Presentation on theme: "Hydrothermal circulation in oceanic crust"— Presentation transcript:
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 sulfidesAt temperatures, they know about 330° C. become white smokersBarium and calcium sulfate
9 The vent systems support ecosystems using chemosynthesis Energy derived from oxidation of sulfideMostly by bacteria – the primary producersSome live symbiotically in multicellular host organismsSome form bacterial mats coding the seabedSome actually live within vent chimneys
13 These vents form as soon as crust has been formed by igneous activity Hydrothermal processes take overEstimated that one third of the entire sea floor has sea water circulating through itHigh temperature vents are confined to spreading axes and active off-axis seamountsThe 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 riversThe 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 systemsHot 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 crestsHot Springs were the obvious explanationThe 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 characteristicsHigh geothermal gradient with hot rocks near the surfacePlumbing system of fracturesDownward circulation occurs slowly over a wide areaUpward 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 waterAnother difference is that the ocean floor is subject to high hydrostatic pressureIn global terms oceanic Hydrothermal circulation is the far more important of the two
26 Heat flow, convection and permeability The thermal gradient is very highUpper 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 waysConduction which is a molecular processConvection which is a bulk processWhich 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-scaleThis 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 motionsThere 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 lavasSpaces between the pillows and among rubble of seismic layer 2AFractures 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 fluidsThe crust becomes covered by thicker sediments
34 Chemical changesDramatic changes were observed in laboratory experiments trying to duplicate conditions under the sea floorAll the magnesium and sulfate in sea water was transferred to rockSignificant 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 rocksbasalt 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 seawaterEven cold seawater can cause chemical changes
38 in cold seawater basalt experiences seafloor weathering Which is similar to what occurs on landDuring 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 mineralsThese depend on the conditions under which metamorphosis occurred
40 Under conditions commonly found Under conditions commonly found. They are metamorphosed to greenschist gradeAt 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."
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 specimensWhat changes most?
49 Changes in seawaterCompare basalts from various bodies in the solar system.They are very similarLow 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 5.2Are 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 sodiumGlobally 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 crustAs 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 sulfideSulfide combines starring and other metals to foreign insoluble metal sulfidesThese are precipitated at the vents, building chimneysSulfides 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 chargeIron can follow magnesium into the new minerals being formedUnder 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 complexHot saline water is a very powerful chemical reagentPressure of about atmospheresPlumbing system can be very complicatedWater 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 dissolvedThe 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 seafloor60-70% SiO2 and 5-7% MgOImplies 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 SeaAccumulations of metal rich muds overlain by concentrated hydrothermal brinesTemperatures of 60° C. and salinities of over 300 ‰
64 Black smokersBoth the temperature and composition of harvestable than solutions are predicted two years before they were sampledFirst Hydrothermal vents found in 1977Low-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 waterOrdinary seawaterNegative correlation found between temperature and concentration of Mg in samples from low-temperature ventsIntercept 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 solutionsNext 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 continuumBlack smokers 350 to 400° C.Precipitates of metal sulfidesWhite smokers 30 to 330° C.Precipitates of sulfates of barium and calcium, and silicaWarm 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 ventsTransitions can happen at any stageTransition may simply result from precipitation of minerals which reduces permeability of the surrounding rockThe 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 chimneyOnce 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 twoWidely dispersed stockworks are characteristic of warm water ventsIsolated 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 manganesePrecipitated when dissolved Fe++ and Mn ++ from the hydrothermal solutions are oxidized on mixing the seawaterNear 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 patchyWarm 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 greatestLayer 2A is believed to have very high permeabilityIn this layer (< 20º C) only seafloor weatheringMetamorphism 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 deeperWidth and spacing a matter of debateProbably only 1-3 mm wide10s of cm to a couple of meters apartCan not penetrate into magma or unsolidified gabbro
85 Assume only latent heat goes into heat flux, what is the lifespan? Based on heat fluxTypical heat flux from a single system is about 200 MW = 2 x 108 Js-1Magma volume is about 10 km3 = ______ m3?Density of gabbro = 2500 kg m3Latent heat of fusion = 4.5 x 105 J kg-1Assume only latent heat goes into heat flux, what is the lifespan?
89 Anatomy of a Vent FieldVents are usually not solitary but occur in clusters – vent fieldA few km across at mostTAG field is 200 m diameter, 50 m high moundCoated by iron and copper sulfidesCluster of chimneys near topCluster of white smokers on one flank
91 TAG mound estimated at 18,000 yr old Compare this with the estimates just madeHow is that?Cores show that the activity has been episodicDuring active phase anhydrite deposited in the mound and sulfides on surfaceDuring 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 moundsAmount of metal sulfide in TAG is estimated at 4 million tonsThis is the largest at a spreading centerException is metalliferous sediments of Red Sea90 million tonsSulfides at fast spreading centers are 2 orders of magnitude lessHigher 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 systemSo a permanent linear heat source over geologic timeVents represent upflow zones –tightly focusedDownflow 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 BiologyWhen 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 larvaeMost die, but enough survive to re-establish new coloniesThe closer two sites are, the more related the speciesSites on EPR 800 km apart – 54 speciesSites 2000 km apart – 11 species in commonOnly 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 resultsArchea may have existed in these environments for 3-4 billion yearsLife may have originated here
100 Hydrothermal PlumesFew vent fields are actually discovered accidentallyMost found by following plume effluentsDetectable overlarge areasPlume several hundred meters above the groundPlume becomes diluted by the surrounding seawater
103 Plume dilution factor is typically 104 Despite the high dilution there are several cluesHeat contentSuspended smoke particlesDissolved 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 outHelium 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 Isotopes4He is most common – product of radioactive decay from uranium3He is rare and has two sourcesCosmic raysPrimordial helium trapped in the earth mantle at the time the formationEscapes during volcanic outgassing
105 The ratio of 3He to 4He is much higher in hydrothermal vent waters than anywhere else
109 CH4, H2 and CO2 can be liberated by partial melting of the mantle, but also by other processes as wellOxidation reduction reactionsMicrobial activity
110 Event plumesSo far, the plumes we’ve discussed of been more or less steady over timeLarge transient plumes have also been observedThese are known as event plumesAlso known as megaphone’sMost probably due to eruptionsEvent plumes rise a thousand meters or more – steady-state ones only a few hundred meters
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 seawaterHowever convection cells seem to remain in the rockUpflow and downflow zones remain fixed and hydrothermal circulation continuesjust diminishing in intensity
122 Extent of Hydrothermal Metamorphism Seismic studies can not show how much metamorphism has taken placeTemperatures in the top 500 m too lowExcept near ventsTemperatures to greenschist grade require º CFound throughout the lower half of layer 2ODP hole 504B showed increased metamorphism from surface to 1.8 kmThis is also the case in ophiolites
123 Mass TransferConvective heat transfer rate (total heat lost by hydrothermal circulation)Estimated at 8 x 1019 J – 3 x 1020 J per yearVolume of the ocean filtered every 106 yrFlow rate F kg yr-1F = 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 ThereforeF = 2 x 1020 J yr-1 / (4.2 x 103 J kg-1 ºC-1(300º – 2º))F = x 1014 Kg yr-1The volume of the oceans is 1.4 x 1021 kgAverage renewal time = V / FTr = 1.4 x 1021 kg / x 1014 Kg yr-1Tr = 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 that1.6 x 1014 Kg yr-1 water circulates through the crustAll of it acquires more Ca++ at 460 ppmHow 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 oceansNot including reprecipitationThat 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 MnIt is also important for Ba, Si, CaOn the other side, it is a major sink forMg2+ and SO42-
128 Why no K in the above analyses? K is leached from rocks at high tempK is added to rock at low temp (< 150º C)