研究者詳細 - 中村　栄三

MISC - 中村　栄三

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Precise determination of Pb isotope ratios by simple double spike MC-ICP-MS technique without Tl addition

Akio Makishima, B. Nagender Nath, Eizo Nakamura

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY 22 ( 4 ) 407 - 410 2007年

記述言語：英語出版者・発行元：ROYAL SOC CHEMISTRY

A double-spike multicollector ICP-MS ( DS-MC-ICP-MS) technique for Pb isotope analysis without T1 addition is established and its analytical performance is examined in detail. This simple DS-MC-ICP-MS technique using 20 ng ml(-1) gave averages of 16.9417, 15.4988 and 36.7196 with reproducibilities of 0.007, 0.008 and 0.009% ( RSD, n = 35), respectively, for Pb-206/Pb-204, Pb-207/Pb-204 and Pb-208/Pb-204 in NIST 981 solution, corresponding to consumption of 4 ng of Pb in a pair of spiked and non-spiked measurements. The average is consistent with those obtained by previous studies by DS-TIMS and DS-MC-ICP-MS with T1 addition. The reproducibility ( RSD %) of Pb-206/Pb-204, Pb-207/Pb-204 and Pb-208/Pb-204 improves from 0.26, 0.54 and 0.69% to 0.0022, 0.0030 and 0.0029% as the Pb consumption during analysis increases from 20 pg to 11 ng. The reproducibility with 11 ng is similar to or even better than those by DS-TIMS, MC-ICP-MS with T1 addition, or with DS-T1 addition. The accuracy of the simple DS-MC-ICP-MS technique for Pb, including the column chemistry, is further confirmed by analysis of actual silicate sample, JB-3.

DOI： 10.1039/b616021b

Li C. Generation of Palaeocene Adakitic Andesites by Magma Mixing, Yanji area, NE China.

Guo, F, Nakamura ,E, Fan, W, Kobayashi, K

J. Petrol. 48, 661 - 692 2007年

Purification of Hf in silicate materials using extraction chromatographic resin, and its application to precise determination of Hf-176/Hf-177 by MC-ICP-MS with Hf-179 spike

Yinghuai Lu, Akio Makishima, Eizo Nakamura

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY 22 ( 1 ) 69 - 76 2007年

記述言語：英語出版者・発行元：ROYAL SOC CHEMISTRY

A two-stage column purification method for Hf from Zr and other elements in silicate samples using the extraction chromatographic resin, UTEVA has been developed. In the first column, using an anion-exchange resin, AG 1X8, Hf, Zr and Ti were collected. In the second, using UTEVA, and after eliminating F- by evaporation with HClO4 and re-dissolution with H2O2 HNO3, Hf was purified from Ti and Zr, based on characteristic of the UTEVA resin that Hf distribution coefficient (K-d) decreases from 770 to 40, while Kd of Zr decreases less from 890 to 200, in 9 and 6 mol l(-1) HNO3, respectively. The recovery yield of Hf was B88% and total blank was similar to 4 pg. A MC-ICP-MS Hf isotope ratio determination method was also developed, which uses 179 Hf spike for simultaneous determination of the Hf concentration and Hf-176/Hf-177 ratio. Combining the chemistry and ICP-MS techniques, Hf-176/Hf-177 ratios in silicate reference materials were determined with and without the spike, which gave identical results, showing the applicability of this method.

DOI： 10.1039/b610197f

Ion microprobe zircon U-Pb dating of the late Archaean metavolcanics and associated granites of the Musoma-Mara Greenstone Belt, Northeast Tanzania: Implications for the geological evolution of the Tanzania Craton

Shukrani Manya, Katsura Kobayashi, Makenya A. H. Maboko, Eizo Nakamura

JOURNAL OF AFRICAN EARTH SCIENCES 45 ( 3 ) 355 - 366 2006年7月

記述言語：英語出版者・発行元：PERGAMON-ELSEVIER SCIENCE LTD

Ion microprobe zircon U-Pb ages from metavolcanic and associated granitic rocks of the late Archaean Musoma-Mara Greenstone Belt (MMGB) of northeast Tanzania reveal that the oldest mafic volcanism in the belt occurred at 2676-2669 Ma followed by felsic volcanism at similar to 2668 Ma. The felsic volcanism was coeval with the emplacement of the oldest pulse of massive granitoids that is dated at 2668 Ma. The youngest volcanic episode, represented by a volcanic horizon in the largely sedimentary Kavirondian Supergroup that overlies the greenstone sequence with a marked unconformity, occurred at similar to 2667 Ma. A younger phase of post-orogenic granites concluded the magmatic evolution of the MMGB at similar to 2649 Ma.
Our age data suggests that the entire volcano-sedimentary sequence in MMGB was emplaced in a relatively short time interval between similar to 2676 and similar to 2667 Ma. It also shows that contrary to arguments based on the degree of deformation, the foliated granites and some amphibolite rafts enclosed in them do not constitute the basement to the greenstone sequence. The data further shows that volcanism in the MMGB was younger than the similar to 2820 Ma age of volcanism in the Sukumaland Greenstorie Belt (SGB) to the far southwest and the similar to 2720 Ma age of volcanism in the nearby Kilimafedha Greenstone Belt (KGB) to the south. The age of granitic magmatism (ca. 2.69-2.55 Ga) in the three belts was, however, largely coeval. Granitic magmatism of this age has also been reported in different parts of the Tanzania Craton suggesting that it was responsible for the late Archaean crustal growth and marks the beginning of a period of stability (or of cratonization). (c) 2006 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jafrearsci.2006.03.004

Elemental mixing systematics and Sr-Nd isotope geochemistry of melange formation: Obstacles to identification of fluid sources to arc volcanics

Robert L. King, Gray E. Bebout, Takuya Moriguti, Eizo Nakamura

EARTH AND PLANETARY SCIENCE LETTERS 246 ( 3-4 ) 288 - 304 2006年6月

記述言語：英語出版者・発行元：ELSEVIER SCIENCE BV

We present major and trace element concentrations in conjunction with Sr-Nd isotope ratios to investigate the geochemical characteristics of melange formation along the subduction zone slab-mantle interface. Melange matrix of the Catalina Schist formed within an active subduction zone of the southern California borderland in Cretaceous time. Melange formed through the synergistic effects of deformation and metasomatic fluid flow affecting peridotite, basaltic, and sedimentary protoliths to form hybridized bulk compositions not typical of seafloor "input" lithologies. In general, all elemental concentrations primarily reflect mechanical mixing processes, while fluid flow mediates all elemental systematios to a varying extent that is largely a function of inferred "mobility" for a particular element or the stability of suitable mineral hosts. Elemental data reveal that mineral stabilities defined by the evolution of bulk composition within melange zones are probably the most important control of solid, liquid, or fluid geochemistry within the subduction system. Sr-Nd isotope ratios are highly variable and reflect contributions of melange protoliths to varying extents. A weak mechanical mixing array present in Sr isotope data is strongly overprinted by a fluid signal that dominates melange Sr systematics. Nd isotope data suggest that Nd is more conservative during metamorphism and is largely controlled by mechanical mixing. We argue that melange formation is an intrinsic process to all subduction zones and that the geochemistry of melange will impart the strongest control on the geochemistry of metasomatic agents (hydrous fluids, silicate melts, or miscible supercritical liquids) progressing to arc magmatic source regions in the mantle wedge. Melange formation processes suggest that comparisons of subduction "inputs" to arc volcanic "outputs" as a means to infer recycling at subduction zones dangerously over-simplify the physics of the mass transfer in subduction zones, as subducted mass is consistently redistributed into novel bulk compositions. Such melange zones along the slab-mantle interface simultaneously bear characteristic elemental or isotopic signals of several distinct input lithologies, while experiencing phase equilibria not typical of any input. We recommend that future studies explore the phase equilibria of hybridized systems and mineral trace element residency, as these processes provide for a physical baseline from which it will be possible to follow the path of subducted mass through the system. (c) 2006 Elsevier B.V All rights reserved.

DOI： 10.1016/j.epsl.2006.03.053

Geochemical evolution of a shallow magma plumbing system during the last 500 years, Miyakejima volcano, Japan: Constraints from U-238-Th-230-Ra-226 systematics

T Yokoyama, T Kuritani, K Kobayashi, E Nakamura

GEOCHIMICA ET COSMOCHIMICA ACTA 70 ( 11 ) 2885 - 2901 2006年6月

記述言語：英語出版者・発行元：PERGAMON-ELSEVIER SCIENCE LTD

In order to unravel magma processes and the geochemical evolution of shallow plumbing systems beneath active volcanoes, we investigated U-series disequilibria of rocks erupted over the past 500 years (1469-2000 AD) from Miyakejima volcano, Izu arc, Japan. Miyakejima volcanic rocks show U-238-Th-230-Ra-226 disequilibria with excess 238 U and Ra-226, due to the addition of slab-derived fluids to the mantle wedge. Basaltic bombs of the 2000 AD eruption have the lowest (Th-230/Th-232) ratio compared to older Miyakejima eruptives, yielding the youngest U-238-Th-230 model age of 2 kyr. This reinforces our previous model that fluid release from the slab and subsequent magma generation in the mantle wedge beneath Miyakejima occur episodically on a several-kyr timescale. In the last 500 years, Miyakejima eruptives show: (1) a vertical trend in a (Th-230/Th-232)-(U-238/Th-232) diagram and (2) a positive linear correlation in a (Ra-226/Th-230)(0)-1/Th-230 diagram, which is also observed in lavas from some of the single eruptions (e.g., 1940, 1962, and 1983 AD). The variations cannot be produced by simple fractional crystallization in a magma chamber with radioactive decay of Th-230 and Ra-226, but it is possibly produced by synchronous generation of melts in the mantle wedge with different upwelling rate or addition of multiple slab-derived fluids. A much more favorable scenario is that some basaltic magmas were intermittently supplied from deep in the mantle and injected into the crust, subsequently modifying the original magma composition and producing variations in (Th-230/Th-232) and (Ra-226/Th-230)(0) ratios via assimilation and fractional crystallization (AFC). The assimilant of the AFC process would be a volcanic edifice of previous Miyakejima magmatism. Due to the relatively short timescales involved, the interaction between the assimilant and recent Miyakejima magmatism has not been recorded by the Sr-Nd-Pb isotopic systems. In such cases, Th isotopes and (Ra-226/Th-230) ratio are excellent geochemical tracers of magmatic evolution. (c) 2006 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.gca.2006.02.027

Determination of Os and Re isotope ratios at subpicogram levels using MC-ICPMS with solution nebulization and multiple ion counting

A Makishima, E Nakamura

ANALYTICAL CHEMISTRY 78 ( 11 ) 3794 - 3799 2006年6月

記述言語：英語出版者・発行元：AMER CHEMICAL SOC

A precise and accurate determination method of Os-187/ Os-188, Os-189/Os-188, and Re-185/Re-187 ratios of down to 0.2 pg of Os and 0.08 pg of Re using multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) with multiple ion counting has been developed. Os and Re were introduced into MC-ICPMS as 0.5 mol L-1 HF solutions through the desolvator, and Re-185, Os-187, Os-188, and Os-189 ions were detected simultaneously by four channeltrons. The Os and Re ratios were determined by the standard bracketing method, in which channeltron yields and mass discrimination factors are corrected together. Os and Re memories after 800-s wash were < 0.1% and negligible, typically 0.02 and 0.03%, respectively. Isotope ratios of Os-187/ Os-188, Os-189/Os-188, and Re-185/Re-187 were constant within error in the concentration ranges of 1-100, 1-40, and 0.4-5 pg mL(-1) with reproducibility (1 sigma) of 2.7-0.14, 0.33-0.10, and 0.41-0.19%, respectively. For analysis of larger amounts of Os and Re, the Faraday cup measurement was employed. The precision and reproducibility obtained in this study are comparable to those of N-TIMS and better than MC-ICPMS achieved so far with a capability of higher sample throughput with simpler sample preparation.

DOI： 10.1021/ac060183t

Petrology and geochemistry of eclogite xenoliths from the Colorado Plateau: Implications for the evolution of subducted oceanic crust

T Usui, E Nakamura, H Helmstaedt

JOURNAL OF PETROLOGY 47 ( 5 ) 929 - 964 2006年5月

記述言語：英語掲載種別：書評論文，書評，文献紹介等出版者・発行元：OXFORD UNIV PRESS

Eclogite xenoliths from the Colorado Plateau, interpreted as fragments of the subducted Farallon plate, are used to constrain the trace element and Sr-Nd-Pb isotopic compositions of oceanic crust subducted into the upper mantle. The xenoliths consist of almandine-rich garnet, Na-clinopyroxene, lawsonite and zoisite with minor amounts of phengite, rutile, pyrite and zircon. They have essentially basaltic bulk-rock major element compositions; their Na2O contents are significantly elevated, but K2O contents are similar to those of unaltered mid-ocean ridge basalt (MORB). These alkali element characteristics are explained by spilitization or albitization processes on the sea floor and during subduction-zone metasomatism in the fore-arc region. The whole-rock trace element abundances of the xenoliths are variable relative to sea-floor-altered MORB, except for the restricted Zr/Hf ratios (36.9-37.6). Whole-rock mass balances for two Colorado Plateau eclogite xenoliths are examined for 22 trace elements, Rb, Cs, Sr, Ba, Y, rare earth elements, Pb, Th and U. Mass balance considerations and mineralogical observations indicate that the whole-rock chemistries of the xenoliths were modified by near-surface processes after emplacement and limited interaction with their host rock, a serpentinized ultramafic microbreccia. To avoid these secondary effects, the Sr, Nd and Pb isotopic compositions of minerals separated from the xenoliths were measured, yielding 0.70453-0.70590 for Sr-87/Sr-86, -3.1 to 0.5 for epsilon Nd and 18.928-19.063 for Pb-206/Pb-204. These isotopic compositions are distinctly more radiogenic for Sr and Pb and less radiogenic for Nd than those of altered MORB. Our results suggest that the MORB-like protolith of the xenoliths was metasomatized by a fluid equilibrated with sediment in the fore-arc region of a subduction zone and that this metasomatic fluid produced continental crust-like isotopic compositions of the xenoliths.

DOI： 10.1093/petrology/egi101

In-situ U-PbSIMS dating and trace element (EMPA) composition of zircon from a granodiorite porphyry in the Wushan copper deposit, China

Ding, X, SY Jiang, KD Zhao, E Nakamura, K Kobayashi, P Ni, LX Gu, YH Jiang

MINERALOGY AND PETROLOGY 86 ( 1-2 ) 29 - 44 2006年1月

記述言語：英語出版者・発行元：SPRINGER WIEN

Zircons from a granodiorite porphyry at the Wushan copper deposit in the Lower Changjiang Metallogenic Belt, east central China, were dated using a Cameca IMS 1270 secondary ion mass spectrometer (SIMS); their chemical compositions and Hf isotopes were analyzed using a JEOL JX A8800 electron microprobe (EMPA) and a Neptune LA-MC-ICP-MS, respectively. The U-Pb dating of zircon reveals two age groups for the granodiorite porphyry; i.e. 144.6 +/- 3.9 Ma and 121.0 +/- 2.5 Ma. Zircons of the two age populations display distinct chemical compositional characteristics with respect to UO2/HfO2 ratios, and show a negative correlation of (UO2 + Y2O3 + ThO2) and HfO2. The older age group of similar to 145 Ma, defined by the majority of the zircons, is interpreted as the magmatic emplacement age, whereas the younger age group of similar to 121 Ma is interpreted as rejuvenation due to subsequent thermal or hydrothermal events. Other interpretations such as presence of two distinct magmatic crystallization stages or inheritance of the older zircons are also discussed in the paper. The similar to 145 Ma magmatism at Wushan is consistent with the ages of most of the granitods in the Lower Changjiang Metallogenic Belt. The Hf isotope composition of zircons (epsilon(Hf) = -2.1 and -7.0; T-DM(Hf) ages = 0.87 and 1.05 Ga), together with other available geochemical and Sr-Nd isotope data suggest that the granodioritic magma was derived from mixing of juvenile mantle with older crustal material. Due to the close spatial and temporal relationship of the granodiorite porphyry and the skarn mineralization at Wushan, the magamatic hydrothermal mineralization likely took place between 145 Ma and 121 Ma, likely around 121 Ma.

DOI： 10.1007/s00710-005-0093-5

Elemental fractionation in lavas during post-eruptive degassing: Evidence from trachytic lavas, Rishiri Volcano, Japan

T Kuritani, E Nakamura

JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH 149 ( 1-2 ) 124 - 138 2006年1月

記述言語：英語出版者・発行元：ELSEVIER SCIENCE BV

Trachytic lavas of Rishiri Volcano, northern Japan, show a peculiar geochemical variation across lava flow units. Samples collected systematically in a vertical cross section from a lava flow unit with a thickness of about 20 in are nearly homogeneous in major element compositions. However, some trace elements, including Li, B and Cs, are considerably depleted in samples collected from the main part of the flow unit, compared to those obtained from the surface of the lava flow (clinker layer). In particular, Cs content of the main flow unit is as low as similar to 30% of the clinker layer. Br-11.B-10 ratios of samples from the main flow unit are also slightly lower than those of the clinker samples, and the isotope compositions positively correlate with boron concentrations. These geochemical variations cannot be explained by magmatic processes in magma chambers, post-eruptive weathering, or alteration process. Rather, we infer these systematics resulted from escape of these elements from the lava flow unit during post-eruptive degassing. Vapor phases in which Li, B and Cs dissolved ire suggested to have been transported through veins formed in the main flow unit as fractures due to slight,,hearing along the flow planes after lava emplacement. In the Tanetomi lava, only rocks of the clinker layer preserve original composition of magmas, although they are porous and brownish due to extensive oxidization. On the other hand, rocks of the main flow unit do not retain original magma compositions, although they are dense and grayish, and seem to tic much fresher compared to the clinkers, A similar geochemical modification of lavas can occur in other volcanic systems, especially for lavas consisting of relatively thick flow units. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jvolgeores.2005.06.008

Determination of major, minor and trace elements in silicate samples by ICP-QMS and ICP-SFMS applying isotope dilution-internal standardization (ID-IS) and multi-stage internal standardization.

Makishima, A, Nakamura, E

Geostandards and Geoanalytical Research 2006年

Carbon self-diffusion in a natural diamond

KT Koga, MJ Walter, E Nakamura, K Kobayashi

PHYSICAL REVIEW B 72 ( 2 ) 2005年7月

記述言語：英語出版者・発行元：AMER PHYSICAL SOC

We report experimentally determined self-diffusion coefficients for carbon in a type-IaA diamond at pressure and temperature conditions within diamond stability field at 10 GPa and 2075-2375 K. The activation energy of diffusion is 6.8 +/- 1.6 eV and a preexponent constant is of 4.1 x 10(-5) m(2)/s [ln(-10.1 +/- 2.0) m(2)/s]. The activation energy is approximately 30% lower than results predicted previously from ab initio calculations.

DOI： 10.1103/PhysRevB.72.024108

Assimilation and fractional crystallization controlled by transport process of crustal melt: Implications from an alkali basalt-dacite suite from Rishiri Volcano, Japan

T Kuritani, H Kitagawa, E Nakamura

JOURNAL OF PETROLOGY 46 ( 7 ) 1421 - 1442 2005年7月

記述言語：英語出版者・発行元：OXFORD UNIV PRESS

Mechanisms of fractional crystallization with simultaneous crustal assimilation (AFC) are examined for the Kutsagata and Tanetomi lavas, an alkali basalt-dacite suite erupted sequentially from Rishiri Volcano, northern Japan. The major element variations within the suite can be explained by boundary layer fractionation; that is, mixing of a magma in the main part of the magma body with a fractionated interstitial melt transported from the mushy boundary layer at the floor. Systematic variations in SiO2 correlate with variations in the Pb, Sr and Nd isotopic compositions of the lavas. The geochemical variations of the lavas are explained by a constant and relatively low ratio of assimilated mass to crystallized mass ('r value). In the magma chamber in which the Kutsogata and Tanetomi magmas evolved, a strong thermal gradient was present and it is suggested that the marginal part of the reservoir was completely solidified. The assimilant was transported by crack flow from the partially fused floor crust to the partially crystallized floor mush zone through fractures in the solidified margin, formed a ny by thermal stresses resultingfrom cooling of the solidified margin and heating of the crust. The crustal melt was then mixed with the fractionated interstitial melt in the mushy zone, and the mixed melt was further transported by compositional convection to the main magma, causing its geochemical evolution to be characteristic of AFC. ne volume flux of the assimilant from the crust to the magma chamber is suggested to have decreased progressively with time (proportional to t(-1/2)), and was about 3 x 10(-2) m/year at t = 10years and I x 10(-2) m/year at t = 100years. It has been commonly considered that the heat balance between magmas and the surrounding crust controls the coupling of assimilation and fractional crystallization processes (i.e. absolute value of r). However, it is inferred from this study that the ratio of assimilated mass to crystallized mass can be controlled by the transport Process of the assimilant from the crust to magma chambers.

DOI： 10.1093/petrology/egi021

Boron isotopic constraints on the source of Hawaiian shield lavas

R Tanaka, E Nakamura

GEOCHIMICA ET COSMOCHIMICA ACTA 69 ( 13 ) 3385 - 3399 2005年7月

記述言語：英語出版者・発行元：PERGAMON-ELSEVIER SCIENCE LTD

Boron isotopic compositions of lavas from three representative Hawaiian shield volcanoes (Kilauea, Mauna Loa, and Koolau) were analyzed by thermal ionization mass spectrometry. The boron isotopic composition of each sample was analyzed twice, once with and once without acid leaching to evaluate the effect of posteruptive boron contamination. Our acid-leaching procedure dissolved glass, olivine, secondary zeolite, and adsorbed boron; this dissolved boron was completely removed from the residue, which was comprised of plagioclase, pyroxenes, and newly formed amorphous silica. We confirmed that an appropriate acid-leaching process can eliminate adsorbed and incorporated boron contamination from all submarine samples without modifying the original B-11/B-10 ratio. On the other hand, when the sample was weathered, i.e., the olivine had an iddingsite rim, B-11/B-10 of the acid-resistant minerals are also modified, thus it is impossible to get the preeruptive B-11/B-10 value from the weathered samples. Through this elimination and evaluation procedure of posteruptive contamination, preeruptive delta(11)B values for the shield lavas are -4.5 to -5.4 parts per thousand for Koolau (N = 8), -3.6 to -4.6 parts per thousand, for Kilauea (N = 11), and -3.0 to -3.8 parts per thousand for Mauna Loa (N = 6).
Historical Kilauea lavas show a systematic temporal trend for B content and Nb/B coupled with other radiogenic isotopic ratios and trace element ratios, at constant delta(11)B, indicating little or no assimilation of crustal materials in these lavas. Uncorrelated B content and delta(11)B in Koolau and Mauna Loa lavas may also indicate little or no effect of crustal assimilation in these lavas. The source of KEA-component (identical to the so-called Kea end member in Hawaiian lavas) of the Hawaiian source mantle, represented by Kilauea, should be derived from lower part of subducted oceanic crust or refractory peridotite in the recycled subducted stab. The systematic trend from Kilauea to Koolau-decreasing delta(11)B coupled with decreasing epsilon Nd as well as increasing Sr-87/Sr-86 and Pb-206/Pb-204-is consistent with involvement of subducted sediment components in the EMK(enriched Makapuu)-component, represented by Makapuu-stage of Koolau lavas. Copyright (C) 2005 Elsevier Ltd.

DOI： 10.1016/j.gca.2005.03.009

Jurassic oceanic lithosphere beneath the southern Ontong Java Plateau: Evidence from xenoliths in alnoite, Malaita, Solomon Islands

A Ishikawa, E Nakamura, JJ Mahoney

GEOLOGY 33 ( 5 ) 393 - 396 2005年5月

記述言語：英語出版者・発行元：GEOLOGICAL SOC AMERICA, INC

The least metasomatized xenoliths of spinel lherzolite and gabbro recovered from ainoite intruding crust of the southern Ontong Java Plateau on the island of Malaita yield an Sm-Nd age of ca. 160 Ma and an initial epsilon(Nd) value of similar to+8. In contrast, the plateau basement is ca. 120 Ma with initial epsilon(Nd) between +3.7 and +6.5. The xenoliths appear to represent normal Pacific oceanic lithosphere (uppermost mantle and lower crust) formed similar to 40 m.y. before the plateau, indicating that the southern part of the plateau was emplaced off axis on mature seafloor. The closest 160 Ma seafloor to Malaita is > 1800 km to the north, off the northeastern margin of the plateau, implying the presence of either an intervening large-offset fracture zone or a triple junction. The presence of significantly older oceanic lithosphere beneath the plateau places restrictions on proposed origins for the plateau.

DOI： 10.1130/G21205.1

U-Pb zircon dating of regional deformation in the lower crust of the Kohistan arc

Yamamoto, H, Kobayashi, K, Nakamura, E, Kaneko, Y, Kausar, A

INTERNATIONAL GEOLOGY REVIEW 2005年

Lithium, boron, and lead isotope systematics of glass inclusions in olivines from Hawaiian lavas: evidence for recycled components in the Hawaiian plume

K Kobayashi, R Tanaka, T Moriguti, K Shimizu, E Nakamura

CHEMICAL GEOLOGY 212 ( 1-2 ) 143 - 161 2004年11月

記述言語：英語出版者・発行元：ELSEVIER SCIENCE BV

Lithium (Li), boron (B), and lead (Pb) isotopic compositions of glass inclusions in olivine phenocrysts from Hawaiian lavas, (Kilauea Iki, Mauna Loa, and Koolau volcano) were measured by high mass resolution ion probe to search for possible signatures of recycled materials in the Hawaiian plume. In order to measure the isotopic compositions, a set of synthetic glass standards with matrices similar to those of the target glass inclusions was prepared. Isotopic variations among these synthetic standards were produced by additions of spikes, and their isotopic compositions were determined by TIMS. Using this set of standards, correction factors for instrumental mass fractionation for Li, B, and Pb isotope measurement were determined with internal precisions of <1.2%, <1.6%, and <0.8% (2sigma) uncertainties for Li, B, and Pb, respectively.
Twenty-eight glass inclusions were measured after homogenization to eliminate dendritic crystals. These glass inclusions showed isotopic variations from -10.2% to +8.4% for delta(7)Li, from -10.5% to +5.2% for delta(11)B, from 0.7994 to 0.8909 for (207) Pb/Ph-206, and from 1.989 to 2.139 for Pb-208/Pb-206, which are considerably larger than those for whole rocks. This suggests that the Hawaiian lavas are mixtures of melts derived from isotopically distinct sources, and that glass inclusions better preserve information regarding source heterogeneity than do whole rocks. In particular, significantly low delta(7)Li and delta(11)B values for the Mauna Loa and Koolau samples indicate an isotopically "light" Li and B source, perhaps containing recycled materials that experienced dehydration during subduction. The extremely low Pb isotope signature, which corresponds to HIMU, also suggests some contribution from recycled materials to the generation of the Hawaiian magma. Our results suggest that recycled materials, which experienced near-surface alteration and then dehydration during subduction, played an important role in creating geochemical heterogeneity in the Hawaiian lavas. (C) 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.chemgeo.2004.08.050

Preface to "Lithium isotope geochemistry"

RL Rudnick, E Nakamura

CHEMICAL GEOLOGY 212 ( 1-2 ) 1 - 4 2004年11月

Experimental constraints on crystallization differentiation in a deep magma ocean

MJ Walter, E Nakamura, RG Tronnes, DJ Frost

GEOCHIMICA ET COSMOCHIMICA ACTA 68 ( 20 ) 4267 - 4284 2004年10月

記述言語：英語出版者・発行元：PERGAMON-ELSEVIER SCIENCE LTD

Major and trace element mineral/melt partition coefficients are presented for phases on the liquidus of fertile peridotite at 23-23.5 GPa and 2300degreesC. Partitioning models, based on lattice-strain theory, are developed for cations in the '8-fold' sites of majorite and Mg-perovskite. Composition-dependant partitioning models are made for cations in the 12-fold site of Ca-perovskite based on previously published data. D(min/melt) is extremely variable for many elements in Ca-perovskite and highly correlated with certain melt compositional parameters (e.g. CaO and Al(2)O(3) contents). The 8-fold sites in Mg-perovskite and majorite generally have ideal site radii between 0.8 and 0.9 Angstrom for trivalent cations, such that among rare-earth-elements (REE) D(min/melt) is maximum for Lu. Lighter REE become increasingly incompatible with increasing ionic radii. The 12-fold site in Ca-perovskite is larger and has an ideal trivalent site radius of similar to1.05 Angstrom, such that the middle REE has the maximum D(min/melt). Trivalent cations are generally compatible to highly compatible in Ca-perovskite giving it considerable leverage in crystallization models. Geochemical models based on these phase relations and partitioning results are used to test for evidence in mantle peridotite of preserved signals of crystal differentiation in a deep, Hadean magma ocean.
Model compositions for bulk silicate Earth and convecting mantle are constructed and evaluated. The model compositions for primitive convecting mantle yield superchondritic Mg/Si and Ca/Al ratios, although many refractory lithophile element ratios are near chondritic. Major element mass balance calculations effectively preclude a CI-chondritic bulk silicate Earth composition, and the super-chondritic Mg/Si ratio of the mantle is apparently a primary feature. Mass balance calculations indicate that 10-15% crystal fractionation of an assemblage dominated by Mg-perovskite, but with minor amounts of Ca-perovskite and ferropericlase, from a magma ocean with model peridotite-based bulk silicate Earth composition produces a residual magma that resembles closely the convectine mantle.
Partition coefficient based crystal fractionation models are developed that track changes in refractory lithophile major and trace element ratios in the residual magma (e.g. convecting mantle). Monomineralic crystallization of majorite or Mg-perovskite is limited to less than 5% before certain ratios fractionate beyond convecting mantle values. Only trace amounts of Ca-perovskite can be tolerated in isolation due to its remarkable ability to fractionate lithophile elements. Indeed, Ca-perovskite is limited to only a few percent in a deep mantle crystal assemblage. Removal from a magma ocean of approximately 13% of a deep mantle assemblage comprised of Mg-perovskite, Ca-perovskite and ferropericlase in the proportions 93:3:4 produces a residual magma with a superchondritic Ca/Al ratio matching that of the model convecting mantle. This amount of crystal separation generates fractionations in other refractory lithophile elements ratios that generally mimic those observed in the convecting mantle. Further, the residual magma is expected to have subchondritic Sm/Nd and Lu/Hf ratios. Modeling shows that up to 15% crystal separation of the deep mantle assemblage froth an early magma ocean could have yielded a convecting mantle reservoir with (143)Nd/(144)Nd and (176)Hf/(177)Hf isotopic compositions that remain internal to the array observed for modern oceanic volcanic rocks. If kept in isolation, the residual magma and deep crystal piles would grow model isotopic compositions that are akin to enriched mantle 1 (EM1) and HIMU reservoirs, respectively, in Nd-Hf isotopic space. Copyright (C) 2004 Elsevier Ltd.

DOI： 10.1016/j.gca.2004.03.014

Discovery of Archean continental and mantle fragments inferred from xenocrysts in komatiites, the Belingwe greenstone belt, Zimbabwe

K Shimizu, E Nakamura, K Kobayashi, S Maruyama

GEOLOGY 32 ( 4 ) 285 - 288 2004年4月

記述言語：英語出版者・発行元：GEOLOGICAL SOC AMERICA, INC

Controversy exists as to whether the 2.7 Ga Belingwe greenstone belt, Zimbabwe, is autochthonous or allochthonous. In this study we report direct evidence for an autochthonous continental setting for the Bellingwe greenstone belt. Garnet and clinopyroxene xenocrysts were discovered in the fresh ultramafic komatiites. Major and trace element compositions of these xenocrysts suggest that they originated from malic lower crust, presumably garnet granulite in composition, at a low temperature of similar to600 or similar to700 degreesC. Furthermore, in a komatiitic basalt sample, we have discovered orthopyroxene and clinopyroxene xenocrysts that may have originated from lithospheric mantle beneath the continental margin. Discoveries of these xenocrysts indicate that the komatiite magma was transported to the surface and quenched rapidly enough to prevent the complete melting of continental fragments.

DOI： 10.1130/G20162.1

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