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GEOCHEMISTRY OF GEOTHERMAL SYSTEMS. WATER CHEMISTRY. Chemical composition of waters is expressed in terms of major anion and cation contents. Major Cations: Na + , K + , Ca ++ , Mg ++ Major Anions: HCO 3 - (or CO 3 = ), Cl - , SO 4 = HCO 3 -  dominant in neutral conditions

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water chemistry
WATER CHEMISTRY
  • Chemical composition of waters is expressed in terms of major anion and cation contents.
  • Major Cations: Na+, K+, Ca++, Mg++
  • Major Anions: HCO3- (or CO3=), Cl-, SO4=
    • HCO3- dominant in neutral conditions
    • CO3=  dominant in alkaline (pH>8) conditions
    • H2CO3  dominant in acidic conditions
  • Also dissolved silica (SiO2) in neutral form

as a major constituent

  • Minor constituents: B, F, Li, Sr, ...
water chemistry3
WATER CHEMISTRY
  • concentration of chemical constituents are expressed in units of
    • mg/l (ppm=parts per million)

(mg/l is the preferred unit)

    • Molality

Molality = no. of moles / kg of solvent

No.of moles = (mg/l*10-3)/ formula weight

water chemistry4
WATER CHEMISTRY
  • Errors associated with water analyses are expressed in terms of CBE (Charge Balance Error)

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

where,

mc is the molality of cation

ma is the molality of anion

z is the charge

  • If CBE  5%, the results are appropriate to use in any kind of interpretation
the constituents encountered in geothermal fluids
The constituents encountered in geothermal fluids

TRACERS

Chemically inert, non-reactive, conservative constituents

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

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

GEOINDICATORS

Chemically reactive, non-conservative species

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

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

water chemistry6
WATER CHEMISTRY
  • In this chapter, the main emphasis will be placed on the use of water chemistry in the determination of :
    • underground (reservoir) temperatures : geothermometers
    • boiling and mixing relations (subsurface physico-chemical processes)
hydrothermal reactions
HYDROTHERMAL REACTIONS
  • The composition of geothermal fluids are controlled by : temperature-dependent reactions between minerals and fluids
  • The factors affecting the formation of hydrothermal minerals are:
    • temperature
    • pressure
    • rock type
    • permeability
    • fluid composition
    • duration of activity
slide8
The effect of rock type --- most pronounced at low temperatures & insignificant above 280C

Above 280C and at least as high as 350C, the typical stable mineral assemblages (in active geothermal systems) are independent of rock type and include

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

At lower temperatures, ZEOLITES and CLAY MINERALS are found.

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

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

slide9

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 reservoir temperatures

ESTIMATION OF RESERVOIR TEMPERATURES

The evaluation of the reservoir temperatures for geothermal systems is made in terms of GEOTHERMOMETRY APPLICATIONS

geothermometry applications12
GEOTHERMOMETRY APPLICATIONS
  • One of the major toolsfor the

exploration & development

of geothermal resources

geothermometry
GEOTHERMOMETRY
  • estimation of reservoir (subsurface) temperatures
  • using
    • Chemical & isotopic compositionofsurface dischargesfrom
      • wellsand/or
      • natural springs/fumaroles
geothermometers
GEOTHERMOMETERS
  • CHEMICAL GEOTHERMOMETERS
    • utilize the chemical composition
      • silica and major cation contents of water discharges
      • gas concentrations or relative abundances of gaseous components in steam discharges
  • ISOTOPIC GEOTHERMOMETERS
    • based on the isotope exchange reactions between various phases (water, gas, mineral) in geothermal systems
focus of the course
Focus of the Course
  • CHEMICAL GEOTHERMOMETERS
  • As applied to water discharges

PART I. Basic Principles & Types

PART II. Examples/Problems

chemical geotheromometers

CHEMICAL GEOTHEROMOMETERS

PART I.Basic Principles &Types

basic principles
BASIC PRINCIPLES

Chemical Geothermometers are

  • developed on the basis of temperature dependent chemical equilibrium between the water and the minerals at the deep reservoir conditions
  • based on the assumption that the water preserves its chemical composition during its ascent from the reservoir to the surface
basic principles18
BASIC PRINCIPLES
  • Studies of well discharge chemistry and alteration mineralogy
    • the presence of equilibrium in several geothermal fields
    • the assumption of equilibrium is valid
basic principles19
BASIC PRINCIPLES
  • Assumption of the preservation of water chemistry may not always hold

Because the water composition may be affected by processes such as

    • cooling
    • mixing with waters from different reservoirs.
basic principles20
BASIC PRINCIPLES
  • Cooling during ascent from reservoir to surface:
    • CONDUCTIVE
    • ADIABATIC
basic principles21
BASIC PRINCIPLES

CONDUCTIVE Cooling

  • Heat loss while travelling through cooler rocks

ADIABATIC Cooling

  • Boiling because of decreasing hydrostatic head
basic principles22
BASIC PRINCIPLES
  • Conductive cooling
    • does not by itself change the composition of the water
    • but may affect its degree of saturation with respect to several minerals
    • thus, it may bring about a modification in the chemical composition of the water by mineral dissolution or precipitation
basic principles23
BASIC PRINCIPLES
  • Adiabatic cooling (Cooling by boiling)
    • causes changes in the composition of ascending water
    • these changes include
      • degassing, and hence
      • the increase in the solute content as a result of steam loss.
basic principles24
BASIC PRINCIPLES

MIXING

  • affects chemical composition
  • since the solubility of most of the compounds in waters increases with increasing temperature, mixing with cold groundwaterresults in thedilution of geothermal water
slide25
Geothermometry applications are not simply inserting values into specific geothermometry equations.

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

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

types of chemical geothermometers
TYPES OF CHEMICAL GEOTHERMOMETERS
  • SILICA GEOTHERMOMETERS
  • CATION GEOTHERMOMETERS (Alkali Geothermometers)
silica geothermometers
SILICA GEOTHERMOMETERS
  • based on the
    • experimentally determined
    • temperature dependent

variation in the solubility of silica in water

  • Since silica can occur in various forms in geothermal fields (such as quartz, crystobalite, chalcedony, amorphous silica) different silica geothermometers have been developed by different workers
silica geothermometers29
SILICA GEOTHERMOMETERS

The followings should be considered :

  • temperature rangein which the equations are valid
  • effects of steam separation
  • possibleprecipitation of silica
    • before sample collection

(during the travel of fluid to surface, due to silica oversaturation)

    • after sample collection

(due to improper preservation of sample)

  • effects of pH on solubility of silica
  • possiblemixingof hot water with cold water
silica geothermometers30
SILICA GEOTHERMOMETERS

Temperature Range

  • silica geothermometers are valid for temperature ranges up to 250 C
  • above 250C, the equations depart drastically from the experimentally determined solubility curves
silica geothermometers temperature range
SILICA GEOTHERMOMETERSTemperature Range

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

At low T (C) 

qtz less soluble

amorph. silica more soluble

Silica solubility is controlled by amorphous silicaat low T (C)quartz at high T (C)

silica geothermometers effects of steam separation
SILICA GEOTHERMOMETERSEffects of Steam Separation
  • Boiling  Steam Separation
  • volume of residual liquid
  • Concentration in liquid
  • Temperature Estimate

e.g.

T =1309 / (5.19 – log C) - 273.15

C = SiO2 in ppm

increase in C (SiO2 in water > SiO2 in reservoir)

decrease in denominator of the equation

increase in T

  • for boiling springs

boiling-corrected geothermometers

(i.e. Quartz-max. steam loss)

silica geothermometers silica precipitation
SILICA GEOTHERMOMETERSSilica Precipitation
  • SiO2
  • Temperature Estimate

e.g.

T =1309 / (5.19 – log C) - 273.15

C = SiO2 in ppm

decrease in C (SiO2 in water < SiO2 in reservoir)

increase in denominator

decrease in T

silica geothermometers effect of ph
SILICA GEOTHERMOMETERSEffect of pH

Fig. 2.Calculated effect of pHupon the solubility of quartz at various temperatures from 25 C to 300 C , using experimental data of Seward (1974). The dashed curve shows the pH required at various temperatures to achieve a 10% increase in quartz solubility compared to the solubility at pH=7.0 (from Fournier, 1981).

  • pH 
  • Dissolved SiO2  (for pH>7.6)
  • Temperature Estimate

e.g.

T =1309 / (5.19 – log C) - 273.15

C = SiO2 in ppm

increase in C

decrease in denominator of the equation

increase in T

silica geothermometers effect of mixing
SILICA GEOTHERMOMETERSEffect of Mixing
  • Hot-Water High SiO2 content
  • Cold-Water  Low SiO2 content

(Temperature  Silica solubility )

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

e.g.

T =1309 / (5.19 – log C) - 273.15

C = SiO2 in ppm

decrease in C

increase in denominator of the equation

decrease in T

silica geothermometers36
SILICA GEOTHERMOMETERS

Process Reservoir Temperature

  • Steam Separation  Overestimated
  • Silica Precipitation  Underestimated
  • Increase in pH  Overestimated
  • Mixing with cold water  Underestimated
cation geothermometers alkali geothermometers
CATION GEOTHERMOMETERS (Alkali Geothermometers)
  • based on the partitioning of alkalies between solid and liquid phases

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

  • majority of are empirically developed geothermometers
    • Na/K geothermometer
    • Na-K-Ca geothermometer
    • Na-K-Ca-Mg geothermometer
    • Others(Na-Li, K-Mg, ..)
cation geothermometers na k geothermometer
CATION GEOTHERMOMETERSNa/K Geothermometer
  • Fig.3. Na/K atomic ratios of well discharges plotted at measured downhole temperatures. Curve A is the least square fit of the data points above 80 C. Curve B is another empirical curve (from Truesdell, 1976). Curves C and D show the approximate locations of the low albite-microcline and high albite-sanidine lines derived from thermodynamic data (from Fournier, 1981).
cation geothermometers na k geothermometer40
CATION GEOTHERMOMETERSNa/K Geothermometer
  • gives good results for reservoir temperatures above 180C.
  • yields erraneous estimates for low temperature waters
    • temperature-dependent exchange equilibrium between feldspars and geothermal waters is not attained at low temperatures andthe Na/K ratio in these waters are governed by leaching rather than chemical equilibrium
  • yields unusually high estimates for waters having high calcium contents
cation geothermometers na k ca geothermometer42
CATION GEOTHERMOMETERSNa-K-Ca Geothermometer
  • Works well for CO2-rich or Ca-rich environments provided that calcite was not deposited after the water left the reservoir

in case of calcite precipitation

Ca 

1647

T= --------------------------------------------------------- - 273.15

log(Na/K)+(log(Ca/Na)+2.06)+2.47

Decrease in Ca concentration (Ca in water < Ca in reservoir)

decrease in denominator of the equation

increase in T

  • For waters with high Mg contents, Na-K-Ca geothermometer yields erraneous results. For these waters, Mg correction is necessary
cation geothermometers na k ca mg geothermometer44
CATION GEOTHERMOMETERSNa-K-Ca-Mg Geothermometer

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

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

underground mixing of hot and cold waters
UNDERGROUND MIXING OF HOT AND COLD WATERS

Recognition of Mixed Waters

  • Mixing of hot ascending waters with cold waters at shallow depths is common.
  • Mixing also occurs deep in hydrothermal systems.
  • The effects of mixing on geothermometers is already discussed in previous section.
  • Where all the waters reaching surface are mixed waters, recognition of mixing can be difficult.
  • The recognition of mixing is especially difficult if water-rock re-equilibration occurred after mixing (complete or partial re-equilibration is more likely if the temperatures after mixing is well above 110 to 150 C, or if mixing takes place in aquifers with long residence times).
underground mixing of hot and cold waters46
UNDERGROUND MIXING OF HOT AND COLD WATERS

Some indications of mixing are as follows:

  • systematic variations of spring compositions and measured temperatures,
  • variations in oxygen or hydrogen isotopes,
  • variations in ratios of relatively *conservative elements that do not precipitate from solution during movement of water through rock (e.g. Cl/B ratios).
silica enthalpy mixing model
SILICA-ENTHALPY MIXING MODEL
  • Dissolved silica content of mixed waters can be used to determine the temperature of hot-water component .
  • Dissolved silica is plotted against enthalpy of liquid water.
  • Although temperature is the measured property, and enthalphy is a derived property, enthalpy is used as a coordinate rather than temperature. This is because the combined heat contents of two waters are conserved when those waters are mixed, but the combined temperatures are not.
  • The enthalpy values are obtained from steam tables.
silica enthalpy mixing model48
SILICA-ENTHALPY MIXING MODEL

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

silica enthalpy mixing model49
SILICA-ENTHALPY MIXING MODEL

A = non-thermal component

(cold water)

B, D = mixed, warm water

springs

C = hot water component at

reservoir conditions

(assuming no steam

separation before mixing)

E = hot water component at

reservoir conditions

(assuming steam separation

before mixing)

Boiling

T = 100 C

Enthalpy = 419 J/g

(corresponds to D in the graph)

Enthalpy values (at corresponding temperatures)

are found from Steam Table in Henley et al.(1984)

silica enthalpy mixing model steam fraction did not separate before mixing
SILICA-ENTHALPY MIXING MODELSteam Fraction did not separate before mixing
  • The sample points are plotted.
  • A straight line is drawn from the point representing the non-thermal component of the mixed water (i.e. the point with the lowest temperature and the lowest silica content = point A in Fig.), through the mixed water warm springs (points B and D in Fig.).
  • The intersection of this line with the qtz solubility curve (point C in Fig.) gives the enthalpy of the hot-water component (at reservoir conditions).
  • From the steam table, the temperature corresponding to this enthalpy value is obtained as the reservoir temperature of the hot-water component.
silica enthalpy mixing model steam separation occurs before mixing
SILICA-ENTHALPY MIXING MODELSteam separation occurs before mixing
  • The enthalpy at the boling temperature (100C) is obtained from the steam tables (which is 419 j/g)
  • A vertical line is drawn from the enthalpy value of 419 j/g
  • From the inetrsection point of this line with the mixing line (Line AD), a horizantal line (DE) is drawn.
  • The intersection of line DE with the solubility curve for maximum steam loss (point E) gives the enthalpy of the hot-water component.
  • From the steam tables, the reservoir temperature of the hot-watercomponent is determined.
silica enthalpy mixing model52
SILICA-ENTHALPY MIXING MODEL
  • In order for the silica mixing model to give accurate results, it is vital that no conductive cooling occurred after mixing. If conductive cooling occurred after mixing, then the calculated temperatures will be too high (overestimated temperatures). This is because:
  • the original points before conductive cooling should lie to the right of the line AD (i.e. towards the higher enthalpy values at the same silica concentrations, as conductive cooling will affect only the temperatures, not the silica contents)
  • in this case, the intersection of mixing line with the quartz solubility curve will give lower enthalpy values (i.e lower temperatures) than that obtained in case of conductive cooling.
  • in other words, the temperatures obtained in case of conductive cooling will be higher than the actual reservoir temperatures (i.e. if conductive cooling occurred after mixing, the temperatures will be overestimated)
silica enthalpy mixing model53
SILICA-ENTHALPY MIXING MODEL
  • Another requirement for the use of enthalpy-silica model is that no silica deposition occurred before or after mixing. If silica deposition occurred, the temperatures will be underestimated. This is because:
  • the original points before silica deposition should be towards higher silica contents (at the same enthalpy values)
  • in this case, the intersection point of mixing line with the silica solubility curve will have higher enthalpy values(higher temperatures) than that obtained in case of silica deposition
  • in other words, the temperatures obtained in case of no silica deposition will be higher than that in case of silica deposition (i.e. the temperatures will be underestimated in case of silica deposition)
chloride enthalpy mixing model
CHLORIDE-ENTHALPY MIXING MODEL
  • Fig.6. Enthalpy-chloride diagram for waters from Yellowstone National Park. Small circles indicate Geyser Hill-type waters and smal dots indicate Black Sand-type waters (From Fournier, 1981).
chloride enthalpy mixing model55
CHLORIDE-ENTHALPY MIXING MODEL

ESTIMATION OF RESERVOIR

TEMPERATURE

  • Geyser Hill-type Waters

A = maximum Cl content

B = minimum Cl content

C = minimum enthalpy at

the reservoir

  • Black Sand-type Waters

D = maximum Cl content

E = minimum Cl content

F = minimum enthalpy at

the reservoir

Enthalpy of steam at 100 C =

2676 J/g (Henley et al., 1984)

chloride enthalpy mixing model56
CHLORIDE-ENTHALPY MIXING MODEL

ORIGIN OF WATERS

  • N = cold water component
  • C, F = hot water components
  • F is more dilute & slightly cooler than C
  • F can not be derived from Cby process of mixing between hot and cold water (point N), because any mixture would lie on or close to line CN.
  • C and F are probably both related to a still higher enthalpy water such as point G or H.
chloride enthalpy mixing model57
CHLORIDE-ENTHALPY MIXING MODEL

ORIGIN OF WATERS

  • water C could be related to water Gby boiling
  • water C could also be related to water H

by conductive cooling

  • water F could be related to water G or water Hby mixing with cold water N
isotope studies in geothermal systems
ISOTOPE STUDIES IN GEOTHERMAL SYSTEMS
  • At Exploration, Development and Exploitation Stages
  • Most commonly used isotopes
    • Hydrogen (1H, 2H =D, 3H)
    • Oxygen (18O, 16O)
    • Sulphur (32S, 34S)
    • Helium (3He, 4He)
isotope studies in geothermal systems62
ISOTOPE STUDIES IN GEOTHERMAL SYSTEMS

Geothermal Fluids

  • Sources
    • Source of fluids(meteoric, magmatic, ..)
    • Physico-chemical processes affecting the fluid comosition
      • Water-rock interaction
      • Evaporation
      • Condensation
    • Source of components in fluids(mantle, crust,..)
  • Ages

(time between recharge-discharge, recharge-sampling)

  • Temperatures (Geothermometry Applications)
sources of geothermal fluids
Sources of Geothermal Fluids
  • Sources of Geothermal Fluids
    • H- & O- Isotopes
  • Physico-chemical processes affecting the fluid composition
    • H- & O- Isotopes
  • Sources of components (elements, compounds) in geothermal fluids
    • He-Isotopes (volatile elements)
sources of geothermal fluids stable h o isotopes
Sources of Geothermal Fluids StableH- & O-Isotopes

1H = % 99.9852

2H (D) = % 0.0148

D/H

16O = % 99.76

17O = % 0.04

18O = % 0.20

18O / 16O

sources of geothermal fluids stable h o isotopes66
Sources of Geothermal Fluids StableH- & O-Isotopes

(D/H)sample- (D/H)standard

 D () = ----------------------------------- x 103

(D/H)standard

(18O/16O)sample- (18O/16O)standard

 18O () = -------------------------------------------- x 103

(18O/16O)standard

Standard = Standard Mean Ocean Water

= SMOW

sources of geothermal fluids stable h o isotopes67
Sources of Geothermal Fluids StableH- & O-Isotopes

(D/H)sample- (D/H)SMOW

 D () = ----------------------------------- x 103

(D/H)SMOW

(18O/16O)sample- (18O/16O)SMOW

 18O () = -------------------------------------------- x 103

(18O/16O)SMOW

sources of geothermal fluids stable h o isotopes68
Sources of Geothermal Fluids StableH- & O-Isotopes

Sources of Natural Waters:

  • Meteoric Water (rain, snow)
  • Sea Water
  • Fossil Waters (trapped in sediments in sedimanary basins)
  • Magmatic Waters
  • Metamorphic Waters
physico chemical processes stable h o isotopes
Physico-Chemical Processes:Stable H- & O-Isotopes
  • Latitute 
    • dD d18O
  • Altitute from Sea level 
    • dD d18O
physico chemical processes stable h o isotopes74
Physico-Chemical Processes:Stable H- & O-Isotopes
  • Aquifers recharged by precipitation from lower altituteshigher dD - d18Ovalues
  • Aquifers recharged by precipitation fromhigher altituteslower dD - d18Ovalues

Mixing of waters from different aquifers

physico chemical processes stable h o isotopes75
Physico-Chemical Processes:Stable H- & O-Isotopes
  • Boiling and vapor separation 

dD d18O in residual liquid

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

monitoring studies in geothermal exploitation
Aquifers recharged by precipitation from lower altituteshigher dD - d18O

Aquifers recharged by precipitation fromhigher altituteslower dD - d18O

Boiling and vapor separation 

dD d18O in residual liquid

Any increase indD - d18O values 

due to sudden pressure drop in production wells

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

subsurface boiling and vapour separation

Monitoring Studies in Geothermal Exploitation
monitoring studies in geothermal exploitation77
Monitoring Studies in Geothermal Exploitation
  • Monitoring of isotope composition of geothermal fluids during exploitationcan lead to determination of, and the development of necessary precautions against
    • Decrease in enthalpy due to start of recharge from cold, shallow aquifers, or
    • Scaling problems developed as a result of subsurface boiling
scaling
(Scaling)

Vapour Separation

  • Volume of (residual) liquid 
  • Concentration of dissolved components in liquid 
  • Liquid will become oversaturated
  • Component (calcite, silica, etc.) will precipitate
  • Scaling
dating of geothermal fluids

Dating of Geothermal Fluids

3H- & 3He-ISOTOPES

dating of geothermal fluids80
Dating of Geothermal Fluids
  • Time elapsed between Recharge-Discharge or Recharge-Sampling points (subsurface residence residence time)
    • 3H method
    • 3H-3He method
tritium 3 h
TRITIUM (3H)
  • 3H = radioactive isotope of Hydrogene (with a short half-life)
  • 3H forms
    • Reaction of 14N isotope (in the atmosphere) withcosmic rays

147N + n 31H + 126C

    • Nuclear testing
  • 3H concentration
    • Tritium Unit (TU)
    • 1 TU = 1 atom 3H / 1018 atom H
  • 3H 3He + 
    • Half-life = 12.26 year
    • Decay constant () = 0.056 y-1
3 h dating method
3H – Dating Method

3H concentration level in the atmosphere has shown large changes

  • İn between 1950s and 1960s (before and after the nuclear testing)
  • Particularly in the northern hemisphere

Before 1953 : 5-25 TU

In 1963 : 3000 TU

3 h dating method83
3H – Dating Method
  • 3H-concentration in groundwater < 1.1 TU
    • Recharge by precipitations older than nuclear testing
  • 3H-concentration in groundwater > 1.1 TU
    • Recharge by precipitations younger than nuclear testing

N=N0e-t 3H0 (before 1963)  10 TU

3H= 3H0e-t  = 0.056 y-1

t = 2003-1963 = 40 years

 3H  1.1 TU

3 h dating method84
3H – Dating Method
  • APPARENT AGE

3H= 3H0e-t

3H = measured at sampling point

3H0 = measured at recharge point

(assumed to be the initial tritium concentration)

 = 0.056 y-1

t = apparent age

3 h 3 he dating method
3H – 3He Dating Method
  • 3He = 3H0 – 3H (D = N0-N)
  • 3H= 3H0 e-t (N =N0e -t)
  • 3H0= 3H et
  • 3He =3H et -3H = 3H (et –1)

t = 1/ * ln (3He/3H + 1)

3He & 3H – present-day concentrations measured in water sample

geothermometry applications86
Geothermometry Applications
  • Isotope Fractionation – Temperature Dependent
  • Stable isotope compositions 

utilized in Reservoir Temperature estimation

  • Isotope geothermometers
    • Based on: isotope exchange reactions between phases in natural systems

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

    • Assumes: reaction is at equilibrium at reservoir conditions
isotope geothermometers
Isotope 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)

S16O4 + H218O = S18O4 + H216O(dissolved sulphate-water)

1000 ln (SO4 – H2O) = 2.88 x 106/T2 – 4.1

(T = degree Kelvin = K)

isotope geothermometers88
Isotope Geothermometers
  • Regarding the relation between mineralization and hydrothermal activities
    • Mineral Isotope Geothermometers
      • Based on the isotopic equilibrium between the coeval mineral pairs
      • Most commonly used isotopes: S-isotopes
suphur s isotopes
Suphur (S)- Isotopes
  • 32S = 95.02 %
  • 33S = 0.75 %
  • 34S = 4.21 %
  • 36S = 0.02 %

(34S/32S)sample- (34S/32S)std.

 34S () = -------------------------------------------- x 103

(34S/32S)sample

Std.= CD

=S-isotope composition of troilite (FeS) phase in Canyon DiabloMeteorite

s isotope geothermometer
S-Isotope Geothermometer

34S = 34S(mineral 1) - 34S(mineral 2)

34S= 34S= A (106/T2) + B