研究者詳細 - 中村　栄三

MISC - 中村　栄三

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Multiple pulses of the mantle plume: Evidence from Tertiary Icelandic lavas

Hiroshi Kitagawa, Katsura Kobayashi, Akio Makishima, Eizo Nakamura

JOURNAL OF PETROLOGY 49 ( 7 ) 1365 - 1396 2008年7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

We present major and trace element concentrations and Sr-Nd-Hf-Pb isotope data for the c. 13-2 Ma Tertiary lavas from eastern Iceland. Our new geochemical results, together with published geological, geochronological, geochemical and geophysical data, are used to evaluate temporal changes in mantle sources contributing to the Tertiary Icelandic magmatism and the relative roles of these sources in magma productivity. The trace element and radiogenic isotopic compositions clearly distinguish three distinct end-member components in the Tertiary magmatism. Temporal variations in lava geochemistry can be attributed to changes in the relative contributions of these three end-member components to the erupted magmas and correlated with temporal variations in magma productivity. The extrusion of lavas with geochemically and isotopically enriched compositions was particularly pronounced at similar to 13-12 and 8-7 Ma, coincident in time with higher magma productivity. However, the geochemical characteristics of the lavas are different during these two periods: the 13-12 Ma lavas have more radiogenic Hf-176/Hf-177 and less radiogenic Pb-206/Pb-204 than those erupted from 8 to 7 Ma. The eruption of relatively depleted lavas, at around 10 Ma and younger than 6.5 Ma, is coincident with lower magma productivity. The correlation between the composition and productivity of the Tertiary lavas from eastern Iceland is probably due to periodic changes in the involvement of the enriched end-member component, followed by a gradation to periods dominated by the signature of the depleted end-member component and lower magma productivity, at an approximate frequency of 5 Myr.

DOI： 10.1093/petrology/egn029

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Gravity-induced diffusion of isotope atoms in monoatomic solid Se

T. Mashimo, M. Ono, X. S. Huang, Y. Iguchi, S. Okayasu, K. Kobayashi, E. Nakamura

EPL 81 ( 5 ) 56002 - 56002 2008年3月

記述言語：英語出版者・発行元：EPL ASSOCIATION, EUROPEAN PHYSICAL SOCIETY

A strong gravitational field resulted in the gravity-induced diffusion (sedimentation) of isotope atoms in monoatomic solid Se. The layer crystalline morphology consisting of three zones of the fine-grained crystals, the long crystals and feather-shaped crystals grown parallel to gravity direction appeared in the specimen ultracentrifuged at (0.8-1) x 10(6) G and at 190 degrees C. Change in the concentration ratio (82)Se/(76) Se of > 0.8% was observed in the grown crystalline region. These results show an evidence for the sedimentation of substitutional atoms in solids via self-diffusion, and suggest the possibility of application to the control of impurity and crystalline states as well as to isotope separation. Copyright (c) EPLA, 2008.

DOI： 10.1209/0295-5075/81/56002

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Crystal-size distribution and composition of garnets in eclogites from the Dabie orogen, central China

Hao Cheng, Zuyi Zhou, Eizo Nakamura

AMERICAN MINERALOGIST 93 ( 1 ) 124 - 133 2008年1月

記述言語：英語出版者・発行元：MINERALOGICAL SOC AMER

Crystal-size and spatial distributions of minerals in metamorphic rocks provide insight into their nucleation, growth environment, and the metamorphic evolution of their host rocks. Episodic nucleation and growth histories for garnets in eclogites from the Dabie orogen arc revealed by variations in crystal size, geochemical zoning, and mineralogy of inclusions in garnets. The studied garnets show pseudo-lognormal crystal-size distributions (CSDs), prograde chemical zoning patterns indexed by mineral inclusions, and are mainly distributed at random. Constant growth rate for each episode is proposed based on the mineralogy of inclusions in garnets and the chemical zoning patterns. The CSD shapes were evaluated in terms of a size-dependent proportionate growth model and a thermally accelerated, diffusion-controlled model but neither is consistent with the geochemical data and the concordant occurrence of mineral inclusions in garnet. Initial increasing nucleation rate followed by a subsequent, medial stage of nearly constant growth and finally declining nucleation rate is inferred from the CSD data. This study suggests that linking chemical analysis with textural analysis is crucial to avoid misleading interpretation solely through CSD shapes of minerals in metamorphic rocks.

DOI： 10.2138/am.2008.2556

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Hawaiian double volcanic chain triggered by an episodic involvement of recycled material: Constraints from temporal Sr－Nd－Hf－Pb isotopic trend of the Loa-type volcanoes

Tanaka, R, Makishima, A, Nakamura, E

Earth Planet. Sci. Lett. 265 ( 3-4 ) 450 - 465 2008年1月

Tourmaline breakdown in a pelitic system: implications for boron cycling through subduction zones

Tsutomu Ota, Katsura Kobayashi, Tomoo Katsura, Eizo Nakamura

CONTRIBUTIONS TO MINERALOGY AND PETROLOGY 155 ( 1 ) 19 - 32 2008年1月

記述言語：英語出版者・発行元：SPRINGER

Pressure-temperature conditions of tourmaline breakdown in a metapelite were determined by high-pressure experiments at 700-900 degrees C and 4-6 GPa. These experiments produced an eclogite-facies assemblage of garnet, clinopyroxene, phengite, coesite, kyanite and rare rutile. The modal proportions of tourmaline clearly decreased between 4.5 and 5 GPa at 700 degrees C, between 4 and 4.5 GPa at 800 degrees C, and between 800 and 850 degrees C at 4 GPa, with tourmaline that survived the higher temperature conditions appearing corroded and thus metastable. Decreases in the modal abundance of tourmaline are accompanied by decreasing modal abundance of coesite, and increasing that of clinopyroxene, garnet and kyanite; the boron content of phengite increases significantly. These changes suggest that, with increasing pressure and temperature, tourmaline reacts with coesite to produce clinopyroxene, garnet, kyanite, and boron-bearing phengite and fluid. Our results suggest that: (1) tourmaline breakdown occurs at lower pressures and temperatures in SiO(2)-saturated systems than in SiO(2)-undersaturated systems. (2) In even cold subduction zones, subducting sediments should release boron-rich fluids by tourmaline breakdown before reaching depths of 150 km, and (3) even after tourmaline breakdown, a significant amount of boron partitioned into phengite could be stored in deeply subducted sediments.

DOI： 10.1007/s00410-007-0228-2

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New sequential separation procedure for Sr, Nd and Pb isotope ratio measurement in geological material using MC-ICP-MS and TIMS

Akio Makishima, B. Nagender Nath, Eizo Nakamura

GEOCHEMICAL JOURNAL 42 ( 3 ) 237 - 246 2008年

記述言語：英語出版者・発行元：GEOCHEMICAL SOC JAPAN

A new 3-step sequential separation chemistry for Sr, Nd and Pb from silicate samples, which is suitable for isotope analysis by MC-ICP-MS as well as TIMS, has been developed. The chemistry is designed to minimize the number of evaporation steps, enabling high throughput especially when MC-ICP-MS is employed. The sample solution in 0.5 mol 1(-1) HNO3 after digestion with HF-HCIO4, which is prepared for trace element analysis, can be directly used in the new chemistry. In the first column using Sr resin, Sr and Pb are collected. The recovered solution for Sr can be directly aspirated into MC-ICP-MS. The Pb solution is dried, re-dissolved, separated into two aliquots and measured by double spike MC-ICP-MS. Subsequently, LREEs are collected by the second column packed with cation exchange resin. Finally, Nd was purified from Sm using Ln resin in the third column. The recovered Nd solution can also be directly nebulized into MC-ICP-MS. The Sr and Nd isotope ratios for the standard material JB-3 (basalt) from the Geological Survey of Japan by MC-ICP-MS and TIMS were reported to verify the chemistry and mass spectrometry developed in this study.

DOI： 10.2343/geochemj.42.237

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Generation of Rear-arc Magmas Induced by Influx of Slab-derived Supercritical Liquids: Implications from Alkali Basalt Lavas from Rishiri Volcano, Kurile Arc

Kuritani, T, Yokoyama, T, Nakamura, E

J. Petrol. ( 49 ) 1319 - 1342 2008年

New preconcentration technique of Zr, Nb, Mo, Hf, Ta and W employing coprecipitation with Ti compounds: Its application to Lu-Hf system and sequential Pb-Sr-Nd-Sm separation

Akio Makishima, Eizo Nakamura

GEOCHEMICAL JOURNAL 42 ( 2 ) 199 - 206 2008年

記述言語：英語出版者・発行元：GEOCHEMICAL SOC JAPAN

A new preconcentration technique of Zr, Nb, Mo, Hf, Ta and W has been invented employing coprecipiation with Ti compounds. Silicate samples were digested by HF with addition of Ti. Subsequent drying with HClO4 resulting in complete elimination of fluorine produces Ti compounds (oxide/hydroxides). The sample was then dissolved with dilute nitric acid and centrifuged, and the residual Ti compounds were collected. The Ti compounds forming from 2 mg of Ti without matrix elements concentrate 84-98% of Zr, Nb, Mo, Hf, Ta and W. In 20 mg basaltic and 50 mg peridotitic matrices, the addition of I nig of Ti was appropriate and gave yields of 46-69 and 54-79%, respectively, for Zr, Nb, Mo, Hf, Ta and W. Exploiting this preconcentration, sequential separation protocols of Hf, Pb, Sr, Lu, Nd and Sm for isotope analysis are developed with total blanks of 16, 11, 60, 2.4, 3 and 0.4 pg, respectively. The method is suitable for Lu-Hf system studies with capability of simultaneous separation of Pb, Sr, Nd and Sm from the same sample digest as well as isotopic anomaly studies of Zr, Mo and W. As application examples, accurate Lu-Hf data and Pb isotope ratios were obtained for basalt (JB-3) and peridotite (JP-1) by MC-ICP-MS.

DOI： 10.2343/geochemj.42.199

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Isotope Fluctuation in Indium-Lead alloy induced by Solid Centrifugation

Ono, M, Iguchi, Y, Okayasu, S, Esaka, F, Kobayashi, K, Hao, T, Bagum, R, Osawa, T, Fujii, K, Nakamura, E, Mashimo, T

J. Nuclear Sci. & Tech. in Press 2008年

Direct measurements of impact devolatillization of calcite using a laser gun.

Ohno, S, Kadono, T, Ishibashi, K, Kawaragi, K, Sugita, S, Nakamura, E, Matsui, T

Geophys. Res. Lett. ( 35 ) 2008年

Precise measurement of (228)Ra/(226)Ra for (226)Ra determination employing total integration and simultaneous (228)Th correction by multicollector ICP-MS using multiple ion counters

Akio Makishima, Takele A. Chekol, Eizo Nakamura

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY 23 ( 8 ) 1102 - 1107 2008年

記述言語：英語出版者・発行元：ROYAL SOC CHEMISTRY

A precise method of measuring (228)Ra/(226)Ra for (226)Ra determination used in U-series disequilibrium studies has been developed using the total integration method with simultaneous 228Th isobaric interference correction by multicollector ICP- MS with multiple ion counters (MIC). A sample solution of 0.2 ml was separately taken into a test tube, and all (226)Ra(+) and (228)Ra(+) signals as well as (229)Th(+) and (228)Th(16)O(+) from beginning to end were integrated simultaneously using four channeltrons, IC1, IC3, IC4 and IC5, respectively. Gains between IC1 and IC3 and the mass discrimination factor for (228)Ra/ (226) Ra determination were canceled out by measurement of a Ra spike solution with an accurately known Ra isotope ratio determined by TIMS, with which samples were bracketed. The isobaric interferences of (228)Th(+) were simultaneously corrected from the signal of (228)Th(16)O(+) and the oxide forming ratio of (228)Th, which was separately determined by simultaneous measurements of (229)Th(+) and 2(29)Th(16)O(+) by IC3 and IC5. The gains between IC3 and IC5 for the Th correction were also canceled out. The ion yield of Ra in MIC- ICP- MS was similar to 1.5%. The intermediate precision using 6.6, 3.3, 1.7, 0.83 and 0.41 fg of (226)Ra with (228)Ra/(226)Ra similar to 5 was 0.33, 0.93, 0.76, 1.6 and 1.5%, respectively. The (228)Ra/(226)Ra ratio showed no systematic change dependent on (228)Th/(228)Ra up to 0.25, indicating the validity of the (228)Th correction. At similar amounts of Ra, our method gives intermediate precision values similar to or better than previous MC-ICP-MS and TIMS studies. The precision of the method was verified using another Ra spike with (228)Ra/ (226)Ra = similar to 1 determined by TIMS. The analytical performance of the method was further investigated using the silicate samples JB-2 and JB-3 basalts issued by GSJ.

DOI： 10.1039/b807431c

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Sedimentation of isotope atoms in monoatomic solid Se

Mashimo, t, Ono, M, Huang, X. S, Iguchi, Y, Okayasu S, Kobayashi, K, Nakamura, E

Europhys. Lett., in press. 2008年

Lithium isotopic systematics of peridotite xenoliths from Hannuoba, North China Craton: Implications for melt-rock interaction in the considerably thinned lithospheric mantle

Yan-Jie Tang, Hong-Fu Zhang, Eizo Nakamura, Takuya Moriguti, Katsura Kobayashi, Ji-Feng Ying

GEOCHIMICA ET COSMOCHIMICA ACTA 71 ( 17 ) 4327 - 4341 2007年9月

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

Li concentrations and isotopic compositions of coexisting minerals (ol, opx, and cpx) from peridotite xenoliths entrained in the Hannuoba Tertiary basalts, North China Craton, provide insight into Li isotopic fractionation between mantle minerals during melt-rock interaction in the considerably thinned lithospheric mantle. Bulk analyses of mineral separates show significant enrichment of Li in cpx (2.4-3.6 ppm) relative to olivine (1.2-1.8 ppm), indicating that these peridotites have been affected by mantle metasomatism with mafic silicate melts. Bulk olivine separates (delta Li-7 similar to +3.3 parts per thousand to +6.4 parts per thousand) are isotopically heavier than coexisting pyroxenes (delta Li-7 similar to -3.3 parts per thousand to -8.2 parts per thousand in cpx, and -4.0 parts per thousand to -6.7 parts per thousand in opx). Such large variation suggests Li elemental and isotopic disequilibrium. This conclusion is supported by results from in situ SIMS analyses of mineral grains where significant Li elemental and isotopic zonations exist. The olivine and opx have lower Li concentrations and heavier Li isotopes in the rims than in the cores. This reverse correlation of delta Li-7 with Li concentrations indicates diffusive fractionation of Li isotopes. However, the zoning patterns in coexisting cpx show isotopically heavier rims with higher Li abundances. This positive correlation between delta Li-7 and Li concentrations suggests a melt mixing trend. We attribute Li concentration and isotope zonation in minerals to the effects of two-stage diffusive fractionation coupled with melt-rock interaction. The earliest melts may have been derived from the subducted oceanic slab with low delta Li-7 values produced by isotopic fractionation during the dehydration of the seawater-altered slab. Melts at later stages were derived from the astheno-sphere and interacted with the peridotites, producing the Li elemental and isotopic zoning in mineral grains. These data thus provide evidence for multiple-stage peridotite-melt interaction in the lithospheric mantle beneath the northern North China Craton. (C) 2007 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.gca.2007.07.006

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Rates of thermal and chemical evolution of magmas in a cooling magma chamber: a chronological and theoretical study on basaltic and andesitic lavas from Rishiri Volcano, Japan

Takeshi Kuritani, Tetsuya Yokoyama, Eizo Nakamura

JOURNAL OF PETROLOGY 48 ( 7 ) 1295 - 1319 2007年7月

記述言語：英語出版者・発行元：OXFORD UNIV PRESS

Rates of magmatic processes in a cooling magma chamber were investigated for alkali basalt and trachytic andesite lavas erupted sequentially from Rishiri Volcano, northern Japan, by dating of these lavas using U-238-Th-230 radioactive disequilibrium and C-14 dating methods, in combination with theoretical analyses. We obtained the eruption age of the basaltic lavas to be 29.3 +/- 0.6 ka by C-14 dating of charcoals. The eruption age of the andesitic lavas was estimated to be 20.2 +/- 3.1 ka, utilizing a whole-rock isochron formed by U-Th fractionation as a result of degassing after lava emplacement. Because these two lavas represent a series of magmas produced by assimilation and fractional crystallization in the same magma chamber, the difference of the ages (i.e. similar to 9 kyr) is a timescale of magmatic evolution. The thermal and chemical evolution of the Rishiri magma chamber was modeled using mass and energy balance constraints, as well as quantitative information obtained from petrological and geochemical observations on the lavas. Using the timescale of similar to 9 kyr, the thickness of the magma chamber is estimated to have been about 1.7 km. The model calculations show that, in the early stage of the evolution, the magma cooled at a relatively high rate (>0.1 degrees C/year), and the cooling rate decreased with time. Convective heat flux from the main magma body exceeded 2W/m(2) when the magma was basaltic, and the intensity diminished exponentially with magmatic evolution. Volume flux of crustal materials to the magma chamber and rate of convective melt exchange (compositional convection) between the main magma and mush melt also decreased with time, from similar to 0.1m/year to similar to 10(-3) m/year, and from similar to 1m/year to similar to 10(-2) m/year, respectively, as the magmas evolved from basaltic to andesitic compositions. Although the mechanism of the cooling (i. e. thermal convection and/or compositional convection) of the main magma could not be constrained uniquely by the model, it is suggested that compositional convection was not effective in cooling the main magma, and the magma chamber is considered to have been cooled by thermal convection, in addition to heat conduction.

DOI： 10.1093/petrology/egm018

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Origin of atoll garnets in eclogites and implications for the redistribution of trace elements during slab exhumation in a continental subduction zone

Hao Cheng, Eizo Nakamura, Katsura Kobayashi, Zuyi Zhou

AMERICAN MINERALOGIST 92 ( 7 ) 1119 - 1129 2007年7月

記述言語：英語出版者・発行元：MINERALOGICAL SOC AMER

Detailed electron- and ion-microprobe analyses were carried out on atoll-shaped and normal garnets in ultrahigh-pressure (UHP) metamorphic eclogite from Dabie, east-central China. Compositional profiles of both normal garnets and rings of atoll garnets show well-preserved growth zoning with a decrease in Mn, Ca, and heavy rare earth elements (HREEs), and an increase in Mg toward rims. Manganese and middle rare earth element (MREE) enrichments are observed near garnet rims. Island- and peninsula-shaped garnet inside atolls are homogeneous in major elements and show the same composition as garnet rims, whereas the HREE concentrations are similar to those of the normal garnet cores. Electron back-scatter diffraction (EBSD) analyses show that the island- and peninsula-shaped garnet fractions inside atolls have crystallographic orientations identical to that of the atoll rings. These observations suggest that the atoll garnets were formed by the consumption of earlier-formed cores by fluid released from both hydroxyl exsolution from the nominally anhydrous minerals (NAMs) and lawsonite decomposition at the onset of exhumation, (i.e., garnet breakdown was from the inside and re-growth from outside to inside). Though somewhat restricted, this study reveals that because garnet and zircon act as sinks for HREEs and Zr, respectively, the majority of released HREEs and Zr were likely re-incorporated into newly grown garnet and zircon during the Dabie UHP metamorphic slab exhumation.

DOI： 10.2138/am.2007.2343

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Ancient recycled crust beneath the ontong Java plateau: Isotopic evidence from the garnet clinopyroxenite xenoliths, Malaita, Solomon Islands

Akira Ishikawa, Takeshi Kuritani, Akio Makishima, Eizo Nakamura

EARTH AND PLANETARY SCIENCE LETTERS 259 ( 1-2 ) 134 - 148 2007年7月

記述言語：英語出版者・発行元：ELSEVIER SCIENCE BV

We present a Sr, Nd, Hf and Pb isotope investigation of a set of garnet clinopyroxenite xenoliths from Malaita, Solomon Islands in order to constrain crustal recycling in the Pacific mantle. Geological, thermobarometric and petrochemical evidence from previous studies strongly support an origin as a series of high-pressure (> 3 GPa) melting residues of basaltic material incorporated in peridotite, which was derived from Pacific convective mantle related to the Ontong Java Plateau magmatism. The present study reveals isotopic variations in the pyroxenites that are best explained by different extents of chemical reaction with ambient peridotite in the context of a melting of composite source mantle. Isotopic compositions of bimineralic gamet clinopyroxenites affected by ambient peridotite fall within the oceanic basalt array, similar to those of Ontong Java Plateau lavas. In contrast, a quartz-gaenet clinopyroxenite, whose major element compositions remain intact, has lower Pb-206/(204) Pb-Nd-143/(144) Nd and higher (87)sr/Sr-86-Pb-207/(204) Pb ratios than most oceanic basalts. These isotopic signatures show some affinity with proposed recycled sources such as the so-called EM-1 or DUPAL types. Constraints from major and trace element characteristics of the quartz-garnet clinopyroxenite, the large extent of Hf-Nd isotopic decoupling and the good coincidence of Ph isotopes to the Stacey-Kramers curve, all indicate that pollution of southern Pacific mantle occurred by the subduction or delamination of Neoproterozoic granulitic lower crust (0.5-1 Ga). This crustal recycling could have taken place around the suture of Rodinia supercontinent, a part of which resurfaced during mantle upwelling responsible for creating the Cretaceous Ontong Java Plateau. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.epsl.2007.04.034

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Boron and lead isotope signatures of subduction－zone me´lange formation: Hybridization and fractionation along the slab-mantle interface beneath volcanic arcs.

King, R. L, Bebout, G. E, Grove, M, Moriguti, T, Nakamura, E

Chem. Geol. 239 ( 3-4 ) 305 - 322 2007年4月

Plume-litho sphere interaction beneath Mt. Cameroon volcano, West Africa: Constraints from U-238-Th-230-Ra-226 and Sr-Nd-Pb isotope systematics

Tetsuya Yokoyama, Festus T. Aka, Minoru Kusakabe, Eizo Nakamura

GEOCHIMICA ET COSMOCHIMICA ACTA 71 ( 7 ) 1835 - 1854 2007年4月

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

Precise measurements of U-238-Th-230-Ra-226 disequilibria in lavas erupted within the last 100 yr on Mt. Cameroon are presented, together with major and trace elements, and Sr-Nd-Pb isotope ratios, to unravel the source and processes of basaltic magmatism at intraplate tectonic settings. All samples possess U-238-Th-230-Ra-226 disequilibria with Th-230 (18-24%) and Ra-226 (9-21%) excesses, and there exists a positive correlation in a (Ra-226/Th-230)-(Th-230/U-238) diagram. The extent of U-238-Th-230-Ra-226 disequilibria is markedly different in lavas of individual eruption ages, although the (Th-230/Th-232) ratio is constant irrespective of eruption age. When U-series results are combined with Pb isotope ratios, negative correlations are observed in the (Th-230/U-238)-(Pb-206/Pb-204) and (Ra-226/Th-230)-(Pb-206/Pb-204) diagrams. Shallow magma chamber processes like magma mixing, fractional crystallization and wall rock assimilation do not account for the correlations. Crustal contamination is not the cause of the observed isotopic variations because continental crust is considered to have extremely different Pb isotope compositions and U/Th ratios. Melting of a chemically heterogeneous mantle might explain the Mt. Cameroon data, but dynamic melting under conditions of high Du and D-U/D-Th, long magma ascent time, or disequilibrium mineral/melt partitioning, is required. The most plausible scenario to produce the geochemical characteristics of Mt. Cameroon samples is the interaction of melt derived from the asthenospheric mantle with overlying sub-continental lithospheric mantle which has elevated U/Pb (> 0.75) and Pb isotope ratios (Pb-206/Pb-204 > 20.47) due to late Mesozoic metasomatism.

DOI： 10.1016/j.gca.2007.01.010

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Trace element fractionation in deep subduction zones inferred from a lawsonite-eclogite xenolith from the Colorado Plateau

Tomohiro Usui, Katsura Kobayashi, Eizo Nakamura, Herwart Helmstaedt

CHEMICAL GEOLOGY 239 ( 3-4 ) 336 - 351 2007年4月

記述言語：英語出版者・発行元：ELSEVIER SCIENCE BV

A coesite-bearing, lawsonite-eclogite xenolith from the Colorado Plateau, interpreted as a fragment of the subducted Farallon plate, is used to characterize trace element behavior in subducted oceanic crust. The xenolith consists of almandine-rich garnet, omphacite, lawsonite, phengite, rutile, pyrite and zircon as the primary mineral assemblage. Garnet crystals are extremely zoned with respect to their Mn contents, with core to rim variation from similar to 1.4 to similar to 0.2 wt.%. The euhedral zoning feature of garnet crystals and its included mineral assemblages suggest that the garnet continued to grow in the coesite stability field during prograde lawsonite eclogite facies metamorphism. In the lawsonite-eclogite xenoliths, garnet dominates the heavy rare earth elements (HREE), and lawsonite dominates both light rare earth elements (LREE) and Sr inventories. Combining the mineralogical and petrographic observations with precise spatial resolution ion microprobe analyses (< 15 mu m) of zoned garnet as well as lawsonite inclusions in garnet, we investigated trace element fractionation in coesite stability field during lawsonite eclogite facies metamorphism. Garnet shows progressive HREE depletion from core to rim, suggesting that HREE, which once partitioned into garnet crystal, would not be involved in postdated metamorphic reactions due to the high partition coefficients of HREE into garnet. Lawsonite inclusions in garnet, which represent lower metamorphic condition relative to lawsonite in the matrix, have LREE concentrations similar to 10 times lower than those of matrix lawsonite. On the contrary, the concentration of Sr in the included lawsonite is (< 20 relative %) lower than that of the matrix lawsonite. Based on constraints from metamorphic history recorded in the prograde-zoned garnet and mass balance among all constituent minerals in the lawsonite-eclogite xenolith, this contrasting feature for Sr and LREE of lawsonite is most plausibly explained by the hypothesis that allanite coexisting with included lawsonite might have decomposed during prograde metamorphism. The LREE released from the decomposing allanite would have been incorporated into lawsonite crystals. Consequently, REE and Sr could be retained in subducting oceanic crust even in the coesite stability field, if the slab is sufficiently cold enough to pass though the lawsonite eclogite facies. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.chemgeo.2006.08.009

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Coprecipitation of Ti, Mo, Sn and Sb with fluorides and application to detennination of B, Ti, Zr, Nb, Mo, Sn, Sb, Hf and Ta by ICP-MS

Yinghuai Lu, Akio Makishima, Eizo Nakamura

CHEMICAL GEOLOGY 236 ( 1-2 ) 13 - 26 2007年1月

記述言語：英語出版者・発行元：ELSEVIER SCIENCE BV

We examined the coprecipitation behavior of Ti, Mo, Sn and Sb in Ca-Al-Mg fluorides under two different fluoride forming conditions: at < 70 degrees C in an ultrasonic bath (denoted as the ultrasonic method) and at 245 degrees C using a Teflon bomb (denoted as the bomb method). In the ultrasonic method, small amounts of Ti, Mo and Sn coprecipitation were observed with 100% Ca and 100% Mg fluorides. No coprecipitation of Ti, Mo, Sn and Sb in Ca-Al-Mg fluorides occurred when the sample was decomposed by the bomb method except for 100% Ca fluoride. Based on our coprecipitation observations, we have developed a simultaneous determination method for B, Ti, Zr, Nb, Mo, Sn, Sb, Hf and Ta by Q-pole type ICP-MS (ICP-QMS) and sector field type ICP-MS (ICP-SFMS). 9-50 mg of samples with Zr-Mo-Sn-Sb-Hf spikes were decomposed by HF using the bomb method and the ultrasonic method with B spike. The sample was then evaporated and re-dissolved into 0.5 mol FI HF, followed by the removal of fluorides by centrifuging. B, Zr, Mo, Sn, Sb and Hf were measured by ID method. Nb and Ta were measured by the ID-internal standardization method, based on Nb/Mo and Ta/Mo ratios using ICP-QMS, for which pseudo-FI was developed and applied. When 100% recovery yields of Zr and Hf are expected, Nb/Zr and TAW ratios may also be used. Ti was determined by the ID-internal standardization method, based on the Ti/Nb ratio from ICP-SFMS. Only 0.053 ml sample solution was required for measurement of all 9 elements. Dilution factors of <= 340 were aspirated without matrix effects. To demonstrate the applicability of our method, 4 carbonaceous chondrites (Ivuna, Orgueil, Cold Bokkeveld and Allende) as well as GSJ and USGS silicate reference materials of basalts, andesites and peridotites were analyzed. Our analytical results are consistent with previous studies, and the mean reproducibility of each element is 1.0-4.6% for basalts and andesites, and 6.7-11% for peridotites except for TiO2. (c) 2006 Elsevier B.V All rights reserved.

DOI： 10.1016/j.chemgeo.2006.08.007

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