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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)