SGC50-01

会場:102A

時間:5 月 25 日 14:15-14:30

海洋地殻の層状構造と２段階脱水：HIMU-FOZO-PREMA の成因に関する解釈
Geochemical connection between HIMU-FOZO-PREMA: two-stage dehydration of oceanic
crust with layered structure
下田 玄 1∗ ; 小木曽 哲 2
SHIMODA, Gen1∗ ; KOGISO, Tetsu2
1

産業技術総合研究所, 2 京都大学人間・環境学研究科
Geological Survey of JAPAN, 2 Graduate School of Human and Environmental StudiesGraduate School of Human and Environmental Studies
1

Isotopic composition of the ocean island basalts (OIBs) can be explained by mixing of isolated reservoirs in the Earth (White,
1985; Zindler and Hart, 1986; Hofmann, 1997; Stracke, 2012). In early research on the mantle reservoirs, the isotopic composition of OIBs was explained by the mixing of depleted MORB mantle (DMM) and three enriched reservoirs, those are HIMU
(high-u: u = 238U/204Pb) EM1 (Enriched Mantle 1) and EM2 (Enriched mantle 2). In addition to these reservoirs, importanceof reservoirs whose isotopic compositions are common and intermediate has been pointed out, such as, FOZO (Focal Zone, Hart
et al., 1992) and PREMA (Prevalent Mantle, Zindler and Hart, 1986). Although these intermediate reservoirs have been used todescribe the isotopic distribution of OIBs, existences of the intermediate reservoirs, themselves, are still in debate. Therefore,
elucidating the origin of the intermediate reservoirs should be also important from the perspective of production of mantle heterogeneity (e.g., Hofmann, 1997; Stracke et al. 2005; Stracke, 2012).
To elucidate the origin of mantle reservoirs, geochemical modeling has been conducted, in which origin of HIMU, FOZO and
PREMA is discussed. The results suggest that MORBs with high frequency (common MORBs) have chemical composition thatis suitable for production of PREMA if chemical modification during subduction is negligible. It may suggest the importanceof recycling of dry MORBs for the production of OIBs source. Dehydration of common MORBs beneath an arc can produce
FOZO isotopic signatures if amount of dehydrated water is large (4 %) which may represent maximum amount of dehydrated
aqueous fluid at sub-arc area. Thus, FOZO-PREMA isotopic array can be explained by recycling of common MORBs that releasevarious amount of aqueous fluid during subduction. For the production of HIMU, additional dehydration (˜2%) at high pressure
is required. As high pressure dehydration release supercritical fluid, chemical fractionation during dehydration is different fromthe sub-arc process. This difference can explain isotopic difference between HIMU and FOZO.
キーワード: HIMU, FOZO, PREMA, MORBs, OIBs
Keywords: HIMU, FOZO, PREMA, MORBs, OIBs

1/1

SGC50-02

会場:102A

時間:5 月 25 日 14:30-14:45

Ocean Basalt Simulator version 1 (OBS1): 輝石岩を含むマントルかんらん岩断熱融解における微量元素マスバランス
Ocean Basalt Simulator version 1 : Trace element mass balance in adiabatic melting of apyroxenite-bearing peridotite
木村 純一 1∗ ; 川畑 博 2
KIMURA, Jun-ichi1∗ ; KAWABATA, Hiroshi211（独）海洋研究開発機構, 2 高知大学
JAMSTEC, 2 Kochi University

We present a new numerical trace element mass balance model for adiabatic melting of a pyroxenite-bearing peridotite for
estimating mantle potential temperature, depth of melting column, and pyroxenite fraction in the source mantle for a primaryocean basalt/picrite. The Ocean Basalt Simulator version 1 (OBS1) uses a thermodynamic model of adiabatic melting of apyroxenite-bearing peridotite with experimentally/thermodynamically parameterized liquidus?solidus intervals and source mineralogy. OBS1 can be used to calculate a sequence of adiabatic melting with two melting models, including (1) melting of
peridotite and pyroxenite sources with simple mixing of their fractional melts (melt-melt mixing model), and (2) pyroxenite
melting, melt metasomatism in the host peridotite, and melting of the metasomatized peridotite (source-metasomatism model).
OBS1 can be used to explore (1) the fractions of peridotite and pyroxenite, (2) mantle potential temperature, (3) pressure oftermination of melting, (4) degree of melting, and (5) residual mode of the sources. In order to constrain these parameters, themodel calculates a mass balance for 26 incompatible trace elements in the sources and in the generated basalt/picrite. OBS1 iscoded in an Excel spreadsheet and runs with VBA macros. Using OBS1, we examine the source compositions and conditionsof the mid-oceanic ridge basalts, Loihi?Koolau basalts in the Hawaiian hotspot, and Jurassic Shatsky Rise and Mikabu oceanic
plateau basalts and picrites. The OBS1 model shows the physical conditions, chemical mass balance, and amount of pyroxenite
in the source peridotite, which are keys to global mantle recycling.
キーワード: かんらん岩, 輝石岩, 断熱融解, 微量元素, マスバランス
Keywords: Peridotite, Pyroxenite, Adiabatic melting, Trace element, Mass balance

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SGC50-03

会場:102A

時間:5 月 25 日 14:45-15:00

Geochemical Composition of Dykes along the Cameroon Volcanic Line (CVL): Constrains on CVL magmatism and tectonism
Geochemical Composition of Dykes along the Cameroon Volcanic Line (CVL): Constrains on CVL magmatism and tectonism
ASAAH, Asobo nkengmatia elvis1∗ ; YOKOYAMA, Tetsuya1 ; AKA, Festus tongwa2 ; USUI, Tomohiro1 ;KURITANI, Takeshi3 ; WIRMVEM, Mengnjo jude4 ; IWAMORI, Hikaru1 ; OHBA, Takeshi4
ASAAH, Asobo nkengmatia elvis1∗ ; YOKOYAMA, Tetsuya1 ; AKA, Festus tongwa2 ; USUI, Tomohiro1 ;KURITANI, Takeshi3 ; WIRMVEM, Mengnjo jude4 ; IWAMORI, Hikaru1 ; OHBA, Takeshi41Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-, 2 Institute of Mining
and Geological Research, P.O. Box 4110, Yaounde, Cameroon, 3 Graduate school of Science, Osaka City University, 3-3-138
Sugimoto, Sumi Yoshi-ku, Osaka 558-8585,, 4 Department of Chemistry, School of Science, Tokai University, Hiratsuka, 2591211, Japan1
Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-, 2 Institute of Mining
and Geological Research, P.O. Box 4110, Yaounde, Cameroon, 3 Graduate school of Science, Osaka City University, 3-3-138
Sugimoto, Sumi Yoshi-ku, Osaka 558-8585,, 4 Department of Chemistry, School of Science, Tokai University, Hiratsuka, 2591211, Japan
Mafic dykes along the Cameroon Volcanic Line (CVL) align in a general N 30?? - N 70?? direction. These alignments are
consistent with the tectonic lineaments and the orientation of the CVL, indicative of a time and space relationship between thedykes and CVL lavas with the tectonic activities of the African plate. However, sub-alkaline volcanic rocks studied in someareas along the CVL are thought to be older than and not related to the CVL magmatism. Mafic dykes outcrop in the Oku
Volcanic Group (OVG) but have not been studied. The aim of this study is to 1) characterize volcanic dykes in the OVG and
Mt. Bambouto, 2) constrain the geochemical relationship between the dykes and CVL alkaline lavas, and 3) establish a tectonic
history of emplacement of these dykes. Three dykes in the OVG and two along the Dschang-Santcho escarpment (flank of Mt.
Bambouto) were systematically studied. An alkaline dyke is observed intruding a sub-alkaline dyke for the first time along theCVL, indicating a time and space relation between the two series. The dykes are weathered and show chemical evidence of
crustal contamination. Contamination of less than 20 % for the alkaline dykes (BD2 and BDS alkaline) and 30 - 40% (BDS
sub alkaline, BD3 and DS), can explain the varying degrees of crustal signatures in the samples. Geochemical characteristics
suggest that the Batibo alkaline intrusions and the sub-alkaline intrusions were derived from a similar mantle source with OVG
lavas. These characteristics are similar to those of the CVL alkaline lavas and OIBs. However, the differences in elemental
concentrations and isotope composition of the sub alkaline dykes and the alkaline dykes in the OVG can be ascribed to varying
degrees of weathering, assimilation of crustal components and different degrees of partial melting. The Low MgO, Ni, Cr, and V
in the Batibo alkaline intrusion are due to the rapid rate of chemical weathering of olivine. The dykes in this study are related to
the CVL magmatism in space and time. Fractures cutting through the basement rocks, but not the dykes suggest that the dykes
are younger than the tectonic activities. The Sub-alkaline intrusion that is also intruded by the alkaline dyke in the BDS is older(43 - 422 Ma). This indicates that its emplacement was earlier than the surface expression of the alkaline CVL magmatism in the
Tertiary (<31 Ma).
キーワード: Oku volcanic group, Dykes, Crustal contamination, Cameroon Volcanic Line, Magmatism, Tectonism
Keywords: Oku volcanic group, Dykes, Crustal contamination, Cameroon Volcanic Line, Magmatism, Tectonism

1/1

SGC50-04

会場:102A

時間:5 月 25 日 15:00-15:15

地球全ケイ酸塩のハフニウム同位体進化：隕石ジルコンからの証拠
The Hf isotope evolution of the bulk silicate Earth: Evidence from meteorite zircon
飯塚 毅 1∗ ; 山口 能央 1 ; 日比谷 由紀 1 ; アメリン ユーリー 2
IIZUKA, Tsuyoshi1∗ ; YAMAGUCHI, Takao1 ; HIBIYA, Yuki1 ; AMELIN, Yuri2
1
1

東京大学, 2 オーストラリア国立大学
The University of Tokyo, 2 The Australian National University
176

Lu-176 Hf 放射壊変系列は，地球の地殻ーマントル分化の時間スケールやメカニズムを制約するのに広く用いられている．しかし，Lu-Hf 同位体データを正確に解釈するには，地球のケイ酸塩部分全体（Bulk Silicate Earth: BSE）の Hf
同位体進化を知る必要がある．Lu と Hf は共に難揮発性かつ親石性元素であるため，BSE の Hf 同位体進化曲線は，未分化隕石コンドライトの現在の 176 Hf/177 Hf 及び 176 Lu/177 Hf から推定されてきた．しかし，変成作用時の Lu と Hf の再分配により，コンドライト中のこれらの同位体比は非常に大きな変動を示すため，現在用いられている BSE モデルの176
Hf/177 Hf 及び 176 Lu/177 の値の妥当性を検証する必要がある．さらに近年，太陽系初期に光励起を介した 176 Lu 壊変の加速がコンドライト中で進んだとする仮説が提唱されており，この仮説が正しければコンドライト隕石の Lu-Hf 同位体
組成は BSE のそれを反映しない．
本研究では，隕石中に含まれるジルコン鉱物の高精度 Lu-Hf 同位体分析を行った．ジルコンは変成作用の影響を受けづらく，非常に低い Lu/Hf をもつために，結晶化時の Hf 同位体組成を保持できる．そこで本研究では，ユークライト
隕石に含まれる初期太陽系に結晶化したジルコンを用いて，太陽系の初生 176 Hf/177 Hf を決定した．また我々は，この太陽系初生 176 Hf/177 Hf を用いて，コンドライト中の 176 Lu 壊変率が太陽系史を通して一定であったことを示し，さらに，
太陽系形成時の Lu-Hf 同位体組成を保持している最も初生的なコンドライト隕石を同定することに成功した．本研究で得られた新しい BSE の Lu-Hf 組成（176 Hf/177 Hf = 0.282793 +/- 11; 176 Lu/177 Hf = 0.0338 +/- 1）から，地球の冥王代ジル
コンの Hf 同位体組成は 45 億年前以前の地球ケイ酸塩分化イベントを反映していることが明らかになった．

キーワード: 地球全ケイ酸塩, ジルコン, ハフニウム同位体, 地殻ーマントル分化, ユークライト, 初期太陽系
Keywords: bulk silicate Earth, zircon, hafnium isotopes, crust-mantle differentiation, eucrite, early solar system

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SGC50-05

会場:102A

時間:5 月 25 日 15:15-15:30

金属−硫化物メルト間の元素分配挙動
Partitioning behavior of elements between metal and sulfide melt
鈴木 敏弘 1∗ ; 平田 岳史 2 ; 高橋 栄一 1
SUZUKI, Toshihiro1∗ ; HIRATA, Takafumi2 ; TAKAHASHI, Eiichi1
11

東京工業大学 地球惑星科学科, 2 京都大学 地球惑星科学
Earth and Planet. Sci., Tokyo Tech., 2 Earth and Planet. Sci., Kyoto Univ.

金属の固相−液相間の元素分配挙動は、地球型惑星の核の進化を解明するための重要な情報である。珪酸塩系の研究
では、分配係数がイオン半径や結晶構造と密接に関連している事が知られている事から、金属系においても原子半径との関連性が議論されている。例えば、 Van Orman et al. (2008) は高圧下で Fe-S 系での Re,Os,Pt の分配係数を測定し、原子半径を用いればこれらの結果を Lattice Strain Model (Blundy & Wood, 1994) で説明できるとした。一方、Stewart et al.
(2009) は原子半径ではなく、Neutral Atom Radius (Clementi et al. 1967) を用いる事により、説明できると主張した。しかしながら、固相の基本構造を形成する Fe よりもかなり大きな Os が最大の分配係数を示す事は奇妙であった。金属系の元素分配係数は、液相中の軽元素濃度（硫黄、リン、炭素）に大きく依存している事が知られており、分配係数が化学的親和性と関連している事が考えられる。本研究では異なる硫黄濃度で FeNi-S 系の高圧融解実験を行い、分配挙動の変化を観察した。
白金属などの 14 元素をそれぞれ約 150ppm 添加した Fe-Ni(95:5) 合金を、アーク放電法を用いて合成した。この合金を少量の FeS 粉末とともに MgO カプセルに入れ、東京工業大学の川井型マルチアンビルを用いて高圧融解実験を行った。
回収した試料の主成分元素組成は EPMA で測定し、微量元素濃度は京都大学の LA-ICP-MS で測定した。
得られた分配係数は、これまでの研究と同様に、液相中の硫黄濃度に大きく依存していた。このため、化学的親和性を排除して原子半径と分配係数の関係のみを観察するためには、純粋な金属だけの系での分配係数から議論する必要がある。本研究では、硫黄濃度が減少するとともに、元素間の分配係数の差が減少する事が観察された。このため、硫黄を含まない系では、Au や W のように Fe よりも非常に大きな元素でも、その分配係数に大きな差は無いと推定される。
Blundy & Wood(1994), Nature 372, 452-454.
Clementi et al.(1967), J. Chem. Phys., 47, 1300-1307.
Stewart et al.(2009), Earth Planet. Sci. Lett. 284, 302-309.
Van Orman et al. 2008), Earth Planet. Sci. Lett. 274, 250-257.
キーワード: 元素分配, 金属, 硫化物, 超高圧
Keywords: element partitioning, metal, sulfide, high pressure

1/1

SGC50-06

会場:102A

時間:5 月 25 日 15:30-15:45

シングルショットレーザーアブレーション多重検出器型 ICP 質量分析法を用いたジルコンの U-Pb 年代分析
Single shot laser ablation-multiple collection-ICP-mass spectrometry for U-Pb age
服部 健太郎 1∗ ; 坂田 周平 1 ; 平田 岳史 1
HATTORI, Kentaro1∗ ; SAKATA, Shuhei1 ; HIRATA, Takafumi1
1
1

京都大学 大学院 理学研究科 地球惑星科学専攻
Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University

Combination of laser ablation sample introduction technique and ICP-mass spectrometry (LA-ICPMS) has been widely used
for element and isotopic analyses for solid samples1) . One of the great advantages of LA-ICPMS is high quantitative capability
achieved by the ”post ionization” configuration. With the post ionization, ionization and atomization of the analytes in the plasma
source will be independently carried out, and therefore, the optimization of the operational conditions for both the sampling andionization can be made. This suggests that the hard ionization can be carried out even under the ”soft” laser sampling. One of the
major source of the analytical error is an elemental fractionation during the laser ablation (down-hole fractionation)2) . In fact,
the measured Pb/U ratio can change with time when the larger ablation was made with ablation spots of higher aspect ratios3) .
To overcome this, we have tried to measure U-Pb ages from the ablation spots of smaller aspect ratio (i.e., shallower pit).
The shallower ablation pit could be obtained when a smaller number of laser shots were adopted. To achieve this, a multiple
collector-ICPMS was adopted to the U-Pb age determinations. With the multiple collector system, precision in the isotope ratio
measurements can be improved even from the transient signals produced by the smaller number of laser shots. Hence, six isotopes
were monitored by the six independent multipliers ; 202 Hg (IC), 204 Pb (IC), 206 Pb (IC), 207 Pb (IC), 208 Pb (IC), 232 Th (F) and235
U or 238 U (IC).
Three natural zircons (OD-34) , Plesovice5) and GJ-16) ) were employed to evaluate the reliability of the resulting U-Pb ages.
Nancy 915007) zircon was used as a calibration standard for the Pb/U ratio measurements. The glass standard (NIST SRM 610)
was used for the calibration in the 207 Pb/206 Pb isotope ratio measurements. Laser ablation system used in this study was a New
Wave Research femto laser ablation system (Fremont, CA 94538, USA), and the ICPMS instrument used in this study was a Nu
Instruments (Wrexham, UK) Nu Plasma II multi-collector ICPMS. For comparison, U-Pb ages were also measured by the single
collector-ICPMS instrument (Nu Instruments AttoM). In our presentation, we will demonstrate the high analytical capability of
the MC-ICPMS technique from the single shot laser sampling.
References
1) J.Koch and D.Guenther, Applied Spectroscopy, 65(5), 155A-162A (2011).
2) S.Eggins, L.Kinsley and J. Shelly, Applied Surface Science, 127-129, 278-286 (1998).
3) I.Horn, R.L Rudnick and W. F. McDonough, Chemical Geology, 164, 281-301 (2000).
4) H.Iwano et al., Island Arc, 22.3, 382-394, (2013).
5) J.Sláma et al., Chemical Geology, 249, 1-35 (2008).
6) S.E.Jackson et al., Chemical Geology, 211, 47-69 (2004).
7) M.Wiedenbeck et al., Geostandard Newsletter, 19, 1-23 (1995).
キーワード: LA-MC-ICPMS, U-Pb 年代, ジルコン, シングルショット, 元素分別, アスペクト比
Keywords: LA-MC-ICPMS, U-Pb age, zircon, single shot, elemental fractionation, aspect ratio

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SGC50-07

会場:102A

時間:5 月 25 日 15:45-16:00

有孔虫の Nd 同位体分析を目的とした極微量 Nd 同位体分析法の開発
Isotope analysis of ng-sized Nd samples: toward the isotopic analysis of foraminifera
extracted from deep-sea sediments
若木 重行 1∗ ; 吉村 寿紘 2 ; 黒田 潤一郎 2 ; 大河内 直彦 2 ; 石川 剛志 1
WAKAKI, Shigeyuki1∗ ; YOSHIMURA, Toshihiro2 ; KURODA, Junichiro2 ; OHKOUCHI, Naohiko2 ;ISHIKAWA, Tsuyoshi1
1
1

海洋研究開発機構 高知コア研究所, 2 海洋研究開発機構 生物地球化学研究分野
KOCHI, JAMSTEC, 2 Department of Biogeochemistry, JAMSTEC

Modern seawater has heterogeneous Nd isotopic compositions. The source rocks of Nd in the ocean, continental rocks andigneous rocks derived from the mantle, show wide range of variation in their radiogenic 143 Nd/144 Nd ratios (˜5 permil). The
short residence time of Nd in the ocean (300-500 yr: Tachikawa et al., 2003; Amakawa et al., 20004; Arouze et al., 2009)
compared with the ocean mixing timescale of 1500 yr (Broecker and Peng, 1982) prevents the homogenization of seawater Ndisotopic compositions. The Nd isotopic variation of the seawater has been used to distinguish different water masses, to trace theocean circulation and to estimate the flow rate and contribution of Nd from various sources (ca. Piepgras and Wassurburg, 1980;
Amakawa et al., 2013).
Calcareous shells of foraminifera are now drawing attention as a proxy of seawater Nd in the past, which can be used to
trace the changes of the oceanic Nd isotopic composition related with paleoclimate changes (Tachikawa et al., 2014). One ofthe difficulties in analyzing Nd isotopic composition of foraminifera comes from the low Nd concentration of the foraminiferal
shells. The low Nd concentration (up to several ppm) of foraminiferal shells requires quite large sample sizes (several mg) tomeet the minimum sample size required for Nd isotope analysis (Nd amount of 2-5 ng). Lowering the sample size limits for
Nd isotope analysis may expand the applicability of the technique to foraminifera-poor sediments derived from oceanic drillingsites.
We have developed a new technique to measure isotope ratios of ng to sub-ng sized Nd samples using total evaporation
normalization method (Wakaki et al., 2007) together with the low-blank Nd separation chemistry. Isotope ratios of Nd aremeasured using thermal ionization mass spectrometer (Thermo TRITON) at Kochi Core Center.
The 143 Nd/144 Nd values of the isotopic reference reagent JNdi-1, measured at sample sizes of 0.5, 1 and 2 ng of Nd, allagreed with the values obtained by large sample measurement (100 ng of Nd) within statistical errors. The reproducibility of
143Nd/144Nd values are 100 ppm (n = 8), 60 ppm (n = 9) and 40 (n = 8) ppm for 0.5, 1 and 2 ng measurements, respectively. The
precision and accuracy of the sample analysis, including chemical procedure, is checked by analyses of two GSJ reference rocks,
JB-2 (basalt) and JCp-1 (coral). The basalt sample was decomposed by mixed HF-HNO3-HClO4, and the coral sample wasdecomposed by HNO3. Sample aliquots, containing 0.5 and 1.2 ng of Nd, respectively, were processed by using the low-blank
Nd separation chemistry. The measured 143Nd/144Nd ratios of JB-2 (n = 9) and JCp-1 (n = 8) samples all agreed with the values
obtained by measurements with large sample sizes within statistical errors. These results demonstrate that our technique gives
reliable results even for samples as small as 0.5 ng of Nd. Preliminary results of subsurface-dwelling planktonic foraminifera
samples, Globorotalia tumida, collected from a Quaternary sediment core from the West Caroline Basin (western equatorial
Pacific Ocean) will be presented.
キーワード: 古環境, Nd 同位体, 有孔虫
Keywords: paleoclimate, Nd isotopes, foraminifera

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SGC50-08

会場:102A

時間:5 月 25 日 16:15-16:30

ベーリング海、Bowers Ridge の起源と漸新世島弧火成作用：IODP exp. 323, Hole
U1342 から得られた結果
Origin of Bowers Ridge and its Oligocene arc magmatism: Results from the IODP Expedition 323, Hole U1342
川畑 博 1∗ ; 佐藤 佳子 2 ; 木村 純一 2 ; 常 青 2 ; 仙田 量子 2 ; 高橋 俊郎 3 ; 平原 由香 4 ; 宮崎 隆 2 ;Scholl David W.5
KAWABATA, Hiroshi1∗ ; SATO, Keiko2 ; KIMURA, Jun-ichi2 ; CHANG, Qing2 ; SENDA, Ryoko2 ;TAKAHASHI, Toshiro3 ; HIRAHARA, Yuka4 ; MIYAZAKI, Takashi2 ; SCHOLL, David W.5
1

高知大学, 2 海洋研究開発機構, 3 新潟大学, 4 愛知教育大, 5 U.S. Geological Survey,
Faculty of Science, Kochi University, 2 Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 3 Faculty of Science, Niigata University, 4 Aichi University of Education, 5 U.S. Geological Survey,
1

The Bowers Ridge is an 800 km-long arcuate aseismic ridge extending northward from the Central Aleutian Ridge. IODP
Expedition 323 drilled a total of 41.54 m of igneous basement at site U1342 in the northwestern Bowers Ridge, where an 800
m-deep wave-eroded platform exist. The recovered basement, composed of andesitic lavas and basaltic to andesitic volcaniclastic
rocks (Kawabata et al., 2011), provide an opportunity to explore temporal change in magma composition at Bowers Ridge justbefore the cessation of arc magmatism in the Oligocene.
The recovered volcanic samples yield 26-34 Ma 40 Ar-39 Ar ages (Sato et al., 2015) and have arc signature characterized bydepletion of HFSE and enrichment of LILE on the multi-element diagrams normalized to NMORB. The trace element features,with the presence of anorthitic plagioclase phenocrysts (An93), confirm the arc-origin of the ridge that was deduced from dredge
samples around Site U1342 (Cooper et al. 1987; Wanke et al., 2012). In arc magmatism, flux from subducted oceanic crust
played an important role as suggested by high Ce/Pb and unradiogenic 206 Pb/204 Pb.
Temporal change of magma chemistry was examined from both the core and reported dredge samples. Lower units of coresamples exhibit low to middle-K series, whereas both the upper units of core and dredge samples show middle to high-K series.
In addition, the dredge samples are more enriched in LREE, Rb, and Ba, and show higher (LREE, MREE)/HREE ratios thanthe core samples. The dredge basalts and andesites fall within the adakite field on the discrimination diagrams of Sr/Y vs Y and
La/Yb vs Yb (Wanke et al., 2012), whereas all the core samples are non-adakitic basalts and andesites. These geochemical results
suggest that the difference in magma compositions between the core and dredge samples could mainly reflect the difference inthe amount of slab-derived flux and/or the flux compositions. In this context, the inferred slab melting event (Wanke et al., 2012)
may not be a unique scenario for producing the adakitic dredge basalts and andesites. Larger amount of slab-derived flux andhigher melting pressure can also explain the change in magma compositions from the non-adakitic core samples to the adakitic
dredge samples. Thus, more careful examination is required to determine whether or not the already inferred extraordinary highgeothermal gradient is essential for the Oligocene magmatism at the Bowers Ridge.

キーワード: アダカイト, バウアーズリッジ, 残存島弧
Keywords: adakite, Bowers Ridge, remnant arc

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SGC50-09

会場:102A

時間:5 月 25 日 16:30-16:45

フッ素含有量から明らかにするチリ南部火山帯の島弧火成活動への新たな知見
Fluorine content reveals new insight into arc magmatism of Southern Volcanic Zone,
Chile.
遠山 知亜紀 1∗ ; 折橋 裕二 2 ; 新正 裕尚 3 ; 角野 浩史 4 ; 村松 康行 5
TOYAMA, Chiaki1∗ ; ORIHASHI, Yuji2 ; SHINJOE, Hironao3 ; SUMINO, Hirochika4 ; MURAMATSU, Yasuyuki51
1

海洋研究開発機構, 2 東京大学地震研究所, 3 東京経済大学, 4 東大院理, 5 学習院大学
JAMSTEC, 2 ERI, Univ. of Tokyo, 3 Tokyo Keizai Univ., 4 Univ. of Tokyo, 5 Gakushuin University

The halogen elements have high partition coefficients in aqueous fluid (Bureau et al., 2000), but the difference in their partition
coefficients for aqueous fluid/silicate melt/silicate minerals/organic matter results in distinct elemental ratios in seawater, pore
fluid, sediment, oceanic crust and mantle (e.g., Johson et al., 2000; Burgess et al., 2002; Palme and O’Neil, 2004). For instance,
fluorine is incorporated into apatite, amphibole and phengite (Svensen et al., 2001) while iodine has a high abundance in organicrich marine sediments because it is an essential element for life. These characteristics make halogens useful to trace water cycling
in subduction zones (e.g., Fehn et al., 2003; Muramatsu et al., 2007; Sumino et al., 2010). Recently, subduction of marine pore
fluid into the mantle has been suggested based on noble gas isotopes in the mantle (Holland and Ballentine, 2006) and halogen
compositions of exhumed mantle wedge peridotites (Sumino et al., 2010). However, only a little is known about the behaviorof halogen elements during subduction processes and their fate in the earth ′ s mantle. Therefore, we newly analyzed halogen
concentrations in regional representative lavas obtained from 13 volcanoes on the Quaternary volcanic front of the Southern
Volcanic Zone (SVZ) of Andean arc in Chile. In this presentation, we focus on fluorine content that has least influence ofdegassing on volcanism.
Fluorine contents of most analyzed samples are 100-300 ppm. To understand behavior of fluorine on fractional crystallization,
the correlations of SiO2 and FeO∗/MgO with fluorine data from a volcano are plotted. This result indicates that the fluorine
dependency with these parameters changes at SiO2 = 55 wt% and FeO∗/MgO = 4.2, resulting from apatite crystallization andremoval from the magma. Thus, the evolved rock samples from which apatite crystallized were excluded from further discussion.
We compared fluorine with major and trace element data obtained by previous study (Shinjoe et al., 2013) and newly revealed
that the fluorine concentrations of selected samples were not correlated with boron concentrations. This means that fluorineshows different behavior in the subduction zone magmatism though both fluorine and boron are fluid-mobile elements. The
basaltic samples from Hudson, the southernmost volcano of SVZ, show very high fluorine concentration (∼1000 ppm). Similar
anomalies were confirmed also with Nb and Ta. The chemical characteristics suggest that the magma source of this region has
been derived from the metasomatized mantle wedge by influence of slab-derived melt (Kilian and Behrmann 2003; Shinjoe etal., 2013). The fluorine concentration possibly becomes new sensitive detector of slab-derived melt in arc volcanism.

キーワード: フッ素, 沈み込み帯, スラブメルト, スラブ流体
Keywords: fluorine, subduction zone, slab melt, slab fluid

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SGC50-10

会場:102A

時間:5 月 25 日 16:45-17:00

阿蘇火山火砕流堆積物中のアパタイトの揮発性元素組成
Volatile components of apatite grains from pyroclastic flow deposits of Aso volcano
道久 真理絵 1∗ ; 佐野 有司 2 ; 高畑 直人 2 ; 石田 章純 2 ; 小池 みずほ 2 ; 吉田 健太 3 ; 東野 文子 3 ;小木曽 哲 1
DOKYU, Marie1∗ ; SANO, Yuji2 ; TAKAHATA, Naoto2 ; ISHIDA, Akizumi2 ; KOIKE, Mizuho2 ;YOSHIDA, Kenta3 ; HIGASHINO, Fumiko3 ; KOGISO, Tetsu1
1

京都大学人間・環境学研究科, 2 東京大学大気海洋研究所, 3 京都大学理学研究科
Graduate School of Human and Environmental Studies, Kyoto University, 2 Atmosphere and Ocean Research Institute, The
University of Tokyo, 3 Graduate School of Science, Kyoto University

1

地球の水は，表層や内部において生命活動やマントルダイナミクスなどに大きな影響を及ぼしている．そのような水の起源の解明のためには，初期地球内部の水の量や，地球の材料物質と考えられている隕石中の揮発性成分量を明らかにすることが重要である．この手がかりの 1 つに，地球最古のジルコンが包有する初生的なアパタイトがある (山本 他,
2013)．アパタイトは結晶中に揮発性成分 (F, Cl, OH) を持つため，このアパタイトから初期地球内部の水，さらには F, Clについての情報を得ることが期待される．また，隕石中のアパタイトから地球の材料物質の水や F, Cl について推測することも可能であると考えられる．
しかしながら，アパタイト-メルト間の揮発性成分量の関係についての知識は不十分であり，特にアパタイト中の水についての先行研究は多くない．Mathez and Webster (2002) や Webster et al. (2009) では，高温高圧実験によってアパタイ
ト-メルト-フルイド間の F, Cl の分配について明らかにしようとしているが，アパタイト中の OH や揮発性元素サイト (Xサイト) 中での OH-F-Cl の交換関係については議論されていない．また，Pan and Fleet (2002) では，アパタイト中の揮発性元素の組成は揮発性元素以外の元素の影響を受けると述べられている．よって本研究では，阿蘇火山火砕流堆積物中のアパタイトと，斜長石中のメルトインクルージョンの揮発性成分を分析し，水も考慮に入れた X サイトでの交換関係
や，揮発性元素組成について影響を与え得るその他の元素について調べることで，アパタイト-メルト間で揮発性成分がどのように分配されるかを明らかにすることを目的とする．
本研究では，Aso-3 と Aso-4 それぞれの安山岩質軽石と流紋岩質軽石の計 4 試料を用いた．岩石試料からアパタイトを取り出し，EPMA(京大理学研究科) で主成分元素と F, Cl 量を、nano-SIMS(東大大気海洋研) では OH 量を組成分析した．斜長石中のメルトインクルージョンは小豆畑 (2009) の試料を用い，EPMA で主成分元素と F, Cl 量を組成分析した．
分析の結果，1 試料中のアパタイトの Cl 量はほぼ一定である一方，F 量に幅があった．これは F と OH が交換関係にあることを示す．また，F, Cl は流紋岩質試料の方に，OH は安山岩質試料の方により多いことがわかった．メルトインク
ルージョンとアパタイトの間に F, Cl の相関性は見られなかったが，温度・圧力・斜長石組成等から見積もられたホスト
マグマ中の H2 O 量 (Kaneko et al., 2007) とアパタイト中の OH 量との間には正の相関がみられた．
また，アパタイト中の OH 濃度と CaO, P2 O5 , MgO 各濃度に正の相関が見られたことから，F または OH の挙動に影響を与える要因として Ca, P, Mg の存在が考えられる．例えば，アパタイト中の Ca がその他の元素 (Na, K, REE など) や空孔に置換されることに伴って X サイトで OH が F に置換される可能性がある．さらに，アパタイト族は M5 (ZO4 )3 X で表されるが，アパタイトの M サイトに入る陽イオン量は酸素 13 に対し 5 と期待されるところ，本試料では 4.66-4.88 と少なかった．そのため，希土類元素など M サイトに入る可能性のある元素についてさらなる分析が必要である．
キーワード: アパタイト, 揮発性元素, 水Keywords: apatite, volatile component, water

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SGC50-11

会場:102A

時間:5 月 25 日 17:00-17:15

先小御岳火山のマグマの分化にかかわるザクロ石・角閃石の分別作用
Garnet-amphibole fractionation generating basalt to dacite magmas of Pre-Komitake volcano, Japan
柴田 知之 1∗ ; 吉本 充宏 2 ; 藤井 敏嗣 2 ; 中田 節也 3
SHIBATA, Tomoyuki1∗ ; YOSHIMOTO, Mitsuhiro2 ; FUJII, Toshitsugu2 ; NAKADA, Setsuya3
1

京都大学大学院理学研究科附属地球熱学研究施設, 2 山梨県富士山科学研究所, 3 東京大学地震研究所
Beppu Geothermal Research Laboratory, Kyoto University, 2 Mount Fuji Research Institute, Yamanashi Prefectural Government, 3 Earthquake Research Institute, University of Tokyo

1

The Pre-Komitake volcano, which was discovered by scientific drilling into Mt. Fuji, is an older volcanic body than Fuji and
Komitake volcanoes buried beneath the NE flank of Fuji volcano (Nakada et al., 2004; Yoshimoto et al., 2010). The Fuji, Komitake, and Pre-Komitake volcanoes are situated in a complex tectonic setting near the junction of three plates: the North American
Plate in the east, the Eurasian Plate in the west, and the Philippine Sea Plate in the south, under two of which the Pacific Plate
subducts from east to west. The magmas of Pre-Komitake are different from those of Fuji and Komitake (Yoshimoto et al., 2010).
According to Yoshimoto et al. (2010), the Pre-Komitake lavas are characterized by hornblende-bearing andesites and dacites,in contrast to the hornblende-free basaltic rocks of Fuji and Komitake. These authors also suggested that the concentrations ofincompatible elements increase with increasing SiO2 in the magmas of Pre-Komitake, whereas those of Fuji increase despite
SiO2 remaining fairly constant (Fujii, 2007). They suggested that the variations in magma chemistry in this area between 250 ka
and recent times might have occurred as a result of a change in regional tectonics that occurred ˜150 ka, although this remains
unproven. To our knowledge, no detailed research has yet been carried out into Pre-Komitake magma genesis on the basis ofcomprehensive major and trace elements, and isotopic compositional data sets. Therefore, we analyzed the trace element andSr-Nd isotopic compositions of the Pre-Komitake magmas in order to better understand their genesis.
The Sr and Nd isotope ratios ranged from 0.703320?0.703476, and 0.512885?0.513087, respectively, which are very similar
to those of the lavas from Fuji and Komitake volcanoes (Nagai et al., 2004) that erupted subsequently. Enrichment of large
ion lithophile elements, Pb and Sr, can be seen in the primitive mantle-normalized multi-element diagram of the Pre-Komitake,
Komitake, and Fuji lavas. These collectively show island arc lava signatures, however, the middle to heavy rare earth elements
are more depleted in the Pre-Komitake lavas, compared to those from Fuji. Positive Eu anomalies are observed, although theextents of these anomalies decrease with increasing SiO2 in the Pre-Komitake lavas, whereas this is not observed in Fuji lavas.
The Sr/Y ratios of Pre-Komitake lavas increase from basalt to basaltic andesite, but decreases through andesite to dacite. Thisoccurs in combination with a rapid increase in La/Yb ratios, followed by a more gradual increase. A gradual decrease in Dy/Yb
ratios is also seen over the entire compositional range. These data suggest deep (>12 kbar) fractionation of garnet and amphibole
followed by shallow (i.e., ˜5 kbar) fractionation of amphibole and plagioclase. Such variations are not observed in the Komitake
and Fuji lavas, for which deep fractionation of clinopyroxene and shallow fractionation of plagioclase have been suggested. All
three lavas, including those from the Pre-Komitake volcano, show similar isotopic, major, and trace element compositions in theunfractionated basalts. The differing geochemical trends found in the Pre-Komitake lavas are likely to be due to different mineral
fractionations occurring in the hydrous Pre-Komitake basalts compared to the dry Fuji and Komitake basalts.

キーワード: 先小御岳火山, 富士山, 微量元素, Sr-Nd 同位体, ザクロ石の結晶分化
Keywords: Pre-Komitake volcano, Fuji volcano, Trace elements, Sr-Nd isotope ratios, Garnet fractionation

1/1

SGC50-12

会場:102A

時間:5 月 25 日 17:15-17:30

小笠原ボニナイト中の Cr スピネルへの白金族元素の分配
Partition of the platinum group elements between Cr spinels and whole rocks in boninites
from Ogasawara islands
仙田 量子 1∗SENDA, Ryoko1∗
1
1

独立行政法人海洋研究開発機構
JAMSTEC

Abundance of the platinum group elements (PGEs) in volcanic rocks are expected to have some information about source
mantle or magma genesis as other elements. Sulfides and Cr spinel are known to important host of PGEs, for example, enrichmentof PGEs are reported in chromitites from layered intrusions, ophiolites and alpine-type peridotites (Ahmed and Arai, 2002, Buchl
et al., 2004). Among volcanic rocks, however, Cr spinel can be observed in mafic-ultramafic magmas as komatiite and boninite.
The correlation between whole rock Cr and PGEs in magmatic rocks are observed (Hamlyn et al., 1985), whether PGEs status in
Cr spinels unresolved. Experimental studies on partitioning of PGEs between spinels and silicate melts are reported and showedsignificant amounts of IPGEs (Iridium-group PGE; Ru, Os, Ir) can be held in solid solution in spinel species as Cr spinels withhigh partition coefficient under oxidized conditions (fO2 >QFM +2). In empirical studies, the partitioning of Rh, Ru and Ir into
Cr spinels increases with increasing oxygen fugacity are shown in komatiites by Jung-Woo et al. (2012). Here newly PGE data of
Cr spinels and whole rocks from boninites Ogasawara and Guam islands are shown and have a discussion about PGE partitioning
in boninite and tholeiite magma.
Keywords: platinum group element, boninite, partition

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SGC50-13

会場:102A

時間:5 月 25 日 17:30-17:45

捕獲岩を用いた日本島弧深部地殻の組成推定
Determining the deep crustal composition of Japan island arc using xenoliths
長尾 潤 1∗ ; 小澤 一仁 2
NAGAO, Jun1∗ ; OZAWA, Kazuhito2
1

東京大学大学院理学系研究科地球惑星科学専攻, 2 東京大学大学院理学系研究科地球惑星科学専攻
Department of Earth and Planetary Science Graduate School of Science, The University of Tokyo, 2 Department of Earth andPlanetary Science Graduate School of Science, The University of Tokyo1

大陸地殻の進化過程を理解する上で、大陸地殻組成を正確に決定することは必要不可欠である。大陸深部地殻の試料を直接入手することは困難なため、その組成は地震波データと変成岩や捕獲岩といった岩石試料から推定される。地震波の速度は密度に左右されるため、地震波データからは大陸地殻の構造や主要元素の情報が得られるが、微量元素については調べられない。一方、変成岩や捕獲岩といった岩石試料からは主要元素だけでなく微量元素についても調べられるが、これらの岩石試料を採取できる地域は非常に限られている。したがって、幅広い範囲の大陸深部地殻の主要元素及び微量元素組成を推定するためには、地震波データと岩石試料から得られた化学的データを併せて解釈する必要がある。
これまでに大陸深部地殻の組成は大陸地域については詳細に調べられてきたが、日本の様な島弧については詳しく調べられていない。地震波データによれば、大陸地域と島弧とでは地殻の厚さ・主要元素組成が異なるので、島弧の深部
地殻の組成を推定する必要がある。そのため、今回は秋田県一ノ目潟と佐賀県高島の捕獲岩合わせて約 100 個について
主要元素を XRF で、微量元素を LA-ICPMS を用いて測定した。
主要元素組成から一ノ目潟の捕獲岩は超塩基性岩から中性岩に、高島の捕獲岩は超塩基性岩から酸性岩に分類された。
また微量元素組成からは両地域の捕獲岩は、Nb,Ta,Zr,Hf といった HFSE に欠如し、Pb といった LILE に富むことがわかった。このことからマントルウェッジでのマントルの部分融解の際に流体が関与し、日本島弧深部地殻が形成されたと考えられる。こうした日本島弧の深部地殻の組成と大陸地域の変成岩や捕獲岩から推定されている大陸地域の深部地殻の組成とを比較すると (図 1)、SiO2 濃度は変わらないにも関わらず、ほぼすべての液相濃集元素について大陸地域に比べて島弧の方では濃度が低いことがわかった。また HFSE については、日本島弧の方がより顕著に欠如していることがわかった。これらのことは、日本の沈み込み帯において流体の影響が強く、マントルの部分融解度が大きかったことを反映していると考えられる。U については島弧と大陸地域とで濃度に差は見られなかった。U には酸化的環境下だと流体に溶けやすくなり、還元的環境下だと溶けにくくなるという性質がある。また、約 22 億年前に地球表層は還元的環境か
ら酸化的環境になったことが知られていることから、大陸地域の古い大陸地殻形成時には還元的環境下で U は HFSE 同様、流体に溶けにくかったが、日本島弧の若い大陸地殻形成時には酸化的環境下で LILE 同様、流体に溶けやすくなったと考えられる。今回用いた捕獲岩の主要元素・微量元素間に弱い相関があることが確認された。今後別の地域の捕獲岩でも同様の相関が見られるか調べる必要があるが、こうした相関から地震波データと組み合わせることで日本島弧深部
地殻の広範囲の組成を推定できることが期待される。
キーワード: 捕獲岩, 一ノ目潟, 高島, ニュートリノ, 島弧, 地殻組成
Keywords: Xenolith, Ichinomegata, Takashima, Neutrino, island arc, crustal composition

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