6. 中央海嶺（１）海洋地殻の形成 Mid-ocean ridge process (1) oceanic crust formation

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

海洋地殻の地震学的構造は？Seismic structure of oceanic crustオフィオライトとは何か，そこから何が分かるのか？ What is “ophiolite”? What does it tell us about oceanic crust formation?中央海嶺の火成活動はなぜ起こるのか？What is the mechanism of mid-ocean ridge magmatism?海洋地殻の組成と中央海嶺プロセス？Mid-ocean ridge process and chemistry of oceanic crust

海底の岩石を実際にどうやって手にするか？How to get rock samples from deep seafloor (and mantle) ?

1

海洋地殻の地震学的構造は？ Seismic structure of oceanic crust オフィオライトとは何か，そこから何が分かるのか？ What is “ophiolite”? What does it tell us about oceanic crust formation?中央海嶺の火成活動はなぜ起こるのか？What is the mechanism of mid-ocean ridge magmatism?海洋地殻の組成と中央海嶺プロセス？Mid-ocean ridge process and chemistry of oceanic crust

海底の岩石を実際にどうやって手にするか？How to get rock samples from deep seafloor (and mantle) ?

2

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

先週の復習

3

海洋におけるＰ波速度構造観測結果 Results of marine seismic survey: P-wave velocity structure

(White et al, 1992）

Moho

high gradient

4

oceanic crust : seismic structure

速度勾配が重要：・第2層は勾配が大きい層として特徴づけられる layer 2 = large velocity gradient

・第2層第3層境界は速度不連続面ではなく速度勾配不連続面 layer 2/3 boundary = discontinuity of velocity gradient (not velocity discontinuity)

lower crustupper crust

(White et al, 1992）

5

地震学的構造と岩石学の対応 seismic layering =?? lithology

classic interpretation

堆積物

斑糲岩

岩脈群

玄武岩質溶岩

橄欖岩

乏しい海底サンプルや陸上情報・実験室情報との対比 Based on poor rock samples and experimental results….(Press, Understanding Earth, 2003）

6

実験室での物性測定との比較 physical property of rocks

Poisson’s ratio ポワソン比

ν = -応力直交の歪み/応力方向の歪み- transverse strain / axial strain

Vp/Vs比からわかる can be measured from Vp/Vs ration

shear-wave speeds, it is necessary to estimate its value from Vp.A direct relation between Vs and Vp, therefore, is highly desir-able for many studies. This is accomplished by using empiri-cally derived Vp–Vs relations that are based on field boreholeand seismic profiling data together with laboratorymeasurements. A recent study (Brocher, 2005) has derivedempirical relations between Vp, Vs, and Poisson’s ratio thatcan be used to estimate the ratio Vp–Vs or, equivalently,Poisson’s ratio from a knowledge of Vp and rock type (i.e.,sedimentary vs. crystalline rock). The empirical and regres-sional fits are only defined for Vp between 1.5 and 8.5 km sand fit the data remarkably well (Figure 8).

Conversely, the mineralogy of the crust can be estimatedwhen both compressional-wave (Vp) and shear-wave (Vs)velocities are measured (Figures 8 and 9). The relationbetween Vp and Vs is commonly expressed by Poisson’s ratio,which varies from 0.23 to 0.32 for most minerals, but quartzhas a value of only 0.08 at room conditions (Christensen,1996). Thus, the measurement of Poisson’s ratio offers themeans of distinguishing between felsic (quartz-rich) andmafic (quartz-poor) rocks.

1.11.3.3.3 Seismic anisotropyMany minerals exhibit birefringence, which is a directionaldependence of the speed of light through the mineral. Thisphenomenon is used by petrologists to identify minerals in athin section (!1 mm) where a sample illuminated by polarizedlight is rotated under a microscope to reveal its birefringence.Likewise, elastic waves show a directional dependence in wavespeed in many minerals. Perhaps, the most prominent exampleis the mineral olivine, which is a major constituent of the uppermantle. The discrepancy between Rayleigh and Love wavespeeds was measured in the early 1960s (Anderson, 1961) andled to the recognition of seismic anisotropy in the mantle lid. Atabout the same time, laboratory measurements of metamorphicrocks demonstrated significant shear-wave anisotropy in thecrust (Christensen, 1966b). These measurements demonstratedthat seismic anisotropy is not only confined to the upper mantlebut also plays a prominent role in the crust (Figure 10). Table 4lists several key papers on the seismic properties, particularlyanisotropy, of the uppermost mantle. Additional references forlaboratory studies of the seismic properties of continental rockscan be found in Table 5.

AbbreviationAGR GGN Granite gneiss

Mafic garnet granuliteGranite-GranodioriteHornblenditeCalcite marbleMafic granuliteMetagraywackeParagranulitePhyllitePyroxeniteMica quartz schistQuartizeSerpentiniteSlate

GGRGRAHBLMBLMGRMGWPGRPHYPYXQSCQTZSERSLT

Anorthositic granuliteAmphiboliteAndesiteAnorthositeBasaltBiotite (Tonalite) gneissGreenschist facies basaltPrehnite-pumpellyite facies basaltZeolite facies basaltDiabaseDioriteDuniteMafic eclogiteFelsic granuliteGabbro-norite-troctolite

AMPANDANOBASBGNBGRBPPBZEDIADIODUNECLFGRGAB

AbbreviationRock type Rock type

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

27002500

Vp

(km

s–1

)

2900

Density (kg m–3)

3100

AND

AN

BAS

BZEQSC

QTZ

BPP

DIA

GABAMP

BGRMGR

GGR HBL

PYXECL

DUN

AGRMBL

MGW

SER

FGR

SLTPGR

DIO

GRAGGN

BGN

PHY

3300 3500

Figure 7 Average velocity versus average density for a variety of rock types at a pressure equivalent to 20 km depth and 309 "C. Rock abbreviationsare as follows. AGR, anorthositic granulite; AMP, amphibolite; AND, andesite; BAS, basalt; BGN, biotite (tonalite) gneiss; BGR, greenschist faciesbasalt; BPP, prehnite–pumpellyite facies basalt; BZE, zeolite facies basalt; DIA, diabase; DIO, dionite; DUN, dunite; ECL, mafic eclogite; FGR, felsicgranulite; GAB, gabbro–norite–troctolite; GGN, granite gneiss; GGR, mafic garnet granulite; GRA, granite–granodiorite; HBL, hornblendite; MBL, calcitemarble; MGR, mafic granulite; MGW, metagraywacke; PGR, paragranulite; PHY, phyllite; PYX, pyroxenite; QCC, mica quartz schist; QTZ, quartzite;SER, serpentinite; SLT, slate. Reproduced from Christensen NI and MooneyWD (1995) Seismic velocity structure and the composition of the continentalcrust: a global view. Journal of Geophysical Research 100: 9761–9788.

Crust and Lithospheric Structure - Global Crustal Structure 349

5.10

8

3.67

3.06

2.69

2.45

2.27

2.14

2.03

1.94

1.87

1.81

1.76

1.71

1.67

1.638.57.56.55.54.53.52.5

λ>µ

λ=µ

λ<µ

1.50.2

0.3

Poi

sson

’s ra

tio

0.4

(1,2)0.5

Brocher et al. (1997a)

Eqn. 7,Castagna et al. (1985)

Eqn. 12,Ludwing

empirical fit

KeyAver. of crystalline rocks (Christensen,1996)

Aver. of sed. rocks (Mavko et al., 1998)

Individual lab. measurement (Calif.)

Individual lab. measurement (Non-Calif.)

Individual borehole measurement (Calif.)

USGS 30-m VSP (Boore, 2003)

Eqn. 8, Mafic line,Mafic and Calcium- rich

rocks

Vp(km s−1)

V p/V

s

Eqn.11, Brocher empirical fit

Figure 8 Poisson’s ratio as a function of Vp for common lithologies. Colored ellipses highlight measurements reported by a single reference: boldnumbers in parentheses link ellipses to similar studies. The thinner horizontal dashed line shows Poisson’s ratio of 0.25 (Vp/Vs¼1.73) commonlyassumed for the crust when the first Lame constant, l, equals the shear modulus, m. Reproduced from Brocher TM (2005) Empirical relations betweenelastic wavespeeds and density in the Earth’s crust. Bulletin of the Seismological Society of America, 95 (6): 2081–2092.

Poi

sson

’s r

atio

Vp

Vs

Felsic

Per

cent

by

volu

me

Mafic

Plagioclase

Hornblende

Ultramafic

Olivine

Min

eral

ogy

20

0

20

30

Biotite

40

QuartzK-F

eldsp

ar 50

Pyroxe

ne

60

10

40

60

80

100

2.0 0.24

0.25

0.26

0.27

0.28

0.29

Vel

ocity

(km

s–1

)

3.0

4.0

5.0

6.0

7.0

8.0

9.0

Gra

nite

Qua

rtz

dior

ite

Dio

rite

Gab

bro

Per

idot

ite

Dun

ite

Oliv

ine

gabb

ro

Gra

nodi

orite

Figure 9 Variations in compressional-wave velocity (Vp), shear-wave velocity (Vs), and Poisson’s ratio (s) with mineral composition (Berry andMason, 1959) for common igneous rock types. Anorthite content of plagioclase feldspar is shown within the plagioclase field. Reproduced fromChristensen NI (1996) Poisson’s ratio and crustal seismology. Journal of Geophysical Research 100: 3139–3156.

. = −F $"$F = ( $"$(

$($= = −(BC

GHGI= +()KL)

)K+L

large variation @ Poisson’s ratio

7

海洋地殻の地震学的構造は？Seismic structure of oceanic crustオフィオライトとは何か，そこから何が分かるのか？ What is “ophiolite”? What does it tell us about oceanic crust formation? 中央海嶺の火成活動はなぜ起こるのか？What is the mechanism of mid-ocean ridge magmatism?海洋地殻の組成と中央海嶺プロセス？Mid-ocean ridge process and chemistry of oceanic crust

海底の岩石を実際にどうやって手にするか？How to get rock samples from deep seafloor (and mantle) ?

8

オフィオライト　Ophiolite• 下位から順番に以下が層状に積み重なったものをオフィオライト層序と呼ぶ ophiolite sequence is stratigraphic-like sequence as follows…

• peridotite/ serpentine 橄欖岩（変成して蛇紋岩の場合もある）• gabbro 斑糲岩• dolerite 粗粒玄武岩• basalt 玄武岩• (deep sea sediment 深海堆積物)

• オフィオライト層序を持つ岩体をオフィオライトと呼ぶ ophiolite: rock body having ophiolite sequence

• 代表的な場所　オマーン、キプロス、幌満famous site : Oman, Cyprus, Horoman

• 海洋地殻（とマントル）の断片が、大陸へ衝上したものと解釈Now, it is interpreted as a part of oceanic crust (with mantle) obducted on land.

蛇 石

9

火成岩の分類 Classification of igneous rocks

volcanic rhyolite andesite basalt komatiite

plutonic granite diorite gabbro peridotite

(%SiO2) acid (ultra)basic

(%dark minerals) fersic (ultra)mafic

海洋性地殻の構成物oceanic crust

マントル mantle

酸性 塩基性

珪長質 苦鉄質

橄欖岩

斑糲岩

玄武岩

深成岩

火山岩 dolerite

10

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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11

Oman : Semail Ophiolite (Nicolas, 1995)

陸上で見るモホ面 MOHO ON LAND!

Moholight-coloured peridotite

dark gabbro layer

12

上部地殻上部（2A層）：　枕状溶岩 upper crust (layer 2A) : pillow lava

(photo by fujii @ Oman in 2013) (photo by okino @ Central Indian Ridge in 2006)

13

(photo by okino @ Cyprus in 1999)

sheet flow with columnar joint

sheeted dyke complexdyke & pillow lava

上部地殻（2A~2B層）：　平行岩脈群 upper crust (layer 2A to 2B): sheeted dyke complex

14

下部地殻（第3層） lower crust (layer 3)

(photo by okino @ Cyprus in 1999)

黒っぽいほうがolivineに富み先に結晶化して下に沈むBlackish layer = Ol. rich

Crystalize and settle (photo by fujii @ Oman in 2013)

layered gabbro下部地殻下部lower layer 3

15

モホ遷移帯 Moho transitions

harzburgite

dunite

(photo by okino @ Cyprus in 1999)

(Oman, Moho transitions: 上杉ほか, 2003)

harzburgiteをmeltが通る(photo by fujii @ Oman in 2013)

16

海洋地殻形成の概念図 Conceptual oceanic crust

(Press, Understanding Earth, 2003）

ただし，こんなマグマだまりが必ずしもあるはいえない． Above illustrated “magma chamber” is rarely detected.

Layer 2

Layer 3

Layer 4

17

典型的なオフィオライト層序ばかりか？ Ophiolite everywhere?

(Dick et al., 2006）Ophiolite model

第8回講義へ

18

海洋地殻の地震学的構造は？Seismic structure of oceanic crustオフィオライトとは何か，そこから何が分かるのか？ What is “ophiolite”? What does it tell us about oceanic crust formation?中央海嶺の火成活動はなぜ起こるのか？ What is the mechanism of mid-ocean ridge magmatism? 海洋地殻の組成と中央海嶺プロセス？Mid-ocean ridge process and chemistry of oceanic crust

海底の岩石を実際にどうやって手にするか？How to get rock samples from deep seafloor (and mantle) ?

19

中央海嶺下での火成活動 Magmatism at mid-ocean ridges

• 広がるプレートの間の隙間を埋めるようにマントル物質が上昇する • 上昇流 があるからプレートが離れていくのではない！(受動的）

passive mantle upwelling beneath divergent plate boundary

• マントル物質は断熱的に地表に向けて上昇する。上昇につれて周囲の圧力は下がり、固体のマントルが融解をはじめる（断熱減圧融解）adiabatic, decompression melting

• マントルは部分融解するので、マントル中の特定の元素（液相濃集元素：K, Naなど）がメルト(液体、マグマ）に選択的に移動する。地殻はメルトが固化したものなので、元のマントルと組成が異なる。

Partial melting -> incompatible elements preferentially move to liquid phase (melt) -> melt is solidified = fractionation of magma

海洋性地殻

マントル

マントル上昇流地温勾配

ソリダスA

A

海洋性地殻

プレートが離れる

マントル AA

マントル上昇流地温勾配

ソリダス

温度

圧力（深さ）

(a) (b)0

Ps

Tact Tp Ts1120°C

20

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メルトの生成と移動 Melt generation and transfer

Tp ポテンシャル温度 (断熱的に地表まで持ってきた場合）

Tact 実際の温度 (潜熱が失われるから）

(Rogers, Our Dynamic Planet, 2007）

potential temperature

latent heat is lost

actual temperature

adiabatic upwelling

22

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liquidus temperature　Cpx.<Opx.<Ol.

+

↓

元のマントル物質

溶け残りマントル

AO: average of ophiolite and fracture zoneAX: xenolith

(Rogers, Our Dynamic Planet, 2007）

23

What is MORB?

volcanic rhyolite andesite basalt komatiite

plutonic granite diorite gabbro peridotite

(%SiO2) acid (ultra)basic

(%dark minerals) fersic (ultra)mafic

海洋性地殻の構成物

酸性 塩基性

珪長質 苦鉄質

橄欖岩斑糲岩

玄武岩

深成岩

火山岩

quartznephelinetholeiitealkali-olivine basalt

olivine-tholeiite

(%SiO2)

olivine

Mid-Ocean Ridge Basalt(MORB)

中央海嶺玄武岩

oceanic crust

24

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固溶体の相平衡図:2成分系

共融系の相平衡図

phase diagram of binary solid solution system

phase diagram of eutectic system

25

中央海嶺下でのマグマの分化 Differentiation beneath mid-ocean ridges

Amphibole

Olivine

Pyroxene

Biotite

Plagioclase

Quartz

Muscovite

Orthoclase

Sodium-rich

Calsium-rich

temperature1400°C

800°C

mafic

intermediate

felsic

結晶分化　fractional crystallization部分溶融　partial melting

26

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マグマの分化におる組成の変化 Trajectory of liquid composition of basalt under fractional crystallization

27

多様性は結晶分化過程か元のマントルか Variation of MORB: fractional crystallization or source

Cayman

4.0

3.0

2.0

6.0 7.0 8.0 9.0

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28

ガブロはメルトレンズでできるgabbro crystallizes @melt lens

決着はついていない

下部地殻の起源 Origin of lower crust

ガブロはモホの下でもできる？Gabbro under Moho?

ガブロはその場でできるgabbro crystallizes @ sills

29

海嶺下メルト生成モデル model of melt generation

部分溶融度小 low degree of meltingメルト少ない：地殻薄い： thin crust液相濃集元素(Naなど）high I.E.

部分溶融度大 high degree of meltingメルト多い：地殻厚い： thick crust液相濃集元素(Naなど）low I.E.

(Langmuir et al., 1992)

30

MELT experiment

of melting are possible in the presence ofwater at depths of 150 to 200 km or more(11). Because the composition of MORBsand the thickness of the oceanic crust arenearly uniform independent of location ofthe spreading center, the common view hasbeen that ridges are generally passive fea-tures that simply tap a well-stirred, nearlyisothermal asthenosphere and that any dy-namic upwelling is simply a local responseto melting. On the other hand, some inves-tigators have suggested that there is a linkbetween mid-ocean ridges and deep struc-tures in the lower mantle mapped withglobal seismic tomography (12), implyingperhaps that upwelling beneath ridges is anactive part of a whole-mantle convectionsystem.

The goals of the MELT Experiment areto use seismic and electromagnetic observa-tions to constrain the pattern of upwellingbeneath a ridge, the geometry of the regionof partial melting, the melt concentrationwithin that region, the distribution of meltwithin the matrix, and the connectednessof melt pockets. Passive arrays of seismom-eters, electrometers, and magnetometerswere deployed on the sea floor across theEPR to record seismic waves from regionaland teleseismic earthquakes and coupledvariations in the electric and magneticfields. In this set of eight reports in thisissue, we report the first results from theseismological component of the MELT Ex-periment, which completed the data acqui-sition phase in May 1996. The electromag-netic data acquisition began at that timeand was completed in June 1997.

Fifty-one ocean-bottom seismometers(OBSs) were deployed across the EPR inNovember 1995 in two linear arrays !800km long (Fig. 1). This site is in the middleof the longest, straightest section of the

global spreading system and, with a totalspreading rate of about 145 km per millionyears, it is one of the fastest spreading partsof the mid-ocean ridge system. This sitethus provides an end-member test of thehypothesis that upwelling should be morelinear at faster spreading rates, where theoryindicates that passive flow is most likely tobe dominant (7). Likely teleseismic earth-quake sources are distributed at a widerange of azimuths, and the oceanic paths arerelatively simple. We were fortunate tohave a good distribution of large eventsduring the 6-month recording period (Fig.2), including some earthquakes at depthsgreater than 100 km. In the stable temper-ature environment of the sea floor, theseismometers (13) recorded signals with re-liable phase and amplitude to a period of50 s and greater, depending on the size ofthe earthquake. The best signal-to-noise ra-tio was typically in the 10- to 30-s band,which is a low-noise “window.”

Each of the MELT reports in this issuedescribes results from different techniquesto address issues of mantle flow and meltproduction. Geophysical mapping surveys(14) show that there is an asymmetry to theridge in this area: the sea floor is subsidingmore slowly on the Pacific Plate to the westthan on the Nazca Plate to the east; thewest side has a greater population of smallseamounts; the Pacific Plate is moving al-most twice as fast to the west as the NazcaPlate is moving to the east in the hot spotcoordinate frame, causing the axis itself tomigrate to the west; and, through a series ofrapidly propagating rifts or overlappingspreading centers, the net accretion of newsea floor or half-spreading rate is more rapidon the Nazca Plate side, causing additionalmigration of the spreading center to the

west. The asymmetry in subsidence and vol-canic activity probably indicates a basicasymmetry in thermal structure and meltproduction.

An asymmetry in seismic structure acrossthe ridge is clear in the pattern of P and Swave delays (15). The average gradient indelay time for S waves is about twice aslarge on the east side as on the west, roughlyproportional to the difference in subsidencerates. Except for the station at the spreadingcenter axis that is probably affected by meltin the crust, the most delayed P and Sarrivals are displaced to the west of the axis.Like the body waves, the pattern of Ray-leigh wave phase velocities is asymmetric.The velocities are low to the west of theaxis and increase rapidly immediately to theeast of the axis (16). Overall, the region oflow velocities is several hundred kilometersacross and is clearly not confined to a nar-row region of concentrated upwelling pre-dicted by some dynamic flow models.

There are at least two components ofmantle structure contributing to the patternof delays and asymmetry. Shear wave split-ting measurements show that the mantle isanisotropic and that there is greater split-ting to the west than to the east of the axis(17). This anisotropy contributes to thepattern; for example, delays for S wavespolarized in the slow direction are moreasymmetric than for those polarized in thefast direction (15). A second contributor tothe delay pattern is variations in theamount of melt present in the upper man-tle. The magnitude of the observed changesare too large to be due to the effect ofvelocity increases caused by solid-statecooling of the lithosphere and astheno-sphere with increasing distance from therise axis (15). The maps of phase velocities

30

60

90

120

150

Fig. 2. Epicenters of earthquakes used as sourc-es in the MELT Experiment. The location of OBSarrays are indicated by two short lines at the cen-ter of the map (azimuthal equidistant projection).

400 200 0 200 400

Distance from axis (km)West East

0

100

200

300

400

Dep

th (k

m)

101 mm/yr 45 mm/yr

LithosphericMantle

EmbeddedHeterogeneity

410 km Discontinuity

Crust

IncipientMelting

Primary Melting

Fig. 3. Schematic cross-section ofthe East Pacific Rise at 17°S. ThePacific Plate, moving at 101 mm/year in the hot spot coordinateframe, is to the left in the diagram.The broad asymmetric region of lowseismic velocities is interpreted tobe the primary melt production re-gion. The region labeled embeddedheterogeneity represents additionalmelting created by anomalously en-riched source material or localizedupwelling; its dimensions are notwell constrained. Although the ex-act flow pattern is not known, thefaster motion of the Pacific Plate isexpected to create greater shear inthe upper mantle, inducing strongeranisotropy. The solid lines continu-ing down from velocity vectors indi-cate possible vertical profiles of horizontal velocities away from the ridge axis, with greater return flowcoming from the west. The small ellipses represent the direction of preferred alignment of olivine a axes,with flatness increasing with increasing degree of alignment.

SCIENCE ! VOL. 280 ! 22 MAY 1998 ! www.sciencemag.org1216 31

海域での岩石採取 Seafloor rock sampling

• 海底面から岩石を採取する

• （船から）ドレッジ・ロックコア（ロックチッパー）

• 潜水船、ROV

• 掘削する

• マリンドリル（浅層掘削）

• 深海掘削

• 海底面の場合は通常は上部地殻の玄武岩しか採取できないが、断裂帯などの特殊な場所（テクトニック・ウィンドウ）で下部地殻や上部マントルの

露頭を探す

rock collected from seafloor (usually only basalt can be obtained)

(from ship) dredge, rock core (rock chipper)

(from submersible & ROV) manipulator, small dredge

drilling oceanic crust

marine drill (shallow drilling)

deep sea drilling

lower crustal rock collected from tectonic window (fracture zone etc.)

32

岩石ドレッジ dredge

• 簡便・安価 easy and low cost

• 量が取れる　 we can get large volume of samples

• 正確にどこで取れたかがわかりにくい not pin-point sampling

camera

recently, with camera, transponder33

rock core (rock chipper)

wax

weight

• 簡便・安価 easy and low cost

• ガラス片のみ取れる　 we can get volcanic glasses

• 正確にどこで取れたかがわかりにくい pin-point sampling

34

有人潜水船・無人探査機 Human Occupied Vehicle・Remotely Operated Vehicle

Shinkai6500#927 2006.1.23 Okinoかいこう7000II（JAMSTEC)

launcher

vehicle

• 肉眼・ビデオで観察して採取可能　 observation + sampling

• 高価・限られた行動範囲 high cost, limited survey area

35

海底掘削 Ocean Floor Drilling

IODP地球深部探査船　「ちきゅう」ChikyuJOGMEC BMS 浅層掘削20ｍBenthic Multi-coring System

36

過去の掘削記録（海洋地殻を掘ったもの） Drill holes (igneous rock)

Expedition 335 Scientists Expedition 335 summary

Proc. IODP | Volume 335 41

Figure F4. Compilation chart showing holes drilled >100 m in intact crust and tectonically exposed lower crustand upper mantle from 1974 to 2010 (drill hole locations in Fig. F3 ). For each hole are indicated the holenumber and the recovery (in percent) for each lithology. This compilation does not include “hard rock” drillholes in oceanic plateaus, arc basement, hydrothermal mounds, or passive margins.

22 cruises, 34 holes

Total cored:12,938 m

(~2.6% of total length cored since DSDP Leg 1)

Total recovered : 5,674 m

(~1.7% of cores recovered since DSDP Leg 1)

Dep

th (

m s

ubba

sem

ent)

Sheeted dikes

Basaltic lavas

Lava-dike transition

Gabbros

Serpentinized peridotites

No recovery

Holes deeper than 100 m:

0

200

400

600

800

1000

1200

1400

1600

1800

2000

38%18% 17% 26% 30% 47%

51%61%

13%23% 31%

27%8%

10%70%

47%

72%18% 21% 30%

25%42%

22%

32%

29%

14%

86%

75%

75%

50%

35%43%46% 15%

35% 29%53% 22%

47%

335

412A

1149

D

1224

F

U13

62A

483B

485B

417A

U13

01B

396B

765D

896A

333A

332A

417D

801C

418A

395A

332B

504B

1256

D

U13

09B

1275

B

894G

1105

A

1275

D

U13

09D

735B

1271

B

920B

1272

A

1268

A

1274

A

920D

(Teagle et al., 2012)

normal? oceanic crust

tectonic window

37

Unexpected drilling results Hole 1256

12-19Ma, rate=220mm/yr superfast

2002 ODP 250m sed.502m crust

(Wilson et al., 2006)

38

gabbro ＝?　layer 3

• Layer2/Layer3境界に達しているはずが.....

• gabbro≠layer3 ?

• 岩石種より空隙率などの差がきく？

緑：坑内計測青点：試料計測赤：屈折法探査

off-axis lava

massive flows

thin sheeted dyke layer

gabbro 1157msb(1407mbsf)

(Wilson et al., 2006)39