研究者詳細 - 中村　栄三

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Experimental Study of the Slab-mantle Interaction and Implications for the Formation of Titanoclinohumite at Deep Subduction Zone

Yoshiyuki Iizuka, Eizo Nakamura

Proceedings of the Japan Academy, Series B 71 ( 6 ) 159 - 164 1995年

記述言語：英語

In order to examine the behavior of elements under the control of aqueous fluid through slab-mantle interactions, a series of high-pressure and high-temperature experiments was carried out at subsolidus temperature, 850°C. The starting materials consist of two portions: metabasite from blueschist as a slab material and olivine as a mantle material. The metabasite was directly placed on the olivine and sealed within an Au tube. At 3 and 6 GPa corresponding to 100 and 200 km in depth, respectively, the slab material was changed from blueschist to rutile-bearing eclogite. Observed phases in the mantle side were primary olivine and enstatite formed by the reaction between olivine and Si02-rich aqueous fluid from the slab side. At 8 GPa, the slab material was further transformed to rutile-free eclogite, and phases occurred in the mantle side were titanoclinohumite in addition to olivine and enstatite. The titanoclinohumite was formed by the reaction between olivine and Ti-bearing aqueous fluid which was supplied from the slab side. The titanoclinohumite is essentially identical in chemical composition to those principally occurring in kimberlite xenoliths. Because the Ti-phase is a host reservoir of HFSE, the formation of titanoclinohumite at 8 GPa corresponding to 250 km in depth suggests that the aqueous fluid can be a carrier of HFSE from the subducting slab to the upper mantle, and thus the source region of kimberlite magma could be impregnated by the interaction between mantle peridotite and the HFSE-bearing aqueous fluid from the subducting slab. © 1995, The Japan Academy. All rights reserved.

DOI： 10.2183/pjab.71.159

SmNd garnet ages from the Uluguru granulite complex of Eastern Tanzania: further evidence for post-metamorphic slow cooling in the Mozambique belt

M. A.H. Maboko, E. Nakamura

Precambrian Research 74 ( 4 ) 195 - 202 1995年

記述言語：英語

Two samples from the Uluguru granulite complex yield garnet SmNd ages of 633 ± 7 and 618 ± 16 Ma, similar to previously published hornblende 40Ar39Ar and KAr ages. The similarity of the SmNd to the KAr age suggests that the closure temperature of garnet to Nd diffusion is similar to that of hornblende to Ar diffusion. Assuming that published zircon UPb ages of about 700 Ma date peak granulite-facies metamorphism, a mean post-metamorphic cooling rate of 2-3°C/Ma can be calculated for the time interval 700 to 630 Ma. Such slow cooling rates imply thermal relaxation with a thickness length-scale greater than the thickness of average continental crust. This, in turn, implies that the thermal perturbation responsible for metamorphism was preceded by regional crustal thickening probably in a collisional orogen. © 1995.

DOI： 10.1016/0301-9268(95)00008-S

GEOCHRONOLOGY OF ALKALI VOLCANISM IN OKI-DOGO ISLAND, SOUTHWEST JAPAN - GEOCHEMICAL EVOLUTION OF BASALTS RELATED TO THE OPENING OF THE JAPAN SEA

K UTO, E TAKAHASHI, E NAKAMURA, KANEOKA, I

GEOCHEMICAL JOURNAL 28 ( 6 ) 431 - 449 1994年

記述言語：英語出版者・発行元：GEOCHEMICAL SOC JAPAN

Geochronological and geochemical studies were performed on alkali volcanic rocks from Oki-Dogo Island, locating about 60 km off the coast of Southwest Japan inside the Japan Sea. There are big differences both in age and in geochemistry between the oldest alkali rocks and the younger rocks; the former having significant Nb- and Ta-deletion and Ba- and K2O-enrichment erupted at 19-18 Ma when the opening of the Japan Sea was ongoing, while the latter with no Ta-depletion were active between 5.5-0.6 Ma considerably after the cessation of the rifting activity. Syn-extensional rocks should not have erupted in the active subduction zone but are probably originated from or at least contaminated by the lithosphere beneath the early Proterozoic crust. The lithosphere is considered to have suffered mantle metasomatism to form phlogopite selectively enriched in Ba and K2O by fluids, which had been released from previous subductions at the Asian continental margin. Partial meting of such lithospheric mantle during the active rifting was essential for the generation of Ba- and K2O-enriched alkali basalt magmas. The younger basaltic rocks are probably originated either from the asthenosphere or the deeper plume mantle. A suite of volcanic rocks from mugearite through trachyte to alkali rhyolite gave concordant ages of 5.5-5.4 Ma. These ages suggest that a previously reported Rb-Sr whole rock age of 6.8 +/- 0.3 Ma does not represent the age of eruption and is probably disturbed by magma mixing between basic and acidic end members.

DOI： 10.2343/geochemj.28.431

Geochronology of alkali volcanism in Oki-Dogo Island, Southwest Japan: Geochemical evolution of basalts related to the opening of the Japan Sea

Kozo Uto, Eiichi Takahashi, Eizo Nakamura, Ichiro Kaneoka

GEOCHEMICAL JOURNAL 28 ( 6 ) 431 - 449 1994年

記述言語：英語

Geochronological and geochemical studies were performed on alkali volcanic rocks from Oki-Dogo Island, locating about 60 km off the coast of Southwest Japan inside the Japan Sea. There are big differences both in age and in geochemistry between the oldest alkali rocks and the younger rocks
the former having significant Nb- and Ta-deletion and Ba- and K2O-enrichment erupted at 19–18 Ma when the opening of the Japan Sea was ongoing, while the latter with no Ta-depletion were active between 5.5-0.6 Ma considerably after the cessation of the rifting activity. Syn-extensional rocks should not have erupted in the active subduction zone but are probably originated from or at least contaminated by the lithosphere beneath the early Proterozoic crust, The lithosphere is considered to have suffered mantle metasomatism to form phlogopite selectively enriched in Ba and K2O by fluids, which had been released from previous subduc-tions at the Asian continental margin. Partial meting of such lithospheric mantle during the active rifting was essential for the generation of Ba- and K2O-enriched alkali basalt magmas. The younger basaltic rocks are probably originated either from the asthenosphere or the deeper plume mantle. A suite of volcanic rocks from mugearite through trachyte to alkali rhyolite gave concordant ages of 5.5-5.4 Ma. These ages suggest that a previously reported Rb-Sr whole rock age of 6.8 ± 0.3 Ma does not represent the age of eruption and is probably disturbed by magma mixing between basic and acidic end members. © 1994, GEOCHEMICAL SOCIETY OF JAPAN. All rights reserved.

DOI： 10.2343/geochemj.28.431

Origin of the slab component in arc lavas from across-arc variation of B and Pb isotopes

Tsuyoshi Ishikawa, Eizo Nakamura

Nature 370 ( 6486 ) 205 - 208 1994年

記述言語：英語

At convergent margins, the subducting oceanic slab is thought to dehydrate, producing fluids which metasomatize the overlying mantle wedge where island-arc magma forms. However, the nature and origin of the metasomatizing fluid, its source composition and its relation to the genesis of the chemical characteristics of arc magmas are largely controversial. Across-arc variation in the chemistry of arc lavas provides a useful key to this problem, because it may reflect the changes in the physical conditions of the subducting slab that control mass transfer from slab to mantle wedge as a function of depth. Here we report clear across-arc variations in the concentrations and isotopic compositions of boronand lead observed in lavas from the Izu arc (Japan). Our data suggest that a homogeneous slab fluid contributes to all Izu volcan-oes, but that the amount of this fluid decreases continuously with increasing depth of the subducting slab. Whereas the Izu slab fluid conies primarily from altered oceanic crust, our data for high-Mg andesites from the Setouchi volcanic belt (a nearby fore-arc) indicate a significant involvement of sediment in the fluid source. © 1994 Nature Publishing Group.

DOI： 10.1038/370205a0

Boron isotope systematics of marine sediments

Tsuyoshi Ishikawa, Eizo Nakamura

Earth and Planetary Science Letters 117 ( 3-4 ) 567 - 580 1993年

記述言語：英語

Boron contents and boron isotopic compositions were determined for modern and ancient (Permian to Miocene) marine sediments, including pelagic clay, calcareous ooze, siliceous ooze and neritic clay sediments. δ11B values of modern marine sediments range from -6.6 to +4.8‰. Isotopic variation is controlled by the simple mixing of four major constituents, detritus of continental origin, marine smectite, biogenic carbonates and biogenic silica. Detritus of continental origin, with an average δ11B value of -13 to -8‰, is the low-δ11B end-member constituent of marine sediments and its boron is largely controlled by the concentration of illite which originates from wind or fluvial transport. Marine smectite, biogenic carbonate and biogenic silica, on the other hand, represent the high-δ11B end members, with δ11B values of +2.3 to +9.2‰, +8.0 to +26.2‰ and +2.1 to +4.5‰, respectively. These high δ11B values are the result of the equilibrium uptake of boron from seawater. Spatial variations in boron isotopes in the Pacific sediments are essentially due to the distribution of the above four constituents. Although ancient argillaceous sediments (shale and slate) have boron contents that are identical with those of modern equivalents, boron contents of limestone and chert are distinctly lower than those of modern calcareous and siliceous oozes. Ancient marine sediments have systematically lower δ11B values (-17.0 to -5.6‰) than those of the modern sediments. The lower δ11B values can be caused by diagenesis, which induces (1) preferential removal of high-δ11B boron in calcium carbonate and silica during recrystallization and (2) boron isotopic exchange in the course of the smectite/illite transition. The observed boron isotopic compositions of ancient argillaceous sediments are distinctly different from those of fresh and altered MORB. Therefore, boron isotope systematics will be useful in identifying components from the descending oceanic slab involved in the formation of island arc magma and in investigating mantle-crust recycling through subduction processes. © 1993.

DOI： 10.1016/0012-821X(93)90103-G

Precise lithium isotopic analysis by thermal ionization mass spectrometry using lithium phosphate as an ion source material

Takuya Moriguti, Eizo Nakamura

Proceedings of the Japan Academy, Series B 69 ( 6 ) 123 - 128 1993年

記述言語：英語

: A high-precision Li isotope analysis was developed for determining the Li+ ion emitted from lithium phosphate as an ion source material by a Re double-filament ionization method in the thermal ionization mass spectrometry. In this method, Li isotopic fractionation is distinctly less sensitive to the filament temperature than those in the previous methods, and stable and high ion beam intensity of more than 8 × 10-11 A for 7Li is obtained. These advantages in determining the 7Li/6Li ratio result in the analytical reproducibility of 0.26%o (1σ). Furthermore, the sample preparation is simple and the low temperature (850°C) required by this method ensures the analysis of the isotopic composition of small amounts of Li with less influence of Li contamination to the sample, compared with the previous methods. © 1993, The Japan Academy. All rights reserved.

DOI： 10.2183/pjab.69.123

Precise isotope determination of trace amounts of Sr in Magnesium-rich samples.

芳川雅子, 中村栄三

岩鉱 88,KJ,-KLU ( 12 ) 548 - 561 1993年

記述言語：英語出版者・発行元：Japan Association of Mineralogical Sciences

H⁺型とピリジン型陽イオン交換クロマトグラフィによる化学分離法を用いることで,超苦鉄質岩やその構成鉱物といったMgに富む試料の,ナノグラム単位のSrの単離を可能にした。総溶離液量はわずか24mlで,この結果,化学分離時のブランクはSrが32pg,Rbが25pgにまで減少した。タンタル酸イオン化活性材を用いた静的マルチコレクター質量分析法で,超苦鉄質岩標準岩石試料PCC-1から単離した10-20 ngのSr試料が,分析精度<0.005%,再現性<0.006%で測定可能にした。同位体希釈法を用いて得られたPCC-1のSr同位体比は,スパイクを加えずに得た値と分析誤差範囲内で一致し,分析精度や再現性もほぼ同じであった。また,ナノグラム単位のRb/Sr比の再現性は2%であった。以上の方法は,Mgに富み微量のSr・Rb試料しか得られない試料のSr同位体比,Rb・Sr濃度を求めるのに有効であり,10 ngという微量Sr試料で従来(>1μg)と同程度の信頼性ある結果を得ることが可能となった。

DOI： 10.2465/ganko.88.548

Precise isotope determination of trace amounts of Sr in magnesium-rich Samples

芳川雅子, 中村栄三

Journal of Mineralogy, Petrology and Economic Geology 88,KJ,-KLU ( 12 ) 548 - 561 1993年

記述言語：英語出版者・発行元：Japan Association of Mineralogical Sciences

H⁺型とピリジン型陽イオン交換クロマトグラフィによる化学分離法を用いることで,超苦鉄質岩やその構成鉱物といったMgに富む試料の,ナノグラム単位のSrの単離を可能にした。総溶離液量はわずか24mlで,この結果,化学分離時のブランクはSrが32pg,Rbが25pgにまで減少した。タンタル酸イオン化活性材を用いた静的マルチコレクター質量分析法で,超苦鉄質岩標準岩石試料PCC-1から単離した10-20 ngのSr試料が,分析精度<0.005%,再現性<0.006%で測定可能にした。同位体希釈法を用いて得られたPCC-1のSr同位体比は,スパイクを加えずに得た値と分析誤差範囲内で一致し,分析精度や再現性もほぼ同じであった。また,ナノグラム単位のRb/Sr比の再現性は2%であった。以上の方法は,Mgに富み微量のSr・Rb試料しか得られない試料のSr同位体比,Rb・Sr濃度を求めるのに有効であり,10 ngという微量Sr試料で従来(>1μg)と同程度の信頼性ある結果を得ることが可能となった。

DOI： 10.2465/ganko.88.548

Boron isotope geochemistry of the oceanic crust from DSDP/ODP Hole 504B

Tsuyoshi Ishikawa, Eizo Nakamura

Geochimica et Cosmochimica Acta 56 ( 4 ) 1633 - 1639 1992年

記述言語：英語

Boron contents and boron isotopic compositions were determined for the uppermost 1.3 km section of typical 6.2 Ma oceanic crust from DSDP/ODP Hole 504B, Costa Rica Rift, Galapagos Spreading Center. Both the boron content and the δ11B value in the oceanic crust are controlled by two types of alteration: 1. (1) low-temperature alteration (0 to 60°C
Zones I and II) and 2. (2) high-temperature hydrothermal alteration (200 to 400°C
Zones III and IV). Basalts subjected to the low-temperature alteration are characterized by their relatively high boron contents (0.69 to 19.3 ppm) and high δ11B values (+2.2 to +10.6%.), indicating uptake of boron into secondary phases in equilibrium with seawater or evolved seawater. Hydrothermally altered basalts contain less abundant boron (0.17 to 0.52 ppm) and relatively constant δ11B values (-0.1 to +1.0%.). Although basalts from the upper part of these hydrothermal zones (&lt
1300 mbsf) show equilibrated boron content and δ11B value with aqueous fluid, effective leaching of boron from basalt is predominant in the lower part (&gt
1300 mbsf). Original boron content and δ11B value of the Hole 504B MORB were 0.35 ppm and +0.2%., respectively. The present data provide fundamental information in understanding of the distribution of boron and boron isotopes in the oceanic crust. © 1992.

DOI： 10.1016/0016-7037(92)90230-G

22. Interaction between Subducted Oceanic Slab and Wedge Mantle Inferred from Across-arc Variations of Pb, Sr and Nd Isotopic Compositions in Northeastern Japan

Tomovuki Shibata, Eizo Nakamura

Proceedings of the Japan Academy Series B: Physical and Biological Sciences 67 ( 7 ) 115 - 120 1991年

記述言語：英語

Isotopic compositions of Pb, Sr and Nd have been determined for island arc basalts from northeastern Japan. The Sr and Pb isotopic ratios decrease, and Nd isotopic ratios increase from the volcanic front towards the back-arc side. The isotopic compositions at the most back-arc side are nearly identical to those of MORB, indicating that the across-arc isotopic variations were caused by a two-component mixing between MORB-type wedge mantle and oceanic sediments with different degrees of sediment involvement which depend on the depth of subducting oceanic slab. The abundance ratios among Pb, Sr and Nd in the subducted oceanic sediment were estimated from the mixing relationships in the isotope compositions. The estimated Sr/Pb ratio of subducted sediments does not change during subduction, while the Nd/Sr and Nd/Pb increase towards the back-arc side. These observations suggest that the subduction component was added to the wedge mantle as a fluid through the dehydration of subducted oceanic sediment and the degree of dehydration decreases with increase in the depth of slab, and that the subducting oceanic slab does not melt beneath northeastern Japan. © 1991, The Japan Academy. All rights reserved.

DOI： 10.2183/pjab.67.115

Suppression of Boron Volatilization from a Hydrofluoric Acid Solution Using a Boron-Mannitol Complex

Tsuyoshi Ishikawa, Eizo Nakamura

Analytical Chemistry 62 ( 23 ) 2612 - 2616 1990年12月

記述言語：英語

Volatilization and mass fractionation of boron during evaporation of the hydrofluoric and hydrochloric acid solutions were investigated with varying mannitol/boron ratios. The degree of volatilization and mass fractionation decreases with increasing mannitol/boron mole ratio, and the boron volatilization is completely suppressed when the ratio is more than unity. These results indicate that the final stable compound is an equimolar complex of boron and mannitol. The formation of this complex in the acid solutions allows the use of hydrofluoric and hydrochloric acids for the dissolution of silicate rock samples and for the subsequent chemical separation of boron from the samples adopting anion-exchange chromatography in F− form. © 1990, American Chemical Society. All rights reserved.

DOI： 10.1021/ac00222a017

Lead isotope analyses of standard rock samples

Koide Yoshiyuki, Nakamura Eizo

Mass Spectroscopy 38 ( 5 ) 241 - 252 1990年

記述言語：英語出版者・発行元：The Mass Spectrometry Society of Japan

New results on lead isotope compositions of standard rock samples and their analytical procedures are reported. Bromide form anion exchange chromatography technique was adopted for the chemical separation of lead from rock samples. The lead contamination during whole analytical procedure was low enough to determine lead isotope composition of common natural rocks. Silica-gel activator method was applied for emission of lead ions in the mass spectrometer. Using the data reduction of "unfractionated ratios", we obtained good reproducibility, precision and accuracy on lead isotope compositions of NBS SRM. Here we present new reliable lead isotope compositions of GSJ standard rock samples and USGS standard rock, BCR-1.

DOI： 10.5702/massspec.38.241

その他リンク： https://jlc.jst.go.jp/DN/JALC/00005568604?from=CiNii

Lead isotope analyses of standard rock samples.

Koide Yoshiyuki, Nakamura Eizo

Journal of the Mass Spectrometry Society of Japan 38 ( 5 ) 241 - 252 1990年

記述言語：英語出版者・発行元：The Mass Spectrometry Society of Japan

New results on lead isotope compositions of standard rock samples and their analytical procedures are reported. Bromide form anion exchange chromatography technique was adopted for the chemical separation of lead from rock samples. The lead contamination during whole analytical procedure was low enough to determine lead isotope composition of common natural rocks. Silica-gel activator method was applied for emission of lead ions in the mass spectrometer. Using the data reduction of "unfractionated ratios", we obtained good reproducibility, precision and accuracy on lead isotope compositions of NBS SRM. Here we present new reliable lead isotope compositions of GSJ standard rock samples and USGS standard rock, BCR-1.

DOI： 10.5702/massspec.38.241

その他リンク： https://jlc.jst.go.jp/DN/JALC/00005568604?from=CiNii

K-Ar ages of basalts from the Higashi-Matsuura district, northwestern Kyushu, Japan and regional geochronology of the Cenozoic alkaline volcanic rocks in eastern Asia.

Nakamura Eizo, McDougall Ian, Campbell Ian H.

GEOCHEMICAL JOURNAL 20,91-99 ( 2 ) 91 - 99 1986年

記述言語：英語出版者・発行元：GEOCHEMICAL SOCIETY OF JAPAN

Seven new K-Ar age determinations are presented on whole rock samples from alkaline and tholeiitic basalts of the Higashi-Matsuura district, northwestern Kyushu, Japan. Ages obtained range from 2.92 ± 0.03Ma to 3.01 ± 0.04Ma; these ages are essentially identical within analytical errors and yield an average age of 2.98 ± 0.03Ma (Late Pliocene). When combined on an isochron type diagram the six Higashi-Matsuura samples give an age of 3.00 ± 0.03 Ma with the composition of nonradiogenic ⁴⁰Ar/³⁶Ar = 294.2 ± 2.0. The excellent age agreement of samples with different K contents and petrographic characteristics provides strong evidence that the tholeiitic and alkaline basalts were erupted for an extremely short period in the Higashi-Matsuura district. A basalt from Ogawashima Island yields a K-Ar age of 3.58 ± 0.04Ma. This study and previously reported data support the hypothesis that alkaline volcanic activity in southwestern Japan commenced some 10Ma ago and continued intermittently until Recent times. Systematic variations of age and distribution of Cenozoic alkaline basalts are recognized in northeastern China, Korea and southwestern Japan. It is suggested that these variations are related to the initiation of "mantle plumes" resulting from convection in the mantle wedge caused or controlled by subduction of the Kula and Pacific plates.

DOI： 10.2343/geochemj.20.91

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