geochemistry of geothermal systems geochemistry of geothermal systems

GEOCHEMISTRY OFGEOCHEMISTRY OFGEOTHERMAL GEOTHERMAL SYSTEMSSYSTEMS

WATER WATER CHEMISTRYCHEMISTRY

Chemical composition of waters is expressed in terms Chemical composition of waters is expressed in terms of major anion and cation contents.of major anion and cation contents.

Major Cations: Major Cations: NaNa++, K, K++, Ca, Ca++++, Mg, Mg++++

Major Anions: Major Anions: HCOHCO33-- (or CO (or CO33

==), Cl), Cl--, SO, SO44==

– HCOHCO33-- dominant in neutral conditions dominant in neutral conditions

– COCO33= = dominant in alkaline (pH>8) conditions dominant in alkaline (pH>8) conditions

– HH22COCO3 3 dominant in acidic conditions dominant in acidic conditions

Also dissolved Also dissolved silica (SiOsilica (SiO22)) in in neutral formneutral form as a as a major constituentmajor constituent

Minor constituents: B, F, Li, Sr, ...Minor constituents: B, F, Li, Sr, ...


concentration of chemical concentration of chemical constituents are expressed in units of constituents are expressed in units of

mg/l mg/l ( (ppmppm==pparts arts pper er mmillion) illion) (mg/l is the preferred unit)(mg/l is the preferred unit)

MolalityMolality

Molality = no. of moles / kg of solventMolality = no. of moles / kg of solventNo.of moles = (mg/l*10No.of moles = (mg/l*10-3-3)) / formula weight/ formula weight


Errors associated with water analyses are expressed in Errors associated with water analyses are expressed in terms of terms of CBECBE ( (CCharge harge BBalance alance EError)rror)

CBE (%) = CBE (%) = ( ( z x mc - z x mc - z x ma ) / (z x ma ) / (z x mc + z x mc + z x ma )z x ma )* 100* 100

where, where,

mcmc is the molality of cation is the molality of cation

mama is the molality of anion is the molality of anion

z z is the chargeis the charge

If CBE If CBE 5%, the results are appropriate to use in any kind of 5%, the results are appropriate to use in any kind of interpretationinterpretation

The constituents encountered in The constituents encountered in geothermal fluidsgeothermal fluids

TRACERSTRACERS

Chemically inert, non-reactive, conservative constituents Chemically inert, non-reactive, conservative constituents

(once added to the fluid phase, remain unchanged allowing (once added to the fluid phase, remain unchanged allowing their origins to be traced back to their source component - their origins to be traced back to their source component - used to infer about the source characteristics)used to infer about the source characteristics)

e.g. He, Ar (noble gases), Cl, B, Li, Rb, Cs, N2e.g. He, Ar (noble gases), Cl, B, Li, Rb, Cs, N2

GEOINDICATORSGEOINDICATORS

Chemically reactive, non-conservative species Chemically reactive, non-conservative species

(respond to changes in environment - used to infer about the (respond to changes in environment - used to infer about the physico-chemical processes during the ascent of water to physico-chemical processes during the ascent of water to surface, also used in geothermometry applications)surface, also used in geothermometry applications)

e.g. Na, K, Mg, Ca, SiO2e.g. Na, K, Mg, Ca, SiO2

In this chapter, the main emphasis In this chapter, the main emphasis will be placed on the use of water will be placed on the use of water chemistry in the determination of :chemistry in the determination of :

underground (reservoir) underground (reservoir) temperatures : temperatures : geothermometers geothermometers

boiling and mixing relationsboiling and mixing relations (subsurface physico-chemical (subsurface physico-chemical processes)processes)


HYDROTHERMAL HYDROTHERMAL REACTIONSREACTIONS

The composition of geothermal fluids are The composition of geothermal fluids are controlled by : temperature-dependent controlled by : temperature-dependent reactions between minerals and fluidsreactions between minerals and fluids

The factors affecting the formation of The factors affecting the formation of hydrothermal minerals are:hydrothermal minerals are:

temperaturetemperature pressurepressure rock typerock type permeabilitypermeability fluid compositionfluid composition duration of activityduration of activity

The effect of rock type --- most pronounced The effect of rock type --- most pronounced at low temperatures & insignificant above at low temperatures & insignificant above 280280CCAbove 280Above 280C and at least as high as 350C and at least as high as 350C, C, the typical stable mineral assemblages (in the typical stable mineral assemblages (in active geothermal systems) are active geothermal systems) are independent of rock type and include independent of rock type and include

ALBITE, K-FELDSPAR, CHLORITE, Fe-EPIDOTE, ALBITE, K-FELDSPAR, CHLORITE, Fe-EPIDOTE, CALCITE, QUARTZ, ILLITE & PYRITECALCITE, QUARTZ, ILLITE & PYRITE

At lower temperatures, ZEOLITES and CLAY At lower temperatures, ZEOLITES and CLAY MINERALS are found.MINERALS are found.

At low permeabilities equilibrium between At low permeabilities equilibrium between rocks and fluids is seldom achieved.rocks and fluids is seldom achieved.

When permeabilities are relatively high and When permeabilities are relatively high and water residence times are long (months to water residence times are long (months to years), water & rock should reach chemical years), water & rock should reach chemical equilibrium.equilibrium.

At equilibrium, ratios of cations in solution are controlled by temperature-dependent exchange reactions such as:

NaAlSi3O8 (albite) + K+ = KAlSi3O8 (K-felds.) + Na+

Keq. = Na+ / K+

Hydrogen ion activity (pH) is controlled by hydrolysis reactions, such as :

3 KAlSi3O8 (K-felds.) + 2 H+ = K Al3Si3O10(OH)2 (K-mica)+ 6SiO2 + 2 K+

Keq. = K+ / H+

where,

Keq. = equilibrium constant,

square brackets indicate activities of dissolved species (activity is unity for pure solid phases)

ESTIMATION OF ESTIMATION OF RESERVOIR RESERVOIR

TEMPERATURESTEMPERATURESThe evaluation of the The evaluation of the

reservoir temperatures for reservoir temperatures for geothermal systems is geothermal systems is

made in terms of made in terms of GEOTHERMOMETRY GEOTHERMOMETRY

APPLICATIONSAPPLICATIONS

GEOTHERMOMETGEOTHERMOMETRY APPLICATIONSRY APPLICATIONS

GEOTHERMOMETRY GEOTHERMOMETRY APPLICATIONSAPPLICATIONS

One of the One of the major toolsmajor tools for thefor the

exploration & development exploration & development

of geothermal resourcesof geothermal resources

GEOTHERMOMETRYGEOTHERMOMETRY

estimation of reservoir estimation of reservoir (subsurface) temperatures(subsurface) temperatures

usingusingChemical & isotopic compositionChemical & isotopic composition

ofof surface dischargessurface discharges fromfrom wellswells and/or and/or natural springs/fumarolesnatural springs/fumaroles

GEOTHERMOMETERSGEOTHERMOMETERS

CHEMICAL GEOTHERMOMETERSCHEMICAL GEOTHERMOMETERSutilize the chemical compositionutilize the chemical composition

silica and major cation contents of silica and major cation contents of water discharges water discharges

gas concentrations or relative gas concentrations or relative abundances of gaseous components abundances of gaseous components in steam dischargesin steam discharges

ISOTOPIC GEOTHERMOMETERSISOTOPIC GEOTHERMOMETERSbased on the isotope exchange based on the isotope exchange

reactions between various phases reactions between various phases (water, gas, mineral) in geothermal (water, gas, mineral) in geothermal systemssystems

Focus of the CourseFocus of the Course

CHEMICAL GEOTHERMOMETERSCHEMICAL GEOTHERMOMETERS

As applied to water dischargesAs applied to water discharges

PART I. Basic Principles & TypesPART I. Basic Principles & Types

PART II. Examples/ProblemsPART II. Examples/Problems

CHEMICAL CHEMICAL GEOTHEROMOMETERSGEOTHEROMOMETERS

PART I.PART I. Basic Principles Basic Principles &Types&Types

BASIC PRINCIPLESBASIC PRINCIPLES

Chemical Geothermometers areChemical Geothermometers are

developed on the basis of developed on the basis of temperature temperature dependent chemical equilibrium dependent chemical equilibrium between the water and the minerals at between the water and the minerals at the deep reservoir conditionsthe deep reservoir conditions

based on the assumption that the based on the assumption that the water water preserves its chemical composition preserves its chemical composition during its ascent from the reservoir to during its ascent from the reservoir to the surface the surface


Studies of well discharge chemistry and Studies of well discharge chemistry and alteration mineralogy alteration mineralogy

the the presence of equilibriumpresence of equilibrium in several in several geothermal fields geothermal fields

the the assumption of equilibrium is validassumption of equilibrium is valid


Assumption of the Assumption of the preservation of water preservation of water chemistrychemistry may may not always holdnot always hold

BecauseBecause the the water composition may be water composition may be affected by processesaffected by processes such as such as coolingcooling mixing mixing with waters from different reservoirs.with waters from different reservoirs.


Cooling Cooling during ascent during ascent from reservoir to surface:from reservoir to surface:CONDUCTIVE CONDUCTIVE ADIABATIC ADIABATIC


CONDUCTIVE Cooling CONDUCTIVE Cooling Heat loss while travelling through cooler Heat loss while travelling through cooler

rocksrocks

ADIABATIC Cooling ADIABATIC Cooling Boiling because of decreasing hydrostatic Boiling because of decreasing hydrostatic

headhead


Conductive coolingConductive cooling does notdoes not by itself by itself change the change the

composition composition of the water of the water but but may affectmay affect its its degree of saturationdegree of saturation

with respect to several minerals with respect to several minerals thus, it may thus, it may bring about a modification bring about a modification

in the chemical compositionin the chemical composition of the of the water water by by mineral dissolution mineral dissolution or or precipitationprecipitation


Adiabatic cooling Adiabatic cooling (Cooling by (Cooling by boiling)boiling)causes changes in the compositioncauses changes in the composition of of

ascending water ascending water these these changeschanges include include

degassingdegassing, and hence, and hencethe increase in the solute contentthe increase in the solute content as a as a

result of steam loss.result of steam loss.


MIXINGMIXINGaffects chemical compositionaffects chemical composition

since the since the solubilitysolubility of most of the of most of the compounds in waters compounds in waters increases with increases with increasing temperatureincreasing temperature, , mixing with mixing with cold groundwatercold groundwater results in theresults in the dilution dilution of geothermal waterof geothermal water

Geothermometry applications are not Geothermometry applications are not simply inserting values into specific simply inserting values into specific geothermometry equations. geothermometry equations.

Interpretation of temperatures obtained Interpretation of temperatures obtained from geothermometry equations requires from geothermometry equations requires a sound understanding of the chemical a sound understanding of the chemical processes involved in geothermal processes involved in geothermal systems. systems.

The main task of geochemist is to verify The main task of geochemist is to verify or disprove the validity of assumptions or disprove the validity of assumptions made in using specific geothermometers made in using specific geothermometers in specific fields.in specific fields.

TYPES OF CHEMICAL TYPES OF CHEMICAL GEOTHERMOMETERSGEOTHERMOMETERS

SILICA GEOTHERMOMETERSSILICA GEOTHERMOMETERS

CATION GEOTHERMOMETERS CATION GEOTHERMOMETERS (Alkali Geothermometers) (Alkali Geothermometers)


based on thebased on the experimentally determinedexperimentally determinedtemperature dependenttemperature dependent

variation in the solubility of silicavariation in the solubility of silica in water in water

Since Since silicasilica can occur in can occur in various formsvarious forms in in geothermal fields (such as geothermal fields (such as quartz, quartz, crystobalite, chalcedony, amorphous silicacrystobalite, chalcedony, amorphous silica) ) different silica geothermometersdifferent silica geothermometers have been have been developed by different workersdeveloped by different workers

SILICA GEOTHERMOMETERSSILICA GEOTHERMOMETERSGeothermometerGeothermometer EquationEquation ReferenceReference

Quartz-no steam lossQuartz-no steam loss T = 1309 / T = 1309 / ((5.19 – log C) - 273.155.19 – log C) - 273.15 Fournier (1977)Fournier (1977)

Quartz-maximum Quartz-maximum steam loss at 100 steam loss at 100 ooCC

T = 1522 / T = 1522 / ((5.75 - log C) - 273.155.75 - log C) - 273.15 Fournier (1977)Fournier (1977)

QuartzQuartz T = 42.198 + 0.28831C - 3.6686 x 10T = 42.198 + 0.28831C - 3.6686 x 10-4 -4 CC2 2 + + 3.1665 x 103.1665 x 10-7 -7 CC3 3 + 77.034 log C+ 77.034 log C

Fournier and Fournier and Potter (1982)Potter (1982)

QuartzQuartz T = 53.500 + 0.11236C - 0.5559 x 10T = 53.500 + 0.11236C - 0.5559 x 10-4 -4 CC2 2 + + 0.1772 x 100.1772 x 10-7 -7 CC3 3 + 88.390 log C+ 88.390 log C

Arnorsson Arnorsson (1985) based on (1985) based on Fournier and Fournier and Potter (1982)Potter (1982)

ChalcedonyChalcedony T = 1032 / T = 1032 / ((4.69 - log C) - 273.154.69 - log C) - 273.15 Fournier (1977)Fournier (1977)

ChalcedonyChalcedony T = 1112 / T = 1112 / ((4.91 - log C) - 273.154.91 - log C) - 273.15 Arnorsson et al. Arnorsson et al. (1983)(1983)

Alpha-CristobaliteAlpha-Cristobalite T = 1000 / T = 1000 / ((4.78 - log C) - 273.154.78 - log C) - 273.15 Fournier (1977)Fournier (1977)

Opal-CTOpal-CT(Beta-Cristobalite)(Beta-Cristobalite)

T = 781 / T = 781 / ((4.51 - log C) - 273.154.51 - log C) - 273.15 Fournier (1977)Fournier (1977)

Amorphous silicaAmorphous silica T = 731 / T = 731 / ((4.52 - log C) - 273.154.52 - log C) - 273.15 Fournier (1977)Fournier (1977)


The followings should be consideredThe followings should be considered : :

temperature rangetemperature range in which the equations are in which the equations are validvalid effects of effects of steam separationsteam separationpossiblepossible precipitation of silicaprecipitation of silica before sample collection before sample collection (during the travel of fluid to surface, due to silica oversaturation) (during the travel of fluid to surface, due to silica oversaturation) after sample collection after sample collection (due to improper preservation of sample)(due to improper preservation of sample)

effects of effects of pH pH on solubility of silicaon solubility of silicapossiblepossible mixingmixing of hot water with cold waterof hot water with cold water


Temperature RangeTemperature Range

silica geothermometers are valid for silica geothermometers are valid for temperature rangestemperature ranges up to 250 up to 250 CC

above 250above 250C, the equations depart C, the equations depart drastically from the experimentally drastically from the experimentally determined solubility curves determined solubility curves

SILICA GEOTHERMOMETERSSILICA GEOTHERMOMETERSTemperature RangeTemperature Range

Fig.1.Fig.1. Solubility of Solubility of quartz (curve quartz (curve A)A) and and amorphous silica (curve C)amorphous silica (curve C) as a function of temperature at the as a function of temperature at the vapour pressure of the solution. vapour pressure of the solution. Curve B shows the amount of silica Curve B shows the amount of silica that would be in solution after an that would be in solution after an initially quartz-saturated solution initially quartz-saturated solution cooled adiabatically to 100 cooled adiabatically to 100 C C without any precipitation of silica without any precipitation of silica (from Fournier and Rowe, 1966, and (from Fournier and Rowe, 1966, and Truesdell and Fournier, 1976).Truesdell and Fournier, 1976).

At low T (At low T (C) C) qtz less solubleqtz less solubleamorph. silica more solubleamorph. silica more soluble

Silica solubility is controlled by Silica solubility is controlled by amorphous silicaamorphous silica at low T (at low T (C)C) quartz at high T (quartz at high T (C) C)

SILICA GEOTHERMOMETERSSILICA GEOTHERMOMETERSEffects of Steam SeparationEffects of Steam Separation

Boiling Boiling Steam Separation Steam Separation volume of residual liquidvolume of residual liquidConcentration in liquidConcentration in liquidTemperature EstimateTemperature Estimate

e.g.e.g.T =T = 1309 / 1309 / ((5.19 – log C) - 273.155.19 – log C) - 273.15C = SiO2 in ppmC = SiO2 in ppmincrease in C increase in C (SiO2 in water > SiO2 in reservoir) (SiO2 in water > SiO2 in reservoir)

decrease in decrease in denominatordenominator of the equation of the equationincrease in Tincrease in T

for boiling springs for boiling springs

boiling-corrected geothermometersboiling-corrected geothermometers (i.e. Quartz-max. steam loss) (i.e. Quartz-max. steam loss)

SiO2

liquid V1

(1)

V2 V1<SiO2 (2) SiO2 (1)>

liquid V2

SiO2 (2)

steam

SILICA GEOTHERMOMETERSSILICA GEOTHERMOMETERSSilica PrecipitationSilica Precipitation

SiO2SiO2 Temperature EstimateTemperature Estimate

e.g.e.g.T =T = 1309 / 1309 / ((5.19 – log C) - 273.155.19 – log C) - 273.15C = SiO2 in ppmC = SiO2 in ppmdecrease in C decrease in C (SiO2 in water < SiO2 in reservoir) (SiO2 in water < SiO2 in reservoir)

increase in increase in denominatordenominatordecrease in Tdecrease in T

SILICA GEOTHERMOMETERSSILICA GEOTHERMOMETERSEffect of pHEffect of pH

Fig. 2.Fig. 2. Calculated Calculated effect of pHeffect of pH upon the upon the solubility of quartzsolubility of quartz at various temperatures at various temperatures from 25 from 25 C to 3C to 3000 0 C , using experimental C , using experimental data of Seward (1974). data of Seward (1974). TheThe dashed curve dashed curve shows the pH required at various shows the pH required at various temperatures to achieve a 10% increase in temperatures to achieve a 10% increase in quartz solubility compared to the solubility quartz solubility compared to the solubility at pH=7.0 (from Fournier, 1981).at pH=7.0 (from Fournier, 1981).

pH pH Dissolved SiO2 Dissolved SiO2 (for pH>7.6) (for pH>7.6)Temperature EstimateTemperature Estimate

e.g.e.g.

T =T = 1309 / 1309 / ((5.19 – log C) - 273.155.19 – log C) - 273.15C = SiO2 in ppmC = SiO2 in ppmincrease in C increase in C decrease in decrease in denominatordenominator of the equation of the equationincrease in Tincrease in T

SILICA GEOTHERMOMETERSSILICA GEOTHERMOMETERSEffect of MixingEffect of Mixing

Hot-WaterHot-Water High SiO2High SiO2 content contentCold-WaterCold-Water Low SiO2Low SiO2 content content

(Temperature (Temperature Silica solubility Silica solubility ))

Mixing (of hot-water with cold-water)Mixing (of hot-water with cold-water)TemperatureTemperatureSiO2 SiO2 Temperature Estimate Temperature Estimate

e.g.e.g.T =T = 1309 / 1309 / ((5.19 – log C) - 273.155.19 – log C) - 273.15C = SiO2 in ppmC = SiO2 in ppmdecreasedecrease in C in C increaseincrease in in denominatordenominator of the equation of the equationdecrease in Tdecrease in T


Process Process Reservoir TemperatureReservoir Temperature

Steam Separation Steam Separation Overestimated Overestimated

Silica Precipitation Silica Precipitation Underestimated Underestimated

Increase in pH Increase in pH Overestimated Overestimated

Mixing Mixing with cold water with cold water Underestimated Underestimated

CATION GEOTHERMOMETERS CATION GEOTHERMOMETERS (Alkali Geothermometers) (Alkali Geothermometers)

based on the partitioning of alkalies between based on the partitioning of alkalies between solid and liquid phases solid and liquid phases

e.g. Ke.g. K++ + Na-feldspar = Na + Na-feldspar = Na++ + K-feldspar + K-feldspar

majority of are empirically developed majority of are empirically developed geothermometersgeothermometers Na/KNa/K geothermometer geothermometer Na-K-CaNa-K-Ca geothermometer geothermometer Na-K-Ca-MgNa-K-Ca-Mg geothermometer geothermometer OthersOthers ((Na-LiNa-Li, , K-MgK-Mg, ..), ..)

CATION GEOTHERMOMETERSCATION GEOTHERMOMETERSNa/K GeothermometerNa/K Geothermometer

Fig.3.Fig.3. Na/K atomic ratios Na/K atomic ratios of well discharges plotted at of well discharges plotted at measured downhole measured downhole temperatures. temperatures. Curve A is Curve A is the least square fit of the the least square fit of the data points above 80 data points above 80 C. C. Curve B is another Curve B is another empirical curveempirical curve (from (from Truesdell, 1976). Curves C Truesdell, 1976). Curves C and D show the and D show the approximate locations of approximate locations of the low albite-microcline the low albite-microcline and high albite-sanidine and high albite-sanidine lines derived from lines derived from thermodynamic data (from thermodynamic data (from Fournier, 1981).Fournier, 1981).


GeothermGeotherm.. EquationsEquations ReferenceReference

Na-KNa-K T=T=[[855.6/855.6/((0.857+log0.857+log(Na/K)(Na/K)))]]-273.15-273.15 Truesdell (1976)Truesdell (1976)

Na-KNa-K T=T=[[833/833/((0.780+log0.780+log(Na/K)(Na/K)))]]-273.15-273.15 Tonani (1980)Tonani (1980)

Na-KNa-K T=T=[[933/933/((0.993+log 0.993+log (Na/K)(Na/K)))]]-273.15-273.15(25-250 (25-250 ooC)C)

Arnorsson et al.Arnorsson et al. (1983)(1983)

Na-KNa-K T=T=[[1319/1319/((1.699+log1.699+log(Na/K)(Na/K)))]]-273.15-273.15(25(2500--3350 50 ooC)C)

Arnorsson et al. Arnorsson et al. (1983)(1983)

Na-KNa-K T=T=[[1217/1217/((1.483+log1.483+log(Na/K)(Na/K)))]]-273.15-273.15 Fournier (1979)Fournier (1979)

Na-KNa-K T=T=[[1178/(1.470+log 1178/(1.470+log (Na/K)(Na/K)))]]-273.15-273.15 Nieva and Nieva Nieva and Nieva (1987)(1987)

Na-KNa-K T=T=[[1390/1390/((1.750+log1.750+log(Na/K)(Na/K)))]]-273.15-273.15 Giggenbach Giggenbach (1988)(1988)


gives good results for reservoir temperatures gives good results for reservoir temperatures above 180above 180C. C.

yields erraneous estimates for low yields erraneous estimates for low temperature waterstemperature waterstemperature-dependent exchange equilibrium temperature-dependent exchange equilibrium

between feldspars and geothermal waters is not between feldspars and geothermal waters is not attained at low temperatures attained at low temperatures andand the Na/K ratio in the Na/K ratio in these waters are governed by leaching rather than these waters are governed by leaching rather than chemical equilibriumchemical equilibrium

yields unusually high estimates for waters yields unusually high estimates for waters having high calcium contentshaving high calcium contents

CATION GEOTHERMOMETERSCATION GEOTHERMOMETERSNa-K-Ca GeothermometerNa-K-Ca Geothermometer

Geotherm. Equations Reference

Na-K-Ca T=[1647/ (log (Na/K)+ (log (CaCa/Na)+2.06)+ 2.47)]-273.15

a)a) if if loglogCa/Na)+2.06Ca/Na)+2.06 < 0, use < 0, use =1/3=1/3 and calculate T and calculate TCCb)b) if if loglogCa/Na)+2.06Ca/Na)+2.06 > 0, use > 0, use =4/3=4/3 and calculate T and calculate TCCc)c) if calculated T > 100 if calculated T > 100C in C in (b)(b), recalculate T, recalculate TC using C using =1/3=1/3

Fournier and Truesdell (1973)

CATION GEOTHERMOMETERSCATION GEOTHERMOMETERSNa-K-Ca GeothermometerNa-K-Ca Geothermometer

Works well for CO2-rich or Ca-rich environments Works well for CO2-rich or Ca-rich environments provided provided that calcite was not deposited after the water left the that calcite was not deposited after the water left the reservoirreservoir

in case of in case of calcite precipitationcalcite precipitationCa Ca 1647T = --------------------------------------------------------- - 273.15

log (Na/K)+ (log (CaCa/Na)+2.06)+ 2.47

Decrease in Ca concentration (Ca in water < Ca in reservoir)Decrease in Ca concentration (Ca in water < Ca in reservoir)decrease in decrease in denominatordenominator of the equation of the equationincrease in Tincrease in T

For waters with high Mg contents, Na-K-Ca For waters with high Mg contents, Na-K-Ca geothermometer yields erraneous results. For these geothermometer yields erraneous results. For these waters, Mg correction is necessary waters, Mg correction is necessary

CATION GEOTHERMOMETERSCATION GEOTHERMOMETERSNa-K-Ca-Mg GeothermometerNa-K-Ca-Mg Geothermometer

Geotherm. Equations ReferenceNa-K-Ca-MgNa-K-Ca-Mg T = TT = TNa-K-Ca Na-K-Ca - - ttMMgg

ooCC

R = (Mg / Mg + 0.61Ca + 0.31K) x 100 R = (Mg / Mg + 0.61Ca + 0.31K) x 100

if R from 1.5 to 5if R from 1.5 to 5

ttMMggooC = -1.03 + 59.971 log R + 145.05 (log R)C = -1.03 + 59.971 log R + 145.05 (log R)2 2 – 36711 – 36711

(log R)(log R)2 2 / T - 1.67 x 10/ T - 1.67 x 1077 log R / T log R / T22

if R from 5 to 50if R from 5 to 50

ttMMggooC=10.66-4.7415 log R+325.87(log R)C=10.66-4.7415 log R+325.87(log R)22-1.032x10-1.032x1055(log (log

R)R)22/T-1.968x10/T-1.968x1077(log R)(log R)33/T/T22

Note: Do not apply a Mg correction if Note: Do not apply a Mg correction if ttMMgg is negative is negative

or R<1.5. or R<1.5.

If R>50, assume a temperature = measured spring If R>50, assume a temperature = measured spring temperaturetemperature. .

T is Na-K-Ca geothermometer temperature iT is Na-K-Ca geothermometer temperature inn Kelvin Kelvin

Fournier Fournier and Potter and Potter (1979)(1979)

CATION GEOTHERMOMETERSCATION GEOTHERMOMETERSNa-K-Ca-Mg GeothermometerNa-K-Ca-Mg Geothermometer

Fig. 4.Fig. 4. Graph for Graph for estimating the estimating the magnesium temperature magnesium temperature correction to be correction to be subtracted from Na-K-Ca subtracted from Na-K-Ca calculated temperature calculated temperature (from Fournier, 1981)(from Fournier, 1981)

R = (Mg/Mg + 0.61Ca + R = (Mg/Mg + 0.61Ca + 0.31K)x1000.31K)x100

UNDERGROUND MIXING OF UNDERGROUND MIXING OF HOT AND COLD WATERSHOT AND COLD WATERS

Recognition of Mixed WatersRecognition of Mixed WatersMixing of hot ascending waters with cold waters at Mixing of hot ascending waters with cold waters at shallow depths is common.shallow depths is common.

Mixing also occurs deep in hydrothermal systems.Mixing also occurs deep in hydrothermal systems.

The effects of mixing on geothermometers is already The effects of mixing on geothermometers is already discussed in previous section.discussed in previous section.

Where all the waters reaching surface are mixed waters, Where all the waters reaching surface are mixed waters, recognition of mixing can be difficult.recognition of mixing can be difficult.

The recognition of mixing is especially difficult if water-The recognition of mixing is especially difficult if water-rock re-equilibration occurred after mixing (complete or rock re-equilibration occurred after mixing (complete or partial re-equilibration is more likely if the temperatures partial re-equilibration is more likely if the temperatures after mixing is well above 110 to 150 after mixing is well above 110 to 150 C, or if mixing C, or if mixing takes place in aquifers with long residence times).takes place in aquifers with long residence times).

UNDERGROUND MIXING OF UNDERGROUND MIXING OF HOT AND COLD WATERSHOT AND COLD WATERS

Some indications of mixing are as follows:Some indications of mixing are as follows:systematic variations of spring compositions systematic variations of spring compositions and measured temperaturesand measured temperatures,,

variations in oxygen or hydrogen isotopesvariations in oxygen or hydrogen isotopes,,

variations in ratios of relatively *variations in ratios of relatively *conservative conservative elementselements that do not precipitate from solution that do not precipitate from solution during movement of water through rock (e.g. during movement of water through rock (e.g. Cl/B ratios)Cl/B ratios)..

SILICA-ENTHALPY MIXING SILICA-ENTHALPY MIXING MODELMODEL

Dissolved silica content of mixed waters can be Dissolved silica content of mixed waters can be used to determine the temperature of hot-water used to determine the temperature of hot-water component .component .

Dissolved silica is plotted against enthalpy of liquid Dissolved silica is plotted against enthalpy of liquid water. water.

Although Although temperature is the measured propertytemperature is the measured property, and , and enthalphy is a derived propertyenthalphy is a derived property, , enthalpy is used as enthalpy is used as a coordinate rather than temperature. This is a coordinate rather than temperature. This is because the combined heat contents of two waters because the combined heat contents of two waters are conserved when those waters are mixed, but the are conserved when those waters are mixed, but the combined temperatures are not.combined temperatures are not.

The enthalpy values are obtained from steam tables.The enthalpy values are obtained from steam tables.


Fig. 5.Fig. 5. Dissolved silica-Dissolved silica-enthalpy diagram showing enthalpy diagram showing procedure for calculating procedure for calculating the initial enthalpy (and the initial enthalpy (and hence the reservoir hence the reservoir temperature) of a high temperature) of a high temperature water that has temperature water that has mixed with a low mixed with a low temperature water (from temperature water (from Fournier, 1981)Fournier, 1981)


A = non-thermal componentA = non-thermal component (cold water)(cold water)

B, D = mixed, warm waterB, D = mixed, warm water springssprings

C = hot water component atC = hot water component at reservoir conditionsreservoir conditions

(assuming (assuming no steamno steam separation separation before mixing)before mixing)

E = hot water component atE = hot water component at reservoir conditionsreservoir conditions

(assuming (assuming steam separationsteam separation before mixing)before mixing)

Boiling Boiling T = 100 T = 100 CCEnthalpy = 419 J/gEnthalpy = 419 J/g(corresponds to D in the graph)(corresponds to D in the graph)

Enthalpy values (at corresponding temperatures)Enthalpy values (at corresponding temperatures)are found from Steam Table in Henley et al.(1984)are found from Steam Table in Henley et al.(1984)

419 J/g(100 C)0

SILICA-ENTHALPY MIXING SILICA-ENTHALPY MIXING MODELMODELSteam Fraction did not separate before Steam Fraction did not separate before mixingmixingThe sample points are plotted. The sample points are plotted.

A straight line is drawn from A straight line is drawn from the point representing the non-the point representing the non-thermal component of the thermal component of the mixed water (i.e. the point with mixed water (i.e. the point with the lowest temperature and the the lowest temperature and the lowest silica content = point A lowest silica content = point A in Figin Fig..), through the mixed ), through the mixed water warm springs (points B water warm springs (points B and D in Fig.).and D in Fig.).

The intersection of this line The intersection of this line with the qtz solubility curve with the qtz solubility curve (point C in Fig.) gives the (point C in Fig.) gives the enthalpy of the hot-water enthalpy of the hot-water component (at reservoir component (at reservoir conditions).conditions).

From the steam table, the From the steam table, the temperature corresponding to temperature corresponding to this enthalpy value is obtained this enthalpy value is obtained as the reservoir temperature of as the reservoir temperature of the hot-water component.the hot-water component.

419 J/g(100 C)0

SILICA-ENTHALPY MIXING SILICA-ENTHALPY MIXING MODELMODELSteam separation occurs before mixingSteam separation occurs before mixing

The enthalpy at the boling The enthalpy at the boling temperature (100temperature (100C) is C) is obtained from the steam obtained from the steam tables (which is 419 j/g)tables (which is 419 j/g)

A vertical line is drawn from A vertical line is drawn from the enthalpy value of 419 j/gthe enthalpy value of 419 j/g

From the inetrsection point of From the inetrsection point of this line with the mixing line this line with the mixing line (Line AD), a horizantal line (Line AD), a horizantal line (DE) is drawn.(DE) is drawn.

The intersection of line DE The intersection of line DE with the solubility curve for with the solubility curve for maximum steam loss (point E) maximum steam loss (point E) gives the enthalpy of the hot-gives the enthalpy of the hot-water component.water component.

From the steam tables, the From the steam tables, the reservoir temperature of the reservoir temperature of the hot-waterhot-water component is component is determined.determined.

419 J/g(100 C)0


In order for the silica mixing model to give accurate results, it In order for the silica mixing model to give accurate results, it is vital that no conductive cooling occurred after mixing. If is vital that no conductive cooling occurred after mixing. If conductive cooling occurred after mixing, then the calculated conductive cooling occurred after mixing, then the calculated temperatures will be too high (overestimated temperatures).temperatures will be too high (overestimated temperatures). This is because:This is because:

the original points before conductive cooling should lie to the the original points before conductive cooling should lie to the right of the line AD (i.e. towards the higher enthalpy values at right of the line AD (i.e. towards the higher enthalpy values at the same silica concentrations, as conductive cooling will the same silica concentrations, as conductive cooling will affect only the temperatures, not the silica contents) affect only the temperatures, not the silica contents)

in this case, the intersection of mixing line with the quartz in this case, the intersection of mixing line with the quartz solubility curve will give lower enthalpy values (i.e lower solubility curve will give lower enthalpy values (i.e lower temperatures) than that obtained in case of conductive temperatures) than that obtained in case of conductive cooling.cooling.

in other words, the temperatures obtained in case of in other words, the temperatures obtained in case of conductive cooling will be higher than the actual reservoir conductive cooling will be higher than the actual reservoir temperatures (i.e. if conductive cooling occurred after mixing, temperatures (i.e. if conductive cooling occurred after mixing, the temperatures will be overestimated)the temperatures will be overestimated)


Another requirement for the use of enthalpy-silica Another requirement for the use of enthalpy-silica model is that no silica deposition occurred before or model is that no silica deposition occurred before or after mixing. If silica deposition occurred, the after mixing. If silica deposition occurred, the temperatures will be underestimated.temperatures will be underestimated. This is because: This is because:

the original points before silica deposition should be the original points before silica deposition should be towards higher silica contents (at the same enthalpy towards higher silica contents (at the same enthalpy values)values)

in this case, the intersection point of mixing line with in this case, the intersection point of mixing line with the silica solubility curve will have higher enthalpy the silica solubility curve will have higher enthalpy values(higher temperatures) than that obtained in case values(higher temperatures) than that obtained in case of silica depositionof silica deposition

in other words, the temperatures obtained in case of no in other words, the temperatures obtained in case of no silica deposition will be higher than that in case of silica deposition will be higher than that in case of silica deposition (i.e. the temperatures will be silica deposition (i.e. the temperatures will be underestimated in case of silica deposition)underestimated in case of silica deposition)

CHLORIDE-ENTHALPY CHLORIDE-ENTHALPY MIXING MODELMIXING MODEL

Fig.6.Fig.6. Enthalpy- Enthalpy-chloride diagram for chloride diagram for waters from Yellowstone waters from Yellowstone National Park. Small National Park. Small circles indicate Geyser circles indicate Geyser Hill-type waters and smal Hill-type waters and smal dots indicate Black Sand-dots indicate Black Sand-type waters (From type waters (From Fournier, 1981).Fournier, 1981).


ESTIMATION OF RESERVOIR ESTIMATION OF RESERVOIR TEMPERATURETEMPERATURE

Geyser Hill-type WatersGeyser Hill-type WatersA = maximum Cl contentA = maximum Cl contentB = minimum Cl contentB = minimum Cl contentC = minimum enthalpy atC = minimum enthalpy at

the reservoirthe reservoir

Black Sand-type WatersBlack Sand-type WatersD = maximum Cl contentD = maximum Cl contentE = minimum Cl contentE = minimum Cl content

F = minimum enthalpy atF = minimum enthalpy at the reservoirthe reservoir

Enthalpy of steam at 100 Enthalpy of steam at 100 C =C =2676 J/g 2676 J/g (Henley et al., 1984)(Henley et al., 1984)


ORIGIN OF WATERSORIGIN OF WATERS

N = cold water componentN = cold water componentC, F = hot water componentsC, F = hot water components

F is more dilute & slightly F is more dilute & slightly cooler than Ccooler than C

F can not be derived from CF can not be derived from C by process of mixing by process of mixing between between hothot and and cold cold water water ((point Npoint N), because any ), because any mixture would lie on or mixture would lie on or close to line CN.close to line CN.

C and F are probably both C and F are probably both related to a still higher related to a still higher enthalpy water such as enthalpy water such as point G or H.point G or H.


ORIGIN OF WATERSORIGIN OF WATERS

water Cwater C could be related could be related to to water Gwater G by boilingby boiling

water C water C could also be could also be related to related to water Hwater H by conductive coolingby conductive cooling

water F water F could be related could be related to to water Gwater G or or water Hwater H by by mixing mixing with with cold water Ncold water N

steamsteam

B ED

C

G F

H

N

H

cold water reservoir

hot water reservoir

steamhot water

mixed water

residual liquid from boiling

B

hot water undergoingconductive cooling

mixed water undergoingconductive cooling

residual liquid undergoingconductive cooling

ISOTOPES ISOTOPES IN IN

GEOTHERMAL GEOTHERMAL EXPLORATION EXPLORATION

& DEVELOPMENT& DEVELOPMENT

ISOTOPE STUDIES IN ISOTOPE STUDIES IN GEOTHERMAL SYSTEMSGEOTHERMAL SYSTEMSAt At ExplorationExploration, , DevelopmentDevelopment and and ExploitationExploitation Stages Stages

Most commonly used isotopesMost commonly used isotopes

– Hydrogen (Hydrogen (11H, H, 22H =DH =D, , 33HH))– Oxygen (Oxygen (1818O, O, 1616OO))– Sulphur (Sulphur (3232S, S, 3434SS))– Helium (Helium (33He, He, 44HeHe))

ISOTOPE STUDIES IN ISOTOPE STUDIES IN GEOTHERMAL SYSTEMSGEOTHERMAL SYSTEMS

Geothermal FluidsGeothermal Fluids

SourcesSources– Source of fluidsSource of fluids (meteoric, magmatic, ..)(meteoric, magmatic, ..)

– Physico-chemical processesPhysico-chemical processes affecting the fluid comosition affecting the fluid comosition Water-rock interactionWater-rock interaction EvaporationEvaporation CondensationCondensation

– Source of components Source of components in fluidsin fluids (mantle, crust,..)(mantle, crust,..)

AgesAges (time between (time between recharge-dischargerecharge-discharge, , recharge-samplingrecharge-sampling))

Temperatures Temperatures (Geothermometry Applications)(Geothermometry Applications)

Sources of Geothermal FluidsSources of Geothermal Fluids

Sources of Geothermal FluidsSources of Geothermal Fluids

H- & O- IsotopesH- & O- Isotopes

Physico-chemical processes affecting the fluid Physico-chemical processes affecting the fluid compositioncomposition

H- & O- IsotopesH- & O- Isotopes

Sources of components (elements, Sources of components (elements, compounds) in geothermal fluidscompounds) in geothermal fluidsHe-Isotopes He-Isotopes (volatile elements)(volatile elements)

Sources of Geothermal Fluids and Sources of Geothermal Fluids and

Physico-Chemical ProcessesPhysico-Chemical Processes

STABLESTABLE

H- & O-ISOTOPESH- & O-ISOTOPES

Sources of Geothermal Fluids Sources of Geothermal Fluids StableStable H- & O-IsotopesH- & O-Isotopes

11H = % 99.9852H = % 99.985222H (D) = % 0.0148H (D) = % 0.0148D/HD/H

1616O = % 99.76O = % 99.761717O = % 0.04O = % 0.041818O = % 0.20O = % 0.201818O / O / 1616OO

Sources of Geothermal Fluids Sources of Geothermal Fluids

StableStable H- & O-IsotopesH- & O-Isotopes (D/H)(D/H)samplesample- (D/H)- (D/H)standardstandard

D (D () = ) = ----------------------------------- x 10----------------------------------- x 1033

(D/H)(D/H)standardstandard

((1818O/O/1616O)O)samplesample- (- (1818O/O/1616O)O)standardstandard

1818O (O () = ) = -------------------------------------------- x 10-------------------------------------------- x 1033

((1818O/O/1616O)O)standardstandard

Standard = Standard = SStandard tandard MMean ean OOcean cean WWater ater

= = SMOW SMOW


StableStable H- & O-IsotopesH- & O-Isotopes (D/H)(D/H)samplesample- (D/H)- (D/H)SMOWSMOW

D (D () = ) = ----------------------------------- x 10----------------------------------- x 1033

(D/H)(D/H)SMOWSMOW

((1818O/O/1616O)O)samplesample- (- (1818O/O/1616O)O)SMOWSMOW

1818O (O () = ) = -------------------------------------------- x 10-------------------------------------------- x 1033

((1818O/O/1616O)O)SMOWSMOW


StableStable H- & O-IsotopesH- & O-Isotopes

Sources of Natural Waters:Sources of Natural Waters:

1.1. Meteoric Water Meteoric Water (rain, snow)(rain, snow)

2.2. Sea WaterSea Water3.3. Fossil Waters Fossil Waters (trapped in sediments in sedimanary basins)(trapped in sediments in sedimanary basins)

4.4. Magmatic WatersMagmatic Waters5.5. Metamorphic WatersMetamorphic Waters


StableStable H- & O-IsotopesH- & O-Isotopes0

0

-40

-80

-120

10 20 30-10-20

O (per mil)18

D (per mil)

+SMOW

Field ofFormationWaters

MagmaticWaters

Most igneousbiotites &hornblendes

MetamorphicWaters



OceanSeepage

precipitation

evaporation

River

H, O1 16

H, O1 16

D, O18

D, O18

D, O18 H, O1 16

H, O1 16 D, O18

D, O18

(D/H) < (D/H)vapor water

vapor<18 16O / O( ) 18 16O / O( )

water

precipitation



0

-40

-80

-120-12 -8 -4 0

del- O (per mil)18

+SMOW

Condensation

Evaporation

Water-RockInteraction

Sources of Geothermal Fluids Sources of Geothermal Fluids StableStable H- & O-IsotopesH- & O-Isotopes

18

MagmatikSular

0

-50

-100

-150-15 -10 -5 0 +5 +10

LarderelloThe Geysers

Iceland

Niland

Lassen Park

Steamboat Kaynakları

O (per mil)

D (per mil)

Physico-Chemical Processes:Physico-Chemical Processes:Stable H- & O-IsotopesStable H- & O-Isotopes

Latitute Latitute DD OO

Altitute from Sea levelAltitute from Sea level DD OO


Aquifers Aquifers recharged by precipitation fromrecharged by precipitation from lower altituteslower altitutes higher higher D - D - OO valuesvalues

Aquifers Aquifers recharged by precipitation fromrecharged by precipitation from higher altituteshigher altitutes lower lower D - D - OO valuesvalues

Mixing of waters from different aquifersMixing of waters from different aquifers


Boiling and vapor separation Boiling and vapor separation DD OO in residual liquid in residual liquid

Possible subsurface boiling as a Possible subsurface boiling as a consequence of pressure decrease consequence of pressure decrease (due to continuous exploitation (due to continuous exploitation from production wells)from production wells)

Monitoring Studies in Monitoring Studies in Geothermal ExploitationGeothermal Exploitation

Aquifers Aquifers recharged by recharged by precipitation fromprecipitation from lower altituteslower altitutes higher higher D - D - OO

Aquifers Aquifers recharged by recharged by precipitation fromprecipitation from higher altituteshigher altitutes lower lower D - D - OO

Boiling and vapor Boiling and vapor separation separation

DD OO in residual in residual liquidliquid

Any increase inAny increase in D - D - O O values values

due to sudden pressure due to sudden pressure drop in production wellsdrop in production wells

recharge from (other) recharge from (other) aquifers fed by aquifers fed by precipitation from lower precipitation from lower altitutes altitutes

subsurface boiling and subsurface boiling and vapour separationvapour separation

Monitoring Studies in Monitoring Studies in Geothermal ExploitationGeothermal Exploitation

Monitoring of isotope composition of Monitoring of isotope composition of geothermal fluids during exploitationgeothermal fluids during exploitation can can lead lead to determination ofto determination of, , and the and the development of necessary precautionsdevelopment of necessary precautions againstagainst

– Decrease in enthalpyDecrease in enthalpy due to start of due to start of recharge from cold, shallow aquifers, orrecharge from cold, shallow aquifers, or

– Scaling problemsScaling problems developed as a result of developed as a result of subsurface boilingsubsurface boiling

(Scaling)(Scaling)

Vapour SeparationVapour Separation Volume of (residual) liquid Volume of (residual) liquid Concentration of dissolved components Concentration of dissolved components

in liquid in liquid Liquid will become oversaturated Liquid will become oversaturated Component (calcite, silica, etc.) will Component (calcite, silica, etc.) will

precipitateprecipitate ScalingScaling

Dating of Geothermal FluidsDating of Geothermal Fluids

33H- & H- & 33He-ISOTOPESHe-ISOTOPES

Dating of Geothermal FluidsDating of Geothermal Fluids

Time elapsed between Time elapsed between Recharge-Recharge-DischargeDischarge or or Recharge-SamplingRecharge-Sampling points (subsurface residence residence points (subsurface residence residence time)time)

–33H methodH method

–33H-H-33He methodHe method

TRITIUM (TRITIUM (33H)H)33H = radioactive isotope of Hydrogene (with a short half-life)H = radioactive isotope of Hydrogene (with a short half-life)33H H formsforms

Reaction of Reaction of 1414N iN isotope (in the atmosphere) withsotope (in the atmosphere) with cosmic rayscosmic rays

141477N + n N + n 33

11H + H + 121266C C

Nuclear testingNuclear testing

33H concentrationH concentration TTritium ritium UUnitnit ( (TUTU) ) 1 TU = 1 TU = 1 atom 1 atom 33H / 10H / 101188 atom H atom H

33H H 33He + He + – Half-life = 12.26 yearHalf-life = 12.26 year– Decay constant (Decay constant () = 0.056 y) = 0.056 y-1-1

33H – Dating MethodH – Dating Method

33H concentration level in the atmosphere has H concentration level in the atmosphere has shown large changesshown large changes

– İn between 1950s and 1960s (before and İn between 1950s and 1960s (before and after the nuclear testing)after the nuclear testing)

– Particularly in the northern hemisphereParticularly in the northern hemisphere

Before 1953 : 5-25 TUBefore 1953 : 5-25 TU

In 1963 : In 1963 : 3000 TU3000 TU


33H-concentration in groundwater < 1.1 TUH-concentration in groundwater < 1.1 TU Recharge by precipitations older than nuclear testingRecharge by precipitations older than nuclear testing

33H-concentration in groundwater > 1.1 TUH-concentration in groundwater > 1.1 TU Recharge by precipitations younger than nuclear testingRecharge by precipitations younger than nuclear testing

N=NN=N00ee--t 3t 3HH0 0 (before 1963) (before 1963) 10 TU 10 TU 33H= H= 33HH00ee--t t = 0.056 y = 0.056 y-1-1

t = 2003-1963 = 40 yearst = 2003-1963 = 40 years

33H H 1.1 TU 1.1 TU


APPARENT AGEAPPARENT AGE

33H= H= 33HH00ee--tt

33H = measured at sampling pointH = measured at sampling point33HH00 = measured at recharge point = measured at recharge point (assumed to be the initial tritium concentration)(assumed to be the initial tritium concentration)

= 0.056 y= 0.056 y-1-1

t = apparent aget = apparent age

33H – H – 33He He Dating MethodDating Method

33HeHe = = 33HH00 – – 33H H (D = N(D = N00-N)-N)

33H= H= 33HH0 0 ee--t t (N =(N = NN00e e --tt))

33HH00= = 33H eH ett

33HeHe = = 33H eH et t -- 33HH = = 33H (eH (et t –– 1)1)

t = 1/t = 1/ * ln ( * ln (33He/He/33H + 1) H + 1)

33He & He & 33H – present-day concentrations measured in water sampleH – present-day concentrations measured in water sample

Geothermometry ApplicationsGeothermometry ApplicationsIsotope Fractionation – Temperature Dependent Isotope Fractionation – Temperature Dependent

Stable isotope compositions Stable isotope compositions

utilized in utilized in Reservoir TemperatureReservoir Temperature estimation estimation

Isotope geothermometersIsotope geothermometers

– Based on:Based on: isotope exchange reactions between phases isotope exchange reactions between phases in natural systemsin natural systems

(phases: watre-gas, vapor-gas, water-mineral.....)(phases: watre-gas, vapor-gas, water-mineral.....)

– Assumes:Assumes: reaction is at equilibrium at reservoir reaction is at equilibrium at reservoir conditionsconditions

Isotope GeothermometersIsotope Geothermometers

12CO2 + 13CH4 = 13CO2 + 12CH4 (CO2 gas - methane gas)

CH3D + H2O = HDO + CH4 (methane gas – water vapor)

HD + H2O = H2 + HDO (H2 gas – water vapor)

SS1616OO4 4 + H+ H221818O = SO = S1818OO4 4 + H+ H22

1616OO (dissolved sulphate-(dissolved sulphate-

water)water)

1000 ln 1000 ln (SO4 – H2O)(SO4 – H2O) = 2.88 x 10 = 2.88 x 1066/T/T22 – –

4.14.1(T = degree Kelvin = K(T = degree Kelvin = K ))

Isotope GeothermometersIsotope Geothermometers

Regarding the relation between mineralization Regarding the relation between mineralization and hydrothermal activitiesand hydrothermal activities

– Mineral Isotope GeothermometersMineral Isotope GeothermometersBased on the isotopic equilibrium between Based on the isotopic equilibrium between the coeval mineral pairsthe coeval mineral pairs

Most commonly used isotopes: Most commonly used isotopes: S-isotopesS-isotopes

Suphur (S)- IsotopesSuphur (S)- Isotopes

3232S = 95.02 %S = 95.02 %3333S = 0.75 %S = 0.75 %3434S = 4.21 %S = 4.21 %3636S = 0.02 %S = 0.02 %

((3434S/S/3232S)S)samplesample- (- (3434S/S/3232S)S)std.std.

3434S (S () = ) = -------------------------------------------- x 10-------------------------------------------- x 1033

((3434S/S/3232S)S)samplesample

Std.= CD Std.= CD

=S-isotope composition of troilite (FeS) phase in=S-isotope composition of troilite (FeS) phase in C Canyonanyon D Diabloiablo MeteoriteMeteorite

S-Isotope GeothermometerS-Isotope Geothermometer

3434S = S = 3434SS(mineral 1) (mineral 1) - - 3434SS(mineral 2) (mineral 2)

3434SS = = 3434SS= A (10= A (1066/T/T22) + B) + B

Pyrite-Galena

800 400 200 150 100 50

0

4

8

12

4

8

0

00 2 4

4

2

6 8 10 12

Sphalerite-Galena

Pyrite-Sphalerite

Temperature C0

10 / T ( K )6 2 -20

geochemistry of geothermal systems geochemistry of geothermal systems

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