5. 沈み込み帯（3 Subduction zones 3 arc magmatismofgs.aori.u-tokyo.ac.jp/.../ofgd18-05subduction3.key.pdf5. 沈み込み帯（3） Subduction zones 3 arc magmatism 海洋底ダイナミクス

5. 沈み込み帯（3） Subduction zones 3 　　arc magmatism

海洋底ダイナミクス 2018Ocean Floor Geodynamics 2018

沈み込み帯のどこで火成活動は起こっているのか？Where can we observe magmatic/volcanic activities along subduction zones?島弧火成活動の特徴は？ Characteristics of arc magmatism (vs. mid-ocean ridge/intraplate magmatism)

島弧の地殻構造はどうなっているのか？どうやって調査するのか？Structure of arc crust and how to know the structure.

島弧火成活動はどのようにして起こるのか？ What is the mechanism of arc magmatism?島弧地殻と大陸の形成arc magmatism and continental formation

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(Understanding Earth, Sliver&Jordan 2003)

沈み込み帯の火成活動：(島弧)火成活動 subduction zone magmatic activity: arc magmatism

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沈み込み帯の概念図 Generalized cross-section

海溝島弧 (火山弧）

背弧 backarc

前弧 forearc

Slab スラブ

( An introduction to our dynamic planet, Rogers ed., 2008)

overriding platesubducted plate down going plate

4

島弧海溝

アウターライズ

背弧

島弧海溝背弧

trencharcbackarc

outer rise

5

東北弧と伊豆弧 Tohoku and Izu arcs

6

Bathymetry data ETOPO1Gray contour Slab1.0 model[Hayes et al., 2012]

volcano locations: Global Volcanism Program database(Smithsonian Institution, National Museum of Natural History)

A: Izu-BoninB: MarianaC: South America

Longitude Latitude Name139.098 34.9 Izu-Tobu139.394 34.724 Izu-Oshima139.279 34.52 Toshima

139.27 34.397 Niijima139.153 34.219 Kozushima139.526 34.094 Miyakejima139.602 33.874 Mikurajima

139.68 33.4 Kurose Hole

演習：島弧の火山の位置をプロットしてみよう Practice: Plot locations of arc volcanoes

7

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火山弧の位置とスラブ傾角 location of volcanic arc and slab dip

（巽「沈み込み帯のマグマ学」, 1994）

volcanic arc

• スラブ傾角がゆるやか：島弧と海溝の距離大

• 火山弧直下でスラブの上面深度がほぼ一定　108km+-14km

• 一般に，スラブ傾角が急なとき火山弧の幅が狭い

Steep slab : large distance between trench and volcanic arc

Depth of upper surface of slab beneath volcanic arc is constant

Generally speaking, steep slab : narrow volcanic arc

11




12

さまざまなテクトニックセッティングでの火山岩の組成 Volcanic rock geochemistry @ different tectonic setting

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Large variation in SiO2 content

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大島 Izu-Oshima basalt-andesite

雲仙岳　Unzen andesite-dacite

口永良部島　Kuchinoerabu andesite

有珠（昭和新山）　Usu dacite

14

水の含有量 Water content in volcanic glasses

(Stern, 2002)

Convergent boundary = ArcDivergent boundary

= Mid-ocean ridge

15

微量元素の違い Trace elements

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island arc basat (IAB): spiky pattern

LILEs= high solubilities, Rb, Cs, Ba, Sr, (U) Th = from subducted sediment

16




17

音波探査（地震波探査） Acoustic (seismic) surveys

• 震源-観測点距離　<~ 水深

• 境界面（地層，断層）の検出

反射波 reflected wave 屈折波 refracted wave

• 震源-観測点距離　>> 水深

• 各層の速度が決まるsource-station distance <~ depth source-station distance >> depth

geometry of layers, faults… seismic velocity structure

マルチチャンネル反射法探査 Multi Channel Seismic reflection

海底地震計 Ocean Bottom Seismometers

18

震源：エアガン source: air-gun

ストリーマーケーブル streamer cable

海洋での観測 Marine seismic surveys

（中西・沖野「海洋底地球科学」）

海底地震計 Ocean Bottom Seismometer

19

• モホロビチッチによる地震学的不連続面の発見

• 地殻・マントル境界の地震学的な定義

• 大陸と海のモホ面の深さの違い

• 大陸 39.17±8.52km (Christensen and Mooney, 1995)

• 海　7.0±0.8km (White et al., 1992)

モホ面がいつも観測できるとは限らないモホ面と岩石境界が一致するとは限らない

地殻の厚さ average thickness of crust

Discovery of Mohorovičić discontinuity (seismic discontinuity)

“seismically” definition of crust/mantle boundary

Depths of Moho discontinuity

continent : thick and large regional variation

ocean floor: thin and homogeneous

attention! Moho can NOT ALWAYS clearly observed.

Moho does NOT coincides with petrological boundary.

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島弧の地殻構造（地震波速度構造） Seismic structure of arc crust

(Suyehiro et al., 1996)

mid-crust 6km/s

海陸的海oceanic oceaniccontinental?

21

日本周辺の島弧地殻の多様性 Variation of crustal structures: around Japan

Arai et al., 2016

South Ryukyu North Ryukyu Southwest Japan Northeast Japan Izu-Bonin

22

島弧方向の変化 along axis variation

GEOLOGY, November 2007 1033

crustal velocities are considered to be strongly affected by several param-eters other than crustal composition, such as variable fracture distribution and porosity (e.g., Carlson and Gangi, 1985; Kelemen and Holbrook, 1995). We recognized the variation of average seismic velocity along the arc, which correlates well with the volume variation of the middle crust. However, the average seismic velocities beneath each basaltic volcano do not vary (Vp = ~6.8 km/s) from the thick Izu arc to the thin Bonin arc (black dots in Fig. 3B). This means that the volume ratios of each crustal component are equivalent in the both thick Izu arc and the thin Bonin arc beneath the basaltic volcanoes (Fig. 4A). It is important to note that those velocities are remarkably higher than the average velocity of the typical continental crust (Vp = 6.4 ± 0.21 km/s) (Christensen and Mooney, 1995).

These observations provide two strong constraints on the growth process of continental crust: (1) the bulk chemical composition of the crust beneath the basaltic volcanoes is the same for thick and thin arc crust, and (2) even though felsic to intermediate crust has been formed beneath the Izu-Bonin arc, the bulk chemical composition of the crust beneath the basaltic volcanoes is still more mafi c than that of typical con-tinental crust. This latter observation suggests that to transform arc crust into continental crust, there must be a process to return the mafi c to ultra-mafi c cumulates to the mantle, such as delamination (Kay and Kay, 1993), foundering (Oliver et al., 2003), or transformation (Takahashi et al., 2007). The requirement for such a process is well demonstrated by the calcu-

lated average seismic velocities excluding the mafi c to ultramafi c cumu-lates (i.e., layer E in Fig. 3B; Vp = 7.2–7.6 km/s). The average velocities derived this way beneath the basaltic volcanoes (blue dots in Fig. 3B) are very close to the range of velocities for typical continental crust, with the exception of the two volcanoes (Nishino-shima and Kaikata seamount). This implies that continued thickening of the Izu-Bonin arc crust, accom-panied by delamination of lowermost crust, can yield typical continental crust velocity structure. Vertical extension of the velocity-depth (V-D) profi le (which represents crustal growth while maintaining constant vol-ume ratios for each crustal component) (Fig. 4) also supports the proposed process. A 250% vertical extension of the V-D profi le beneath the middle crust at the Suiyo seamount of the Bonin arc shows a similar pattern to that of the Izu arc at Aoga-shima. In addition, the 150% vertical extension of the V-D profi le at the Izu arc agrees well with a typical continental crust (Christensen and Mooney, 1995; Rudnick and Fountain, 1995), except for ~10 km of the mafi c to ultramafi c cumulates layer.

Another important outcome of this study is the fi nding of a unique structure beneath the rhyolite volcanoes that is predominantly observed between the large basalt volcanoes in the Izu arc among the Izu-Bonin arc. Peak to peak values of the variation curve of the average velocity in the Izu arc are larger (e.g., 0.25 km/s between Aoga-shima and Myojin-knoll ) (Fig. 3B) than those of the Bonin arc (e.g., 0.12 km/s near Kayo seamount). This difference is mainly attributed by the difference of vol-

Figure 3. A: Seismic velocity image along volcanic front from Izu to Bonin arc obtained by seismic refraction tomography. Seismic image of Izu section was reported on by Kodaira et al. (2007). Modeling procedure, including refl ectivity imaging, traveltime fi tting, and resolution of the model, is shown in the Appendix, Figures DR2–DR4 (see footnote 1). Checkerboard test (Fig. DR3) shows that the structure shal-lower than the dashed lines is well resolved. Layers A–E indicate geological interpretations of seismic image: A—Upper crust consisting of sediment, volcaniclastics, and volcanic rocks. B—Felsic composition plutons. C—Intermediate composition plutons. D—Mafi c plutons. E—Mafi c to ultramafi c cumulates. F—Upper mantle. See also the Appendix. B: Average crustal seismic velocity (black line) and thickness of the middle crust (Vp = 6.0–6.8 km/s) (red line), which is interpreted to be plutonic rocks of felsic to intermediate composition. Black and red dots indicate average seismic velocities and thicknesses of the middle crust, respectively, beneath basaltic volcanoes. Blue dots show average crustal seismic velocities beneath basaltic volcanoes, but excluding the Vp = 7.2–7.6 km/s component. Orange shading shows the velocity range of typical continental crust (Christensen and Mooney, 1995). C: Average wt % SiO2 of volcanic rocks sampled and dredged from Quaternary volcanoes (Bloomer et al., 1989; Yuasa and Nohara, 1992; Kodaira, et al., 2007). Abbreviations as in Figure 1.

on February 2, 2012geology.gsapubs.orgDownloaded from

(Kodaira et al., 2007)23




24

この図は何を示している？ What does this illustration indicate?

(Understanding Earth, Sliver&Jordan 2003)

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ソリダス温度に対する水の影響 influence of H2O on solidus temperature

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水が存在すると，岩石のソリダス（溶け始め）温度は急激に下がる

26

沈み込んだ海洋地殻は溶けるのか？ Can subducted oceanic crust melt?


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プレートが非常に若く沈み込み速度が遅いときだけ海洋地殻は溶ける

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スラブとマントルウェッジの熱構造 Thermal structure of slab and mantle wedge

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海洋地殻からの脱水 Dehydration from altered oceanic crust

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The role of altered oceanic crust in most arcs is to give up H2O progressively as the slab dehydrate under the forearm and volcanic front.

多くの島弧で，海洋地殻そのものは溶けない．が，沈み込んだ海洋地殻（変質）からの脱水が前弧～火山フロントで起きる

amphibole 角閃石　安山岩や斑糲岩中に多く含まれる．水酸基を持つ含水鉱物

hydrous mineral: including OH-

H2O saturated basalt

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堆積物がとけることもあり得る沈み込みに伴い当然脱水が連続的に起こる

Subducted sediment continuously dehydrates as the slab descends. Sediment can melt under certain condition.

スラブのマントル部分はまず溶けない脱水は起きるIt is highly unlikely that hydrous mantle in slab ever melts. Mantle in slab potentially dehydrate and flush H2O.

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水を受け取ったマントルウェッジは溶けるか？ Can mantle wedge receiving a large amount of H2O melt?


•島弧の玄武岩マグマはおよそ2GPa, 1300-1400°Cで生成

•角閃石の脱水によって生じる部分溶融度は約8%, 一方採取された島弧の溶岩は部分溶融度30%以上を示す

Island arc basalt are produced @ 2Gpa, 1300-1400°C

Amphibole breaks down causes 8% partial melting : arc lavas are generated by at least 30% partial melting

マントルウェッジはより高温? High temperature in mantle wedge?

31

マントルウェッジの温度構造 thermal structure of mantle wedge

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34

大陸地殻と海洋地殻の組成 chemistry of continental / oceanic crust

upper crust lower crust whole crust oceanic

SiO2 66.6 53.4 60.6 50.5

TiO2 0.64 0.82 0.72 1.6

Al2O3 15.4 16.9 15.9 15.3

FeO 5.04 8.57 6.7 10.4

MgO 2.48 7.24 4.7 7.6

CaO 3.59 9.59 6.4 11.3

Na2O 3.27 2.65 3.1 2.7

K2O 2.8 0.61 1.8 0.2

(Rudnick and Gao, 2005) (Condie, 1997))

andesitic crust: unique in the solar system

andesite basaltmantle-derived

安山岩質の地殻の存在は地球に独特

35

大陸の形成は連続的ではない Not constant growth rate through earth history

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(Hawkesworth and Kemp, 2006)episodical?continuous

36

現在の大陸地殻生産 Continent formation today

• Island arc and Andean margins (convergent boundary) are recognized as the current sites of continental crust formation

• andesitic rocks in upper and middle layer of arc crust

• Current estimate of crustal growth rate is 1.6km3/year (ranging from 1 to 4 km3/year)

• too small to have generated the total volume of the continent over the age of the Earth

• Mantle-derived magmas in the modern arc are mostly mafic or basaltic

• mass balances of current crustal compositions require some crustal material to have been returned to the mantle

37

IODP expeditions Izu-Bonin-Mariana arc crust

1

OCEAN CREATES CONTINENT: Ultra-deep drilling to the middle crust of the IBM arc

Yoshiyuki Tatsumi

Kobe Univ. and IFREE/JAMSTEC

Continental crust, andesite, intra-oceanic arc, Izu-Bonin-Mariana

One characteristic feature of the planet Earth is the bimodal height distribution at the surface. This is caused

by the presence of two types crust with different density and thickness, i.e.� the oceanic and continental

crusts. The oceanic crust having basaltic compositions have formed at divergent plate boundaries, whereas

the average continental crust possesses intermediate compositions that typify arc magmatism and as a result

it is believed to have been created at convergent plate boundaries. However, mantle-derived magmas

produced in the modern arc-trench system are mostly mafic or basaltic. This is probably the greatest dilemma

facing those interested in the origin of continental crust and more generally in the Earth evolution.

The Izu-Bonin-Mariana (IBM) arc system,

extending 2800km to the south of Honshu (Fig. 1), is

uniquely suited to the study of arc evolution and

continental crust formation, because it is a juvenile

intra-oceanic arc with no pre-existing continental crust,

yet a thick middle crust layer with 6.0-6.8 km/s Vp

identical to the average Vp of the continental crust is

widely distributed in this arc (Fig. 2). The primary

goals of sampling the in situ arc crust through drilling

are: (1) to identify the structure and lithologies of the

upper and middle crust, (2) to test seismic models of

arc crustal structure, (3) to constrain the petrologic and

chronological relationship of the middle crust to the

overlying upper crust, (4) to establish the evolution of

arc crust by relating this site with other regional drill

sites and exposed arc sections, and (5) to test competing

hypotheses of how the continental crust forms and

evolves in an intra-oceanic arc setting. These objectives

address questions of global significance, but we have specifically identified the IBM arc system as an ideal

locale to conduct this experiment. The composition of the pre-subduction upper plate was normal oceanic

crust, and the tectonic and temporal evolution of this arc system is well-constrained. Moreover, the IBM

system is considered as the best-studied intra-oceanic arc on Earth by extensive sampling of the slab inputs

Fig. 1. Location map of the Philippine Sea region. The IBM arc-trench system forms the convergent margin between Pacific and Philippine Sea plates. Backarc basins such as Shikoku Basin, Parece Vela Basin and Mariana Trough were created by seafloor spreading between the formerly contiguous remnant arc (Kyushu-Palau and West Mariana ridges) and the active IBM arc. At its northern tip, the IBM arc has collided with the Honshu since 15 Ma. The red lines locates the along-arc t refraction and wide-angle reflection seismic data shown in Fig. 2. Numbers show a series of proposed drilling sites as Project IBM. Sites 1-3 are schedule to be drilled by JR.

WP007_CF02_Tatsumi.pdf

Exp.350 IBM rear-arc

history of across-arc variation in magma composition during arc evolution

Exp.352 IBM forearc

early processes in magmatic evolution associated with subduction initiation

Exp.351 IBM arc-origins geochemistry of the mantle prior to IBM arc inception ~the source of arc foundation

38

５万分の１地質図幅「父島列島」産総研 Geologica Sheet Map

1:500,000 Chichijima Islands (GSJ/AIST)

西之島 Nishinoshima

39

西之島新島噴1973-74 Nishinoshima eruption 1973-1974

５万分の１沿岸海の基本図「西之島」海上保安庁 Basic Map of coastal waters 1:500,000 Nishinoshima (Japan Coast Guard)

40

西之島 2013.11

２日目

2日目10ヶ月後

2017.7

41

Documents

5. 沈み込み帯（3 Subduction zones 3 arc magmatismofgs.aori.u-tokyo.ac.jp/.../ofgd18-05subduction3.key.pdf5. 沈み込み帯（3） Subduction zones 3 arc magmatism 海洋底ダイナミクス