm). We also found low contents of Os (~35ppt) with an isotopic composition of ¹⁸⁷Os/¹⁸⁸Os=~1.00. These results are consistent with previously reported chemical compositions of hydrothermal Mn crusts. δ^98/95Mo values in the sample ranged from -0.56‰ to -0.66‰, which are ~2.7‰ lighter than the present-day seawater δ^98/95Mo but similar to those in modern hydrogenous Fe-Mn crusts and Mn nodules. Although current data do not preclude the possible contribution of hydrothermally derived Mo, the light δ^98/95Mo values can be explained by isotope fractionation associated with a change in coordination number during the adsorption of Mo from seawater onto Mn oxides. We suggest that the δ^98/95Mo data are useful for constraining the source of Mo in hydrothermal Mn crusts.

DOI： 10.1016/j.margeo.2015.08.007

記述言語：英語   掲載種別：研究論文（学術雑誌）  

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. Key Points: The isotope geochemistry of the western Pacific Plate slab is reported The Pacific Plate originated from both Indian and Pacific mantle The Indian-Pacific mantle boundary has been stationary in the western Pacific at least since the Cretaceous.

DOI： 10.1002/2015GC005911

記述言語：英語   掲載種別：研究論文（学術雑誌）  

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 fore arc. 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. Key Points: The Japan Sea back-arc basin basalt originated from a high-temperature hydrous mantle Subducted sediments on the Pacific Plate slab were part of the mantle source High magma productivity generated unusually thick oceanic crust in the Japan Sea

DOI： 10.1002/2015GC005720

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The youngest geomagnetic polarity reversal, the Matuyama-Brunhes boundary (MBB), provides an important datum plane for sediments, ice cores, and lavas. Its frequently cited age of 780 ka is based on orbital tuning of marine sedimentary records, and is supported by ⁴⁰Ar/³⁹Ar dating of Hawaiian lavas using recent age calibrations. Challenging this age, however, are reports of younger astrochronological ages based on oxygen isotope stratigraphy of high-sedimentation-rate marine records, and cosmogenic nuclides in marine sediments and an Antarctic ice core. Here, we present a U-Pb zircon age of 772.7 ± 7.2 ka from a marinedeposited tephra just below the MBB in a forearc basin in Japan. U-Pb dating has a distinct advantage over ⁴⁰Ar/³⁹Ar dating in that it is relatively free from assumptions regarding standardization and decay constants. This U-Pb zircon age, coupled with a newly obtained oxygen isotope chronology, yields an MBB age of 770.2 ± 7.3 ka. Our MBB age is consistent with those based on the latest orbitally tuned marine sediment records and on an Antarctic ice core. We provide the first direct comparison between orbital tuning, U-Pb dating, and magnetostratigraphy for the MBB, fulfilling a key requirement in calibrating the geological time scale.

DOI： 10.1130/G36625.1

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The Louisville Seamount Chain (LSC) is, besides the Hawaiian-Emperor Chain, one of the longest-lived hotspot traces. We report here the first Re-Os isotope and platinum group element (PGE) data for Canopus, Rigil, and Burton Guyots along the chain, which were drilled during IODP Expedition 330. The LSC basalts possess (¹⁸⁷Os/¹⁸⁸Os)_i = 0.1245-0.1314 that are remarkably homogeneous and do not vary with age. A Re-Os isochron age of 64.9 ± 3.2 Ma was obtained for Burton seamount (the youngest of the three seamounts drilled), consistent with ⁴⁰Ar-³⁹Ar data. Isochron-derived initial ¹⁸⁷Os/¹⁸⁸Os ratio of 0.1272 ± 0.0008, together with data for olivines (0.1271-0.1275), are within the estimated primitive mantle values. This (¹⁸⁷Os/¹⁸⁸Os)_i range is similar to those of Rarotonga (0.124-0.139) and Samoan shield (0.1276-0.1313) basalts and lower than those of Cook-Austral (0.136-0.155) and Hawaiian shield (0.1283-0.1578) basalts, suggesting little or no recycled component in the LSC mantle source. The PGE data of LSC basalts are distinct from those of oceanic lower crust. Variation in PGE patterns can be largely explained by different low degrees of melting under sulfide-saturated conditions of the same relatively fertile mantle source, consistent with their primitive mantle-like Os and primordial Ne isotope signatures. The PGE patterns and the low ¹⁸⁷Os/¹⁸⁸Os composition of LSC basalts contrast with those of Ontong Java Plateau (OJP) tholeiites. We conclude that the Re-Os isotope and PGE composition of LSC basalts reflect a relatively pure deep-sourced common mantle sampled by some ocean island basalts but is not discernible in the composition of OJP tholeiites.

DOI： 10.1002/2014GC005629

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The Asahi Mountains, located along the border between Yamagata and Niigata Prefectures, lies to the northwest of the Tanagura Shear Zone, which represents the boundary of pre- Neogene basement rocks. The Asahi Mountains are comprised of Cretaceous plutonic rocks of various lithologies, of which the Nishiasahi basic intrusive complex composed of quartz dioritic rocks and gabbroic rocks represents the first phase of intrusive activity. Geochemical modeling suggests that the diŠerentiation of the quartz dioritic rocks resulted from the fractional crystallization of 25.4% amphibole, 23.0% plagioclase, 8.2% biotite, and 1.1% magnetite, whereas the gabbroic rocks were formed by the accumulation of these fractionated minerals. The gabbroic rocks have slight geochemical variations, which can be explained by the selective accumulation of the minerals. The initial Sr and Nd isotopic ratios of the Nishiasahi basic intrusive complex, corrected using an age of 120 Ma, are in the range of 0.70697-0.70797 and 0.51223-0.51229, respectively. This suggests the existence of an enriched source material, resembling the case of the Ryoke basic intrusive rocks in the Southwest Japan arc. The primitive magma of the quartz dioritic rocks geochemically corresponds to the experimental melt composition obtained by Rapp and Watson (1995) for amphibolite under the conditions of 1075 °C and 0.8GPa. Therefore, an enriched mafic lower crust (i.e., enriched amphibolite) would be appropriate for the source material of the Nishiasahi basic intrusive complex.

DOI： 10.2465/gkk.140331

記述言語：英語  

The few geological and geophysical studies of the Lyra Basin at the western margin of the Ontong Java Plateau (OJP; Pacifi c Ocean) revealed that it is underlain by thicker than normal oceanic crust. The unusually thick oceanic crust is attributed to the emplacement of massive lava fl ows from the OJP. Dredging was conducted to sample the inferred OJP crust on the Lyra Basin but instead recovered younger extrusives that may have covered the older plateau lavas in the area. The Lyra Basin extrusives are alkalic basalts with (⁸⁷Sr. ⁸⁶Sr. _t = 0.704513-0.705105, (¹⁴³Nd/¹⁴⁴Nd)_t = 0.512709-0.512749, ε_Nd(t) = +3.0 to +3.8, (²⁰⁶Pb/²⁰⁴Pb)_t = 18.488-18.722, (²⁰⁷Pb/²⁰⁴Pb)_t = 15.558-15.577, and (²⁰⁸Pb/²⁰⁴Pb)_t = 38.467-38.680 that are distinct from those of the OJP tholeiites. They have age-corrected (^187Os/¹⁸⁸Os)_t = 0.1263-0.1838 that overlap with the range of values determined for the Kroenke-type and Kwaimbaita-type OJP basalts, but their (¹⁷⁶Hf/¹⁷⁷Hf)_t = 0.28295-0.28299 and ε_Hf(t) = +7.9 to +9.3 values are lower. These isotopic compositions do not match those of any Polynesian ocean island volcanics. Instead, the Lyra Basin basalts have geochemical affi nity and isotopic compositions that overlap with those of some alkalic suite and alnoïtes in the island of Malaita, Solomon Islands. Although not directly related to the main plateau volcanism at 120 Ma, the geochemical data and modeling suggest that the origin of the Lyra Basin alkalic rocks may be genetically linked to the mantle preserved in the OJP thick lithospheric root, with magmatic contribution from the Rarotongan hotspot.

DOI： 10.1130/2015.2511(14)

記述言語：英語  

Two hydrothermal fields, the Kairei and Edmond hydrothermal fields, are known in the southern Central Indian Ridge (CIR). The Kairei hydrothermal field at 25°19′S is associated with hydrogen-rich hydrothermal activity, whereas the Edmond hydrothermal field at 23°52′S is recognized in the typical mid-ocean ridge type hydrothermal activity. Differences of lithology and geological background between two hydrothermal fields are reflected in the different type of hydrothermal activity. We recovered more than 870 kg of rock samples by dredging from the southern part of the CIR adjacent to the Kairei and Edmond hydrothermal fields during the KH-10-6 cruise. Here, we present new petrological and geochemical data for MORB samples taken between the CIR-S1 and CIR-S4 segments with the aim of constraining distributions of lithology at the southern CIR, and discuss the petrogenesis and the mantle source for these basalts. The MORB melts that formed rocks within the CIR-S1, CIR-S2, and CIR-S4 segments equilibrated with mantle olivine at approximately 10 kbar, and were erupted after undergoing only minor fractionation. MORB samples from the CIR-S4 segment have slightly depleted trace element compositions, whereas MORB samples from the off-ridge part of the CIR-S1 segment are highly depleted. MORB samples from the Knorr seamount have enriched compositions involved a minor amount of hotspot-derived material, as indicated by previous isotope analyses. The presence of a depleted MORB source beneath the off-ridge section of the CIR-S1 segment indicates that the older mantle material at the boundary between the CIR-S1 and CIR-S2 segments was highly depleted. In turn, this suggests that the source mantle beneath the southern CIR is heterogeneous both along and across the present spreading axis, and that the composition of the mantle in this area is a function of the degree of mixing between depleted and enriched sources.

DOI： 10.1007/978-4-431-54865-2_13

記述言語：英語   掲載種別：研究論文（学術雑誌）  

In reconstructions of the Gondwana supercontinent, correlations of Archean domains between Madagascar and India remain debated. In this paper, we aim to establish correlations among these Archean domains using whole-rock geochemistry and U-Pb zircon geochronology of meta-granitoids from the Masora and the Antananarivo domains, central-eastern Madagascar. A meta-granitoid from the central part of Masora domain is dated at 3277 Ma and shows a typical Archean tonalite-trondhjemite-granodiorite composition, whereas a tonalitic gneiss from the southeastern part of the Antananarivo domain gives an age of 2744 Ma. The geochemical signature of this tonalitic gneiss differs from that of the-2500 Ma granitoids of the northwestern part of Antananarivo domain. In addition, the geochemical composition of the-760 Ma granitic gneisses is consistent with a volcanic-arc origin for the protolith. Based on the geochemical and geochronological results, along with existing data, we identified three episodes of granitic magmatism in central-eastern Madagascar at-3300, 2700, and 2500 Ma. These three magmatic events are consistent with those reported for the Dharwar Craton in India, suggesting that the Archean Masora and Antananarivo domains in Madagascar were part of the Greater Dharwar Craton during the period of 3300-2500 Ma. The 700-800 Ma volcanic arc granites identified in eastern Madagascar have not been reported in India. Therefore, the subduction of the oceanic plate that led to the formation of these granites likely took place at the western margin of the Greater Dharwar Craton, which included part of eastern Madagascar.

DOI： 10.2465/jmps.141225

記述言語：英語   掲載種別：研究論文（学術雑誌）  

HIMU (high-μ ²³⁸U/²⁰⁴Pb) is a mantle reservoir that has been thought to form by subduction and subsequent storage of ancient oceanic crust and lithosphere in the mantle. In order to constrain the processes that acted on subducted materials over several billion years, we present precise Pb-Sr-Nd-Hf-He isotopic data together with ⁴⁰Ar/³⁹Ar and K/Ar ages of HIMU lavas from St. Helena in the Atlantic. Clinopyroxene separates were analyzed together with whole-rock samples to better describe the geochemical characteristics of the HIMU component. Although isotopic variations are small in the St. Helena lavas (20.6-21.0 for ²⁰⁶Pb/²⁰⁴Pb) between 12 and 8Ma, the younger lavas have more HIMU-like isotopic compositions than the older lavas. The mixing arrays defined by these lavas are remarkably similar to those observed in HIMU lavas from Austral Islands in the Pacific, suggesting that the two HIMU reservoirs located in different mantle domains are characterized by similar isotopic compositions with radiogenic ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb, enriched Nd and Hf isotopes, depleted Sr isotopes, and radiogenic ³He/⁴He. However, there is a significant difference between the St. Helena and Austral Islands lavas in ²⁰⁷Pb/²⁰⁴Pb. The St. Helena lavas show systematically higher ²⁰⁷Pb/²⁰⁴Pb for a given ²⁰⁶Pb/²⁰⁴Pb. Lead isotope evolution models suggest that both HIMU reservoirs formed around 2Ga; however, the HIMU reservoir for St. Helena is about 0.3Ga older than that for Austral Islands. The relation between ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb could reflect the time-integrated κ (²³²Th/²³⁸U) in the components. The HIMU components for St. Helena and Austral Islands have κ values between 3.3 and 3.7, which are intermediate between the present-day fresh mid-ocean ridge basalts (MORB; 2.6-3.2) and the chondritic silicate Earth (~4). This is consistent with the model that the HIMU precursor is subducted oceanic crust created around 2Ga from depleted upper mantle, in which κ monotonously decreased from the chondritic to the present-day values since late Archean or early Proterozoic, because of enhanced U recycling from the Earth's surface back to the mantle in response to the increasing oxygen levels in the hydrosphere. Moreover, the fact that the HIMU components have much higher κ than the present-day hydrothermally altered MORB (0.2-2) suggests that either the HIMU precursor was an unaltered ancient oceanic crust, or more likely, an altered oceanic crust with minimal U enrichment by hydrothermal fluids in the less oxic marine environment of the late Archean or early Proterozoic. The unradiogenic ⁸⁷Sr/⁸⁶Sr of the HIMU components also suggests formation of ancient oceanic crust altered with hydrothermal fluids having much lower ⁸⁷Sr/⁸⁶Sr in that eon than at present, followed by removal of Rb from it by subduction dehydration.

DOI： 10.1016/j.gca.2014.03.016

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Here we evaluate measurement procedures currently used for the combined determination of highly siderophile element concentrations (HSEs: Re, Ir, Os, Ru, Pt and Pd) and Os isotope ratios in geological samples by isotope dilution mass spectrometry (ID-MS) applying high-temperature sample digestion using inverse aqua regia in closed glass vessels, such as Carius tubes or a high-pressure asher system (HPA). In particular, we address the question of whether an additional hydrofluoric acid dissolution step is required to release HSEs hosted in the silicate portions of rock samples. To do so, we conducted systematic experiments for basaltic (TDB-1 and BIR-1), ultramafic (UB-N and JP-1) and sedimentary (SCo-1, SDO-1 and JCh-1) reference materials with and without HF desilicification. Through the extensive tests on TDB-1-varying apparatus (microwave, Carius tube, high-pressure asher), conditions (temperature, duration, sample size) and protocol (HF desilicification before or after aqua regia attack)-we find the optimum digestion method is to use inverse aqua regia for digesting 1-2g of powder over long durations, such as Carius tubes heated to 240 °C for 72h, followed by an HF desilicification step after CCl₄ solvent extraction of Os. For basaltic reference materials, the method provides significantly improved HSE recoveries-particularly Ru-largely due to the use of HF. Strong linear correlations between Os and Ir-Ru-Pt concentrations and isochronous behavior on a ¹⁸⁷Re/¹⁸⁸Os vs. ¹⁸⁷Os/¹⁸⁸Os diagram (1240±59Ma, MSWD=1.4) are observed for repeat dissolutions of TDB-1, reflecting sample heterogeneity due to minor minerals enriched in Os, Ir, Ru and Pt-most likely sulphides. In comparison, excellent reproducibilities were obtained for BIR-1: relative standard deviations (RSDs) for 1-2g aliquots were 6.9% Os, 5.1% Ir, 2.0% Ru, 5.1% Pt, 1.5% Pd and 0.7% Re (n=9). Thus, BIR-1 might be a useful candidate reference material for obtaining certified values with small measurement uncertainties. Unlike the basaltic reference materials, extraction of HSEs from ultramafic and sedimentary reference materials is largely independent of the use of HF. The data obtained by Carius tube for UB-N and JP-1 agree well with the literature data obtained using apparently more aggressive digestion techniques. The precision of our data for 0.5-2g aliquots of the sedimentary reference materials also compares favourably with limited data available in the literature.

DOI： 10.1016/j.chemgeo.2014.06.013

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Mantle-derived spinel peridotite and spinel plagioclase pyroxenite xenoliths hosted in Cenozoic alkali basalts (20Ma) are found in the Xikeer area, western Tarim Block, NW China. Based on petrographic and geochemical characteristics, the peridotite xenoliths are divided into three groups. Group 1 peridotite xenoliths have experienced high degree melt extraction (~17% fractional melting) and weak, incipient metasomatism. Group 2 and 3 peridotite xenoliths have undergone extensive silicate melt metasomatism, resulting in clinopyroxenes with spoon-shaped and highly fractionated REE patterns respectively. Although their present texture is metamorphic, the pyroxenite xenoliths were initially of metasomatic origin, with high pressure protolith mineral assemblage of orthopyroxene+clinopyroxene+spinel±olivine. Numerical modeling of the Mg-number shows that the Xikeer pyroxenites may have resulted from reaction between Group 1 peridotite and a primitive Permian picritic melt at a high melt/rock ratio (>15) and that the host basanite is incapable of being the metasomatic melt. The Re-Os isotopic systematics of the Xikeer peridotites and pyroxenites yield an apparent isochron of ~290Ma, virtually identical to the age of Tarim flood basalts. Their ε_Nd(t=290Ma) ranges from +9.5 to +14.3, typical of convecting mantle. We propose that the Xikeer xenolith suite may have been initially formed by melt extraction from the convecting mantle and, shortly after, was refertilized by Tarim mantle plume melts during the Early Permian, a process which is referred to as mantle 'auto-refertilization'.

DOI： 10.1016/j.lithos.2014.01.005

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Major and trace element and Sr-Nd-Pb isotope data for wholerocks and major element data for minerals within basalt samples from the Chugaryeong volcano, an intra-plate back-arc volcanic centre in the central part of the Korean Peninsula, are used to address the process of magma genesis in the deep back-arc region of eastern Asia.There are two lava flow units at Chugaryeong volcano: the Chongok (0·50Ma) and the Chatan (0·15Ma) basalts. These basalts have similar MgO (9·1-10·4 wt %) but exhibit differences in their major and trace element and isotope compositions. The Chongok basalt has higher TiO₂, Al₂ O₃, Na₂O, K₂O, P₂O₅, Cr₂ O₃, large ion lithophile elements (LILE), high field strength elements (HFSE), and rare earth elements (REE), and lower FeO*, SiO₂, and CaO than the Chatan basalt. In addition, the Chongok basalt has more radiogenic ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁶Pb/²⁰⁴Pb, and less radiogenic ⁸⁷Sr/⁸⁶Sr and ²⁰⁸Pb/²⁰⁴Pb than the Chatan basalt. Chi-square tests for the major elements indicate that crystal fractionation can explain the chemical variations within each basalt suite; intra-crustal processes, including crystal fractionation and assimilation of continental crust, cannot result in the formation of one basalt suite from the other.The Sr-Nd-Pb isotopic compositions of the Chongok and Chatan basalts plot on mixing hyperbolae between peridotite mantle xenoliths from the area and a fluid flux derived from a mixture of ancient and recent sediments. The trace element compositions of the estimated primary melts for the two basalt suites suggest different degrees of partial melting of a common enriched mantle source that was metasomatized by a Ba-, K-, Pb-, and Sr-rich fluid. The estimated degree of melting increased with time from ̃7·5% for the Chongok basalt to ̃10% for the Chatan basalt. The source mantle for the Chatan basalt is more enriched in Ba and Pb, indicating a greater fluid flux than for the Chongok basalt. This suggests that melting of the source mantle increased with time, sustained by an increased sediment-derived fluid flux from the deeper upper mantle.

DOI： 10.1093/petrology/egt074

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The conditions under which rear-arc magmas are generated were estimated using primary basalts from the Sannome-gata volcano, located in the rear of the NE Japan arc. Scoriae from the volcano occur with abundant crustal and mantle xenoliths, suggesting that the magma ascended rapidly from the upper mantle. The scoriae show significant variations in their whole-rock compositions (7.9-11.1 wt% MgO). High-MgO scoriae (MgO > ~9.5 wt%) have mostly homogeneous ⁸⁷Sr/⁸⁶Sr ratios (0.70318-0.70320), whereas low-MgO scoriae (MgO < ~9 wt%) have higher ⁸⁷Sr/⁸⁶Sr ratios (>0.70327); ratios tend to increase with decreasing MgO content. The high-MgO scoriae are aphyric, containing ~5 vol% olivine microphenocrysts with Mg# [100 × Mg/(Mg + Fe²⁺)] of up to 90. In contrast, the low-MgO scoriae have crustal xenocrysts of plagioclase, alkali feldspar, and quartz, and the mineralogic modes correlate negatively with whole-rock MgO content. On the basis of these observations, it is inferred that the high-MgO scoriae represent primary or near-primary melts, while the low-MgO scoriae underwent considerable interaction with the crust. Using thermodynamic analysis of the observed petrological features of the high-MgO scoriae, the eruption temperature of the magmas was constrained to 1,160-1,220 °C. Given that the source mantle was depleted MORB-source mantle, the primary magma was plausibly generated by ~7 % melting of a garnet-bearing spinel peridotite; taking this into consideration, and considering the constraints of multi-component thermodynamics, we estimated that the primary Sannome-gata magma was generated in the source mantle with 0.5-0.6 wt% H₂O at 1,220-1,230 °C and at ~1.8 GPa, and that the H₂O content of the primary magma was 6-7 wt%. The rear-arc Sannome-gata magma was generated by a lower degree of melting of the mantle at greater depths and lower temperatures than the frontal-arc magma from the Iwate volcano, which was also estimated to be generated by ~15 % melting of the source mantle with 0.6-0.7 wt% H₂O at ~1,250 °C and at ~1.3 GPa.

DOI： 10.1007/s00410-014-0969-7

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The Mikabu and Sorachi-Yezo belts comprise Jurassic ophiolitic complexes in Japan, where abundant basaltic to picritic rocks occur as lavas and hyaloclastite blocks. In the studied northern Hamamatsu and Dodaira areas of the Mikabu belt, these rocks are divided into two geochemical types, namely depleted (D-) and enriched (E-) types. In addition, highly enriched (HE-) type has been reported from other areas in literature. The D-type picrites contain highly magnesian relic olivine phenocrysts up to Fo_93.5, and their Fo-NiO trend indicates fractional crystallization from a high-MgO primary magma. The MgO content is calculated as high as 25 wt%, indicating mantle melting at unusually high potential temperature (T _p) up to 1,650 °C. The E-type rocks represent the enrichment in Fe and LREE and the depletion in Mg, Al and HREE relative to the D-type rocks. These chemical characteristics are in good accordance with those of melts from garnet pyroxenite melting. Volcanics in the Sorachi-Yezo belts can be divided into the same types as the Mikabu belt, and the D-type picrites with magnesian olivines also show lines of evidence for production from high T _p mantle. Evidence for the high T _p mantle and geochemical similarities with high-Mg picrites and komatiites from oceanic and continental large igneous provinces (LIPs) indicate that the Mikabu and Sorachi-Yezo belts are accreted oceanic LIPs that were formed from hot large mantle plumes in the Late Jurassic Pacific Ocean. The E- and D-type rocks were formed as magmas generated by garnet pyroxenite melting at an early stage of LIP magmatism and by depleted peridotite melting at the later stage, respectively. The Mikabu belt characteristically bears abundant ultramafic cumulates, which could have been formed by crystal accumulation from a primary magma generated from Fe-rich peridotite mantle source, and the HE-type magma were produced by low degrees partial melting of garnet pyroxenite source. They should have been formed later and in lower temperatures than the E- and D-type rocks. The Mikabu and Sorachi Plateaus were formed in a low-latitude region of the Late Jurassic Pacific Ocean possibly near a subduction zone, partially experienced high P/T metamorphism during subduction, and then uplifted in association with (or without, in case of Mikabu) the supra-subduction zone ophiolite. The Mikabu and Sorachi Plateaus may be the Late Jurassic oceanic LIPs that could have been formed in brotherhood with the Shatsky Rise.

DOI： 10.1007/s00410-014-1019-1

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Alkali fusion after acid digestion method (AFAD) was used for determination by ICP-MS of twenty-five trace elements (Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, Pb, Th, U, and rare earth elements) in four silicate reference materials (JG-1a, JG-2, and JG-3 granitoids and JB-2 basalt) from the Geological Survey of Japan (GSJ). Results were compared with those obtained by acid digestion (AD) method. Our results are in excellent agreement with previously reported data obtained by fusion methods. The reproducibility of replicate analyses is better than 5% (1ó) for all the elements, except Rb, Cs, and Pb in JG-2.

DOI： 10.2343/geochemj.2.0280

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We have developed a protocol for Os isotope analysis employing a sparging method coupled with an enhanced-sensitivity multiple Faraday collector-inductively coupled plasma-mass spectrometry (MFC-ICP-MS) technique. The enhanced-sensitivity ICP interface with 10¹² Ω high-gain amplifiers allowed for the stable and precise isotopic ratio analysis of Os by sparging in a very wide concentration range of 15-4000 pg. The analytical reproducibilities of Johnson Matthey Chemical (JMC) Os standards at 50, 100, 200, 400, and 2000 pg Os were 0.8, 0.5, 0.2, 0.1, and 0.02% within two standard deviations (2SD), respectively. The low Os (50-200 pg) results compared with those obtained by sparging multiple-ion counter (MIC)-ICP-MS and high Os (400-2000 pg) results rivalled those of desolvating nebulisation MFC-ICP-MS and negative thermal ionisation mass spectrometry (N-TIMS). The analysed geological standards consisting of JCh-1 (chert; ∼15 pg, n = 3), JMS-2 (marine sediment; ∼150 pg, n = 5), UB-N (lherzoritic peridotite; ∼4 ng, n = 4), and JP-1 (harzburgitic peridotite; ∼3 ng, n = 5) showed ¹⁸⁷Os/ ¹⁸⁸Os = 0.657 ± 0.065, 0.842 ± 0.053, 0.12752 ± 0.00016, and 0.12071 ± 0.00069 (errors are in 2SD), respectively; these results are comparable with those obtained by MIC-ICP-MS and N-TIMS. The results showed that the sparging method coupled with enhanced-sensitivity MFC-ICP-MS is a strong tool for determining Os isotope ratios in natural samples over a wide range of Os concentrations. Simple sample digestion and low procedural blanks using Carius tube digestion alone without any further element separation provide an additional advantage for Os isotope analysis by the method. This journal is

DOI： 10.1039/c4ja00092g

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The feasibility of oceanic igneous crust melting in stagnant slabs has previously been proposed from experimental petrological research. However, geochemical evidence for such melting has not yet been found from igneous rocks. We present evidence to suggest that melts from the igneous layer in the stagnant Pacific slab contributed to the source composition of basalts erupted in eastern China. Fe-rich (> 13 wt%), Si-poor (< 43 wt%) basalts occur only above the leading edge of the stagnant Pacific slab in eastern China, ~. 2000. km west of the Pacific Plate trench. The source of these basalts has Nd-Hf and Sr-Nd-Pb isotopic compositions akin to those of the igneous layer in the Pacific slab, and their extremely low Rb and Pb contents suggest that their source material was modified by subduction processes. Together with forward modeling calculation on trace elements and isotope compositions, these geochemical characteristics imply that they received a contribution from fluid released from hydrated transition zone and dehydrated carbonate-bearing oceanic crust in the stagnant slab, without a long time-integrated ingrowth of Sr-Nd-Hf-Pb isotope systems, almost at the leading edge of the stagnant Pacific slab.

DOI： 10.1016/j.chemgeo.2013.09.012

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We present the first report of geochemical data for submarine basalts collected by a manned submersible from Rurutu, Tubuai, and Raivavae in the Austral Islands in the South Pacific, where subaerial basalts exhibit HIMU isotopic signatures with highly radiogenic Pb isotopic compositions. With the exception of one sample from Tubuai, the ⁴⁰Ar/³⁹Ar ages of the submarine basalts show no significant age gaps between the submarine and subaerial basalts, and the major element compositions are indistinguishable at each island. However, the variations in Pb, Sr, Nd, and Hf isotopic compositions in the submarine basalts are much larger than those previously reported in subaerial basalts. The submarine basalts with less-radiogenic Pb and radiogenic Nd and Hf isotopic compositions show systematically lower concentrations in highly incompatible elements than the typical HIMU basalts. These geochemical variations are best explained by a two-component mixing process in which the depleted asthenospheric mantle was entrained by the mantle plume from the HIMU reservoir during its upwelling, and the melts from the HIMU reservoir and depleted asthenospheric mantle were then mixed in various proportions. The present and compiled data demonstrate that the HIMU reservoir has a uniquely low ¹⁷⁶Hf/¹⁷⁷Hf decoupled from ¹⁴³Nd/¹⁴⁴Nd, suggesting that it was derived from an ancient subducted slab. Moreover, the Nd/Hf ratios of the HIMU basalts and curvilinear Nd-Hf isotopic mixing trend require higher Nd/Hf ratios for the melt from the HIMU reservoir than that from the depleted mantle component. Such elevated Nd/Hf ratios could reflect source enrichment by a subducted slab during reservoir formation.

DOI： 10.1007/s00410-013-0926-x

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The Paleoproterozoic (2.5-2.0 Ga) is one of the most important periods in Earth's history, and was characterized by a rise in atmospheric oxygen levels and repeated (at least three) severe glaciations (the Huronian glaciations). In this study, we investigate redox conditions in the atmosphere and in shallow-marine environments immediately after the first Huronian glaciation based on the isotopic composition of Os, and the abundance of redox-sensitive elements (Os, Re, and Mo) in sedimentary rocks from the Huronian Supergroup, Canada. We found no significant authigenic enrichment of Os in the sedimentary rocks deposited during the first Huronian deglaciation. The initial isotopic composition of Os in the sediments was close to that of chondrite at the time of deposition (Os187/188Os=~0.11). These results suggest that atmospheric O₂ levels were insufficient to mobilize radiogenic Os through continental weathering (p_O2 < ^{10 -5}-^{10 -3} present atmospheric level (PAL)). In contrast, we found enrichment of Re in the sedimentary rocks, which suggests the occurrence of oxidative weathering of Re under mildly oxidizing conditions (>^{10 -8}-^{10 -5} PAL). Despite the Re enrichment, low abundances of Mo imply possible non-sulfidic conditions in shallow-marine environments at the time of deposition. Together with the results of organic carbon and sulfur analyses, we suggest that atmospheric O₂ remained at relatively low levels of around ^{10 -8}-^{10 -5} PAL after the first Huronian deglaciation, which contrasts with proposed dramatic increases in O₂ after the second and third Huronian deglaciations. These results imply that the second and third Huronian glaciations may have been global events, associated with climatic jumps from severe glaciations to super-greenhouse conditions and the subsequent blooming of photosynthetic cyanobacteria in the glacial aftermath.

DOI： 10.1016/j.epsl.2013.06.018

記述言語：英語   掲載種別：研究論文（学術雑誌）  

It has long been thought that the low-K tholeiitic (TH) and medium-K calc-alkaline (CA) lavas in the NE Japan arc were produced by fractional crystallization from mantle-derived basalt magmas, and that the latter formed from mixing of mafic and felsic magmas, both derivedfrom a common primaryTH basalt through fractionation. An alternative view was recently proposed on the basis of Sr isotope microanalysis of plagioclase phenocrysts from the Zao volcano, suggesting that (1) the low-K TH basaltic andesites formed by melting of lower crustal amphibolite and that (2) the medium-K CA basalts to andesites formed by mixing of mantle-derived basalt and crustal TH melts. To investigate further the origin of the 'primary' low-K TH and medium-K CA basalts, we investigated basalts and andesites from Azuma volcano. Azuma is a Quaternary eruption center at the volcanic front in the NE Japan arc that has erupted two types of basalt: (1) radiogenic-Sr (⁸⁷Sr/ ⁸⁶Sr=0.7058-0.7062) low-K TH basalt lavas without evidence of magma mixing and assimilation; (2) unradiogenic-Sr (⁸⁷Sr/⁸⁶Sr=0.7039-0.7041) medium-K CA basalt lavas with subtle evidence for magma mixing. Associated intermediate lavas are voluminous and are all (3) mildly radiogenic-Sr (⁸⁷Sr/⁸⁶Sr=0.7044-0.7055) medium-K andesites, all ofwhich have CA affinities with evidence for rigorous magma mixing but no crustal assimilation. The low-K TH basalt has an isotopic composition similar to that of crustal granitoids beneathAzuma and has a composition indicating that it potentially formed from a high-degree lower crustal amphibolite melt. The medium-KCA basalt has a basaltic groundmass with Mg-rich olivine (Fo₈₉) and calcic plagioclase phenocrysts (An₉₀) and the most unradiogenic Sr (⁸⁷Sr/ ⁸⁶Sr=0.7037-0.7038), suggesting that it originated from a primary mantle melt. Major and trace elementmicroanalysis of the basaltic groundmass indicates that the primary magma composition iscloseto high-K. We conclude that the mantle-derived basalt at Azuma is the result of a high- to medium-K magma that was later mixed with a low-K TH basalt melt from the amphibolitic lower crust to form medium-K CA basalts and andesites. This supports the view of a lower crustal origin of the low-KTH basalts and simultaneously requires a reappraisal of the origin of theacross-arc variation in K contents of the mantle-derived primary arc basalts, as the high- to medium-K CA basalt is geochemically fairly similar to the high-K rear-arc basalt in the NE Japan arc.

DOI： 10.1093/petrology/egs065

記述言語：英語   掲載種別：研究論文（学術雑誌）  

[1] The Sangihe arc, northern Indonesia, is an oceanic arc that is the western half of the only active arc - arc collision on Earth. To elucidate magma genesis and slab thermal structure in such a setting, we have obtained new geochemical data for lavas from the entire Sangihe arc. In the southern arc, the volcanic front lavas are enriched in fluid-mobile elements, while the rear arc lavas are more enriched in melt-mobile elements. The proportion of sediment versus altered oceanic crust in the slab component is only ∼20% but still larger in Sangihe than other arcs in the western Pacific such as Izu, suggesting more subduction of the thick sediments in the narrowing Molucca Sea. The slab component changes in character across the arc from low-temperature fluid, through high-temperature fluid, to partial melt during progressive subduction. The geochemical systematics, the estimated mass fraction of the slab component, and the inferred stability of accessory rutile, zircon, and phengite in the slab are all similar between the southern Sangihe and Izu arcs, indicating that the thermal structure of the slab is not affected by impending collision. In contrast, lavas from the dormant northern Sangihe arc show geochemical characteristics similar to the Quaternary rear arc rather than the Quaternary volcanic front lavas in the southern arc. This may be related to advanced collision in the northern arc that could have slowed the subduction rate and heated the slab in the Pliocene followed by cessation of volcanic activity in the Quaternary.

DOI： 10.1029/2012GC004346

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The peri-Arabian ophiolite belt, from Cyprus in the west, eastward through Northwest Syria, Southeast Turkey, Northeast Iraq, Southwest Iran, and into Oman, marks a 3000km-long convergent margin that formed during a Late Cretaceous (ca 100Ma) episode of subduction initiation on the north side of Neotethys. The Zagros ophiolites of Iran are part of this belt and are divided into Outer (OB) and Inner (IB) Ophiolitic Belts. We here report the first Nd-Hf isotopic study of this ophiolite belt, focusing on the Dehshir ophiolite (a part of IB). Our results confirm the Indian mid-oceanic ridge basalt (MORB) mantle domain origin for the Dehshir mafic and felsic igneous rocks. All lavas have similar Hf isotopic compositions, but felsic dikes have significantly less-radiogenic Nd isotopic compositions compared to mafic lavas. Elevated Th/Nb and Th/Yb in felsic samples accompany nonradiogenic Nd, suggesting the involvement of sediments or continental crust.

DOI： 10.1111/j.1440-1738.2012.00815.x

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Shatsky Rise consists of thick (∼30km 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 (<200MPa). Model calculations of immobile trace elements estimate that the normal type basalt can be formed by ∼15% melting of a depleted mantle source in the presence of residual garnet. This degree of melting is similar to N-MORB, but the larger effect of residual garnet during petrogenesis implies that a greater depth of melting.

DOI： 10.1029/2012GC004235

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The Sorachi-Yezo belt, central Hokkaido, Japan, is composed of voluminous tholeiitic basaltic volcanics, and has been thought to be accreted fragments of an oceanic plateau formed in the Late Jurassic Pacific Ocean. Picrites have been reported as pillow lava and hyaloclastite from the Sorachi-Yezo belt. These picrites are characterized by very magnesian olivine phenocrysts (up to Fo [=100* Mg/ (Mg+Fe)] = 94.1), which indicate that the primary magma was unusually Mg rich. The estimated MgO of the primary magma and the mantle potential temperature are as high as 29 wt% and 1700 °C, respectively, comparable to those of the Neoarchean komatiites, and higher than those of the Gorgona komatiites and picrites. The rare earth element patterns of the Sorachi-Yezo picrites are divided into two groups that are chemically akin to the Neoarchean komatiites and Gorgona komatiites and picrites, indicating different melting regimes in an extremely hot mantle plume. The Sorachi-Yezo picrites provide evidence for extremely high temperature magmatism, like that of Archean komatiite, caused by melting of the hottest mantle plume among the Phanerozoic oceanic large igneous provinces.

DOI： 10.1130/G32752.1

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The Miocene Kaikomagatake pluton is one of the Neogene granitoid plutons exposed in the Izu Collision Zone, which is where the juvenile Izu-Bonin oceanic arc is colliding against the mature Honshu arc. The pluton intrudes into the Cretaceous to Paleogene Shimanto accretionary complex of the Honshu arc along the Itoigawa-Shizuoka Tectonic Line, which is the collisional boundary between the two arcs. The pluton consists of hornblende-biotite granodiorite and biotite monzogranite, and has SiO ₂ contents of 68-75 wt%. It has high-K series compositions, and its incompatible element abundances are comparable to the average upper continental crust. Major and trace element compositions of the pluton show well-defined chemical trends. The trends can be interpreted with a crystal fractionation model involving the removal of plagioclase, biotite, hornblende, quartz, apatite, and zircon from a potential parent magma with a composition of ~68 wt% SiO ₂. The Sr isotopic compositions, together with the partial melting modeling results, suggest that the parent magma is derived by ~53% melting of a hybrid lower crustal source comprising ~30% Shimanto metasedimentary rocks of the Honshu arc and ~70% K-enriched basaltic rocks of the Izu-Bonin rear-arc region. Together with previous studies on the Izu Collision Zone granitoid plutons, the results of this study suggest that the chemical diversity within the parental magmas of the granitoid plutons reflects the chemical variation of basaltic sources (i. e., across-arc chemical variation in the Izu-Bonin arc), as well as a variable contribution of the metasedimentary component in the lower crustal source regions. In addition, the petrogenetic models of the Izu Collision Zone granitoid plutons collectively suggest that the contribution of the metasedimentary component is required to produce granitoid magma with compositions comparable to the average upper continental crust. The Izu Collision Zone plutons provide an exceptional example of the transformation of a juvenile oceanic arc into mature continental crust.

DOI： 10.1007/s00410-011-0689-1

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Combined Pb-Sr-Nd-Hf-Os isotopes, together with major and trace element compositions, were determined from clinopyroxene and olivine phenocrysts, along with whole rocks, for ocean island basalts with high μ(μ = ²³⁸U/²⁰⁴Pb) (HIMU) and enriched mantle isotopic characteristics from Cook-Austral Islands. Clinopyroxene and olivine separates record reliable isotopic information of the sources because of minimized in situ radiogenic ingrowth and their lower susceptibility to crustal contamination. Coherent isotopic systematics in multi-isotope spaces defined by the HIMU samples are best explained by recent mixing of melts derived from the HIMU reservoir and the local shallow mantle. The isotopic compositions of the HIMU reservoir are constrained to be low ε_Nd (≤+4), low ε_Hf (≤+3), and moderately radiogenic ¹⁸⁷Os/ ¹⁸⁸Os (0.14-0.15) in association with radiogenic Pb isotopes ( ²⁰⁶Pb/²⁰⁴Pb ≥21.5). Since ancient oceanic crust would have had exceptionally radiogenic ¹⁸⁷Os/¹⁸⁸Os, moderately high ¹⁸⁷Os/¹⁸⁸Os precludes recycled oceanic crust as the only contributor to the HIMU reservoir. Instead, mantle metasomatized with partial melts from subducted oceanic crust is a likely candidate for the HIMU reservoir. Moreover, partial melting of oceanic crust in equilibrium with Mg perovskite would fractionate U/Pb, Sm/Nd, and Lu/Hf, which are in accordance with the time-integrated U/Pb, Sm/Nd, and Lu/Hf deduced from Pb, Nd, and Hf isotopic compositions of the HIMU reservoir, respectively, with a formation age of 2-3 Ga. We thus propose that the HIMU reservoir was formed by hybridization of a subducted oceanic crust-derived melt with the ambient mantle and then stored for several billion years in the lower mantle.

DOI： 10.1029/2010GC003252

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The Os isotopic system is a potential tracer of a recycled crustal component in mantle or volcanic rocks because of the significant contrast between Os isotope ratios of crust and mantle. However, the potential for crustal contamination overprinting this possible signal has hampered the utility of Os isotopic ratios. We explored the use of Os isotopes in chromian spinel (Cr-spinel) as a discriminator of primitive magma Os compositions in the Izu-Bonin arc, using beach sands as composite samples of the boninite and tholeiite magmas in three different islands. Cr-spinel is an early-stage crystal that preserves its isotopic composition even during later crustal contamination of the bulk rock. We found highly unradiogenic Os isotopic compositions in Cr-spinels from boninites, suggesting that they represent primitive magmas with slight or no Os contribution from the subducting slab during the generation of boninites in the infant arc stage (48-46 Ma). Conversely, the radiogenic Os isotopic ratios in Cr-spinels from tholeiites most likely refl ect the contribution from a slab-derived component, because more oxidative conditions in the subarc mantle probably allowed Os to mobilize from the subducting slab during the transitional arc stage (45-41 Ma). Although shallow-level assimilation of crustal components with radiogenic Os may overprint the original Os signature of tholeiite magma during its ascent, Cr-spinel allows us to compensate such possibilities.

DOI： 10.1130/G32044.1

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Early Palaeoproterozoic (2.5ĝ€"2.0 billion years ago) was a critical phase in Earth's history, characterized by multiple severe glaciations and a rise in atmospheric O 2 (the Great Oxidation Event). Although glaciations occurred at the time of O 2 increase, the relationship between climatic and atmospheric transitions remains poorly understood. Here we report high concentrations of the redox-sensitive element Os with high initial 187 Os/ 188 Os values in a sandstoneĝ€"siltstone interval that spans the transition from glacial diamictite to overlying carbonate in the Huronian Supergroup, Canada. Together with the results of Re, Mo and S analyses of the sediments, we suggest that immediately after the second Palaeoproterozoic glaciation, atmospheric O 2 levels became sufficiently high to deliver radiogenic continental Os to shallow-marine environments, indicating the synchronicity of an episode of increasing O 2 and deglaciation. This result supports the hypothesis that climatic recovery from the glaciations acted to accelerate the Great Oxidation Event.

DOI： 10.1038/ncomms1507

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We constructed a new age model based on high-time-resolution ¹⁴C data sets from three sediment cores from Academician ridge, Lake Baikal, for reconstruction of environmental and biological changes in southern Siberia during the last ca. 30 kyr. We used ¹⁴C ages of total organic carbon (TOC) for the model, because terrestrial plant residues and biogenic carbonate were not observed in the sediments. For accurate dating and age models based on ¹⁴C ages of TOC, the freshwater ¹⁴C reservoir effect and the effect of dead carbon from land-derived organic materials must be estimated. In this study, we estimated the correction factor for these effects to be 2100 ± 90 yr, on the basis of a key layer, the "¹⁴C plateau", caused by changes in the atmospheric ¹⁴C concentration during the Younger Dryas (YD) cooling event. The new age scale, along with the TOC mass accumulation rate (MAR_TOC) and stable carbon isotope ratio in the sediment cores, clearly indicate a rapid decrease in lake productivity and reduced influx of terrestrial organic materials into the lake during the YD (12.8-11.6 cal ka BP). Productivity was high (MAR_TOC, up to 19.7 mg/cm²·kyr) in and around Lake Baikal during 9.3-6.4 cal ka BP (Holocene climate optimum). Moreover, paleoproductivity changes during the last ca. 30 kyr in and around the Lake Baikal were clearly associated with fluctuations in the East Asian monsoon intensity, as inferred from the δ¹⁸O record from Sanbao and Hulu caves, China, during the late Quaternary (Wang et al., 2008. Nature 451, 1090-1093).

DOI： 10.1016/j.epsl.2009.06.046

記述言語：英語   掲載種別：研究論文（学術雑誌）  

High-time resolution ¹⁴C dating of Lake Baikal sediment cores indicates negative and positive anomalies of calculated linear sedimentation rate (LSR; 1.1 and 35.6 cm/ka, respectively) during the period of climate transition from the last glacial to Holocene. The timing of the Lake Baikal apparent LSR anomalies is consistent with that of the changes in the atmospheric radiocarbon concentration (Δ¹⁴C) during Younger Dryas rapid cooling event. ¹⁴C dating of lipids in the Lake Baikal surface sediments revealed that the sources of sedimentary lipids were different in each basin. In the Northern Basin of Lake Baikal, the ¹⁴C age of total lipids from the surface sediment (4.0 ¹⁴C ka) was found to be older than that of TOC (1.6 ¹⁴C ka). By contrast, the ¹⁴C age of total lipids in the Southern Basin was younger than that of the TOC by ca. 0.7-3.0 ka. In the Lake Hovsgol sediment cores, ages of the main lithologic boundaries during the last glacial-interglacial transition were estimated based on new ¹⁴C data sets. TOC concentration in the cores started to rapidly increase at 13.8 ± 0.3 ¹⁴C ka at the base of the basinwide finely laminated layer deposited during Bølling/Allerød. The base of the layer diatomaceous mud corresponds to the end of Younger Dryas event (10.6 ± 0.1 ¹⁴C ka).

DOI： 10.1016/j.quaint.2009.02.002

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We examined seven ultramafic xenoliths from 1~3 Ma alkali olivine basalt reefs near the Eurasian continent and one sample of the host alkali basalt to identify the mantle wedge material and to constrain the origin and evolution of mantle beneath SW Japan. Six xenoliths are from Kurose and one xenolith is from Takashima, northern part of the Kyushu islands, SW Japan. The Sr and Nd isotopic ratios vary from 0.70416 to 0.70773 and from 0.51228 to 0.51283, respectively. The Kurose and Takashima xenoliths have higher Sr isotopic ratios and lower Nd isotopic ratios than those of the peridotite xenoliths from the other arc settings such as Simcoe and NE Japan. The Kurose xenoliths have less radiogenic Os isotopic ratios (¹⁸⁷Os/¹⁸⁸Os = 0.123-0.129) than the primitive upper mantle (PUM) estimate and limited variation compared to the other arc xenoliths. Their Os isotope compositions are rather similar to the ultramafic xenoliths from NE and east China. In addition, the samples of the Kurose and Takashima xenoliths plot along a mixing line between ultramafic xenoliths from SE and NE China and a slab component in Sr-Nd-Os isotopic space. Our results suggest that fragments of continental lithospheric mantle from the China craton may exist beneath Kurose and Takashima after the Sea of Japan expansion when the Japanese islands were rifted away from the Eurasian continent during Miocene. Later magmatism due to subduction of the Philippine Sea Plate beneath the SW Japan arc around 15 Ma ago may have introduced fluids or melts derived from slab component, interpreted to be oceanic sediments rather than altered oceanic crust, that possibly modified the original composition of the lithospheric mantle sampled by the peridotite xenoliths from Kurose and Takashima.

DOI： 10.1016/j.lithos.2006.07.014

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The isotopic compositions of Os, Nd, and Sr, as well as major elements of dunites, spinel lherzolites, websterite, and serpentinite from the Nikubuchi ultramafic cumulous complex in the Sambagawa metamorphic belt (SMB) were investigated. The Nikubuchi ultramafic complex is a layered cumulate and is surrounded by a metagabbro massif in the SMB. We obtained a Sm-Nd whole-rock isochron of the Nikubuchi complex with an age of 138 ± 18 (2σ) Ma and an initial ¹⁴3Nd/¹⁴⁴Nd ratio of 0.51270 ± 3. The Nikubuchi complex contains well preserved protogranular textures and cumulous layering structures, and has good correlation coefficients among the major element abundances, which indicate an event before the Sambagawa metamorphism. The ε_Nd values of 138 Ma (+3.5 to +5.3) correspond to the isotope signatures of ocean island basalt (OIB) or island are basalt (IAB), and not mid-ocean ridge basalt (MORB). Taking the results of previous studies into consideration, the most likely origin of the Nikubuchi complex is that of a fragmented block of residual cumulate formed by the OIB magma chamber. The Rb-Sr isotope system has been disturbed by hydration and/or metamorphism after 138 Ma, and the Re-Os isotope system also has no valid isochron. One of the possible causes is that rhenium addition by hydration has occurred subsequent to 60 Ma. The results of the isotope systems show that hydration can cause disturbance in Re-Os systems but not in Sm-Nd systems. This probably reflects differences between these systems in resistance of the host phase.

DOI： 10.2343/geochemj.40.135

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Platinum group elements (PGE), gold, and rhenium are the siderophiles which show contrasting geochemical distribution with lithophile elements such as rare earth elements. The PGE abundances are generally very low on the order of ppb or less in felsic rocks, and are relatively high in ultramafic rocks. The PGE pattern is the plot of abundances of PGE and Au, normalized by CI-chondrite value or primitive mantle value, in descending order of melting point, i.e., Os, Ir, Ru, Rh, Pt, Pd, and Au. The pattern shows characteristic varieties on their rock species or tectonic settings. The PGE and Au can be classified into two sub-groups on the basis of their geochemical behavior: The Ir group (I-PGE; Os, Ir, and Ru) and the Pd group (P-PGE; Rh, Pt, Pd, and Au). Compared with the I-PGE, the P-PGEs tend to be disturbed during metasomatism or hydrothermal events. Investigation of the PGE behavior in ultramafic xenoliths and their host rocks would clarify the interactions which xenoliths have experienced in upper mantle materials or during the uplift.

DOI： 10.2465/gkk.34.195

開催年月日： 2018年12月

記述言語：英語   会議種別：口頭発表（一般）  

国名：アメリカ合衆国  

開催年月日： 2018年9月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：琉球大学   国名：日本国  

開催年月日： 2018年5月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：幕張メッセ   国名：日本国  

開催年月日： 2017年9月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：東京工業大学   国名：日本国