図版12

海洋底ダイナミクス2014(12)
hydrothermal activity
Indian Ocean : Kairei Hydrothermal Field : max 400°C
Courtesy of Dr. K. Takai
1
海洋底ダイナミクス2014(12)
Discovery of hydrothermal vent in 1977
(courtesy of C. German)
2
LO OR
DS
CK
he oceanic
urs because
ce (magma
a permesured igneturates the
By comparing the fluids and deposits formed in distinct
geologic and tectonic settings, it is possible to examine the role
that specific factors play in determining fluid composition ...
海洋底ダイナミクス2014(12)
and mineral deposit size, shape, and composition ...
Known site of hydrothermal venting (2006)
crust (seawater). The composition of hot
fluids that exit at vent fields reflects a
These factors affect the depth and scale
of fluid circulation, the temperature and
number of factors: the initial fluid compressure at which water-rock reactions
position (seawater); the composition of
take place, and whether the fluid underFigure 1. Known sites of hydrothermal venting along mid-ocean ridges, in back-arc basins, rifted arcs, and at submerged island-arc
the rock that reactsvolcanoes
with(red),
theandfluid
as it
goes phase separation. Most mid-ocean
areas of activity as indicated by mid-water chemical anomalies (yellow). EPR= East Pacific Rise. TAG= Trans
Atlantic Geotraverse, MEF = Main Endeavour Field, and GR-14 = Sea Cliff hydrothermal field on the northern Gorda Ridge. Figure
circulates and the structure
of German
that and
rock
ridge
vent
fields
areet al.,hosted
within basalt,
after Baker et al., 1995;
Von Damm, 2004;
Hannington
et al., 2005;
Koschinsky
2006
(Tivey, 2007, Oceanography)
(e.g., the distribution of fractures and
and chemical reactions occur as fluids
fissures, the depth to the brittle/ductile
circulate, first at low temperatures in the
Oceanographyzone,
March 2007海洋底ダイナミクス2014(12)
51
transition); and the depth, size, and
down-flowing limb or “recharge”
shape of the heat source (Figure 2a).
then at much higher temperatures in the
3
Hydrothermal system
drawing of a hydrothermal sys(a)
Components/processes involved
rust showing the different comin generating reduced fluid
that can affect the composition
at the seafloor (e.g., initial fluid
trate composition, permeability
Mixing of two fluids - chemical rxns Initial fluid
te, and geometry and nature of
mineral precipitation/dissolution
(e.g., seawater)
contribute to the temperatures
eactions occur). (b) Elaboration
contribute to formation of midModified fluid
s. As seawater penetrates down
lass, olivine, and plagioclase are
Discharge
smectite, and Fe-oxyhydroxides
°C–60°C). As the modified fluid
Recharge
he crust and is heated to higher
Porous media composition (e.g., basalt,
peridotite, andesite, rhyolite, dacite ±
itation of smectite and chlorite
sediment) and structure
f Mg from the fluid in exchange
2+
=
Ca and SO4 are lost from the
O4) precipitates at temperatures
(temperature-pressure of fluid-rock
interaction; geometry of heat source)
eeper in the system, anorthite is
cess called albitization, with Na
to the crust in exchange for Ca,
(b)
the rock into the fluid. The sum
Generic ridge vent system
(Tivey, 2007, Oceanography)
lts in a fluid that is slightly acid,
4
d Mg-poor relative to seawater.
Diffuse,
Focused, high-temp flow
low-temp
hes S and metals from the rock.
through chimneys
flow
海洋底ダイナミクス2014(12)
水の流れやすさ
deepest portions of the circulation sys-
fractures and scarps that expose deeper
tem (the “root” or “reaction” zone), and
lastly as the hot, buoyant fluid rises rap-
(Figure 3a), with alkali metals
Cs), B, and H2O removed from
parts of the crust, from drill cores, and
Spinelli, Giambalvo, and Fisher
堆積物は最上部(数十メートル）以深は流れにくい
to the altered minerals and Si
from
ophiolites (slices
of oceanic crust
0
e
some cases, Mg, lost from the
that have been thrust
idit onto land by plateooze
idly through the “up-flow” zone to exit at
the seafloor (Alt, 1995) (Figure 2b).
Fine-grain
ed tu
rb
Hemip
172
tectonic processes).
At temperatures up
20
to about 40°C
to 60°C, reactions of sea40
MARGARET KINGSTON TIVE
curring in the down-flowing limb relies
largely on data and observations of al-
water with basalt result in the alteration
60
of basaltic glass, olivine, and plagioclase
whoi.edu) is Associate Scientist,
ment of Marine Chemistry and
teration mineral assemblages within oceanic crust recovered by submersible from
80
by oxidation
to ferric micas and smectite,
100
Mg-rich smectite,
and Fe oxyhydroxides
try, Woods Hole Oceanographic
Woods Hole, MA, USA.
Silice
ous o
oze
120
Pela
gic
c lay
Depth (m)
elag
Calc.
ic
Our understanding of processes oc-
140
10−17
10−16
10−15
10−14
10−13
10−12
2
Permeability (m )
(a)
Figure 3. (a) Altered basalt recovered on O
Fig. 6.7 Sediment permeability profiles derived from porosity versus depth and permeability versus
porosity relationships (see Table 6.2 for equations and references). Permeability
with depth,
Drillingdecreases
Program
(ODP) Leg 51 from Hole 4
weakly within siliceous and calcareous oozes, and most dramatically in shallow, fine-grained, terrigecomposed of ferric smectites and Fe-oxyhy
nous sediments.
(c)
(Spinelli et al., 2004)
ides (red-brown) that replace plagioclase, o
(川田ほか, 2009)
phase separation
and basaltic glass; and veins of carbonate (
impacts consolidation behavior by influencing the interaction between
as grains
(b) Theparticles
two-phase
boundary, critical point
are rearranged and realigned. In the central equatorial Pacific, carbonate sediments have reldensity surfaces for seawater as a function
atively low porosity, as platy nannofossils are easily aligned during consolidation, whereas
perature
siliceous sediments at the same sites have relatively high porosity
due to theand
openpressure,
structure or depth beneath th
of the constituent radiolarians (Wilkens and Handyside, 1985). floor,
Throughout
consolidation,
assuming
hydrostatic5pressure (after
platy clay particles progress from random orientations, providing
open spaces
andnumerous
Rosenbauer,
1985). The red parallelogr
and high porosity, to tightly packed, well-aligned arrangements with low porosity (e.g.
indicates
temperatures
Bennett et al., 1981; Mitchell, 1993). Grain size can also 海洋底ダイナミクス2014(12)
affect sediment consolidation, and pressures of ve
ing observed
at the
seafloor in different loc
with finer sediments generally consolidating more easily (Lambe
and Whitman,
1969;
Mitchell, 1993). Less consolidation in silts than in clays may result
from
a greater
strength
along
the
world’s
spreading centers. (c) A p
of the framework of silty sediments, due to the way the larger (and more rounded) silt
stockwork,
or chloritized basalt breccia, rec
grains interact (Dewhurst and Aplin, 1998). Within terrigenous (turbidite
and hemipelagic)
from
116 mwith
beneath
sediments on the Juan de Fuca Ridge flank, the ease of consolidation
increases
increas-the TAG active hydro
ing proportion of hemipelagic sediment, which is characteristically
finer on
grained
(Spinelli
mound
ODP
Leg 158. The sample is com
1 cm
et al., 2003).
1 cm
(b)
図 3 上部海洋地殻の浸透率．（a）浸透率深さ分布．
ここで深さは基盤岩の上面からの距離を表す．
浸透率は，最上部 200 ∼ 300 m で最も大きく，
それ以深では非常に小さい．（b）最上部 300 m
の浸透率と地殻年代との関連．浸透率は，地
殻年代が増加すると急激に小さくなる．数字
は ODP 掘削孔の名前を表す．（Fisher（1998）
と Becker and Davis（2003）に基づく）
Fig. 3 Estimated permeability of upper oceanic crust.
（a）Depth variation. Here depth is measured
with reference to the top of the basaltic layer.
Permeability is large within at the uppermost
200-300 m, and i s small at greater depths.
（b）Permeability within the uppermost 300 m
with respect to the crustal age. Permeability
decreases with increasing age. Numbers denote
ODP drilling sites.（based on Fisher（1998）and
Becker and Davis（2003））
of highly altered chloritized basalt (gray-gr
iron sulfide veins (gold), and quartz cemen
courtesy of S. Humphris (Woods Hole Ocean
Institution and Ocean Drilling Program)
(Tivey, 2007, Oceanography)
Oceanography6
March 2007
mectite, and Fe-oxyhydroxides
–60°C). As the modified fluid
crust and is heated to higher
ation of smectite and chlorite
Mg from the fluid in exchange
a2+ and SO4= are lost from the
precipitates at temperatures
per in the system, anorthite is
ss called albitization, with Na
the crust in exchange for Ca,
e rock into the fluid. The sum
in a fluid that is slightly acid,
Mg-poor relative to seawater.
s S and metals from the rock.
CO2, CH4, H2) may be added,
id composition. Further fluid
r from separation of the fluid
-rich phase and a brine phase
pressures exceed those of the
e hot, buoyant fluids rise rape may be some equilibration,
precipitation and/or dissolue fluid rises. Quartz becomes
ecipitate due to kinetic barrihe fluid may exit directly into
in the subsurface if seawater
ed in the vicinity of the vents.
Porous media composition (e.g., basalt,
peridotite, andesite, rhyolite, dacite ±
sediment) and structure
海洋底ダイナミクス2014(12)
(temperature-pressure of fluid-rock
Generic MOR vent system
interaction; geometry of heat source)
(b)
Generic ridge vent system
Diffuse,
low-temp
flow
Focused, high-temp flow
through chimneys
Seawater
Low-T alt.
Mg
Ca 2+ +S O 4=
smectite/chlorite
H +, Ca2+, Na+
anhydrite
Albitization
MORB
S , Cu, Fe , Mn,
Zn, etc.
≥ 350° vent fluid
water-rock rxn ( ? )
( e.g., S i, Cu, H2 )
Phase separation/segregation
3
zone”
} “reaction
or “root zone”
He, CO2, CH 4, H 2
~1200°C
Heat source =
magma or hot rock
(Tivey, 2007, Oceanography)
No. 1
7
海洋底ダイナミクス2014(12)
hydrothermal plume
8
海洋底ダイナミクス2014(12)
event plumes @ northern Lau spreading center
(VENTS program,http://www.pmel.noaa.gov/vents/PlumeStudies/ne-lau/index.html )
9
海洋底ダイナミクス2014(12)
熱水系の果たす役割
• 地球を冷やす
• マグマの冷却と固化を熱水循環によって促進
• 海嶺軸付近では、熱水循環によって運ばれる熱が全体の3/4
• 海洋と固体地球の間の物質のやりとり
• 海水中の元素（Mg, Pなど）を固体地球へ、岩石中の元素（Mn, Feなど）
を熱水プルームを通じて海洋へ
• 特異な生態系を維持
• 熱水の含む物質と熱水循環の生み出す温度場が、光合成に頼らない「地球
を食う」生態系を支える
10
海洋底ダイナミクス2014(12)
地球を冷やす
conductive cooling (theoretical)
convection
observed
5
10
Seafloor Age (Ma)
20
(Gamo, 1996; Modified Wolery and Sleep, 1976)
11
海洋底ダイナミクス2014(12)
物質を運ぶ
hydrotheramal dominant
hydrothermal
heat flow (10-6cal/cm2/sec)
fast spreading ridges
fluvial
12
海洋底ダイナミクス2014(12)
生態系を育む：地球を食べるものたち
酸化還元電位高
酸化還元電位低
e-
H2S
H2
NO2SO32Fe2+
ATPの合成
電子供与体
電子受容体
O2
NO3CO2
* 光合成のように外部エネルギーを必要としない
ごはん
13
海洋底ダイナミクス2014(12)
資源を産する
377
SCHEMATIC DISTRIBUTION OF HYDROTHERMAL
磁硫鉄鉱
閃亜鉛鉱
硫黄
BLACK SMOKE
Pyrrhotile, Pyrite,
Sphalerite
DEAD CHIMNEY
INTERIOR Spholerlte, Sulfur, Pyrite,
~
FALLOUT ~'~t#'~t ~
t
SURFACES
Fe-Mn Oxyhydroxide
BASALT
/
,,_~a~
~ I ~ P ~
2m I
bornite
硬石膏
黄銅鉱
WHITE SMOKER CHIMNEY
WORMTUBESINASULEIDEMATRIX:
AmorphOUSWurt
M.....
zSilica,it
ite, . . . .Sul
. . f.ur,dum(?)PyrHe'Borile, Spholerile,
~
WHITE~SMOKE
~.~,~.%=b'~ ~morphous Silica
~t%~PyriteChol-~.i( ~il(~lJ . ~ ' ~ . ~ /
c~copyrite \
. 'IE~ ~
u r t z ~ ~ ' ~ .
s
MINERALS
INTERIORCholcopyrite, cubenite,
/
~,~,~'~ } / ~ , ~
golena,bornite,cubanite,choleocite ~%, " ~ J
' " ~
EXTERIORAmorphousSilica,Borite,Goethite, ~zfl C ~
~. i ;~.....3 /
Jarosite,natrojarosite,~.c°rundum1?1 L ~ ., ~L. ~f ' ~~' I\%
~ . ~~.~
2 ~.o.
Chalcopyrite, Wurtzite. Marcasite,
黄鉄鉱
EXTERIOR
VENT
BLACK SMOKER CHIMNEY
Anh2drito, Mg-HydroxysulfQte-Hydrote,Gypsum,
Sphalerite, Pyrlte,pyrrhotite, wurtzite, covellite
Barite
, ,...t~. ~
非晶質シリカ
/
重晶石、黄鉄鉱
~
WHITE SMOKER "SNOWBALL"
INTERIORSUBSTRATEFORWORMTUBES
Anhydrite,
Pyrite
DISAGGREGATED MOUNDS
2;;"o:'
~
I
f~'
e,gypsum, Fe-oxyhydroxide
鉄マンガン水酸化物
閃亜鉛鉱
黄銅鉱
黄鉄鉱
PILLOW
BASALT
Fig. 10. A composite sketch illustrating the variety of structures observed at the different RISE vent sites and the mineral distributions
associated with these structures. Note that anhydrite and Mg-hydroxysulfate-hydrate are found only in active, black smokers, and
pyrrhotite occurs mainly in black smoke or in black fallout sediment. Note also that Mn-oxyhydroxides precipitate on basalt surfaces
slightly away from vents. Main phases are italicized; minor phases are capitalized; accessory phases are printed in lower case.
(Haymon and Kastner, 1981)
5. Evolution of the hot spring deposits
The characteristics of the RISE sulfide samples
suggest the following generalized history of development for vent deposits at EPR 21°N. Hot,
altered seawater-bearing dissolved metals, gases,
solution removes anhydrite, and most probably
also "caminite", entirely from the vent deposits.
After construction of the outer walls; the chimney
interior is radially filled with sulfide minerals. In
cooler chimneys ( < ~ 250°C) the interior mineral
assemblage is dominated zy ZnS and FeS2 _+
14
海洋底ダイナミクス2014(12)
black smoker chmineys
(a) black smoker
(a) Photograph of a black smoker
om the southern East Pacific Rise,
e submersible Alvin on Dive 3296
Woods Hole Oceanographic Institu; M. Lilley and K. Von Damm chief
nd a schematic drawing showing a
n of a black smoker chimney, and
ons of fluid flow through the coner advection and diffusion occure walls. (b) Photograph of a 284°C
ting spire from the Vienna Woods
n the Manus Basin taken on Jason
tesy of WHOI; M. Tivey chief scienhematic drawing of the cross secEast Pacific Rise diffusely venting deepest portions of the circulation sysomposed of an inner, very porous
tem (the “root” or “reaction” zone), and
rhotite (Fe1-xS), wurtzite (Zn,Fe)S,
nite (CuFe2S3); a less-porous mid- lastly as the hot, buoyant fluid rises rapite, pyrite (FeS2) and chalcopyrite
idly through the “up-flow” zone to exit at
an outer layer of marcasite (FeS2)
s et al., 2006). (c) Photograph of a the seafloor (Alt, 1995) (Figure 2b).
he Tui Malila vent field, Lau Basin,
(b) ocdiffuser
Our understanding of processes
ason Dive 134 (courtesy of WHOI;
f scientist), and a schematic draw- curring in the down-flowing limb relies
g a cross section of a flange with a
l of high-temperature fluid. Fluids largely on data and observations of althrough the porous flange layers, teration mineral assemblages within oceminerals as they traverse the steep
radient, or “waterfall” over the lip anic crust recovered by submersible from
e. (d) Photograph of diffuse warm
the top of a “crust” sample on the
he TAG mound, taken from Alvin
Thompson, WHOI). Textures of resamples indicate that much hotooled beneath these crusts within
and that the hot fluids percolate
(c) flange
gh cracks. (e) Photograph of low< 1– 2) venting from the sides of
he North Su vent field in the easttures
andonscarps
that 221
expose
deeper
(Figure 3a), with alkali metals (K, Rb,
in, taken
Jason Dive
(courIt has
beendrill
proposed
sald,
of WHOI).
the crust,
from
cores, and
Cs), B, and H2O removed from seawater
ow pH results from input of magto the altered minerals and Si, S, and, in
m ophiolites (slices of oceanic crust
volatiles (e.g., Gamo et al., 1997).
Alteration by hydrothermalism
(a)
have been thrust onto land by plateonic processes). At temperatures up
bout 40°C to 60°C, reactions of seaer with basalt result in the alteration
asaltic glass, olivine, and plagioclase
xidation to ferric micas and smectite,
rich smectite, and Fe oxyhydroxides
eanography
fractures and scarps that expose deeper
(Figure 3a), with alkal
parts of the crust, from drill cores, and
Cs), B, and H2O remov
to the altered minerals
from ophiolites (slices of oceanic crust
some cases, Mg, lost fr
that have been thrust onto land by platetectonic processes). At temperatures up
MARGARET KINGSTO
to about 40°C to 60°C, reactions
of sea(Tivey, 2007,
Oceanography)
whoi.edu) is Associate S
water with basalt result in the alteration
15
ment of Marine
Chemis
of basaltic glass, olivine, and plagioclase
by oxidation to ferric micas and smectite, try, Woods Hole Oceano
海洋底ダイナミクス2014(12)
Woods Hole, MA, USA.
Mg-rich smectite, and Fe oxyhydroxides
(c)
some cases, Mg, lost from the minerals
MARGARET KINGSTON TIVEY (mktivey@
whoi.edu) is Associate Scientist, Department of Marine Chemistry and Geochemistry, Woods(d)
Holecrust
Oceanographic Institution,
Woods Hole, MA, USA.
(e) low pH fluids
1 cm
1 cm
(b)
Vol. 20, No. 1
Figure 3. (a) Altered basalt recovered on Ocean
Drilling Program (ODP) Leg 51 from Hole 417a,
composed of ferric smectites and Fe-oxyhydroxides (red-brown) that replace plagioclase, olivine,
and basaltic glass; and veins of carbonate (white).
(b) The two-phase boundary, critical point, and
density surfaces for seawater as a function of temperature and pressure, or depth beneath the seafloor, assuming hydrostatic pressure (after Bischoff
and Rosenbauer, 1985). The red parallelogram
Figure 3. (a) Altered basalt recover
Drilling Program (ODP) Leg 51 fro
composed of ferric smectites and
ides (red-brown) that replace plag
and basaltic glass; and veins of car
(b) The two-phase boundary, criti
density surfaces for seawater as a f
perature and pressure, or depth b
floor, assuming hydrostatic pressu
and Rosenbauer, 1985). The red pa
indicates temperatures and pressu
ing observed at the seafloor in diff
along the world’s spreading center
stockwork, or chloritized basalt br
from 116 m beneath the TAG acti
mound on ODP Leg 158. The sam
of highly altered chloritized basalt
iron sulfide veins (gold), and quar
courtesy of S. Humphris (Woods Ho
Institution and Ocean Drilling Prog
(Tivey, 2007, Oceanography)
16
海洋底ダイナミクス2014(12)
metal sulfide ores
黄鉄鉱 pyrite FeS2
硬石膏 anhydrite CaSO4
閃亜鉛鉱 sphalerite (Zn, Fe)S
黄銅鉱 pyrite CuFeS2
磁硫鉄鉱 pyrrhotite Fe1-xS
重晶石 barite BaSO4
写真はWikiから
17
海洋底ダイナミクス2014(12)
Kuroko Deposits 黒鉱
• Mainly Miocene age in the reen Tuﬀ
volcanic sequences
• massive, stratiform sulphide ore
• pyrite, chalcopyrite, sphalerite,
galena, barite and quartz
• origin: hydrothermal deposits
http://staff.aist.go.jp/y-watanabe/plate.htm
18
海洋底ダイナミクス2014(12)
Kuroko:
JAMSTEC ウェブサイトから
Chikyu Exp331+ C0016 (artificial chimney)
sphalerite, pyrite, galena, wurtzite
IHEYA
systems where magma is more siliceous
and richer in H2O, very-low-pH fluids
are observed, consistent with addition
of magmatic SO2 that disproportionates
to form sulfuric acid (e.g., Gamo et al.,
1997). Some of these back-arc- and arcrelated magmatic fluids may also con-
tribute metals to the hydrothermal system (e.g., Cu, Zn, Fe, As, Au) (Ishibashi
and Urabe, 1995; Yang and Scott, 1996;
Hannington et al., 2005).
The evolved fluid in the root or reaction zone is very buoyant relative to cold
seawater (Figure 3b) and thus rises at a
Variation of chemistry
rapid rate to the seafloor. Observations
of the rock record, which integrate the
effects of water-rock interaction over
long time periods, combined with results of thermodynamic calculations that
consider the measured compositions of
fluids sampled at vents, indicate that the
19
海洋底ダイナミクス2014(12)
Table 1. Compositions of fluids venting from different settings.
Mid-Ocean
Ridge
Back-Arc
Rainbow
Lost City
SedimentHosted
Seawater
T (°C)
≤ 405
278–334
365
≤ 91
100–315
2
pH (25°C)
2.8–4.5
< 1–5.0
2.8
10–11
5.1–5.9
8
30.5–1245
255–790
750
548
412–668
545
Cl, mmol/kg
Na, mmol/kg
10.6–983
210–590
553
479–485
315–560
464
Ca, mmol/kg
4.02–109
6.5–89
67
< 30
160–257
10.2
K, mmol/kg
-1.17–58.7
10.5–79
20
-
13.5–49.2
10.1
Ba, µmol/kg
1.64–18.6
5.9–100
> 67
-
> 12
0.14
H2S, mmol/kg
0–19.5
1.3–13.1
1
< 0.064
1.10–5.98
-
H2, mmol/kg
0.0005–38
0.035–0.5
13
< 1–15
-
-
CO2, mmol/kg
3.56–39.9
14.4–200
na
bdl
-
2.36
CH4, mmol/kg
0.007–2.58
.005–.06
0.13–2.2
1–2
-
-
NH3, mmol/kg
< 0.65
-
-
-
5.6–15.6
-
Fe, µmol/kg
7–18700
13–2500
24000
-
0–180
-
Mn, µmol/kg
59–3300
12–7100
2250
-
10–236
-
Cu, µmol/kg
0–150
.003–34
140
-
< 0.02–1.1
-
Zn, µmol/kg
0–780
7.6–3000
160
-
0.1–40.0
-
Pb, µmol/kg
0.183–0.1630
0.036–3.900
0.148
-
< 0.02–0.652
-
Co, µmol/kg
0.02–1.43
-
13
-
< 0.005
Cd, µmol/kg
0–0.910
-
0.130
-
< 0.01–0.046
-
Ni, µmol/kg
-
-
3
-
-
-
SO4 , mmol/kg
0
0
0
1–4
0
28
Mg, mmol/kg
0
0
0
<1
0
53
tectonic and geological
setting controls the fluid
chemistry
Data from Von Damm et al., 1985; Welhan and Craig, 1983; German and Von Damm, 2003; Trefry et al., 1994; Ishibashi and Urabe,
1995; Jeffrey S. Seewald, Woods Hole Oceanographic Institution, pers. comm., 2006; Douville et al., 2002; Kelley et al., 2001, 2005;
Proskurowski et al., 2006.
20
Oceanography
March 2007
55
海洋底ダイナミクス2014(12)
Black smoker and clear (white) smoker
3
Geochemistry
Geophysics
Geosystems
G
baker et al.: hydrothermal venting in magma deserts 10.1029/2004GC
(Indian Ridge：JAMSTEC)
• 270-403°C
• mature hydrothermal system
• heavy metals etc. are dispersed by
plume
• 150-290°C
• young system and/or sedimented ridges
• hydrothermal ore deposits (Au, Ag, Zn,
Cu, Sn)
c.f. 黒鉱鉱床
21
Geochemistry
Geophysics
Geosystems
3
G
海洋底ダイナミクス2014(12)
baker et al.: hydrothermal venting in magma deserts 10.1029/2004GC
熱水活動は火成活動に規制されているのか？
Baker(2004)
熱水サイト数/1000km3 Myr
熱水サイト数/100km
y=0.98+0.015x Global Trend
両側拡大速度[mm/yr]
両側拡大速度[mm/yr]
21. (a) Plume incidence for selected ridge sections where these statistics have been compiled (not in
マグマ生産量が少ない割に低速系では熱水活動がさかん
-affected Reykjanes
and Southeast Indian Ridges) (solid symbols indicate mean value, and open
minimum and maximum estimates, where applicable: red for ultraslow ridges and blue
22 for all
d plume incidences include 1, EPR 13! – 18!S [Urabe et al., 1995]; 2, EPR 27!– 32!S [Baker et al., 2
海洋底ダイナミクス2014(12)
構造運動の卓越する拡大系での熱水系の発見
Escartin et al.(2008)
玄武岩マグマ以外の熱源
ガブロ貫入岩の冷却
•
上部マントルの冷却
•
断層増加による浸透率の変化
蛇紋岩化
23
海洋底ダイナミクス2014(12)
熱水系を規制するものは何か
熱源
母岩
循環経路
マグマ、地下深部、潜熱
玄武岩、超苦鉄質岩、堆積物
断層分布、堆積層
地質・地球物理学的背景が
熱水系の多様性を生み出している
24
海洋底ダイナミクス2014(12)
テクトニックセッティングの例その１
海嶺軸の中心部
• Neo Volcanic Zone: 最も新しい火山活動の場
• マグマ活動に主に規制される熱水活動
• 比較的小規模短命
25
海洋底ダイナミクス2014(12)
3 ferrini et al.: submeter bathymetric mapping on east pacific rise 10.1029/2006GC001333
G
East Pacific Rise 9°N
Geochemistry
Geophysics
Geosystems
t al.: epr ridge segmentation
10.1029/2006GC001407
R.M. Haymon, S.M. White / Earth and Planetary Science Letters 226 (2004) 367–382
379
510N, and samples of the hydrothermal fluids and
biology were recovered [Haymon et al., 1993;
Rubin et al., 1994; Lutz et al., 1994; Gregg et
al., 1996; Shank et al., 1998; Von Damm, 2000].
High-temperature vents within the floor and along
the walls of the AST are primarily located along the
trace of the 1991–1992 eruptive volcanic fissures
or along fractures above the dike swarm located
beneath the AST that tap the vigorous hydrothermal system at the EPR axis [Haymon et al., 1993;
Fornari et al., 1998a, 2004; Shank et al., 1998; Von
Damm, 2000; Von Damm and Lilley, 2004]. The
hydrothermal systems in this area are believed to
be driven by a shallow magma lens at !1.5 km
[Detrick
et al.,at 3rd1987;
Kent
al.,
1994]
Fig. 6. 3D sketch depth
of fast-spreading
ridge crest segmented
and 4th order
scales: aet
single
3rd order
segment,and
divided by a 4th order RAD
into two 4th order segments, is illustrated here. Chimney icons illustrate distribution of high-T vents; vertical arrows illustrate magma supply
focused
by supply
vertical
toand
near-vertical
dike(s)
(bold arrows indicate
more robust magma
at mid-segment,
lighter
weight arrows indicate
diminution in that
magma supply near segment
Haymon
White
(2004)
ends).
sourced the 1991–1992 eruptions and permeable
zones created by drain backconsistent
of magma
along the
inflated, bwaxingQ stage described by Cormier et al.
with the relative rapidity of 4th order crack
portions
of the
fissureand
[Fornari
et al.,in2004].
[35] in whichwidest
small, volcanic
axial collapse
troughs
dike propagation
comparison to 3rd order
sometimes form. Hence, Fig. 6 is a depiction of a 3rd
order segment 9in the bwaxingQ stage of evolution.
Because we used a method of stacking data to
enhance the signal present in inherently noisy
volcanic and hydrothermal data, it is advantageous
that both survey areas are on bwaxingQ portions of the
EPR, and thus have comparable patterns of hydrothermal activity and magma supply that are unaffected
by the added complexity of permeability effects on
hydrothermal circulation introduced by ridge crest
faults that may form during the bwaningQ stage
[35,36]. Since our study areas do include multiple
segments with and without unfaulted, volcanic axial
collapse troughs, our study should be fully representative of bwaxingQ fast-spreading ridge crests.
magmatic inflation and deflation of the axial high.
The distribution of 4th order segment boundaries and
hydrothermal vents will be reconfigured many times
(probably tens to hundreds of times) during the life
0 order events can
span of a 3rd order segment. 4th
cause short-term temporal variations in hydrothermal
flow, and can shift locations of hydrothermal seafloor
vents, but the data show that on average the majority
of vents remain clustered in the midsections of 3rd
order segments.
Recent work along a 610-km-long portion of the
ultrafast-spreading southern EPR crest has shown a
robust spatial correlation between hydrothermal
plume intensity, cross-sectional area of the axial high
(binned at 3-km intervals) [22], and areas of flat-lying
Blacksheet flows [37]. These results suggest that hydro5.2. Implications for EPR hydrothermal systems
thermal vent distribution is governed by heat supply,
smoker
der and (right) fourth-order ridge discontinuities. Contour interval is
and they are generally consistent with our findings
4th order segments persist for short periods of time
that hydrothermal vents cluster in ~3–10-km-long10
The third-order offset, at 8!370N, separates an unusual segment with
Figure 1b. High-resolution map (2 m east – west "(decades
5 m to millennia) relative to 3rd order segments
central portions of 3rd order volcanic segments where
0
5
ment to the south. The fourth-order discontinuity at 9!52 N is a very
apparent effusion rates indicate maxima in magma
north – south) generated from previously reported (millennia
ABE to 10 years). These time scales are
Three flowを伴う中軸fissure内
primary environments for high-tempera•[ture]lava
(hi-T) and low-temperature (low-T) hydrotherventing have been observed at the EPR 9!50 N
高温小規模熱水噴出孔
•mal
area.
In the first environment, both high- and low-T
Ferrini et
White et al.(2006)
riptions of these and all other ridge discontinuities in the survey area.
Imagenex sonar data [Fornari et al., 2004]. In this map,
EM300
egment
f axial
more details of the complex AST bathymetry can be
discerned. Until now, these data have only been gridded
at 5 m " 5 m resolution (see Figure 1a).
where such data already exists [Fornari et al.,
2004; Haymon et al., 1991; White et al., 2002].
1.1. Ridge Crest Setting
4. Development of Very-Near-Axis
[8] The northern EPR in the 9!500N area has been
venting occurs within and along the trace of the
primary eruptive fissure system on the floor of the
AST. Tica, Bio9/9prime, P vent, and Tubeworm
Pillar are all hi-T areas that occur in this type of
(a) EPR Vent Site
environment, while Bio119, Bio141 are low-T
diffuse sites also located along the primary fissure
system (Figures 1a and 1b). The second venting
environment is primarily along the east wall of the
al.(2007)
AST where both low-T diffuse venting (e.g., East
Wall Riftia site, Figures 1a and 1b) and high-T
focused flow occur (e.g., Alvinellid
Pillar,
5–10
m Figures 1a
and 1b; M and Q vents (see Haymon et al. [1993],
Shank et al. [1998], VonTivey(2007)
Damm [2000], and Fornari
et al. [2004] for vent locations). The third area of
venting is low-T diffuse flow immediately outside 26
the AST on lobate or sheet lava where extensive
collapse is observed (e.g., Mussel Bed, 5–10
Figures
m 1a
and 1b). The collapsed nature of the volcanic
(b)
m
海洋底ダイナミクス2014(12)
テクトニックセッティングの例その２
中軸谷壁~オフアクシス
segment end
segment center
N VZ
中軸谷内で断層崖に沿って
segment end
NTO
(non-transform offset)
断裂帯に沿ったオフアクシス域
27
海洋底ダイナミクス2014(12)
Mid-Atlantic Ridge : TAG
350
0
00
5
5
8
9
44° 55′
70°
44° 50′
3
4
5
6
00
8
km/s
A′
0
1
2
7
6
7
LTZ
3
4
44° 45′
5
Mean
EQ uncertainty
Figure 1. Area map of Mid-Atlantic Ridge at 26°N,
10
A
and Trans-Atlantic Geotraverse (TAG) hydrother11A: Bathymetry (100 m contour interval)
mal field.
26°
of TAG segment with microearthquake epicenters
12
10′
(black dots), ocean bottom seismometer (OBS)
network (white circles), active TAG mound (brown
star), and neovolcanic
zones (red patches). Note
B
that five 0OBSs were deployed in small cluster
around active TAG mound and are within brown
No V.E.
-2.0
-1.0
0
1
30
star. Composite
focal plane solutions (lower
Locus of hydrothermal
00
B
hemisphere)
are shown for three Neovolcanic
event groups discharge
2 A
A′
26°
along A-A′ cross
section. Black shading repT
zone
3
350
05′
resents
compressional
fi
rst
arrivals
and
white
0
shading 4
denotes dilatational first arrivals. Average 95% confidence level epicenter microearth5 uncertainties (±1.0 km east-west,
quake (EQ)
±0.9 km 6north-south) are shown for reference.
B: Relationship of TAG hydrothermal field to sur7
face geology.
Active TAG mound (brown star) is
Antithetic normal
located between
neovolcanic zone (red patches)
8
26° N
Maximum
depth (orange square)
faulting
and eastern valley
wall. Gabbro
9 (gray
of hydrothermal
and
diabase
square) exposures have been
EQ uncertainty
circulation
observed
and
sampled
along
prominent
fault
10
scarp ~2.5 km east of active mound (Reves-Sohn
11 Zonenshain
Melt zone
Weak
zone
et al., 2004;
et al., 1989),
and
low44° 55′
44° 50′
44° 45′
45° 00′ W
44° 40′
(b) MEF Vent Structure
temperature
12 alteration products and venting have
been observed between
the arrows labeled
“LTZ”
(a) EPR Vent Site 5
-10
-5
0
10
for low-temperature zone (Rona et al., 1993). Relict high-temperature zones (brown patches; M—Mir zone, A—Alvin zone) are located northDistance along cross section (km)
east of active TAG mound. OBSs (white circles) used for seismic refraction velocity inversion are shown along cross section.
6
7
8
Diffuser
Flange
Black
smoker
•
•
•
•
712
高温熱水
大規模正断層の存在
3
4
5
6
P-velocity
model (km/s)
4
6
7
8
8
Depth below active TAG mound (km)
0
1
2
15
Impermeable
silicified pipe
10 m
5–10 m
(C) TAG Active Hydrothermal Mound
100 m
Black smoker complex
Anhydrite-rich zone
Seawater
entrainment
Crust
Zn-rich
white smokers
Seawater
entrainment
Sulfide talus
マグマ活動の中心から離れた軸谷の縁
巨大・長寿命（間欠的か）
A′
Figure 2. Depth sections (no vertical exaggeration) across axial valley on A-A′ (from Fig. 1).
A: P-wave velocity model and hypocenters from events within 1 km of cross section. TomoDISCUSSION
graphic
model uncertainties are typically <0.1 km/s, but can be as large as 0.2 km/s at eastern
provide
important
constraints zone, hydrothermal deposits, fault termination, and
endOur
ofresults
model.
Position
ofnew
neovolcanic
for the geometry
faulting and the crustal
archi-(white circles) used for velocity inversion along cross
ocean
bottomofseismometers
(OBSs)
tecture inare
the also
TAG region.
WeAverage
find that lithosection
shown.
95% confidence level hypocentral uncertainties (projected
sphericcross
extension
on the east
of thefor
axialreference (depth uncertainty is ±1.1 km). B: Schematic
along
section)
areside
shown
valley is of
being
accommodated
on a curved
normodel
crustal
accretion,
deformation,
and hydrothermal circulation at Trans-Atlantic
mal fault with a(TAG)
steep (~70°)
over the depth and P-wave velocity model used to define earthquake
Geotraverse
withdip
hypocenters
interval of ~3–7 km
below the seafloor.
We didare shown as perturbations against one-dimensional
hypocenters.
Tomographic
results
not detectmodel,
significant
of seismic
activity velocities. Extension on east side of spreading axis
average
aslevels
opposed
to absolute
the fault at depths <~3
km, but the shallow
isonaccommodated
on dome-shaped
detachment fault and on two distinct planes of antithetic
fault geometry
is delimited
our tomographic
normal
faulting
(yellowbylines).
New lithosphere is formed by exhuming lower crustal material
velocity
Seismic
(P-wave) velocities
on exposed at seafloor from termination (T) to breakaway
on
faultmodel.
footwall.
Detachment
fault is
the as
eastidentifi
side of the
valley
exceed 6.5
km/s(Tivey et al., 2003). Long-lived, high-temperature hydro(B)
edaxial
from
magnetic
data
at depths circulation
as shallow as ~1
km beneath
the seathermal
at TAG
hydrothermal
field requires magmatic heat source (e.g., Cann and
floor, and1982;
indicateHumphris
that the faultand
exhumes
lower2000), but our seismic velocity model and earthquake
Strens,
Cann,
crustal rocks oneffectively
a low-angle interface
dipping
hypocenters
preclude
presence of crustal magma chamber. This suggests that
at ~20°must
towardbethedeep
spreading
consistent
there
meltaxis,
reservoir
beneath neovolcanic zone, which may also root highwith geological
at other
detachangle
normal observations
fault. Gabbros
crystallizing
from this reservoir would then be accreted onto
ment faults(orange
(e.g., Dickarrow)
et al., 1981;
Cannextension.
et al.,
footwall
during
Hydrothermal fluids may flow through hanging
1997;atSmith
et al., 2006).
The inferred
footwall–
wall
shallow
depths,
but they
must be focused on detachment deeper in crust, and they
hanging-wall
interface
intercepts
thekm
westmust
penetrate
to depths
of ≥7
to extract high-temperature heat from base of fault.
deMartin et al.(2007)
constructed Mid-Atlantic Ridge segments
(e.g., Hooft et al., 2000; White et al., 1992),
but the eastern flank is underlain by a large,
high-velocity anomaly. Seismic velocities at
depths below 1 km on the eastern flank exceed
6.5 km/s (compared to 4.5 km/s beneath the
neovolcanic zone), indicating the presence
of lower crustal and/or serpentinized upper
mantle rocks at anomalously shallow depths.
The velocity anomaly dips toward the spreading axis at an angle of 20° ± 5°, passes under
the active TAG mound at a depth of ~1 km
(Fig. 2A), and intersects the hypocenter trend
of the west-dipping fault plane at a depth of
~3 km. Although our cross-axis tomography
model is limited to the upper ~2.5 km of the
crust, additional tomography models obtained
along the axial valley (Canales et al., 2005) do
not contain any low-velocity zones indicative
of crustal melts at the TAG segment, in contrast
to the Kong et al. (1992) seismological study of
this same area.
2.0 km/s
10 m
5–10 m
arrival polarities from ~300 individual events
are consistent with normal faulting along the
hypocentral trend (dip 80°). Seismic activity
on this fault decreased dramatically at depths
<~3 km (detection threshold of ML ≥ 1). The
ridge-parallel microearthquakes beneath the
eastern valley wall form two spatially distinct
clusters at depths of ~2–5 km below the seafloor.
The spatial association of these events with discrete fault planes is somewhat ambiguous, but
composite focal mechanisms for both clusters
are consistent with antithetic normal faulting on
planes dipping eastward (away from the spreading axis) at 65° (Fig. 1A).
We inverted P-wave first arrival traveltimes
using the method of Korenaga et al. (2000)
to generate a tomographic model of seismic
velocities across the ridge axis (Fig. 2; GSA
Data Repository). We find that crustal structure
is highly asymmetric across the axial valley.
The western side of the axial valley has a
seismic structure similar to other volcanically
1.0
bro
Gab
Depth below sea level (km)
250
400
0
450
0
25
00
0
300
0
30
7
4
5
6
0
3
G
6
7
G
250
4
Hydrothermal
deposits
Fault
intersects 2
seafloor
Depth below active TAG mound (km)
26°
05′
2500
4
00
26°
15′
A Neovolcanic
zone
M
40
26°
10′
Mid-Atlantic
Ridge
A
0
A No V.E.
1
2 A
3
30
00
40
Atlantic
Ocean
TAG
Hydrothermal
Field
Depth below sea level (km)
00
00
B
35
35
A
26°
20′
Zn
mobilization
Massive pyrite and
pyrite breccias
Pyrite and
silica zone
Demagnetized zone
58
Oceanography
Silicified and pyritized
stockwork
Vol. 20, No. 1
same place where gabbro and diabase outcrops
have been observed and/or sampled on the
seafloor (Reves-Sohn et al., 2004; Zonenshain
et al., 1989) (Fig. 1B). The low-angle fault
geometry inferred from our seismic velocity
model is required to reconcile our microearthquake patterns with the magnetics data, because
any surface connecting the fault termination
with the arc of seismicity will necessarily have a
dip ≤30° (Fig. 2B). A low-angle fault geometry
in the shallow crust is also required to reconcile
the footwall vertical relief (~1.5 km) with the
horizontal extension required by the magnetics
data (~3.9 km). Alternative models without
significant rollover of the west-dipping normal
fault are effectively precluded by dive observations (Karson and Rona, 1990; Zonenshain
et al., 1989) and high-resolution sidescan sonar
images (Kleinrock and Humphris, 1996) of the
axial valley; these images provide no evidence
of large-offset fault scarps or hydrothermal discharge within the neovolcanic zone. We cannot
provide a plausible faulting model that satisfies
the combination of our seismic data, the magnetics data, the sidescan sonar data, the bathymetric data, and the dive observations from the
TAG region without invoking a rollover to a
low-angle geometry at shallow depths.
We estimate a total fault rollover of ~50°
(i.e., 70° to 20°) when all the relevant information is considered. This amount of rollover
is consistent with paleomagnetic rotations
measured at the Fifteen-Twenty and Atlantis
massif oceanic core complexes (Blackman
et al., 2006; Garcés and Gee, 2007), as well
as rotations deduced for some continental core
complexes (e.g., Manatschal et al., 2001).
Highly rotated crustal blocks, however, have
not been observed in the footwall east of TAG
(e.g., Karson and Rona, 1990; Zonenshain
et al., 1989), which is consistent with the fact
that the detachment fault has only been active
long enough (~350 k.y.) to exhume ~1.5 km
of largely unrotated upper crust. Although the
footwall exhibits a dome-shaped morphology,
it lacks the surface corrugations associated
with more advanced stages of core complex
evolution (e.g., Smith et al., 2006), also supporting a young age for the detachment.
We conclude that the TAG hydrothermal field
28 but
is located on the hanging wall of an active,
young, oceanic detachment, and we use the
kinematic and geometric data provided by our
seismic study to develop a schematic model of
courtesy of Dr. R. Sohn
dipping normal fault just above the depth where
2007
it becomes aseismic GEOLOGY,
(Fig. 2B),August
suggesting
that
the change in seismic behavior is associated
1989) and global seismicity compilations (e.g.,
Wernicke, 1995).
Ridge, reveal positive magnetic anomalies, and therefore a strong
magnetization at the largest sites. This observation reflects the presence of a wide mineralized zone beneath these sites, the stockwork,
where several chemical processes concur to create and preserve
strongly magnetized magnetite. Beyond pointing out the importance
of subsurface chemical processes in hydrothermal activity, the aging
of oceanic lithosphere, and the ocean chemical budget, our results
have immediate application for detecting and characterizing economically valuable deep-sea mineral deposits.
align along the western side of a 200-m-long north-northeast tectonic depression separating a gabbro massif from serpentinite (Fig. 3C). Hydrothermal activity is found in the depression, on a crater-shaped structure,
~5 m high and 25 m in diameter, with numerous
venting small chimneys
海洋底ダイナミクス2014(12)
within the crater. This smoking crater suggests explosive episodes of
hydrothermal discharge (Fouquet et al., 2010; Ondréas et al., 2012).
Site Logachev is located south of the Fifteen-Twenty Fracture Zone,
on the eastern flank of the MAR at 14°45′N, at depths ranging from 3060
to 2910 m (Krasnov et al., 1995). Seven active hydrothermal areas are
aligned along a southeast-northwest trend over a distance of 500 m on the
flank of a westward slope (Fig. 3D). These sites are <15 m high and <50 m
in diameter; five of them are smoking craters (Petersen et al., 2009).
Mid-Atlantic Ridge Rainbow
ARCH
ARTICLES
Eúlalia et al.(2000)
INTRODUCTION
Hydrothermalism at mid-oceanic ridges and backarc basins contributes significantly to the dissipation of the Earth internal heat and tolike
the any hydrothermal system found to date,
Segment境界にある高まりの西斜面
ANDventing
METHODS
ocean chemical budget. After the first direct observations of hydrotherhostingDATA
diffusely
carbonate monoliths
High-resolution
bathymetric
and magnetic data were acquired in the
mal systems on the Galapagos Rift (Corliss et al., 1979), the discovery
of
towering
tens
of蛇紋岩が母岩（高まりの頂部は古い玄武
meters above
the seafloor
fourcomposition
study areas using
remotely
hydrothermal site TAG (Trans-Atlantic Geotraverse) on the Mid-Atlantic
(4). The
of its
fluidsoperated
derivesvehicle (ROV) Victor of Ifremer
(Institut between
Français
de
Recherche
pour
Ridge (MAR) (Rona et al., 1986) revealed that slow-spreading ridges
from reactions
seawater
and
uplifted
岩の報告あり） l’Exploitation de la Mer) during
cruises
MoMARDREAM
andinteractions
Serpentine (Fig. 3). Samples collected at
also support high-temperature venting. As a result of a more complex
mantle
peridotite
rather than from
2
3
geologyA.(Gente
et al., Gretchen
1995) characterized
by a limited magma supply
Karson,
ey,1* Jeffrey
L. Fru¨h-Green,
between seawater
and cooling basalts (4).
高温熱水あり、1.2万年前∼
5
(Cannat, M.
1993)
and a 4dominant
activity (Tucholke
et al., 1998;
ger,4 Timothy
Shank,
David tectonic
A. Butterfield,
Subsurface, exothermic, mineral-fluid re4 Escartín et al., 2008), mantle
1 outcrops are frequently
1
observed at slowactions associated with the oxidation of iron
ayes, Matthew O. Schrenk, Eric J. Olson,
Bathymetry
N
spreading ridges, 6and at least six hydrothermal
sites 1on such a basement
4
in cooling mantle peridotite produce alkaline
owski,1 have
Mikebeen
Jakuba,
Al
Bradley,
Ben
Larson,
identified
on
the
MAR
(Fouquet
et
al.,
2010,
and
references
1
4
fluids rich in hydrogen and methane at
udwig,1 Deborah
Glickson,
Kate
Buckman,
therein). While
the magnetic
response
of basalt-hosted hydrothermal venting
systemperatures 3up
7
1
5
km to 90-C (4, 5). These
Magnetization
S. Bradley,
J. Brazelton,
Kevin
Roe,
0.
tems William
is well known
(Tivey et al., 1993;
Tivey
and Johnson, 2002; Tivey
m
high-pH
volatile-rich
fluids trigger carbonate
1
3
3
R
E
S
E
A
R
C
H
A
R
T
I
C
L
E
S
and Dyment
, 2010; Stefano
Zhu et al., 2010;
Caratori-Tontini et al., 2012; Honsho
´lie
nd, Ade
Delacour,
M. Bernasconi,
–2200 upon mixing with seawater and
precipitation
1 et al., 2013; Szitkar et
1 al., 2014), the magnetic response
7
of high-temperaLCHF.
Near the
summit
of the massif,
Here we present an integrated
summary
ization reactions
last for hundreds
Lilley,
John may
A. Baross,
RogerS1).
E. Summons,
servetheas–2250
important
energy
sources
for micro4 hydrothermal
ultramafic-hosted
sitesstudies
remains
documented
we identified a È50-m-thick mylonitic-toof our field
and poorly
laboratory
findings.
thousands ture
of Sean
years
(11,
12). Alkaline
P.
Sylva
organisms
that
thrive
in
the
porouslenses
chimney
–2300 shear zone that includes
cataclastic
Our
results
provide
a
comprehensive
overstems such as
the
LCHF
may
have
been
(Tivey and Dyment, 2010), despite their important mineral potential
walls.
New site?
deformed material (Fig. 3 and fig. S1).
view of
geological the
and magnetic
structural controls
aracteristic of
early Earth
–2350
et al.,hydrothermal
2010).field
In thisispaper
wetheinvestigate
signature of less
osted
Lost(Fouquet
City the
hydrothermal
aonremarkable
peridotite-hosted
biotopes
This shear
zone is probably the
long-liveddiffer
fluid flow at submarine
Lost City and elucidate the These
vironments,
where
eruption of Mg-rich
four
ultramafic-hosted
sitesprocesses
discovered are
on
MAR.interaction in the detachment fault that exposed the mantle
Active site
consequences
of the
fluid/rock
lavasofwas
common
(13).biological
hmatiitic
geological,
chemical,
and
intimately
ntinite-Hosted Ecosystem:
t City Hydrothermal Field
•
•
•
substantially
from axial, magmatically drivand 36°13.80′
lower crustal rock sequences that make
basement
for fluid produce
chemistry and chimney
In 2003,
a secondseawater
expedition returned
to the mantle
ons
between
and upper
peridotite
en
vent
systems
in which carbon dioxide and
–33°54.10′
Inactive
up the massif
(15). This zone grades downgrowth
and for the life that can be supported
CHF to studyGEOLOGICAL
the linkages between
hydro- OF
SETTING
ULTRAMAFIC-HOSTED
SITES
36°13.75′
rogen-rich
fluids,
with
temperatures
ranging
from G40- to
hydrogen
sulfide
are
the
dominant
volatile
site
ward
into
massive
jointed
rocks
that
lack
a
in
these
environments.
rmal alteration ofSites
mantle
peridotite,
fluid
Rainbow, Ashadze 1, Ashadze 2, and Logachev are high-temperand carbonate
chimneys
30The
tofield
60 meters
tall. A low
diversity
–33°54.20′
species (5, 6). Exposed serpentinized
peridThe basement rocks
Geologic
setting.
Alvin and ABE
ochemistry,
and biological
activity.
ature active vents
with average
fluid
temperatures
of The
296–370
°C (Fouquet strong deformation fabric.36°13.80′
in are
the vicinity
of the field
are cut and
by veins
of
surveys
delineate
the southern
ogram
included
dives with the submersrelated
to 19
methane-cycling
Archaea
thrive
in the
warm scarp of the
otites
widespread
(7–10),
reactions
No vertical
etanal.,unconventional
2010; Charloumapping
et al., 2010,
and and
references
therein).
aragonite, which 36°13.75′
derive from
massif
faultoflineaments
e)2
Alvin
and
–33°54.30′
the
Macrofaunal
showthe@ABCDAEC9F
a major
degree
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exaggeration
similar
toofand
those
producing
the
Lostat City
Site Rainbow
a 300 ×it200
m area
on
the
eastern
flank
of
some
the
oldest
hydrothermal
activity
(Figs.
1BR Eand
2
and
fig.
S1).
ort with the Autonomous
Benthic extends
Exploreroverintersect
Rainbow
SEARCH A RTICLES
tBE)
as high
as extreme
that oftopography
black :;RS
smoker-"01-7/G
ventfoliated
sites
along
the talc-amphibole
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018"74
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Bathymetry
hydrothermal
(LCHF)
are probably
this
site (11).
serpentinite,
in this the
area
of "6/#10
Mfield
MAR
at 36°13′N,
33°54′W,Variably
at a depth
of-)G/+
~2300
m
(German
et al.,
1996;
in length at a water depth of 800
to 900 m
system is in marked
to the vertical
of
carbonate
veins and
subhorizontal-to2350
2300
2250 m
A contrast
e,
but
they
lack
the
high
biomasses
of
chemosynthetic
(markers 7that
and
fig. S1). In this area, the
conduits
thatthese
typifyRblack
smoker
environsubvertical
open
fractures
in
the
basement
%LM$Ten
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4.1insummit
common
implying
ofareas,
the
massif
is capped
byH,1there
schist,
andwestern
metagabbroic
make
up10the
4).
active
vent
sites
Szitkar
et
al.(2014)
Magnetization
Fouquet
et al.,
1997).
It were
is located
on the
siderock.
ofrocks
a
large
(10
×
km)
vertical serpentinite cliffs are draped with
ments and continental sulfide deposits.
These are actively leaking hydro10
0
20
30
40 A/m
3 The
mcoreofof theflat-lying
sedimentary
breccias
nearly
continuous
atfluid
theandtop
of young
the
mpled
for
thevolcanically
first time2/>*4
for driven
co-registered
typical
of
systems.
deposits
that haveof
astoundingly
field unexplored
is dominated
by the
thermal
feeding
whiteto
may
beactively
extensive
regions
the diverse morG>"45
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ultramafic
hill,
within
non-transform
site
consists
of overlain
phologies. Fluids weeping from the scarp face
venting carbonate monolith called
hydrothermal
precipitates
(Fig. 4).
by
variably
lithified,
fossiliferous
scarps to discontinuity.
the northwest
and
northeast
of
ids, rocks, and
biota (Figs.
1 and
2 anda fig.
have produced
clusters
of delicate multiPoseidon (fig.harboring
S1), an edifice that rises
960forms
m
The white hydrothermal
deposits
range basins
ocean
life
that
are
not
7/*1 "- 2+1Y>1G75
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HLKBKIEK%OK%IM
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three parts,
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150-m-wide
hydroFigure
1. (È800
Ultramafic-hosted
siteout-Rainbow (Mid-Atlantic
prongedhydrothermal
carbonate
above
the
seafloor
m of water
depth).bedrock
fromand
arraystectonized
of delicate fingerlike
crystals topelagic
limestone
with
sparse
clasts.growths that extend
ward
like the driven
fingers
of upturned
hands.
This
composite
structure is
composed
of four
beehivelike
massesLost
that are a fewsolely
tens of
supported
by
magmatically
In 2000,
hydrothermal
field
called
astounded
by the
-)
"G0"7/410$
location
shown
in inset.
Left: Bathymetry in
Ridge
[MAR]
at
36°13′N);
breccias
probably
debris
slide
thermal
mound bounded
by aa50-m-high
ridge-facing
faultacross,
scarp;
a 150-mMultipinnacled
chimneys, some reaching
large
columns that
are several represent
meters in
centimeters
growing
directly out ofThe
10detachment
m in height,
grow
vertically out of the cliff hydrothermal mound
diameter
(Figs.
2 and
4B). These
pinnacles
cracks (Fig. 4E).
Individual conduits
aredeposits
three-dimensional
(3-D)
view.
The
westernmost
hydrothermal
flow.
In
addition,
peridotiteCity
was
serendipitously
discovered
more
mal
chimneys
and
shed
onto
the
sloping
. 2. Three-dimensional
wide, ~25-m-high fault-controlled active hydrothermal
mound
by coalesce at their base to form a massive east- faces (Fig. 4A), and single chimneys that
sealed by
growths ofcovered
calcium carbonate.
is
cut
bystructure
acan
fault
its
western
flank,
resulting
in stockwork mineralizalooking
the of
14
海洋底ダイナミクス2014(12)
surface
when
defined
the from
median
sprout
gently
dipping bedrock
west
trending
extending
at least
Overgrowths and
swathsof
of youngerhosted
carbonatefault
systems
beiton
long-lived:
Cmoreheight
dating
than 15 km away from the spreading
axis
nw,
by
thetoward
cooling
sediments;
reach 30 m indebris
(Fig.in
4D the
and movie
50
m. Parasitic
chimneys, resembling
inverted
across older and
materialseveral
suggest retheast, of theblack
LCHF. smokers, fallen chimneys, and hydrothermalcutting
tion
outcrops
hydrothermal
talus. Right: Equivalent
valley
wall.
As thatand
surface
moved
offstructures
axis form
S1). These
a nearly continustalactites,
are particularly abundant activity
on the
activation
and
multiple
stages of fracturing.
the
Mid-Atlantic
Ridge
(MAR)
at
a
water
n
ridges
and
hostshows
that
hydrothermal
at
Lost
City
s image is based
on groups of active black smokers without significant
isolated
sulfide
accumumagnetization
draped
onsubhorizontal
bathymetry
in can
3-D
view.
ous
carbonate deposit that
be traced
for Strong positive magface, as are
activelyits
venting
flanges,
Control of the
hydrothermal
outflow byandnorthwest
flattened
into
present
ABE
missions
using
over
200 m in length
along the 850- to 900-m
or ledges
that protrude several
from
fractures is particularly
obvious where
haswalls
been
ongoing
for meters
at are
least
30,000
years
depth
of 750 tomound
900 m(Fouquet
(Fig. 1).
studies
synthetically
based
netization
contrasts
associated
with
the two hydrothermal mounds.
lation,
suggesting
an immature
et Initial
al.,
2010)
(Fig.
1). have
sedimentary
regime
contourchanged
(fig. S1).
the main trunk the
(Fig. 4C).
of massive
shingled
carbonate
beenorientation,
SM2000 sonar sysThe northern portion
of the field is bounded
The most visually
is just
deposited on
the steep scarps, highlighting
based on a single dive by the deep
submer1).in Since
that piv(11).from
Modeling
ofstriking
thisregion
system
suggests
clastic to pelagic.
Exiting
high-pH
hydro- that
m
a down-looking
by a shallow depression (Fig. 1B) that is closed
northeast of Poseidon, in an area about 70 m
the fact that much of the LCHF plumbing
to the east and gradually deepens to the west.
thermal
fluids
generated
by
serpentinization
mode.
hydrothermal
activity
sustained
by
serpengence
vehicle
Alvin
showed
that
it
was
un0d side-looking
vent fields
have
GEOLOGY, August 2014; v. 42; no. 8; p. 1–4; Data Repository item 2014266 | doi:10.1130/G35729.1 | Published online
XX ‘‘Chaff
Month
2014
Nicknamed
Beach’’
(fig. S1), this
Fig. 4. Hydrothermal
e LCHF is in the foreprobably enhanced carbonate precipitation
gently slopingand
area is covered by variably
at Lost City.
ocean
basins
©
2014
Geological
Society
For et
permission
todeposits
Copyright Permissions, GSA, or [email protected].
GEOLOGY
2014of| America.
www.gsapubs.org
1
Kelley
al.(2005)
| August
und;
at depths
less (2).
cemented
foraminifera,
pteropod shells, coral
(A)copy,
Graceful, contact
10-m-tall,
cementation
of
these
sediments
(16).
The
reactively venting cardebris, urchin spines, and hydrothermally
n È900 m,
the area
posits
and
diverse
bonate chimney growsulting
cap
rock
was
important
in
the
formaderived
carbonate
crust.
Concentrations
of
Oﬀ-axisin the(断裂帯沿）
ing directly out of a
haracterized by nearwater column here are 30 times
serpentinite cliff on
ms,
andcarbonate
swarming
tion of the LCHF, acting as an CH
impervious
(55 nM) those in background ocean water at
continuous
the eastern side of the
depths È50 m above the seafloor [supporting
field.
The
small
carbonlid
trapping
both
fluids
and
heat.
mneys,
spires,
and
familiar
hallmarks
online material (SOM) text and fig. S1].
ate deposits in the
蛇紋岩が母岩
The LCHF. The LCHF
lies
atopconcentrations
the are over 100 times
bris. The massive
background mark sites
Hydrogen
mal
vent
systems.
of active and inactive
(349 nM) those of background seawater
nacles
at
the
sumsedimentary
cap
rock,
on
a
triangular
downA,A,OO (5//G </#1G4/ ^11* _U/+4
seeps along the steep
values. This ground fog of volatile-enriched
walls. (B) One of four
erature
systems
t of this platform
are are
dropped block that forms低温(~90°C)
a terracefluids
on may
the represent
edge the leaking of diffuse
actively venting peaks
alkaline fluids through the underlying serpencomposite,
actively
that make up the mase
global
mid-ocean
of the south wall (Figs. 1B and
3). Our
tinites, leading
to rapid lithification of the pesive hydrothermal strucnting edifices that
ture called Poseidon.
lagic
sediments.
Hydrocasts conducted È75
mapping indicates that the largest
and most
Young and/or actively
kewhere
up the more
massivethan
to 100 m east of the main field also detected
venting material is white
active
vents
are
along
an
east-west
volatiletrending
enrichment up to 150 m above the
-m-tall
structure
in color; inactive areas
output is localized
seafloor; the highest concentrations of CH
are brown to creamlineament more than 300 m long were
(Fig.measured
1B and
ed
Poseidon.
The area is
characterized
by fi
extreme
topographic relief,
here,TAG-type
reaching values vents
of
colored.
This
is
ingin with
wallvertical-to-subvertical
ofm across.
thepinnacle
fault
andcliffs
intruded
by basalts
ultamafi
c-hosted
Rainbow
vent
eld. Rainbow
A
researchers
to/4
d-led
overhanging
ledges
in the
serpentinite
bedrock. The
smooth)2
surface
the background
is the
of
180 nM.
È4
(C)
The summit
fig.
S1).
The
lineament
is
intersected
by a fault
)2
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2+)6
base of the ‘‘IMAX’’
Carbonate
fluid chemistry. Geochemiunder
greenschist
facies
conditions
(MacLeod
fl
uids
are
characterized
by
low
pH,
and
high
Atlantis
massif. (G1 to
flange, which is a threerow
corridor
trending approximately north-south,
which
cal and petrologic
analyses of the carbonate
Extrusives
spirelike growth
*"G1- 018"74of/+1/"6/#10 >-"G# :;RS -"01-7/G story-tall
-)G/+
HIAKIBM$
-4/+rocks reveal distinct differences between the
Escartín
et al.,T."41
2003).
Thismassive
is7)++1,
thejointed outcrops
on the side
of Poseidon.
exposes
of relatively
Gabbros
sabundances
of the ridge. Fe, Mn, Ca, Y, and REEs com- et al., 2002;
active and extinct structures. Actively venting
This area is actively
Ultramafics
JK%LM$
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chimneys and flanges
undeformed
serpentinized
harzburgites
that are highly porous, friable
55-C fluids that
recharge
distance
for
the
fl
uid,
and
the
pared to mafic-hosted vent systems (Douville minimumventing
formations composed predominantly of aragosupport dense microbial
. 3. Diagrammatic
form
the been
major
north-southFluid
ridge
justandsouth
communities.
(D) Activenite
brucite of
(Fig. 5). Brucite commonly
recharge
time-integrated
fl
ux
must
have
at
et
al.,
2002).
In
terms
of
Ca
and
Si
content,
paths
G4"G>"45
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tch showing geologic
ly venting 50-C pinnacle
the7 field. Toward
the topDetachment
of the encloses
scarp,dendritic
thesefeathery growths of aragoon the east side of the
–2
of
Washington,
nite, showing that it is a later phase probably
dversity
tectonic
relation~1 km
least
comparable
to that of /G0
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these
fluids
would be in@ODBOF$
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with
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Dikes
rocks are strongly foliated and areformed
overlain
throughby
mixing of magnesium-rich
feathery growths of carps at LostofCity.
Hy- and 0.12/3. 4225 67.83
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seawater and hydrothermal fluid. Distinct
)&*&*++
,-../
bonate. the
This edifice
is
and tremolite
in the reaction
zone
(Seyfried
culated
for
Troodos
ophiolite
(Bickle
and
the
sedimentary
cap
rocks.
This
distinctive
Fig.
1.
(A)
The
Atlantis
massif
is
located
È15
km
to
the
west
of
the
MAR
axial
valley.
The
/*-)
"G0"7/410$
othermal
structures
annular
orifices,
which
are
common
in
black
growing
in
a
large
vertiersity, Durham, NC
B
29
Mid-Atlantic Ridge Lost City
•
•
•
4
NVZ
4
NVZ
NVZ
cal crevice within
thesignificantly higher
smokernodal
systems,
rare at Lost City.vents
Inet
al., 2004).
Textures
our fault rocks
clearly
Teagle,
1992),
and
than
is seen
the higher scarps
toRainbow-type
theare
located
on a faulted
of the
Atlantis
Fracture
Zone
and
the MAR issurface
marked
bythe
a inÈ6000-m-deep
basin.
wall. Actively
venting
Earth
Sciences,
ETH- inintersection
stead, fluids emerge from complex networks
wn-dropped
block of
6 carbonate flanges,
–2 which
4indicate replacement
northeast
and
northwest.
The
difference
in
of
centimeter-sized
channels.
Within
a
horizontal
distance
of
È20
km,
the
seafloor
rises
to
within
700
m
of
the
surface.
On
the Petrographic
of
harz
burgite
by
talc
+
2
×
10
kg
m
calculated
for
ODP
Hole
504B
Woods
Hole
Oceanoare several meters across,
iably altered and deanalyses of the carbonate chimneys show
form overhanging ledges
depth
between
equivalent outcrops
indicates
basis
of magnetic
data,(Teagle
the massif
has
uplifted
atless
rates
comparable
tofinethose
theof carbonate lined
anastomosingof
networks
oods
Hole,
MA
tremolite,
and02543,
basalt by
chlorite
± tremoetaboveal.,
which
is much
altered
and 2003),
to
the sidesbeen
med
crust
composed
withof
brucite,
indicating mixing of seawater
of the chimney. The
that
there
has
been at
least
150
m
vertical
Himalayan
mountains
(15).
Well-developed
corrugations
on
the
surface
of
the
massif
are
believed
udy
of
the
Atmosphere
dominantly
of
sergreen-capped
black
and
hydrothermal
fluids within the interior
lite and/or actinolite (Escartín et al., 2003). for both Sr
and O isotopes (Fig. 1).
displacement
on
a
fault
that
trends
westcylinder at the notch
walls.
ntinite.
It is and
likelyNational
that
hington,
be tracesCa
of and
a long-lived
detachment
dips experiment
gentlyplaced
beneath
axis
between the twofault
pinnaclesthat
was a biological
in active flow;the
it is È20
cm inof the MAR (15).
In contrast
to the active structures, extinct
These coupled
reactionstoinvolving
Si
Mature
northwest.
Thisthefault
and
faults chimneys
with
similar
diameter. (E) Feathery carbonate growth rising
from a vein within
serpentinite
bedrock.
drothermal
flow is fodministration
Pacific
are lessof
porous,
well lithified, and
The flat elongate bench on the eastern
side
ofseveral
thecentimeters
massif
is
interpreted
the
hanging
wall
the
Many of these
veins are
across.
Near
the summit of the as
massif
they form
detachment
trends
are systems,
essentially
the
nearby
ed
by transfer
intersecting
a higher
abundance of calcite. Prolonged
mass
will
occurfault
wherever
fl
uids
pass
Evolution
the
Hydrothermal
System
denseof
cross-cutting
networks that mark fossilized
stockwork
which fed parallel
past sites of tohave
oratory,
Seattle
WA
and is composed of volcanic
material. Lost City Atlantis
is located
within
the
small
box
near
the
lower-temperature
interactions
with seawater
venting.
ctures associated with
fault. Transform-parallel
between
mafiWHOI,
c and ultramafi
rocks of
under
Figure
shows how
different map
types based
ofTransform
hydroint
Program,
centralcsummit
the massif.
(B) 3Shaded
bathymetric
on ABE
dataoffsets
for the
LCHF and
nsform
fault developLost
City–type
vents
faults
with
extensive
vertical
are
typical
7
C
4 MARCH
2005
VOL
307 SCIENCE
www.sciencemag.org
1430
Department
oftheEarth,
greenschist
conditions.
circulation
could bestructures
associated
with
difnt
and uplift offacies
adjacent terrain. Most of thermal
the small
mound-shaped
are
individual
chimneys
or clusters of
of many
other
oceanic massifs
(7, 17).
ssif. A
In addition,
an up of our carbonate
ciences,
Massachusetts
chimneys
delineate
trend
marks
a
major
lineament.
A
zone
of
key
feature
model is that
the mix-thatferent
stagesanineast-west
detachment
faultthat
evolution.
In
The fracture and fault network in the
40% increase
in rockUSA.
ridge
MA 02139,
continuous
carbonate
of multiple
edificesultramafi
forms
the
core
field. Itpathways
extendsthat
nearly
ture resulting
of mafifrom
c and ultramafi
c rock types
in the composed
the early stages
of extension,
c rocks
areof the
basement
provides
permeable
ume
200active
m in deformation,
length and several
tens of on
meters
down
face
to
the
south.
should
be may
addressed.
control
outflow
at the
main Hydrothermal
vent sites. In addi-fluids
pentinization
enfault zone,
coupled
with
not exposed
the seafl
oor the
and cliff
TAG-type
vents
nce
fracturing, signifion.edu
are weeping actively
steep cliffs.
S1,
S1.
tionfig.
to the
subvertical faults
that channel flow
Inactive
metamorphic
recrystallization,
releasefrom
aremany
hostedofinthese
hanging-wall
basalts.
There
may
be
tenhances
mass wasting
(in a
to the largest structures,
much of the subdetachment
analogous(5//G
to
Enner
</#1G4/
_U/+4
of%A,A,OO
chemical
components
to the fluid,^11*
and isolittle penetration of fluid into the surface
footwall
of emanates
the
flow
from surfaces that are
baerial frost action),
MARCH
2005
VOL studies
307 SCIENCE
www.sciencemag.org
topic alteration.4 In
particular,
previous
fault at this
stage. Continued extension
to foliation
parallel toleads
basement
d hydrothermal flow.
Figureand
3.subparallel
Model to
for fluid circulation and
gently
west-dipping faults
3). The steep venting in and around
ocean-fl
oor reaction
zones
(Bach
et al.,breccias
1996)associated
exhumation
of peridotite
hosting
Rainbow-type
eof
networks
of carbonate
veins and
carbonate
cemented
with this field
are very similar
to
types(Fig.
of hydrothermal
faultsofexpose
relativelydetachment
old inactive stockworks
hicalcite
in Alpine
other more
ophiolites.
faults. A: Early intense circulahave deposits
failed found
to find
the ophiolites
source and
of inpositive
Euancientvents.
Further extension and cooling
the foot-
McCaig et al.(2007)
anomalies in vent fluids (Fig. 2). In our case the wall sees a change to Lost City–type peridotiteSCIENCE VOL 307 4 MARCH 2005
Eu anomaly is easily explained www.sciencemag.org
by plagioclase hosted
venting at low temperatures. We suggest
tion at high temperature is driven by gabbroic intrusions into
variably serpentinized
1429
ultramafic footwall. Flow is focused into
30
海洋底ダイナミクス2014(12)
超苦鉄質岩ホストの熱水系は水素とメタンに富む
hydrogen
pH
methane
Rainbow（ Atlantic）
16mM
2.8
2.5mM
Logatchev（ Atlantic）
12mM
3.3
2.1mM
LostCity（Atlantic）
10mM
~11
<1mM
Nibelungen (S Atlantic)
11.4mM
2.9
1.4mM
TAG（ Atlantic）
0.1~0.4mM
3.1
~0.1mM
Edmond (CIR-S3)
0.06-0.1mM
3.2
0.3mM
31
海洋底ダイナミクス2014(12)
水素とメタンの起源
5Mg2SiO4 + FeSiO4 + 9H20
olivine/カンラン石
3Mg3SiO5(OH)4 + Mg(OH)2 + 2Fe(OH)2
serpentine
蛇紋石
brucite
magnetite
3Fe(OH)2
Fe3O4 + H2↑ + 2H2O
catalyst Ni-Fe alloy
4H2 + CO2
CH4↑ + 2H2O
32
海洋底ダイナミクス2014(12)
テクトニックセッティングの例その３
島弧と背弧
• 島弧の熱水系
• 島弧火山のカルデラ内が多い
• 高温熱水だまりが地下浅部に（水曜火山）
• 背弧熱水系
• 背弧拡大軸では中央海嶺と共通の背景
• 島弧の火成活動の影響（スラブ起源物質の関与）
• 大陸縁辺の場合は堆積物の影響下に
33
Izu arc/backarc
海洋底ダイナミクス2014(12)
Myojin
Bayonnaise knoll
Myojin-sho
(Honsho, personal comm.)
34
Okinawa Trough
海洋底ダイナミクス2014(12)
• sedimented backarc rift
• CO2と微生物起源CH4に富む熱水
35
海洋底ダイナミクス2014(12)
テクトニックセッティングの例その4
古い海洋底
• 堆積物に覆われた場所に海
山が点在する
• 海山が出入り口となる
源流
河口
流域
• 低温湧水の広がりは？
熱源
イメージ図
36
301 Scientists
Expedition 3
, given typical
sediment
recharge velocity of at least 10 cm yr21Expedition
thickness. Geochemical analyses of pore fluids collected well away
from basement highs in this area indicate
diffusive
and Ridge
reactive
Figure
F3. Second
maps. A. Topographic map showing Second Ridge and surrounding regio
海洋底ダイナミクス2014(12)
Expedition 301 Scientists
Expedition
summary Locations of ODP and IODP holes are shown, as are locations of outcrop
from
Fisher
et no
al.,301evidence
2003).
conditions
throughout the thick sediment
layer,
with
12,21
trate
regionally
continuous
. These
analyses
sensitive
to fluid sediment cover. B. Basement map of Second Ridge drilling area, showi
for
advection
Figurevertical
F1. Regional bathymetric
map showing
major tectonic
features
andare
the locations
of IODP
Expedition
301 drill sites and the ODP Leg 168 drilling
transect. Bathymetry from Smith and Sandwell (1997).
= First
hole
locations
(Zühlsdorff et al., this volume). Data are based on bathymetry shown in A a
, and
thus an IODP
area of
1,500
km2 FRaround
velocities
$0.1
yr21
Ridge, SR = Second
Ridge,mm
DR = Deep
Ridge.
b
tation
of
~25
seismic
lines
Baby Bare outcrop (a radial distance of 22 km) would be required
to collected during the 2000 Sonne expedition (ImageFlux). Holes at Site
21
drilled
Va
a during
rechargea subsequent
velocity of expedition.
support discharge of 5 l s . Even assuming
nc
ou
ve
rI
0.1 mm yr21, sediment properties8,12 would
require
a
much
greater
sla
48°00′
nd
8,9,22
Abottom
to draw
differential pressure than observed regionally
N
sea water downward through the sediment layer and into basement.
Olympic
Sulphate compositions of Baby Bare ventPeninsula
fluids require recharge
through basaltic outcrops21. There is no thermal or geochemical
Papa
Mama
Bare
Bare
evidence for hydrothermal recharge through outcrops
close to Baby
North
outcrop
outcrop
American
Bare, but a consistent trend Juan
in dethe
plate of basement pore
Fuca composition
Juan de Fuca eastern flank
23,:564
50°
N
130°W
128°
126°
BECKER
ET AL.'
BABY BARE GEOLOGY
124°
ODP Leg 168
transect
Sites 1026,
1027, and U1301
48°
SR
FR
e
uca
Rid
ge
DR
Cascadia
Basin
ODP Site 1032
Jua
n
plate
ODP Hole
1026B
IODP Holes
U1301A, B
44°
4000
3500
3000
2500
2000
1500
1000
500
47°40′
0
e
Depth (m)
3.5Maの海底
ODP Hole
1027C
2800
Baby Bare
outcrop
fFigure F3B
2600
dire
ctio
n
低温熱水と生物群
flow
Grinnin' Bare
outcrop
infe
rred
fluid
2400
Depth (mbsl)
de F
46°
Figure 3 Calculated pressures available to drive large-scale lateral fluid flow, thermal
profiles within outcrops, and lateral bulk permeabilities within basement
between
2200
47°20′ outcrops. a, Pressures available to drive large-scale lateral fluid flow within basement
Grizzly Bare
between Grizzly Bare (recharge)
outcrop and Baby Bare (discharge) outcrops, based on the
difference between pressures at the base of recharging and discharging columns of fluid.
and for recharge of 5 l s21 (solid
Calculations were completed for discharge of 5 l s21
10 ,km
line) and 50 l s21 (dashed line). The lower values in each case indicate results for fluid
Plate 1. Alvin
of seafloor
featuresrepresentative
of map
units
at Baby
Bare.values
Brittlestars
are -15-20
cm for flow at a
circulation
tophotos
the top
ofunit.
regional
basement,
and
the
higher
show
results
across.(a) Smoothsediment
(b) Slopebreakmarkingboundary
betweensmoothsediment
andexposedrock
128°00′W
127°40′
212 2
units. (c) Exposedrockunit;photograph
showstypicalsediment
coverfor areaswith visibleexposed
rock (d)
m would
depth
of 1and
kmfilter
below
top
of regional
Vertical
permeabilities
,10
Suspension
feeders
unit,with
golfball-andbasement.
bird's-nest-shaped
sponges.(e
andf) Vent faunaunit;
white
objects
areclamshells.
Arrowindicates
1ow-te•
Jperature
hydrothermal
ventat Marker15(seetext). (g) Faultscarp
Second
B Depth
Proc. IODP | Volume 301
25
(mbsl)
result
in ofconsiderable
energysediment
beingonlost
during
ascent
and
leaving
composed
partiallyindurated
carbonate
thewestern
sideof
BabyBare:
notedescent,
thelackof a thick
cover less
of organisms.
(h) Faultscarpontheeastern
sideRidge
of BabyBarewithanoctopus
guarding
itseggs.
Figure 2 Heat-flow values, isotherms and seismic profiles across the Baby Bare 2400
and
differential pressure to drive lateral flow at depth. Shaded band
range
of driving
Papa indicates
Bare
37
212 2
outcrop
m . Higher
discharge values19 would
Grizzly Bare outcrops. Heat-flow values are those that are not on the outcrops and are
pressures based on vertical permeability .10
Mama Bare
3200is
move the curves to the
right, as indicated by the arrow, but would not change the pressure
within 100 m of the seismic lines. Typical sediment thickness away from the outcrops
outcrop
available to drive large-scale, lateral flow. b, Vertical thermal profiles at recharge and
about 500 m. Heat-flow values away from the outcrops are consistent with conductive
Hole
sites were calculated using a one-dimensional model of heat and fluid flow24,
cooling models for 3.5-Myr-old sea floor, after correcting for sedimentation20, and upper discharge
1026B
basement temperatures are about 65 8C. a, Isotherms are subparallel to the sediment–50′ and volume fluxes of 5 l s21 (solid line) or 50 l s21 (dashed line). The calculated discharge
47°
Sitemore
SR-2isothermal than the recharge profiles because the fluid is assumed to
basement interface in the vicinity of the Baby Bare outcrop, rising with basement abruptly N profiles are
Holes
a much smaller cross-sectional area at Baby Bare outcrop than at Grizzly
near the outcrop. Isotherm locations are approximate, being based on the assumption of pass through
U1301A, B
Hole
Bare outcrop. c, Lateral bulk permeability required
between the outcrops as a function of
locally conductive conditions. b, Heat flow decreases near the edge of Grizzly Bare
1027C
volume flow/area. Travel
times and average linear velocities shown on upper axis are for
outcrop, and basement temperatures remain depressed out to a distance of several
Baby
, of 5%; higher or lower values would shift the upper axis as
an effective porosity, neBare
kilometres from the area of basement exposure. Local variations in heat flow—for
outcrop
example, the single elevated value adjacent to the northwest side of Grizzly Bare
indicated with the arrows. The actual fluid travel time between outcrops cannot be greater
outcrop—are likely to result from irregularities in aquifer geometry and properties, and are than that the 14C age of 4.3 kyr (ref. 14), but considerably shorter times are indicated once
47°40′ loss during flow is considered25–27.
most common where sediment cover is thin.
dispersive
620
127°40′
© 2003 Nature Publishing Group127°50′W NATURE | VOL 421 | 6 FEBRUARY 2003 | www.nature.com/nature
Proc. IODP | Volume 301

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