Examensarbete vid Institutionen för geovetenskaper ISSN 1650-6553 Nr 243
Numerical Model of a Fossil Hydrothermal System in the Southern East Pacific Rise Exposed at Pito Deep
Numerical Model of a Fossil Hydrothermal System in the Southern East Pacific Rise Exposed at Pito Deep
Páll Halldór Björgúlfsson
Páll Halldór Björgúlfsson
Uppsala universitet, Institutionen för geovetenskaperExamensarbete E1, Hydrogeologi, 30 hpISSN 1650-6553 Nr 243Tryckt hos Institutionen för geovetenskaper, Geotryckeriet, Uppsala universitet, Uppsala, 2012.
The Mid Ocean Ridge system with its volcanism and related hydrothermal activity has been a subject for many studies since the discovery of high temperature hydrothermal vents at the ridge surfaces in the 1970´s. This thesis focuses on deep sea hydrothermal activity on a superfast spreading ridge, the Southern East Pacific Rise (SEPR).The ridge is located in the South Pacific, off the coast of South America, and separates the Nazca Plate and the Pacific Plate. A fossil high temperature hydrothermal zone hosted by a fault was sampled 80 m below the lava/dike transition zone in the Pito Deep (a tectonic window into the SEPR). Geochemical data from the fault zone indicates that cold (<150°C) and hot (<390°) fluids coexisted at the same time whilst the hydrothermal system was active. A numerical model (HYDROTHERM) developed by the USGS was used to recreate the geological settings in the SEPR in order to try to model the hydrothermal activity and fluid flow. The model solves two governing partial differential equations numerically, the water component flow equation (Darcy law for flow in porous media) and the thermal energy transport equation (conservation of enthalpy for the water component and the porous media). The result of the modeling indicates that cold seawater can penetrate from the relatively permeable volcanic material into a highly permeable fault zone in the sheeted dike unit. The cooler seawater fluid flows down the fault zone, reheats and flows up again in a narrow upflow zone at the edge of the fracture/sheeted dike boundary. The result is a horizontal temperature gradient created in the fractured zone supporting the theory that hot and cold fluids can coexist in a fault hosted hydrothermal zone.
Examensarbete vid Institutionen för geovetenskaper ISSN 1650-6553 Nr 243
Numerical Model of a Fossil Hydrothermal System in the Southern East Pacific Rise Exposed at Pito Deep
Páll Halldór Björgúlfsson
Copyright © Páll Halldór Björgúlfsson and Department of Earth Sciences, Air, Water and Landscape Sciences, Uppsala University. Printed at the Department of Earth Sciences, Geotryckeriet, Uppsala University, Uppsala 2012.
Numerical Model of a Fossil Hydrothermal System in the Southern East Pacific Rise Exposed at Pito Deep
Páll Halldór Björgúlfsson
Abstract The Mid Ocean Ridge system with its volcanism and related hydrothermal activity has been a subject
for many studies since the discovery of high temperature hydrothermal vents at the ridge surfaces in
the 1970´s. This thesis focuses on deep sea hydrothermal activity on a superfast spreading ridge, the
Southern East Pacific Rise (SEPR).The ridge is located in the South Pacific, off the coast of South
America, and separates the Nazca Plate and the Pacific Plate. A fossil high temperature hydrothermal
zone hosted by a fault was sampled 80 m below the lava/dike transition zone in the Pito Deep (a
tectonic window into the SEPR). Geochemical data from the fault zone indicates that cold (<150°C)
and hot (<390°) fluids coexisted at the same time whilst the hydrothermal system was active. A
numerical model (HYDROTHERM) developed by the USGS was used to recreate the geological
settings in the SEPR in order to try to model the hydrothermal activity and fluid flow. The model
solves two governing partial differential equations numerically, the water component flow equation
(Darcy law for flow in porous media) and the thermal energy transport equation (conservation of
enthalpy for the water component and the porous media). The result of the modeling indicates that
cold seawater can penetrate from the relatively permeable volcanic material into a highly permeable
fault zone in the sheeted dike unit. The cooler seawater fluid flows down the fault zone, reheats and
flows up again in a narrow upflow zone at the edge of the fracture/sheeted dike boundary. The result
is a horizontal temperature gradient created in the fractured zone supporting the theory that hot and
cold fluids can coexist in a fault hosted hydrothermal zone.
Numerical Model of a Fossil Hydrothermal System in the Southern East Pacific Rise Exposed at Pito Deep
Páll Halldór Björgúlfsson Referat
Mittoceaniska ryggar med tillhörande vulkanism och hydrotermisk aktivitet, har studerats sedan
1970‐talet då man upptäckte vätska med hög temperatur som flödar upp från jordskorpan på
Mittoceanryggen. Denna uppsats fokuserar på vätska med hög temperatur lokaliserade djupt under
havsytan på en mittoceanisk rygg i Stillahavet, kallat Southern East Pacific Rise (SEPR), vars
spridningshastighet klassificeras som väldigt snabb (>160 mm/år). Ryggen ligger i södra Stillahavet,
väster om Sydamerikas kust och skiljer Nazcaplatten och Stillahavsplatten. Prover från ett tektoniskt
fönster i Pito Deep (SEPR) 80 m under gränsen mellan vulkanisk material och gångformationen,
härstammar från ett tidigare aktivt hydrotermiskt system i sprickzonen. Geo‐kemisk data från
proverna indikerar att kalla (<150°C) och varma (<390°) vätskor fanns då systemet var aktivt. En
numerisk modell (Hydrotherm) som utvecklats av USGS (U.S. Geological Survey) används här i ett
försök att modellera vätskans flöde i bergtyperna i SEPR. Modellen löser två ekvationer numeriskt;
flöde i poröst material (Darcys lag) och transport av värmeenergi. Resultaten från modelleringen
indikerar att kallt havsvatten kan flöda från relativt permeabelt vulkaniskt material, ner i en mer
permeabel sprickzon som ligger i gångformationen. Kallt havsvatten flödar ner sprickzonen, värms
och flödar upp igen i en trång zon som ligger på gränsen mellan gångformationen och
spricksystemet. Resultaten blir en horisontal temperaturgradient i vätskan som cirkulerar i
spricksystemet och styrker teorin att varma och kalla vätskor kan förekomma samtidigt i
hydrotermiska spricksystem.
Acknowledgements I like to thank my wife Gunnhildur, and my two children Úlfur and Dísa for all the patience they have
shown me during the last year whilst writing this thesis. I also like to thank my supervisor Abigail
Barker for suggesting the subject and for all the assistance and helpful comments, as well as Zhibing
Yang for his helpful comments. I also like to thank all the people in Geo‐centrum whom I have come
to know during my studies in Uppsala and have made my stay enjoyable.
Contents
Abstract ................................................................................................................................................... iii
Acknowledgements .................................................................................................................................. v
List of figures .......................................................................................................................................... vii
1. Introduction ......................................................................................................................................... 1
2. The Mid Ocean Ridge system .............................................................................................................. 2
2.1 The East Pacific Rise, geology and location ................................................................................... 3
2.2 Tectonics and volcanism in the East Pacific Rise ........................................................................... 5
3. SEPR Hydrothermal systems and their hydrogeology ......................................................................... 6
3.1 Distribution of vents and relation to tectonics and volcanism ..................................................... 6
3.2 Venting and Black smokers ........................................................................................................... 8
3.3 Alteration in the sheeted dike complex ...................................................................................... 10
4. Permeability of Oceanic crust ........................................................................................................... 12
4.1 Oceanic crust permeability: In Situ measurements .................................................................... 12
4.2 Oceanic crust permeability: Laboratory testing .......................................................................... 13
4.3 Indirect measurements/Thermal measurements of permeability ............................................. 13
5. Description of software used for modeling ....................................................................................... 14
6. Model Setup and domain. ................................................................................................................. 16
6.1 Rock units .................................................................................................................................... 16
6.2 Rock Parameters .......................................................................................................................... 16
6.2.1 Thermal conductivity ............................................................................................................ 17
6.2.2 Porosity ................................................................................................................................. 17
6.2.3 Permeability ......................................................................................................................... 17
6.2.4 Density .................................................................................................................................. 18
6.2.5 Specific heat ......................................................................................................................... 18
6.2.6 Rock compressibility ............................................................................................................. 18
7. Modeling results ................................................................................................................................ 19
Sc1. .................................................................................................................................................... 19
Sc2. .................................................................................................................................................... 21
Sc3. .................................................................................................................................................... 22
8. Discussion .......................................................................................................................................... 25
9. Conclusions ........................................................................................................................................ 28
References ............................................................................................................................................. 29
List of figures
Figure 1: Cross axis bathymetric profiles of mid ocean ridges…………………………………………2
Figure 2: Depth to the top of a magma lens at MOR by multichannel seismic data…………2
Figure 3: Mid ocean ridges along axis minimum bathymetric profiles…………………………….3
Figure 4: Location of the Pito Deep and the East Pacific Rise………………………………………….4
Figure 5: Lithology from the Pito Deep……………………………………………………………………………4
Figure 6: A hydrothermal model proposed for a fast spreading ridge……………………………7
Figure 7: A histogram from 7 third order segments at the NEPR and the SEPR……………….8
Figure 8: Thermodynamics of black smoker formations………………………………………………….10
Figure 9: A model proposed by Heft et al 2008 based on dike and lava alteration………….11
Figure 10: The temperature evolution and the plume formations for Sc1……………………..20
Figure 11: Flow diagram for Sc1 and liquid water mass fluxes and flow vectors…………….20
Figure 12: The temperature evolution and the plume formations for Sc2……………………..21
Figure 13: Flow diagram for Sc2 and liquid water mass fluxes and flow vectors…………….22
Figure 14: The temperature evolution and the plume formations for Sc3……………………..23
Figure 15: Flow diagram for Sc3 and liquid water mass fluxes and flow vectors……………24
Figures 1 – 8 are reprinted with the permission of the authors.
1
1. Introduction The Mid Ocean Ridge system with its volcanism and related hydrothermal activity has been a subject
for many studies since the discovery of high temperature hydrothermal vents at the ridge surfaces in
the 1970´s. Studies from different ridges located around the world have shown that the difference in
ridge topography as well as its along axis profile can be contributed to different behavior of the
volcanism associated to spreading ridges. The difference (or variety) in the volcanism also creates
different geological and hydrological conditions for high temperature venting to occur. The nature
and behavior of hydrothermal vents and geothermal areas is therefore highly related to the tectonics
and volcanism of an area.
One of the ridges which have been studied frequently during the last two decades is the East Pacific
Rise (EPR). The East Pacific Rise (EPR) is located in the Pacific, off the coasts of Central America and
South America, and is classified as a fast – superfast spreading ridge. Often there is a distinction
made between the North and the South EPR, respectively NEPR and SEPR. The NEPR separates the
Cocos Plate and the Pacific Plate, whilst the SEPR separates the Nazca Plate and the Pacific Plate, but
further south the Easter Microplate replaces the Nazca Plate (see figure 4). The Pito Deep is believed
to be a tectonic window into the super‐fast spreading SEPR. Magnetic anomaly 2A, which can be
extrapolated across the Pito Deep Rift (PDR) from the Nazca plate to the north, suggests that the
exposed Nazca crust was generated about 3 Ma ago near 23°S (Martinez et al., 1991; Naar et al.,
1991).Results from investigations in the Pito Deep have given valuable insight in the behavior of a
super‐fast spreading ridge. One of the observations from the Pito Deep is an exposed fault zone
which extends over 40 m, with six 1 m wide highly deformed faults within relatively undeformed
dikes. A detailed study published by Barker et al. (2010) based on geochemical data from rock
samples collected from the exposed fault zone suggests that cold (<150° C) and hot (<390°C) fluids
coexisted at the same time whilst the fault zone hosted an active geothermal system. The data is
recorded in the alteration formed in a fault believed to be created at or close to a fast spreading
ridge.
The aim of this thesis is to discuss and explain with numerical modeling the results from Barker et al.
(2010), that is the co‐existence of hot and cold fluids observed in fossil high temperature fault zones
in the East Pacific Rise. The current depth of the East Pacific Rise is about 2700m, or about 27 MPa.
This simplifies the modeling by assuming a constant single fluid phase during the last 3 million years.
The software used for modeling was created by the US Geological Survey and is named Hydrotherm
Interactive. It’s a numerical model and can handle temperatures up to 1200° C and pressures up to
109 Pa. The results of the modeling are presented graphically showing temperature variations over
time. This thesis will also discuss theories and research on deep see hydrothermal systems located at
spreading ridges, mostly focusing on the East Pacific Rise.
2. The As ment
Rise, wh
therefor
The Mid
been ma
greatly, a
of sprea
reinjecti
that con
gradient
chamber
axial ma
Mid Oceaioned before
ich is a part
re necessary
d Ocean Ridg
apped in det
and the alon
ding (see fig
on in the sys
ditions with
t, as well as r
rs has also b
gma chambe
an Ridge se this thesis
of the Mid O
and this cha
ge system is o
ail (about 1 %
ng axis profile
ure 1 and 3)
stem, the fas
fast rate of
rock chemist
een shown t
er lies (Philip
system concentrate
Ocean Ridge
apter briefly
over 70,000
%) with visua
es as well as
. The rate of
ster the reinj
magma injec
ry is more co
to vary with t
ps Morgan an
2
es on discuss
(MOR) syste
discusses th
km in length
al survey. Th
cross axis p
f spreading h
jection the fa
ction are mo
onstant over
the rate, the
nd Chen 199
ing the findin
em. Some ba
e MOR syste
h, and only a
he spreading
rofile also va
has been link
aster the rat
ore stable in t
r time. The d
e faster the s
3).
ngs in the So
sic knowledg
em.
fraction of t
rate of mid o
ary greatly de
ed with the
e of spreadin
the sense th
epth of the a
preading rat
Figure
bathym
mid oce
Differe
form/ca
profiles
ridges (
Chadw
from M
Figure 2: De
magma lens
the brittle li
function of
Squares are
to magma le
multichanne
whilst dots
earthquake
Chadwick 1
Philips Morg
1993).
outhern East
ge of the MO
these system
ocean ridges
epending on
rate of magm
ng. This also
at the therm
axial magma
te the shallo
1: Cross axis
metric profiles
ean ridges.
nt spreading
ause differen
s on mid ocea
(Perfit and
ick 1998, mod
Macdonald 198
epth to the to
s, or the dept
ithosphere as
spreading rat
e determined
enses by
el seismic dat
are axis
e depths (Perfi
998, modified
rgan and Chen
Pacific
OR is
ms have
s varies
n the rate
ma
implies
mal
a
wer the
s of
rates
t
an
dified
86).
op of a
h of
s a
tes.
depth
ta,
it and
d from
n
Figure 3:
the botto
2.1 The
The East
coasts of
often the
The NEP
and the
The Pito
anomaly
north, su
(see figu
spreadin
Mid ocean rid
om (Perfit and
e East Pacif
t Pacific Rise
f Central Am
ere is a distin
PR separates
Pacific Plate
Deep is beli
y 2A, which c
uggests that
ure 4) is just n
ng portion of
dges along ax
d Chadwick 19
fic Rise, ge
(EPR) is loca
merica and So
nction made
the Cocos P
, but further
eved to be a
can be extrap
the exposed
north of the
f the SEPR (M
is minimum b
998).
eology and
ated in the Pa
outh America
e between th
late and the
r south the E
a tectonic wi
polated acro
d Nazca crust
East Pacific
Martinez et a
3
bathymetric p
d location
acific, as one
a. It is classif
he North and
Pacific Plate
aster Microp
ndow into th
oss the Pito D
t was genera
Rise –Easter
al., 1991; Naa
rofiles. Slow s
e could guess
fied as a fast
d the South E
e, whilst the
plate replace
he super‐fast
Deep Rift (PD
ated about 3
r Microplate
ar et al., 199
spreading rate
s from the na
– superfast s
PR, respectiv
SEPR separa
es the Nazca
t spreading S
DR) from the
Ma ago nea
junction alon
1).
es on top and
ame itself, o
spreading rid
vely NEPR an
ates the Nazc
Plate (see fi
SEPR. Magne
Nazca plate
ar 23°S. The l
ng the fastes
fast at
off the
dge and
nd SEPR.
ca Plate
gure 4).
etic
to the
ocation
st
This win
to be an
dominat
dikes we
thicknes
transitio
Figure 5:
represent
complex
dow allowed
alyzed and m
ted by pillow
ere generally
ss of 700 m. T
onal in nature
Lithology from
t lavas. The th
zone also vari
d a cross sect
mapped (figu
w lavas, and n
y 700 – 1000
The transitio
e. (Pollock et
m the Pito De
hickness of the
ies greatly in t
tion of a 3 m
ure 5). The vo
not so much
m thick whi
on zone is ge
t al., 2009)
eep (Morgan e
e volcanic pill
thickness, fro
4
million year o
olcanic units
by lobate lav
lst the gabbr
nerally abou
et al., 2005). S
ow lava units
m few tens of
old crust assu
s are general
vas or sheet
ro units ther
ut 200 m thic
Solid circle sho
range from 5
f meters to ov
Fi
an
re
se
re
fig
ab
et
umed to orig
ly 100 – 500
flows. The u
eunder have
ck in both cas
ows in situ dik
0 – 300 m. Th
ver 400 m (Po
gure 4: Locati
nd the East Pa
econstruction
eems to sugge
epresenting ar
gure 5 were lo
bout 3 million
t al., 2009).
ginate from t
m thick, and
underlying s
e a maximum
ses and is
kes, whilst ope
he lava/sheete
llock et al., 20
ion of the Pito
acific Rise. Pla
(Naar et al., 1
est that the cr
reas A and B i
ocated on the
n years ago (Po
he SEPR
d are
heeted
m
en circles
ed dike
009).
o Deep
te
1991)
rust
n
e SEPR
ollock
5
2.2 Tectonics and volcanism in the East Pacific Rise
The connection between hydrothermal vents, volcanism and tectonics on the EPR has been studied
over the last decades. The tectonic features recognized in the EPR play an important role in trying to
understand the complex relation between tectonics, volcanism and hydrothermal systems. In
Macdonald (1998) and White et al. (2002) the segmentation of the EPR is discussed and explained.
The segments are classified ranging from 1 to 4, based on the length of the segments.
For fast spreading ridges the first order segments are considered to be 600 +/‐ 300 km long, cut by
large transform faults forming a type of large propagating rift with an offset bigger than 30 km. The
lifetime estimated for these segments is estimated > 5x106 years. For the second order segments the
length is estimated 140 +/‐ 90 km, and thought to represent overlapping spreading centers often
characterized by shear zones at the boundaries of each segment with an offset ranging 2 – 30 km.
The longevity of these segments is considered to be 0,5 – 5 x 106 years. The third order segments are
estimated 50 +/‐ 30 km in length with overlapping spreading centers thought to represent the gaps
between volcanoes(volcanic systems). The offset is only 0,5 – 2 km between third order segments.
The timescale considered for each segment is thought to range from 104 – 105 years. The smallest
segments are the fourth order segments, only 14+/‐ 8 km in length and are considered to be offsets
from the axial summit caldera (graben). The offset is less than 1 km and span the timescale from 102
– 104 years.
White et al. (2002) studied in detail a section of the NEPR and conclude that third order segments are
basically composed of volcanic units as described by i.e. Gudmundsson (1995), resembling volcanic
units in Iceland . An example of such a volcanic unit would be the Krafla volcanic system in Iceland.
Each fissure swarm or a volcanic unit is defined by a central crater/volcano with an underlying
magma chamber and an underground plumbing system running out from the center of the system
along the ridge/fissures. Dike and sill injection can happen along the ridge (fissure system) and cause
eruptions anywhere in the swarm but the most frequent eruptions take place in the center of the
system. The tendency of lava domes to form at the end of a system in the EPR indicate a lower
infusion rates or more viscous lava resulting in pillow lava being the dominant rock formation, rather
than sheet flow or lobate lava in the center of a system. This suggests that the eruptive style is
significantly different at the edge of a system than in the center.
The rifting and related volcanism is not considered to happen constantly, rather thought to be more
episodic and each unit within the third order segment is believed to have the lifetime of 103 – 105
years. That is considerable longer lifetime than the 102 ‐ 103 years that volcanism related to fourth
order segments is believed to have (Sinton et al., 2002).
A multi‐channel seismic profile along the EPR between 8°50’ N and 13°30’N is indicates that the top
of a crustal magma chamber is located at 1.2 km – 2.4 km below the sea floor. The magma chamber
is also relatively narrow, less than 4 – 6 km wide. The observations also indicate that the magma
chamber does not widen with depth, but remains narrow. Seismic data from the SEPR have even
reported a narrow melt sill, as narrow as 1 km located only about 1000 m below the seafloor (Detrick
et al., 1993).
6
3. SEPR Hydrothermal systems and their hydrogeology As discussed in chapter 2 the layering of the oceanic crust has been defined from direct observations
from the Pito Deep and geophysical measurements. Studies of ophiolites have also contributed
significantly in understanding the ocean crust layering. Generally the upper part of the crust consists
of volcanic material, either sheet flows or pillow lava, followed by the sheeted dike complex with the
gabbro unit at the base (see figure 4 and 5).
One basic aspect about fluid flow in the upper oceanic crust is the highly fractured porosity of this
part of the crust. That implies that most of the fluid travels in channels along fractures. The upper
part of the crust (the upper 200 – 300 m) illustrates a more horizontal layering compared to a more
vertical layering in the sheeted dike complex (Fisher, 1998). This is not surprising when comparing
the processes responsible for creating these layers. Dikes usually penetrate near vertical from a
magma source, whilst extrusive material (sheet flow or pillow lava) is laid out in horizontal units with
new units covering old units and even sediments formed between those two units.
To estimate fluid volumes, water/rock ratios in oceanic crust have been calculated in various studies.
The water/rock ratios generally require some basic assumptions like the initial composition of both
the rock and the circulating fluid, is the system open or closed and the rapid exchange between liquid
and solid phases. The general trend indicates greater water/rock ratios in the upper part of the crust
(the extrusive rocks) compared to the lower part of the extrusive and the sheeted dike complex. This
is in accordance with permeability measurements as discussed in chapter 3 (Fisher 1998 and
references therein).
It should also be kept in mind that fluid compressibility as well as fluid expansivity contribute to
storage in systems that undergo changes in pressure and temperature. All water that comes into a
system has to be either stored in the system or go out (discharge). At steady state recharge and
discharge should be balanced while in transient systems the difference must be balanced by a change
in storage (Fisher, 1998).
3.1 Distribution of vents and relation to tectonics and volcanism
Haymon et al. (1991) visually and acoustically surveyed a 80 km section of the EPR in order to map
the occurrence and distribution of high temperature vents (T>200° C) along the ridge (black, white
and gray smokers). Their results showed a correlation between the occurrence of high temperature
vents on the second order scale with the shallowing of the axial magma chamber (< 1,7 km beneath
the ocean bottom) and the presence of a well‐developed axial summit caldera (also referred to as
axial summit graben). In fact most of the high temperature vents within the second order segments
were located on the edge of the caldera margin. In the case of the 4th order segments (here the 4th
order segments also include the 3rd order segments) the occurrence of high temperature vents was
correlated to narrow axial summit caldera (40 – 70 m wide), shallow axial magma chamber (<1.55 km
beneath the axial zone) and also in the youngest and least fissured lavas. To try and explain the
distribution of the vents a model is proposed (see figure 7).
In their research the absence of hydrothermal venting in areas with older, highly fissured lavas
supports the fact that high temperature venting is most likely related to volcanic events, and that the
amount of vents in a system represent the cycle a hydrothermal area undergoes. The onset of a
hydrothermal area is probably started with a volcanic event. The volcanism continues in that area,
resulting
(also ref
that time
topograp
topograp
area. The
surface a
colder fl
along wi
model fo
Figure 6:
observati
and 3rd or
A test on
segment
the age o
associate
A later st
order se
well as b
often for
diffuse f
g in the form
erred to as a
e dike inject
phically high
phy so that t
e rock failure
and as a resu
uids in the e
ith a deeper
or the 4th ord
A hydrotherm
ions from the
rder segment
n sulfide dep
ts hydrother
of 0 – 78 yea
ed with olde
tudy from H
gments at th
bio communi
rm around a
low shut dow
mation of a su
axial summit
ion is contin
est under th
the dikes are
e and faultin
ult increased
extrusive rock
circulation i
der segments
mal model pro
Venture hydr
s are classifie
posits in the D
mal systems
ars are assoc
er and more f
aymon and W
he NEPR and
ities was mo
low diffuse
wn the bio co
ummit, and la
graben) as r
uously going
he axial summ
e topographic
ng along the c
d venting in t
k (pillow and
n the dikes is
s hydrotherm
oposed by Ha
rothermal fiel
d as one.)
Deval on the
s undergo a h
ciated with fr
fissured lava
White (2004
the SEPR sh
re frequent
of hot fluids
ommunities
7
ater a gravit
result of the
g on in pulse
mit caldera,
cally lower u
caldera rim h
the caldera s
d lobate lava
s proposed b
mal venting (
aymon et al. (1
lds 9°09’ – 54´
e EPR crest 1
hydrotherma
resh lavas, w
a (Kalou et al
4) comparing
howed that t
at the cente
s, that often
around the
ational colla
deflating of
s and resulti
but a caldera
under the cal
help to creat
ystem. A thr
s) perpendic
by Hayman e
(Haymon et
1991) for a fas
´ N on the EPR
2° 46´N supp
al/tectonic cy
whilst south o
., 1985).
locations of
he location o
r of the segm
aren’t obser
vent disappe
pse creating
an axial mag
ng in a dike s
a collapse m
ldera compa
te a hydrothe
ree dimensio
cular as well
et al. (1991) a
al., 1991).
st spreading r
R. (In this stud
ports the the
ycle. North o
of Deval inac
f hydrotherm
of high temp
ments. The b
ved visually,
ear. Sheet flo
g a summit ca
gma chambe
swarm that
ight change
ared to surro
ermal flow t
onal circulati
as along axia
as hydrother
ridge, based o
dy 4th order se
eory that 4th
of Deval dep
ctive deposits
mal vents wit
perature ven
bio communi
, and when t
ows also pro
aldera
er. During
is
the
unding
o the
on of
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figure 7)
Figure 7:
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agma into the
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that the mag
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ctivity and th
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that the 4th o
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n this chapte
more detail.
cean ridges is
a.
es at the EPR
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(2008)
er the
. As
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R 21° N
tion of
9
barite and silicia. In the EPR these smokers build chimneys with the diameter of 2‐ 12 centimeters.
Black smokers also build high sulfide chimneys with the diameter measured in centimeters and the
temperature measured between 350 – 400°, with sulfide precipitates contributing to the black color.
In the same article calculations based on the amount of flow as well as temperature from venting
areas was used to show how hydrothermal circulation effectively plays an important role in cooling
of the ocean crust. That is to say cold seawater percolates into the newly formed crust, absorbs heat
and cools down the ocean crust in the process.
Von Damm (2000) presents data from a relatively fresh volcanic site in the EPR. Also there the
temperatures measured in the vents rarely reach temperature over 400° C.
Three mechanisms have been proposed why black smoker temperatures are limited with the
maximum temperatures measured at a little over 400° C (Jupp & Adam, 2000 and references
therein).
• High ductile rock (higher than 500° C) is essentially impermeable as the ductile
nature prevents formation of cracks
• High temperature rock becomes impermeable over time either by mineral
precipitation or by thermal expansion, or a combination of both factors
• A temperature cap is imposed by the thermodynamics of water.
A model proposed by Jupp & Adam (2000), tries to explain the thermodynamics of black smoker
formations, examining the internal temperature of a porous medium. The aim of the research was to
explain why black smoker temperatures aren’t higher than 350 – 400° C, whilst the heat source can
be up to 1200° C. A numerical simulation was used to show how the thermal structure of a
convection cell works.
When analyzing the governing equations in the modeling software (Hydrotherm) it was revealed that
the thermodynamic properties of the water are the governing factors, rather than the parameters
used for the modeling. Hence their study excluded all the temperature limiting mechanism and used
a homogeneous, isotropic medium representing the crust. A gauss bell shaped heat profile was used
to represent the heat source from beneath, with 1100° at the center and cooling down to 10° C at
the edges. Their model stretched 1700 m from the center to left and right.
The results showed that for pressures according to seafloor crust plumes of fresh hot water start to
form at 400° C, for any heat source higher than 500° C (see figure 8). This implies that venting
temperature remain steady whilst the magma source supplying the heating is cooling and solidifying.
A high temperature reaction zone with incoming horizontal flow at the base of the bottom boundary
was also showed to occur in their study.
Figure 8:
formation
never exc
temperat
3.3 Alte
Heft et a
conclude
heteroge
is amphi
systema
and >450
permeab
systems)
timescal
suggesti
areas qu
the uppe
and mixi
Pito Dee
from the
Results from
ns. Figure a is
ceed 400° in t
ture and the f
eration in
al. (2008) stu
ed that most
eneous and s
ibole domina
tically with d
0° C (amphib
bility and hig
) along the E
e of 5 – 20 k
ng three diff
uartz veins as
er part of the
ing with hott
ep those elem
e sheeted dik
the model wo
the large sca
heir model. F
fluids mass flu
the sheete
udied the alte
t of the alter
shows no sp
ated with ch
depth. The te
bole). The pr
gher water/r
EPR create a
ka. A model
ferent proce
s well as met
e dikes, with
ter fluids in t
ments were n
ke complex (
ork of Jupp &
le plume form
igure b is zoo
ux vectors.
ed dike com
eration in Pit
ation develo
atial trends o
lorite rich di
emperature
esence of fa
ock interacti
distinctive ch
based on the
sses based o
tal enrichme
one fluid or
the sheeted
not observed
see figure 9)
10
Adam, (2000)
mation created
med into the
mplex
to Deep dike
oped within a
on the scale
ke sporadica
estimated to
ults near the
ions. They pr
haracter of a
ese observat
on the geoch
ent suggests
riginated in t
dike comple
d, probably r
).
), explaining t
d in the mode
center lower
es and the di
a broad hot
of ten to hu
ally distribute
o create thes
e chlorite alt
ropose that m
alteration on
tions is prese
emical obse
the mixing o
the volcanic m
x. In anothe
representing
he thermodyn
el showing tha
part of figure
ke/lava trans
upwelling zo
ndreds of m
ed and they
se alteration
ered dikes in
migration of
n the scale 1‐
ented in figu
rvations. In o
of two differe
material flow
er study area
g a more dire
namics of blac
at surface tem
e a, showing th
sition zone a
one. The alte
meters. The a
are not distr
are <300° (c
ndicates high
f cells (hydro
‐ 2 km and th
re 10. The m
one of the st
ent kinds of f
wing down fr
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ect upwelling
ck smoker
mperature
he
and
ration is
lteration
ributed
chlorite)
her
othermal
he
model
tudied
fluids in
ractures
d in the
g zone
Figure 9:
Deep wer
observati
goes all t
well as m
C), while
sheeted d
A model prop
re compared,
ion areas with
he way throug
metal enrichme
the other stu
dyke complex
posed by Heft
suggesting th
hin the Pito De
gh the extrusi
ent suggests t
dy area lacked
x and alters th
t et al 2008 ba
hree different
eep, probably
ive unit and e
the mixing of d
d those eleme
e sheeted dyk
11
ased on dike a
processes. Fl
y representing
exits at the oce
different kind
ents complete
ke complex in
and lava altera
uid path A wa
g a more direc
ean bottom. I
d of fluids in th
ely. Fluid path
to amphibole
ation. Two are
as observed in
ct upwelling zo
n one of the a
he upper part
B depicts how
e or chlorite do
eas within the
n both of the
one where th
area quartz ve
of the dikes (
w the fluid sta
ominated ass
e Pito
e fluid
eins as
(fluid path
ays in the
emblies.
12
4. Permeability of Oceanic crust Permeability can best be described as the ease which rock fluids can move through porous rocks.
Permeability of rock is probably the key factor in understanding flow in rock units. Permeability of
the oceanic crust seems to be highly heterogeneous and anisotropic as within crystalline rock in
general (Fisher, 1998). Understanding how permeability is created and destroyed in rocks is also of
equal importance, such as the creation of highly permeable zones followed by tectonic processes, or
the precipitation in cracks and voids that diminish or destroy the permeability in a layer.
In the case of oceanic crust, measuring permeability is not a simple task. In‐Situ measurements,
laboratory testing of cores and various indirect geophysical measurements are all different methods
to estimate the permeability of oceanic crust. The following subchapters discuss each of these
methods. The values obtained from core scale measurements indicate a permeability range from 10‐
22 – 10‐17 m2 whilst borehole measurements (bulk rock scale) indicate a higher permeability of 10‐18 –
10‐13 m2 with the higher values usually located in the upper part of the crust, and lower values in the
lower part (Fisher, 1998).
4.1 Oceanic crust permeability: In Situ measurements
Mostly two kind of packer tests are used, pressure slug test and constant rate injection test. Values
for bulk permeability for oceanic crust mostly come from DSDP (Deep See Drilling Project) and ODP
(Ocean Drilling Project) boreholes. All the interpretations are based on following assumptions:
1) The permeable zone is horizontally oriented and has infinite lateral extent and constant thickness
2) The head is the same everywhere in the permeable zone prior to pumping
3) The well has a small diameter compared to the depth of influence of the test and is 100% efficient
4) The well fully penetrates the aquifer
5) The aquifer is considered to be isotropic and homogeneous
6) Fluid flow to and from the well is radial
7) The fluid properties do not vary with time or place during testing
These assumptions are in accordance with the REV (representative element volume), indicating that
the testing place is large enough to represent a porous medium. But if the transmissivity measured in
a deep borehole is concentrated to a few thin layers the permeability of the bulk will depend on the
thickness of the tested interval (Fisher, 1998).
Bulk rock permeability from the Middle Valley, Juan de Fuca Ridge in young crust < 0.2 Ma at depth
of 11.0 – 173.6 m into the crust show the value 8.0 x 10‐14 m2, but at the interval of 61.0 – 91.0 m has
the value of 2.5 x 10‐13 m2 (Becker et al,. 1994). These values are probably the most relevant values to
compare to the permeability in the upper crust of the EPR since very few tests (if any) have been
conducted on ridge centers.
13
4.2 Oceanic crust permeability: Laboratory testing
Laboratory testing of basalt core samples have generally showed a lot lower values of permeability
than in situ measurements. Values from laboratory testing are ranging from 10‐18 – 10‐19 m2at low
confining stresses and room temperature, but increase up to 7x 10‐18 m2when heated up to 600° C
(28Ma old crust) and under confining pressure (30 – 100 MPa) (Aksyuk et al., 1992). Younger crust
from pillow lavas from Juan de Fuca Ridge tested under confined pressure ( 5 – 40 MPa) gave values
up to 1.5 x 10‐18 m2 (Christensen and Ramananantoandro, 1988).
4.3 Indirect measurements/Thermal measurements of permeability
There are other ways to measure or infer the permeability in the ocean crust. One of the methods
involves measuring the temperature with temperature logs in an open borehole. From temperature
measurements it can be established whether ocean water is flowing into the borehole, or if there is
water flowing from the borehole into the ocean. In most cases cold denser seawater is flowing into
the boreholes. The cold dense seawater and the percolating fluids in the crust have different
pressure, and the pressure difference induces flow which can be used to estimate permeability of the
surrounding crust. The calculations are somewhat similar to the ones used in packer tests.
Values estimated from the East Pacific Rise, at the west flank in a 20 – 50 Ma year old rock, gave
permeability values between 10‐12 – 10‐10 m2 (Baker et al. 1991). Another study from the East Pacific
Rise located at the ridge crest in a 0 – 100 ka year old rock estimated permeability 1000 m into the
crust to be 5 x 10‐12 m2 (Evans, 1994).
14
5. Description of software used for modeling The Hydrotherm simulator is versatile software created by the US Geological Survey. The version
used in this thesis is version nr 3. The simulator is a numerical model and can handle temperature up
to 1200° C and pressures up to 109 Pa. It only simulates the ground water flow of pure water
components.
A graphical Interface, Hydrotherm Interactive, is available to help with the modeling but the use of
that is restricted to two dimensional Cartesian or cylindrical coordinates. It should though be noted
that the model is still in three dimensions, but the third dimension is a unit vector chosen by the
scale of modeling. In this thesis the Hydrotherm Interactive graphical interface was chosen to model
as it is relatively user friendly and less time consuming, and helps to visualize the results of the
modeling.
Two governing partial differential equations are solved numerically, the water component flow
equation (Faust and Mercer,1979a; Huyakorn and Pinder,1983) coupled with Darcy law for flow in
porous media with the pressure chosen as the dependent variable, and the thermal energy transport
equation based on the conservation of enthalpy for the water component and the porous
media(Faust and Mercer 1977; Huyakorn and Pinder 1983).
For more details regarding the equations and their limitations readers are referred to the
Hydrotherm Guide version 3 (Kipp et al., 2008). Most of the limitations regarding the model work in
this thesis are addressed later in this chapter.
The equations are coupled through the dependence of advective heat transport on the fluid velocity
field, and the dependence of fluid density, viscosity and saturation on pressure and temperature.
Finite difference techniques are then used for the spatial and temporal discretization of the
equations.
The primary dependent variables are pressure and enthalpy, and numerical solutions are obtained
simultaneously for them. The secondary dependent variables are computed from the primary
variables. They include temperature, saturation, fluid density, fluid viscosity and interstitial fluid
velocity.
There are limitations since the range of thermodynamic tables used do not cover regions with super
critical pressure and low enthalphy. Therefore the Hydrotherm is not applicable for low temperature
systems in the deep ocean floor. In this study the problem is solved by setting the temperature at the
ocean/rock boundary at 7° Celsius. The simulator also has problems dealing with injection of cold
fluids/cold precipitation penetrating several kilometers below the land surface creating a plume of
cold water in high pressure areas.
In order to apply the simulator in highly fractured volcanic environments Darcy’s equation for flow in
porous medium has to be assumed. Therefore flow in fractured rock will only be realistically
represented when the scale of the region is large relative to the fracture spacing.
The simulation region is spatially discretized using a cell centered grid. Since the code uses finite‐
difference techniques for spatial and temporal derivative approximations there are some numerical
15
limitations. Those problems can be avoided in a simple model of a system where the main purpose is
to investigate mechanisms and testing hypothesis as is the case in this study.
Another limitation is that finite‐difference grids do not conform to boundaries that are not parallel to
the coordinate axes. Stair‐step approximations to angular boundaries, such as sloping land surfaces,
are inconvenient to specify and can cause local variations in the ground‐water flow‐field that are not
realistic. The topography as well as boundaries between layers has therefore been set as either
horizontal or vertical in the model represented in this paper to avoid these complications.
Also the thermal energy transport equation neglects pressure volume, therefore it is possible that
temperatures in the liquid phase at large depths become lower than any boundary condition, source
temperature or initial condition temperature. But in most cases this approximation is only a few
degrees Celsius lower than if it were not neglected.
16
6. Model Setup and domain. The modeling results presented in chapter 7 are the result of 3 different modeling scenarios. Two of
the models have the same initial conditions but different geological setup (Sc2 and Sc3), whilst one of
them has different initial temperature conditions (Sc1).
The model domain is two dimensional, 1.2 km wide (x‐axis) and 0.99 km deep (z‐axis) with a 60 x 50
cells grid. The hydrostatic pressure set at the top of the domain is 270 bars. The boundary conditions
are set as constant from initial conditions, using hydrostatic pressure and a geothermal temperature
profile created from initial conditions
• The boundary condition used is known as a Dirichlet boundary condition
• Specified fluid pressure condition for the ground‐water flow equation, and a specified
enthalpy or specified temperature condition for the heat‐transport equation. These
conditions can be specified independently as functions of location and they can also vary
independently with time.
• The surface is predefined as a closed aquifer.
• The run time of the model was set at 1500 years.
6.1 Rock units
Four rock units were created in the model to represent the geological conditions in the SEPR based
on observations presented in chapter 2.1. One of the units is 665 m thick and is considered to
represent the sheeted dike complex. Another unit 335 m thick is considered to represent the
extrusive volcanic material. Those two units are the basic units in the model. Two other units were
also created and were thought to represent more fractured areas (fault zones) within the basic units.
One of the units represents a fractured area within the extrusives and the other unit represents a
fractured area that extends into the sheeted dike complex. The thickness of the fractured unit in the
extrusive material is identical to the depth of the extrusive material unit, but the unit representing
fractured material in the sheeted dike complex extends about 380 m into the sheeted dike complex
from the boundary between the sheeted dikes and the extrusive material (see figures in chapter 7).
6.2 Rock Parameters
In order to run the Hydrotherm software some basic rock parameters have to be established for
every rock unit. Those parameters are:
• Porosity
• X permeability
• Z permeability
• Thermal conductivity
• Specific heat of rock
• Rock density
• Rock compressibility
17
6.2.1 Thermal conductivity
One of the parameters required for running the Hydrotherm program is thermal conductivity of the
rock being modeled. The Si unit is W/(mK) and describes the rate which energy will cross an area
with the driving potential of a unit gradient perpendicular to an area. Since many rocks are
anisotropic it follows that the thermal conductivity is a tensor, and therefore dependent on direction.
Measuring thermal conductivity is not easy, but many measurements from basalt indicate a range
between 2.38 and 1.12, but for gabbro the values range from 3.58 – 1.98. The variation with
temperature and pressure is relatively small for temperatures less than 1200° C (Jessop, 1990). This
model uses an average value of 1.9 W/(mK) for all rock units, consistent with a model in a similar
setting (Jupp & Adam, 2000).
6.2.2 Porosity
All solid rocks contain holes, and the amount of holes per volume defines the porosity of a rock. The
porosity of rock is therefore expressed as a percentage. For dense crystalline rocks the porosity
varies from 0 – 5 %, but basalt shows a wider range from 3 – 35%. Pezard (1990) defines 3 types of
primary porosity that are most common in igneous seafloor crust:
• Primary porosity such as vesicles
• Micro‐cracks with limited extent
• Various macro‐features such as boundaries between layers, tectonic fractures and
collapse structures.
Resistivity measurements conducted at DSDP hole 504B revealed layered structure correlated with
basement lithology. Interpretation of apparent resistivity values showed a decrease from 11 – 14%
porosity in the upper 150 m, and 7 – 10% porosity in the next 500 m, dropping to 1‐3% porosity in
the sheeted dike complex below (Becker 1985). This data were later reanalyzed by (Pezard 1990),
indicating that free water, that is water not bound to minerals, is significantly lower than previously
estimated. This layering in porosity with depth is in accordance with results from permeability
measurements conducted in boreholes in the ocean crust (see discussion on permeability in chapter
4.1). In this study the sheeted dike unit porosity was set at 2%. For the fractured dikes and volcanic
extrusive unit the value was set at 10%. The values for the fractured extrusive unit were set at 20%.
6.2.3 Permeability
The connectivity of voids (porosity) in rock or rock units controls the permeability of a rock or a rock
unit. The permeability of oceanic rocks was discussed in more detail in chapter 4 in this thesis.
Permeability in a rock unit does not have to be isotropic (a constant for all directions), but when the
permeability varies with directions the rock is referred to be anisotropic. All of the rock units used in
this model were defined as anisotropic. The values chosen can be seen in table 1. The permeability in
the dike formations in Z direction (vertical) is chosen to be higher than in the X direction (horizontal)
as a result of fissure and dike orientation along the ridge axis. For the pillow lava the values for
permeability in the X direction are higher based on horizontal flows in lavas and the horizontal
layering between lava sequences as discussed in chapter 3.
18
Table 1: Permeability of rock units used in the model. X is horizontal permeability whilst Z is vertical
permeability (see references in chapter 4).
X (m2) Z (m2)
Sheeted dike 10‐17 10‐16
Fractured dike 10‐15 10‐14
Extrusive volcanics 10‐13 10‐14
Fractured volcanics 10‐13 10‐12
6.2.4 Density
Density of basalt is temperature dependent, which has been experimentally determined to be 2760.4
kg/m3 , but 2704.7 kg/ m3 at room temperature (Yan et al., 2001). Melting of basalt can happen over
a wide range of temperature, between 1000° and 1260° and in that temperature interval liquid and
solid phase can coexist (Yan et al., 2001). Basalts can also range in chemical compositions as well as
the porosity of the rock. For this model the temperature never exceeds 900° and the value of basalt
was set at 2690 kg/m3 for the fractured rock, 2700 kg/m3 for extrusive material and 2730 kg/m3 for
dikes. These values are within the range of the basalt wet bulk density which varies from 2831 kg/m3
for flow basalts down to 2196 kg/m3 for vesicular basalts (Christensen et al., 1980).
6.2.5 Specific heat
Specific heat of an object is one of its physical properties. It can be established by direct
measurements, simply by taking a known mass of matter, and adding heat into the system and
carefully measuring the heating of the object. Specific heat of basalt has been measured 0.84 kJ/kg K
and that is the parameter chosen for the model (Davis, 1982).
6.2.6 Rock compressibility
Rock compressibility is the ability of rock to reduce in volume when pressure is applied. It is the
inverse of bulk modulus whose base unit is Pa. The base unit for compressibility is therefore Pa‐1. A
rock compressibility value of 0.15 x 10‐10 Pa‐1 is used in this study for extrusive material, whilst 0.5 x
10‐10 Pa‐1 is used for denser dike material. These values are based on the average values from
Christensen et al 1980 from initial reports of the Deep See Ocean Drilling Project.
19
7. Modeling results This chapter presents the result from 4 different geological setups named Sc1 – Sc3. The results are
presented graphically showing the temperature profile evolution over time, followed with a short
description.
The setup/initial conditions are expressed with the yellow color represents the volcanic extrusive
unit, the green represents fractured area (fault zones) within the extrusive unit. Blue color shows
fractured area (fault zones) within the sheeted dike unit and purple color represents the sheeted
dike unit.
In all of the scenarios presented the model domain is 1.2 km wide and 1.0 km deep. The thickness of
the extrusive unit is 335 m, and the sheeted dike complex is 665 m thick. The width of the fractured
zone (fault zone) is 160 m. The rock parameters are represented in chapter 6.
Sc1.
The initial temperature is represented by 4 temperatures at different levels. At the top of the domain
the temperature is set at 7° C, followed by an 80° C boundary located 260 m from the top of the
domain. Just below the boundary between the extrusive unit and the sheeted dike unit, 340 m from
the top of the domain a 400°C boundary is set. 40 m from the bottom of the domain the temperature
is set 900° C.
When following the process over time in figure 10 it can be noted that 2 small plumes start
developing at the boundary between the dike formation and the extrusive units around the 400° C
boundary, later developing into multiple plumes. The plumes rise higher into the extrusive unit and
after 500 years they form one powerful high temperature plume with temperatures between 307° ‐
357° C at the center of the domain. The plume slowly diminishes in strength whilst the temperature
in the upper part of the sheeted dike unit slowly starts to cool down. This scenario shows a
circulation in the extrusive unit, gaining heat from the sheeted dike whilst the dike unit slowly cools
down. The majority of the fluids contributing to the plume are reheated seawater from the extrusive
unit (see figure 11).
Figure 10
Figure 11
mass flux
Initia
100 ye
500 ye
0: The temper
1: To the left a
xes and flow v
al Conditions
ars
ars
rature evolutio
a simplified flo
vectors from H
on (°C) and th
ow diagram fo
Hydrotherm. I
0 years
250 yea
1500 ye
20
he plume form
or Sc1 at the e
n the color sc
ars
ears
mations during
end of the mo
cale blue show
g the model ru
del run. To th
ws low fluxes a
un for Sc1.
he right liquid
and red high.
water
The red
dotted ar
water in t
boundary
Sc2.
The initia
presente
from the
have bee
scenario
permeab
Figure 12
In the fir
dike form
weak an
Initial Co
100 ye
800 ye
rea in the shee
the plume ori
y, gain heat an
al temperatu
ed in Sc1. Th
e top of the d
en added to
o is because t
bility.
2: The temper
rst stage afte
mation and w
d short‐lived
onditions
ears
ears
eted dike unit
ginated from
nd rise in the
ure is repres
e only differ
domain. In th
the domain
the Hydrothe
rature evolutio
er the model
works its way
d and starts t
t shows where
the extrusive
center to crea
ented by 4 te
ence is that
his scenario t
(table 1). Th
erm had som
on (°C) and th
is started a
y up into the
to move to t
0 years
300 yea
1500 yea
21
e the fluid is s
e unit. Cooler
ate a hot plum
emperatures
the 400° bou
two fault zo
he reason wh
me problems
he plume form
plume starts
e fractured ro
he right and
ars
ars
supercritical. T
liquids flow al
me.
s at different
undary has b
nes with diff
hy the 400° C
running hig
mations during
s forming fro
ock formatio
creates a th
The model sho
long the extru
t levels, muc
been moved
ferent perme
C boundary w
h temperatu
g the model ru
om the botto
on (see figure
hin upwelling
ows that mos
usive /sheeted
ch the same
and is locate
eability para
was set so low
ures at high
un for Sc2.
om of the fra
e 12). The pl
g zone in the
t of the
d dike
as
ed 730 m
meters
w in this
actured
ume is
fractured
formatio
tempera
formed i
the 400°
low perm
fractured
Howeve
zone bou
fractured
formatio
Figure 13
mass flux
dotted ar
water in t
boundary
fractured
from the
fluxes tha
Sc3.
In this sc
formatio
The abse
forming
the Sc2 c
liquid sta
therefor
move up
sheeted
d sheeted di
on from the e
ature in the s
in the extrus
° C boundary
meable fract
d zone whils
r a large por
undary whils
d dike unit c
on, with grad
3: To the left a
xes and flow v
rea in the shee
the plume is o
y, gain heat an
d extrusive par
extrusive unit
an the cooler
cenario all th
on has been
ence of the f
than in Sc2 w
case the plum
art to flow fr
re works as c
p in an upflow
dike comple
ke unit. At th
extrusive roc
surrounding
sive rock out
y below the f
ured area. In
st the hotter
rtion of the h
st at the opp
auses a horiz
dient as big a
a simplified flo
vectors from H
eted dike unit
originated fro
nd rise in the
rt. In a large p
t, but on a sm
downflow zon
he same cond
removed.
fractured zon
with temper
me in the fra
rom the extr
circulation w
w zone on th
ex have gaine
he same tim
ck. Water co
sheeted dike
side of the fr
fractured are
nstead the co
fluids slowly
hotter fluids a
osite end co
zontal tempe
as 130° C ove
ow diagram fo
Hydrotherm. I
t shows where
m the extrusiv
center to crea
portion of the
mall part of the
ne.
ditions as in
ne in the ext
ratures betw
actured part
usive unit in
here cold flu
he other side
ed more hea
22
e colder wat
ntinues to p
es starts to r
ractured zon
ea does not s
older and de
y make their
also rise at t
older fluids a
erature grad
er the 160 m
or Sc2 at the e
n the color sc
e the fluid is s
ve unit. Coole
ate a plume, w
fractured par
e unit a thin u
Sc2 apply, b
rusive unit a
ween 157° – 2
of the sheet
to the fractu
uids are brou
e. The fluids f
at compared
ter starts pen
enetrate the
rise and wide
ne. In this sce
supply enoug
enser fluids s
way through
he edge of t
re moving do
dient to form
wide fractu
end of the mo
cale blue show
supercritical. T
er liquids flow
whilst cooler l
rt of the sheet
upflow zone is
ut here the f
llows a more
207° C in the
ted dike unit
ured sheeted
ught down at
flowing into
to the fluids
netrating int
e fractured d
e low temper
enario it is ob
gh heat to su
tart flowing
h the sheete
he sheeted d
own. This cir
m in the fractu
re zone (see
del run. To th
ws low fluxes a
The model sho
along the ext
iquids mostly
ted dike unit f
observed wit
fault zone in
e powerful p
center of th
shifts to the
d dike unit. T
t the one side
the fracture
s flowing dow
to the fractu
dike, but the
rature plume
bvious that p
ustain a plum
down into t
ed dike comp
dike and frac
rculation in t
ured sheeted
figure 13).
he right liquid
and red high.
ows that mos
trusive /sheet
y flow down in
fluids are flow
th higher wate
the extrusiv
plume to star
he plume. Bu
e right as coo
The fractured
e, gain heat
ed part of the
wn in Sc2 sin
red dike
es are
placing
me in the
he
plex.
ctured
the
d dike
water
The red
t of the
ted dike
n the
wing down
er mass
ve
rt
t as in
oler
d unit
and
e
nce they
are most
15).
Figure 14
Initial Co
200 ye
1000 y
tly originated
4: The temper
onditions
ears
years
d from fluids
rature evolutio
s that flowed
on (°C) and th
0 yea
500
150
23
d along the e
he plume form
rs
0 years
00 years
extrusive /sh
mations during
eeted dike b
g the model ru
boundary (se
un for Sc3.
ee figure
Figure 15
mass flux
dotted ar
between
relatively
more hea
along the
5: To the left a
xes and flow v
rea in the shee
Sc1 and Sc2.
y strong plume
at compared t
e extrusive /sh
a simplified flo
vectors from H
eted duke uni
The lack of hi
e. The fluids fl
to the fluids fl
heeted dike bo
ow diagram fo
Hydrotherm. I
it shows wher
gh permeable
lowing into th
owing down i
oundary.
24
or Sc3 at the e
n the color sc
re the fluid is
e fracture zon
he fractured p
in Sc2 since th
end of the mo
cale blue show
supercritical.
e in the extru
part of the she
hey are mostly
del run. To th
ws low fluxes a
The fluid flow
sive part help
eeted dike com
y attributed fr
he right liquid
and red high.
w can be seen
ps to create on
mplex have ga
rom fluids tha
water
The red
as mix
ne
ained
at flowed
25
8. Discussion The modeling work represented in chapter 7 can be thought to represent the geological conditions
on a fast spreading ridge in the SEPR. The thickness of the formations is close to what is described by
Morgan et al. (2005). The choice of 900° C at 1 km depth is in accordance with a shallow magma
chamber observed under active hydrothermal areas in the SEPR (Detrick et al 1993).
The main purpose of the modeling was to investigate the coexistence of hot (<390°) and cold
(<150°C) fluids in fracture zones as observed by Barker et al 2010. In the study by Barker et al. from
2010 rock samples were collected 80 m below the lava/dike transition zone in the Pito Deep and
analyzed. Titanium in quartz thermometry indicate temperature of 392° C +/‐ 33° C., and higher 87Sr/86Sr in fault breccias than in average vent fluids at the seafloor suggest an input from a higher 87Sr/86Sr source, perhaps seawater (Barker et al 2010). Also fault Breccia enriched in MgO indicates
presence of cold seawater, since magnesium is completely removed as seawater is heated up to 150°
C (Seyfried 1987).
One of the observations from the modeling results that could help explain the coexistence of hot
and cold fluids is the circulation that is created in the sheeted dike fracture zone observed in Sc2 and
Sc3 (see figure 13 and 15). Cooler fluids from the extrusive unit flow down into the fractured area,
reheat and flow up again in an upflow zone at the other side of the fracture zone creating a
circulation, with very small contribution from the fluids entrapped in the low permeable sheeted dike
unit due to the large difference in permeability parameters chosen.
In the study by Barker et al. (2010) the fault zone extends over 40 m, with six 1 m wide highly
deformed faults within relatively undeformed dikes. However the fracture zone in the model
presented in this thesis was set 160 m wide. That is considerable wider than the fault zone observed
by Barker et al. (2010). The reason for choosing such a wide fracture zone was the relative course
grid used for the modeling.
The circulation in the fractured part of the sheeted dike complex observed in the modeling causes a
horizontal temperature gradient to form in the fractured sheeted dike formation, with gradient as
big as 130° C over the 160 m wide fracture zone in Sc2 (see figure 11). Also in Sc2 the 150° C
boundary in the fractured downflow zone observed in the end of the model run is located around
100 – 150 m below the sheeted dike – extrusive boundary. In Sc3 the temperatures observed in the
sheeted dike fracture zone were considerably higher than in Sc2 although the water penetrating into
the fractured sheeted dike complex in both cases originated as cold seawater as explained in figure
13 and 15. This circulation of colder seawater being reheated in the fracture zone supports the
theory that MgO enriched breccia observed in the fracture zone studied by Barker et al 2010 could in
fact be contributed by cold seawater.
These results can also be compared to the model that Heft et al 2008 proposed in their study on
alteration in the sheeted dike complex in the EPR and is discussed in chapter 3.3. In one of the area
studied in the Pito Deep quartz veins as well as metal enrichment suggests the mixing of different
kind of fluids in the upper part of the dikes related to mixing through faults (see figure 9). Sc2 and Sc3
could be considered representing similar conditions where mixing of different fluids occur in the
upper part of the dikes close to fractures, but the mixing of fluids occurring in the modeling scenarios
26
presented here mostly occurs above the boundary between the sheeted dike complex and the
overlying volcanic material.
But how reliable are the modeling results presented in this thesis? The answer to that is the
modeling results presented in this paper should not be interpreted to literally. The model is a
numerical simulation and there are many other factors that affect hydrothermal systems that the
model doesn’t take into considerations.
In chapter 3.2 the model work of Jupp and Adam (2000) was presented and comparison to their
result could help to give some insight into the reliability. They used the same model, Hydrotherm, as
the model presented in this thesis. But there are some differences between the model presented in
this thesis and the model of Jupp and Adam’s. They start with a bell shaped heat source, 1200° in the
center and cooling down to 10° in the edges. Whilst the model presented in this paper uses a
constant heat source through the whole model regime, and the hydrothermal gradient is established
from predefined initial conditions. That is thought to represent the conditions in the EPR with
constant heat flow and constant source of magma. Jupp and Adam (2000) assume isotropic material,
whilst the models presented in this thesis contain up to four different rock units each with different
anisotropic properties. The model presented by Jupp and Adam mostly emphasizes on observations
of Black smoker’s temperatures not exceeding 400° C. But the results from Sc1 are not so different
from the results presented by Jupp and Adams supporting their finding that when the governing
equations in the modeling software (Hydrotherm) were analyzed it was revealed that the
thermodynamic properties of the water are the governing factors, rather than the parameters used
for the modeling.
The time factor cannot be regarded as real time, since there are many other factors that control the
fluids attributes. The time factor only represents the time for these particular modeling scenarios as
calculated in the model and is a result of initial temperature conditions. The choice of initial
temperature conditions was highly limited by the models incapability to run high temperature fluids
in low permeable formations, such as fractured material in the dike complex at considerable depth.
Hence in scenario 2 – 3 the 400° temperature is set below the fractured dike formation.
The permeability of the area modeled in this study is also quite hard to establish as mentioned in
chapter 3. The permeability chosen for every formation in the model is thought to represent the
permeability of the whole formation, the REV (Representative element volume) which in some sort
of an average permeability over an area observed. This scalar thinking in the REV concept will
drastically diminish the effect of highly permeable fractures created by contraction/expansion or
even tectonic events. Those kinds of highly permeable fractures on a smaller scale could easily move
hotter liquids, as observed in Black smokers, to the surface, but the model does not cache those
moments.
Another factor that largely affects hydrothermal systems is precipitation in fractures, and the
cooperation between precipitation and thermal expansion/contraction of the country rock. A study
by Lowell et al 2007 investigating crack closure in hydrothermal upflow zones indicates that thermal
expansion happens about ten times faster than the silica precipitation, resulting in rather thin layer
of silica in fractures that were perhaps a lot wider whilst active. The cooling of the rock
(thermoelestic stress) would then result in reopening the fracture, but chemical bonding between
27
silicia and country rock could make the process more difficult. A fracture located at the outer regions
of a hydrothermal upflow zone would be sealed by the effect of thermal expansion and chemical
precipitation, and then reopened again to allow cooler fluids to enter the space, and perhaps come in
contact with hotter fluids and reheated. This would lead to the rock expansion and chemical
precipitation closing the facture again. This could explain many thin bands of silica precipitation
observed in cracks, as well as explain while cold and hot fluids seem to coexist at the same time.
These results from Lowell et al 2007 are applicable in the deeper parts of a hydrothermal system,
where thermal expansion is the governing part. In shallower regions the chemical precipitation can
be seen as the more governing factor. In the model presented here none of these factors, that is
thermal contraction/expansion as well as chemical precipitation, are taken into consideration.
High temperature plumes that are created are often a result of some disturbance in the geology of a
region, seismicity and or volcanic activity as discussed in chapter 3 (see references in the chapter).
Then the local temperature profile is temporarily displaced and plumes of high temperature can be
created. Injecting dikes and sills add temperature to a system, as well as disturb the local previously
established flow of water in the crust, allowing short lived phenomena such as black smokers to be
created. This of course is hard to model since this is not a system in balance and cannot be sustained
over longer time periods and the model does not take any of those factors into consideration.
The Hydrotherm model also specifically warns about limitation that finite‐difference grids do not
conform to boundaries that are not parallel to the coordinate axes. Stair‐step approximations to
angular boundaries, such as sloping land surfaces, are inconvenient to specify and can cause local
variations in the ground‐water flow‐field that are not realistic. This explains the box like shape of the
formations in the model.
Another shortcoming of the model is the choice of a closed aquifer. It is not a very realistic to
assume a closed aquifer in the center of the SEPR ridge. Although, observations from the Sea Cliff
hydrothermal field in the northern Gorda Ridge in the Northeast Pacific Ocean, show a crust that has
an impermeable layer caused by silica precipitation. According to calculations the capping of the
crust is a process that can take only decades (Rona et al 1990). The choice of a closed aquifer
probably resulted in wider plumes and higher temperature in the extrusive formation, and could be
responsible for driving some of the cooler fluids down the fractured dike formation.
Also this is model running non brine liquid, fresh water. That is rarely the case with seawater derived
fluids.
All in all, despite the shortcomings of the model mentioned above the modeling of EPR with such a
crude method and relatively large grid used can aid to try and understand some aspects of the
relative complex nature of the EPR hydrothermal systems. Therefore the results of the model should
be viewed with that in mind.
28
9. Conclusions Results from the modeling work in Sc2 and Sc3 as presented in chapter 7 supports the theory that
hot and cold fluid can coexist in an active fault hosted hydrothermal system. The main reason for the
coexistence of the hot (<390°C) and cold fluid (<150°C) is suggested to be the permeability
parameters used in the modeling, that is to say the huge contrast in permeability between the
sheeted dike complex and the fault zone as well as the extrusive material. This huge contrast drives
the circulation of cold fluids down into the fault and then up again after gaining heat, resulting in a
horizontal temperature gradient observed in the fracture zone.
29
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