Download Hydrothermal circulation in oceanic crust

Hydrothermal circulation in
oceanic crust
Chapter five
• Deep-sea Hot Springs discovered during
the 1970s
• Black smokers, where hot water gushes
out at of vents at temperatures of 350 to
400° C.
– Smoke is actually minute particles of metal
sulfides
– At temperatures, they know about 330° C.
become white smokers
• Barium and calcium sulfate
• Black smoker
• Black Smoker You-tube
• Warm water vents, sometimes known as
diffuse vents, water emerges at about 1025° C.
– Less spectacular but equally important
• Surrounding water is one to 3° C.
• The vent systems support ecosystems
using chemosynthesis
– Energy derived from oxidation of sulfide
– Mostly by bacteria – the primary producers
• Some live symbiotically in multicellular
host organisms
• Some form bacterial mats coding the
seabed
• Some actually live within vent chimneys
Photo – Ruth Turner
• These vents form as soon as crust has
been formed by igneous activity
– Hydrothermal processes take over
• Estimated that one third of the entire sea
floor has sea water circulating through it
– High temperature vents are confined to
spreading axes and active off-axis seamounts
• The rate is sufficient to circulate the entire
ocean volume in 10 million years
• The crust is therefore an important buffer
of the chemical composition of seawater
– For some elements, it is a more important
source than rivers
• The precipitation of metal sulfide is one of
the Earth’s principal mechanisms of ore
generation.
– Sulfide ore deposits in ophiolites
Massive sulfide ore from the Photo Lake Cu-Zn-Au VMS deposit that is mostly
chalcopyrite with cubes of pyrite in grayish pyrrhotite.
Gold ore in thin section. Field of view is 1 mm. Snow Lake, Canada.The host rock is a
fine grained schist dominated by amphibole, biotite, calcite, and quartz, with lesser
epidote, pyrite, arsenopyrite, and oxides. This is a typical assemblage in greenschistfacies gold deposits.
• Hydrothermal circulation through the crust
was predicted even before vents were
discovered
– In the mid-1960s, Hydrothermal systems in
volcanic areas on land led to the proposal that
similar system should be found along Ocean
Ridge systems
• Hot Springs and geysers of Iceland provided
visible evidence of Hydro thermal activity at the
Ridge crest
• Analysis of seafloor samples showed a
systematic increase of concentrations of
iron, manganese, and other metals (Ag,
Cr, Pb, Zn) towards Ridge crests
– Hot Springs were the obvious explanation
– The basaltic rock dredged from Ridge axes
showed clear evidence of having been altered
and metamorphosed by reaction with hot
seawater
• Studies of ophiolites showed that large
volumes of seawater can penetrate more
than 5 km into oceanic crust and circulate
within it at high temperatures
The nature of Hydrothermal
circulation
• To basic characteristics
– High geothermal gradient with hot rocks near
the surface
– Plumbing system of fractures
• Downward circulation occurs slowly over a
wide area
• Upward flow is concentrated in a limited
number of channels
• In terrestrial Hydrothermal systems
groundwater circulates through the cracks
– In the oceans it is sea water
• Another difference is that the ocean floor
is subject to high hydrostatic pressure
• In global terms oceanic Hydrothermal
circulation is the far more important of the
two
Heat flow, convection and
permeability
• The thermal gradient is very high
– Upper boundary of crust is one to 3° C.
– Lower boundary of crust may be >1000° C.
• He is transferred from hot to cold in two
ways
– Conduction which is a molecular process
– Convection which is a bulk process
• Which is most important?
• How do measurements of conductive heat
flow near spreading axes differ from
theoretically predicted heat flow?
• What is the significance of the shaded
area in the figure?
• The heat loss deficit provided scientists
with the first evidence that Hydrothermal
circulation through the oceanic crust must
occur on a very large-scale
– This was even before vents had actually been
discovered
• Convection requires to important
conditions
– Thermal gradient is high enough to overcome
forces acting against fluid motions
– There must be channels in the rock through
which the water can move
• Permeability
– Major faults and fractures
– Smaller fractures in the rock, especially in
below lavas
– Spaces between the pillows and among
rubble of seismic layer 2A
– Fractures within and between dikes
• Fracturing likely to the greatest near active
ridges
– As crust moves away from the Axis channels
become progressively clogged by minerals
precipitated from the circulating fluids
– The crust becomes covered by thicker
sediments
Chemical changes
• Dramatic changes were observed in
laboratory experiments trying to duplicate
conditions under the sea floor
– All the magnesium and sulfate in sea water
was transferred to rock
– Significant amounts of potassium, calcium
and silicon were leached from the rock
• It became clear that Hydrothermal activity,
must have been a major unconsidered
contributor to chemical mass balance of
the oceans
Changes in the rocks
• basalt becomes completely crystallized by
the time it’s cool to 900° C.
– They consist of mixtures of mineral crystals
that are chemically unstable in the presence
of seawater
– Even cold seawater can cause chemical
changes
• in cold seawater basalt experiences
seafloor weathering
– Which is similar to what occurs on land
• During Hydrothermal circulation, they are
metamorphosed into different rock types
by reaction with heated seawater
• Hydrothermal metamorphosis changes,
the basic appearance of the seafloor
rocks, very little
• Closer examination reveals that the
original crystals have been replaced by
new mixtures of minerals
– These depend on the conditions under which
metamorphosis occurred
• Under conditions commonly found. They
are metamorphosed to greenschist grade
• At higher temperatures and pressures.
they are metamorphosed to amphibolite
grade
"Copyright 2000 by Andrew Alden, geology.about.com, reproduced under
educational fair use."
Amphibolite - http://csmres.jmu.edu/geollab/fichter/MetaRx/Rocks/Amphibol1.html
If we observe a terrane of increasing metamorphic intensity, beginning with
a mafic parent, like basalt or gabbro, the parent undergoes a systematic
sequence of mineralogic and textural changes, as shown below.
• Chemical changes due to metamorphosis,
are best monitored by making bulk
chemical analyses of representative rock
specimens
• What changes most?
Changes in seawater
• Compare basalts from various bodies in
the solar system.
– They are very similar
– Low Fe in terrestrial basalts indicates a large
core.
– Large variations in Na reflect differences in
initial volatile element inventory.
Science 12 April 2002:
Vol. 296. no. 5566, pp. 271 - 273
DOI: 10.1126/science.1070768
• Look at table 5.2
– Are hydrothermal solutions more acidic?
• Some elements that are important in
seawater are only trace constituents in
rocks (Mg, SO4)
• The exception is sodium
– Globally hydrothermal circulation, removes
sodium from seawater into rock
• Potassium is significantly higher in
hydrothermal solutions only where
temperatures are in excess of about 150°
C.
– At lower temperatures, potassium is taken up
by the rocks from seawater
• Concentration of silicon is also much
higher in some hydrothermal fluids
– reaches saturation in the solution the
prevailing temperatures and pressures of the
system within the crust
– As the fluids rise to the surface, temperature
and pressure fall, and silica precipitates in the
form of mineral quartz
• Other precipitates are formed close to the
sea floor
– Most important are sulfates, which are
typically reduced to sulfide
– Sulfide combines starring and other metals to
foreign insoluble metal sulfides
• These are precipitated at the vents, building
chimneys
– Sulfides are also precipitated within the upper
part of the crust
• Calcium is enriched in hydrothermal
solutions relative to sea water
– As hydrothermal solutions rise through the
crust and mix with normal seawater. Some of
the dissolved calcium reacts with sulfate and
bicarbonate to precipitate the minerals
anhydrite (CaSO4) and calcite (CaCO3)
• Magnesium is entirely absent from
hydrothermal solution
– Removed from seawater and added to rock to
form magnesium rich metamorphic minerals
• Iron and manganese are both soluble
under acidic reducing conditions found in
hydrothermal solutions
• Iron (Fe++)and magnesium (Mg++) ions can
occupy the same sites because they have
the same size of charge
– Iron can follow magnesium into the new
minerals being formed
• Under oxidizing conditions, both Fe and
Mn are insoluble – form hydrous residues
(rust)
Variability in hydrothermal systems
• The principle of hydrothermal circulation is
simple but the reality is more complex
– Hot saline water is a very powerful chemical
reagent
– Pressure of about 500-1000 atmospheres
– Plumbing system can be very complicated
• Water they reach equilibrium with rocks in
one part of the system. Then react with
rocks in another part
• Further solution and precipitation of
elements can occur, including solution of
elements previously precipitated and
Precipitation of elements recently
dissolved
• The interaction between water and rock
may also be influenced by the total
amount of water, which is moved through
the system. How fast it is moved
• Greenschist grade rocks consisting of
almost entirely quartz and chlorite have
been recovered from seafloor
– 60-70% SiO2 and 5-7% MgO
– Implies that silica is added to the rocks and
magnesium has been leached
• Some variability is to be expected at vents
• The most atypical vents occur in the Red
Sea
– Accumulations of metal rich muds overlain by
concentrated hydrothermal brines
– Temperatures of 60° C. and salinities of over
300 ‰
Black smokers
• Both the temperature and composition of
harvestable than solutions are predicted
two years before they were sampled
– First Hydrothermal vents found in 1977
– Low-temperature, six to 20° C.
– Result of simple mixing between high
temperature fluid and ordinary seawater
• Pain magnesium low-temperature springs
likely to originate from mixing
– Magnesium free hot hydrothermal water
– Ordinary seawater
• Negative correlation found between
temperature and concentration of Mg in
samples from low-temperature vents
– Intercept is at 350° C. indicating that this is
the temperature of hot hydrothermal solutions
• Similar extrapolations for other
constituents allowed predictions to be
made about the composition of hightemperature solutions
– Next step is to find where these vented
• In 1979 Black smokers were first found on
the crest of the specific rise
• Chemical analysis of the fluid confirmed
the compositional characteristics that had
been predicted from the low-temperature
Galapagos vents
Black smokers White smokers in
warm water vents
• Black smokers in warm water vents
appear to be the extremes of the
continuum
– Black smokers 350 to 400° C.
• Precipitates of metal sulfides
– White smokers 30 to 330° C.
• Precipitates of sulfates of barium and calcium, and
silica
– Warm water that’s less than 30° C.
• Table 5.2 shows the concentration of
barium is two orders of magnitude less
than that of calcium
– So why is barium sulfate precipitated
alongside calcium sulfate at White smokers?
• Barium sulfate is much more insoluble
than calcium sulfate and precipitates
readily when vent solutions mix with
normal seawater
• The next figure illustrates of possible
relationship between black smokers, white
smokers and warm water vents
– Transitions can happen at any stage
– Transition may simply result from precipitation
of minerals which reduces permeability of the
surrounding rock
• The precipitated minerals included silica (SiO2),
anhydrite (CaSO4), barite (BaSO4), calcite
(CaCO3), and sulfides of iron (FeS, FeS2, Fe2S3)
copper (Cu2S, CuS2) in zinc (ZnS)
– Zinc Sulfide (ZnS) is used as a transmission window for
IR spectroscopy.
• The precipitated minerals for in a sealed
lining around the conduit that eventually
reaches the seabed and builds a chimney
• Once isolated the vent waters cannot mix
and therefore emerge at very high
temperatures
• These events lead to a stockwork or
network of pipes below the hydrothermal
vents
– In seismic layer two
– Widely dispersed stockworks are
characteristic of warm water vents
– Isolated stockworks are found below Black
smokers
• Particles around the vents may be
dispersed by currents
– Widely dispersed particles are mostly oxides
and hydroxide of iron and manganese
• Precipitated when dissolved Fe++ and Mn ++ from
the hydrothermal solutions are oxidized on mixing
the seawater
– Near the vents to particles are mostly sulfides
• The stockworks are a way to explain the
ore deposits associated with the ophiolites
found on land
• Black-and-white smokers may exist within
100 m of each other
– Indicates that the stockwork is locally patchy
• Warm water vents may occur in close
proximity to smoking vents and others may
represent the waning phase of
hydrothermal activity
• In the cartoon of vent development note
the sharp temperature change below the
top 0.5 km
– This is where fractured rock is the greatest
– Layer 2A is believed to have very high
permeability
– In this layer (< 20º C) only seafloor weathering
• Metamorphism does not occur at < 1 km,
except near hydrothermal conduits.
Lifetimes of hydrothermal systems
• Circulation gets deeper over time as rocks
cool and cracks are able to penetrate
deeper
– Width and spacing a matter of debate
– Probably only 1-3 mm wide
– 10s of cm to a couple of meters apart
• Can not penetrate into magma or
unsolidified gabbro
• Magma bodies are discontinuous as
discussed previously
• Magma bodies are also episodic wherever
hydrothermal cooling is sufficient to
crystallize the gabbro layer
• So what is the lifespan?
• Based on downward propagation of the
cracking front
– Max depth is 5 km
– Migration of cracking front estimated at a few
meters/year (about 3)
– How long will it take?
• 5000m/(3 m/yr) = 1666.6 yr
• Based on heat flux
– Typical heat flux from a single system is about
200 MW = 2 x 108 Js-1
– Magma volume is about 10 km3 = ______ m3?
– Density of gabbro = 2500 kg m3
– Latent heat of fusion = 4.5 x 105 J kg-1
• Assume only latent heat goes into heat
flux, what is the lifespan?
•
•
•
9 3
5
10
m
2500
kg
4
.
5

10
J
10km3 


km3
m3
kg
Lifespan 
 1757.8 yr
8
6
2  10 J 32  10 s

s
yr
• It is still too premature to come up with
accurate estimates of lifespans for vent
systems
• Temperatures have been seen to change
over 3 years
• H2S concentrations also evolved over that
time
Anatomy of a Vent Field
• Vents are usually not solitary but occur in
clusters – vent field
– A few km across at most
• TAG field is 200 m diameter, 50 m high
mound
– Coated by iron and copper sulfides
– Cluster of chimneys near top
– Cluster of white smokers on one flank
• TAG mound estimated at 18,000 yr old
– Compare this with the estimates just made
– How is that?
• Cores show that the activity has been
episodic
– During active phase anhydrite deposited in
the mound and sulfides on surface
– During inactive periods the mound collapses
disrupting the layering into a mixture of
anhydrite and sulfide
• At some distance from the axis, TAG will
probably go extinct
– As have nearby mounds
• Amount of metal sulfide in TAG is
estimated at 4 million tons
– This is the largest at a spreading center
• Exception is metalliferous sediments of Red Sea
– 90 million tons
• Sulfides at fast spreading centers are 2
orders of magnitude less
– Higher rate of magma means field can’t
persist very long before moved away or
buried by lava
Extent of Hydrothermal Activity
• Evidence shows that hydrothermal activity
must occur over the entire 50,000 km
length of the mid ocean ridge system
– So a permanent linear heat source over
geologic time
• Vents represent upflow zones –tightly
focused
• Downflow zones draw seawater from a
wide area
• Off-axis hydrothermal vents have also
been identified at seamounts
– Several ore deposits in ophiolites formed this
way
• Majority of known vents are in shallow
parts of the ridge
Vent Biology
• When vent fields die, the energy source
for the ecosystem is removed and the
organisms die as well.
• Most of the organisms are slow moving or
sessile.
– So how do they colonize new vents?
• Planktonic larvae
– Most die, but enough survive to re-establish
new colonies
• The closer two sites are, the more related
the species
– Sites on EPR 800 km apart – 54 species
– Sites 2000 km apart – 11 species in common
• Only 5 species shared between EPR and
western Pacific
• Bacteria and other unicellular organisms
may provide even more interesting results
• Archea may have existed in these
environments for 3-4 billion years
– Life may have originated here
Hydrothermal Plumes
• Few vent fields are actually discovered
accidentally
• Most found by following plume effluents
– Detectable overlarge areas
– Plume several hundred meters above the
ground
– Plume becomes diluted by the surrounding
seawater
• Plume dilution factor is typically 104
• Despite the high dilution there are several clues
– Heat content
– Suspended smoke particles
– Dissolved gases (methane(CH4), hydrogen (H2),
hydrogen sulfide (H2S), carbon dioxide (CO2), carbon
monoxide CO, nitrous oxide (N2O), helium (He)
• These are the most persistent since they do not settle out
• Helium concentration is several orders of
magnitude less than methane and hydrogen, but
it is a more potent tracer
• Helium is rare in the Earth’s atmosphere
because it escapes very easily to space
• Isotopes
– 4He is most common – product of radioactive
decay from uranium
– 3He is rare and has two sources
• Cosmic rays
• Primordial helium trapped in the earth mantle at
the time the formation
– Escapes during volcanic outgassing
• The ratio of 3He to 4He is much higher in
hydrothermal vent waters than anywhere
else
• The figure shown previously with the
aluminum with sediments, showing
asymmetry between 5°S and 40°S.
• Before observations of the helium which
plume circulation models did not account
for this
• CH4, H2 and CO2 can be liberated by
partial melting of the mantle, but also by
other processes as well
– Oxidation reduction reactions
– Microbial activity
Event plumes
• So far, the plumes we’ve discussed of
been more or less steady over time
• Large transient plumes have also been
observed
– These are known as event plumes
– Also known as megaphone’s
• Most probably due to eruptions
• Event plumes rise a thousand meters or
more – steady-state ones only a few
hundred meters
Gorda Ridge
• March 10-11, 1996.
• SEM analysis of the first sample from the
Megaplume site at GR-14.
– Have seen Fe oxides, Zn sulfides and what appears
to be bacterial aggregates (not sure yet).
– The Fe oxides are in two distinct forms.
• One form contains Phosphorus and the other does not.
• Perhaps we are seeing Fe oxides that are formed
subseafloor (no phosphorus) and within the megaplume
(enriched in phosphorus).
– The Zn sulfides are very pure (i.e., no Fe). This
sample appears to be similar to the plume samples
seen over the flow site in 1993.
Off-axis hydrothermal circulation
• If a half spreading rate of the Ridge is 2
cm per year. It will travel 2 km in 100,000
years.
• At this time it will be in the down flows
down, reacting only with cold seawater
– However convection cells seem to remain in
the rock
– Upflow and downflow zones remain fixed and
hydrothermal circulation continues
• just diminishing in intensity
Circulation Flow Model
• The cases shown are for varying depths of
fluid penetration.
• All have exponentially decreasing
permeability with depth though the value
at upper surface is varied:
Extent of Hydrothermal
Metamorphism
• Seismic studies can not show how much
metamorphism has taken place
– Temperatures in the top 500 m too low
• Except near vents
– Temperatures to greenschist grade require
200-400º C
• Found throughout the lower half of layer 2
– ODP hole 504B showed increased
metamorphism from surface to 1.8 km
• This is also the case in ophiolites
Mass Transfer
• Convective heat transfer rate (total heat
lost by hydrothermal circulation)
– Estimated at 8 x 1019 J – 3 x 1020 J per year
– Volume of the ocean filtered every 106 yr
• Flow rate F kg yr-1
–
F = H / (Cw (T2 – T1))
• H= convective heat transfer (2 x 1020 J yr-1)
• Cw = specific heat of seawater (4.2 x 103 J kg-1 ºC-1)
• T = temperature (1= 2º, 2 = 300º C)
• Therefore
• F = 2 x 1020 J yr-1 / (4.2 x 103 J kg-1 ºC-1(300º – 2º))
• F = 1.598 x 1014 Kg yr-1
• The volume of the oceans is 1.4 x 1021 kg
• Average renewal time = V / F
– Tr = 1.4 x 1021 kg / 1.598 x 1014 Kg yr-1
– Tr = 8,761,200 yr
• These calculations based on data from
near the spreading center
• Difficult to estimate flow rates in low
temperature zones away from the
spreading centers
• Assuming that
– 1.6 x 1014 Kg yr-1 water circulates through the
crust
– All of it acquires more Ca++ at 460 ppm
– How much Ca++ is to the ocean yearly?
• = 460 x 10-6 x 1.6 x 1014 Kg yr-1
• = 7.4 x 1010 Kg Ca++ yr-1 added to the oceans
– Not including reprecipitation
• That is only somewhat less than comes in
from rivers, 5 x 1011 kg
• More sophisticated analyses show that
hydrothermal activity is the major source
of Li, Ru, and Mn
– It is also important for Ba, Si, Ca
• On the other side, it is a major sink for
– Mg2+ and SO42-
• Why no K in the above analyses?
– K is leached from rocks at high temp
– K is added to rock at low temp (< 150º C)