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 10-25° 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 greenschist-facies 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 - 273DOI: 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 high-temperature 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?

•

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yr

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s

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J

m

kg

km

mkm

Lifespan 8.17571032102

105.425001010

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• 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)