1. Hydrothermal circulation in oceanic crust
Chapter five

2. 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

4. Black smoker
Black Smoker You-tube

6. Surrounding water is one to 3° C.
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.

9. 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

10. Photo – Ruth Turner

13. 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

14. 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

15. Massive sulfide ore from the Photo Lake Cu-Zn-Au VMS deposit that is mostly chalcopyrite with cubes of pyrite in grayish pyrrhotite.

16. Gold ore in thin section. Field of view is 1 mm. Snow Lake, Canada
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.

17. 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

19. 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

21. 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

22. 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

25. 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

26. 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?

28. 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?

29. 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

30. 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

31. 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

33. 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

34. 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

35. It became clear that Hydrothermal activity, must have been a major unconsidered contributor to chemical mass balance of the oceans

36. 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

38. 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

39. 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

40. Under conditions commonly found
Under conditions commonly found. They are metamorphosed to greenschist grade
At higher temperatures and pressures. they are metamorphosed to amphibolite grade

41. "Copyright 2000 by Andrew Alden, geology. about
"Copyright 2000 by Andrew Alden, geology.about.com, reproduced under educational fair use."

42. Amphibolite - http://csmres. jmu

45. 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.

46. Chemical changes due to metamorphosis, are best monitored by making bulk chemical analyses of representative rock specimens
What changes most?

49. 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.

50. Science 12 April 2002: Vol. 296. no. 5566, pp. 271 - 273 DOI: 10
Science 12 April 2002: Vol no. 5566, pp DOI: /science

51. Look at table 5.2
Are hydrothermal solutions more acidic?

53. The exception is sodium
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

54. 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

55. 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

56. 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

57. 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)

58. Magnesium is entirely absent from hydrothermal solution
Removed from seawater and added to rock to form magnesium rich metamorphic minerals

59. 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)

60. 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 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

61. 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

62. 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

63. 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 ‰

64. 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

65. 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

66. 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

67. 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

68. 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.

69. 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?

70. Barium sulfate is much more insoluble than calcium sulfate and precipitates readily when vent solutions mix with normal seawater

71. 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.

73. 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

74. 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

75. 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

77. The stockworks are a way to explain the ore deposits associated with the ophiolites found on land

78. 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

79. 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.

80. 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

82. 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?

83. 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?

84. 5000m/(3 m/yr) = yr

85. Assume only latent heat goes into heat flux, what is the lifespan?
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?

87. 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

89. 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

91. 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

92. 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

93. 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

94. Off-axis hydrothermal vents have also been identified at seamounts
Several ore deposits in ophiolites formed this way

96. Majority of known vents are in shallow parts of the ridge

97. 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?

98. The closer two sites are, the more related the species
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

99. Archea may have existed in these environments for 3-4 billion years
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

100. 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

103. 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

104. 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

105. The ratio of 3He to 4He is much higher in hydrothermal vent waters than anywhere else

107. 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

109. 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

110. 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

112. Gorda Ridge
March 10-11, 1996.

116. 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.

118. 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

120. 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:

122. 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 º 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

123. 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)

124. The volume of the oceans is 1.4 x 1021 kg Average renewal time = V / F
Therefore
F = 2 x 1020 J yr-1 / (4.2 x 103 J kg-1 ºC-1(300º – 2º))
F = 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 / x 1014 Kg yr-1
Tr = 8,761,200 yr

125. These calculations based on data from near the spreading center
Difficult to estimate flow rates in low temperature zones away from the spreading centers

126. That is only somewhat less than comes in from rivers, 5 x 1011 kg
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

127. On the other side, it is a major sink for
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-

128. 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)