Isotope Analysis of Geothermal Waters

Hans-Wolfgang Hubberten

Alfred Wegener Institute for Polar and Marine Research, Telegrafenberg A43, D-14473Potsdam, Germany

1. Introduction

Stable isotopes are one of the most useful tools for the study of geothermal waters.Especially the variation of the stable isotopes of Hydrogen (H) and Oxygen (O), the twocomponents building up the water molecule, provide important information on thegenesis of hydrothermal solutions. Understanding of the isotopic fractionation occurringalong processes such as evaporation, condensation, precipitation, infiltration, andothers, stable isotopes tell us a long story on the history of hydrothermal watersespecially, when applied together with other geochemical parameters. The aim of thislecture is to show the basic principles of isotope fractionation, the different isotopeeffects occurring in the water cycle, and the application of stable isotopes to the study ofgeothermal waters.

2. Basic principles of isotope geochemistry

Isotopes are defined as atoms whose nuclei contain the same number of protons but adifferent number of neutrons.

It is usual to denote isotopes in the form of

Mass number A 16 17 18 O O O

Number of protons Z 8 8 8

1 2or H D

1 1

Where A = Z + N (number of neutrons)

It is convenient to write 16O 17O 18O

or H D

Isotopes can be divided into stable and unstable (radioactive) species. This paperconcentrates on stable isotopes only, mainly on the stable isotopes of H and O.

DEFINITION OF ISOTOPE EFFECTSIsotope fractionations are the result of isotopes´ differences of an element in theirphysical and chemical properties that are caused by the mass differences of theseisotopes. As a general rule it is assumed, that the number of electrons of an element isresponsible for its chemical behavior, whereas the core is responsible for the physicalproperties (Table 1)

Constant H216O D2

16O H218O

Density (20°, in g cm-1) 0.9979 1.1051 1.1106Melting point (760 Torr, in °C) 0.00 3.81 0.28Boiling point (760 Torr, in °C) 100.00 101.42 100.14

Table 1: Characteristic constants of H216O, D2

16O, and H218O (after Hoefs, 1997).

In Table 1, significant differences can be observed for the physical properties of light orheavy isotopes. In the context of the water cycle, differences in vapor pressure forexample may lead to significant fractionation during evaporation from water (ocean,lakes, etc.).

THEORY OF ISOTOPE EFFECTSDifferences in physicochemical properties of isotopes arise as a result of quantummechanical effects. This is shown schematically in Fig. 1, where the energy of adiatomic molecule is plotted as a function of the distance between the two atoms.According to the quantum theory, the molecule cannot retain any energy on thecontinuous curve, but is restricted to certain discrete energy levels. The lowest level isnot at the minimum of the energy curve, but above of it by an amount of 1/2hν, where his the Planck’s constant, ν is the vibration frequency in which the atoms of themolecule vibrate with respect to each other. The vibration frequency of a moleculedepends inversely on the mass of the atoms. Therefore, different isotopic species, i.e.different masses will have different zero point energies. This results in different zeropoint energy levels as shown in Fig. 1, with a higher energy level for the light isotope(EL) and the lower one for the heavy isotope (EH). It is obvious from this figure that thebonds of the lighter isotope is weaker than that of the heavier, and this results in the factthat molecules bearing the light isotope will react slightly more readily than those withthe heavier isotope. In general, during chemical reactions, the lighter isotopes reactfaster than the heavier.

ISOTOPE FRACTIONATION PROCESSESDue to the differences in physicochemical properties transitions or transformations ofisotopic substances are accompanied by isotope fractionation effects. The partitioning ofisotopes between different substances is called isotope fractionation. It is conventionalto distinguish between- equilibrium isotope exchange, and- kinetic isotope exchange

Fig. 1: Schematic potential energy curve for the interaction of two atoms in a molecule(after Bigeleisen 1965)

Equilibrium isotope exchangeEquilibrium isotope exchange reactions can be described similar to chemicalequilibrium reactions:

aA1 + bB2 aA2 + bB1

where the subscripts indicate that species A and B contain either the light (1) or theheavy (2) isotope as for example

H232S + 34SO2 H2

34S + 32SO2

Fractionation factor αIsotope reactions at equilibrium are described by the fractionation factor α which isdefined as the ratio of the numbers of any two isotopes in one chemical compound Adivided by the corresponding ratio for another chemical compound B

RA

αA-B = ------- RB

as for example

H218O + 1/3CaC16O3 ‡ H2

16O + 1/3CaC18O3

(18O/16O)CaCO3

αCaCO3 – H2O = --------------------- = 1.032 at 25°C(18O/16O)H2O

If the isotopes are randomly distributed over all possible positions in the compounds Aand B, a is related to the equilibrium constant K by

α = K1/n

n is the number of exchanged atoms. If only one atom is exchanged α = K

Fig. 2: Fractionation factor α for H/D in the system H2O – H2 versus temperature (afterRichet et al., 1977)

Temperature dependence of αThe temperature dependence of the fractionation factor α is shown in Fig. 2. In thisgraph α is shown on the left side in relation to the temperature at isotopic equilibrationbetween water and hydrogen gas. The various curves result from different methods ofcalculation. It can be seen, that the fractionation becomes greater with lowertemperatures and vice versa. From this temperature dependence of the isotopefractionation it can be seen that equilibrium isotope fractionation can serve as ageological thermometer

Kinetic isotope fractionationKinetic isotope fractionation occurs during unidirectional reactions in which thereaction product always shows a lighter isotope composition compared to the initialeducts. This is the result of the fact that the bonds of the lighter isotopes are more easilybroken than those of the heavier (compare Fig. 1).Kinetic isotope effects are of high importance for example in evaporation, diffusion andbiological processes.

The δ−valueIn isotope geochemistry the isotope ratios are usually not presented as absolute valuesbut in the form of a δ-value. The isotope composition of two substances A and Bmeasured in the laboratory are expressed in relation to a standard

RA

δA = (---------- - 1) x 103 (‰) RSt

and

RB

δB = (---------- - 1) x 103 (‰) RSt

Where RA is the isotope ratio of substance A and RB that of substance B. RSt is theisotope ratio of a standard.

δ-values are related to α- and ∆-values by the approximation

δA – δ B ∆A-B 103 lnαA-B

In practice, this means that approximations are excellent for differences in δ-values ofless than 10, i.e. α-values and differences in δ-values can directly be compared.

3. Isotopes, standards, occurrences, fractionation processes

HYDROGEN

Isotopes 1H = 99.9844 %2D = 0.0156 %(3H = Tritium, radioactive isotope with T1/2 = 12.5 years)

For stable isotope measurements the ratio of H/D is used. The radioactive isotopetritium is very useful in hydrology and in dating of younger geothermal waters but willnot be considered in this lecture.

Standards V-SMOW = 0 ‰Per definition the isotopic composition of V-SMOW (Vienna-Standard Mean Ocean Water) is taken as the zero point of the δ-reference scale.SLAP = - 428 ‰SLAP (Standard Light Antarctic Precipitation) is used as acalibration standard of the δ-scale

Occurrence H2O, OH-, H2, CH4

In the hydrosphere and lithosphere (crystal water and OH- groupsof clay minerals)

Processes Differences in vapor pressure (evaporation, precipitation)Equilibrium fractionation (p.e. H2O – H 2, H2O – CH 4)Kinetic fractionation (diffusion, bacteriogenic processes)

OXYGEN

Isotopes 16O = 99.763 %17O = 0.0375 %18O = 0.1995 %

For isotope hydrology the ratio of 16O and 18O are commonly used.

Standards V-SMOW = 0 ‰SLAP = - 55.5 ‰(V-PDB = 30.86 ‰)V-PDB is mainly used for carbonates and paleoclimate research.It can be converted to V-SMOW byδ18O(V-SMOW) = 1.03086 (δPDB) + 30.86

Occurrence H2O, OH-, CaCO3, SilicatesIn the hydrosphere and lithosphere

Processes Differences in vapor pressure (evaporation, precipitation)Equilibrium fractionation (e.g., H2O – carbonates)Kinetic fractionation (diffusion, bacteriogenic processes,photosynthesis)

4. Isotopes in the water cycle

General: The water cycle is considered as a system in which the water is continuously inmovement because of processes such as evaporation, condensation, precipitation,infiltration, ground water movement, river run off, etc., i.e. the water is again and againreinvolved in the process.Isotope fractionation occurs in all steps of the water cycle. The most importantfractionation processes are due to the difference in vapor pressure of the isotopesforming the water, i.e. the isotopes of H and O.

H216O - lower vapor pressure

D218O – higher vapor pressure

It follows: the light isotopes are enriched in the vapor phase – the remaining water isrelatively enriched in the heavy isotopes, i.e. is isotopically heavy

The isotopic fractionation between water and water vapor depends on the temperature asshown in Fig. 3. At lower temperature the fractionation is higher than at highertemperatures. Note that α serves as an approximation of the δ-difference between bothphases (remember: δA – δ B ∆A-B 103 lnαA-B), i.e. that for example at 20°C thedifference in O-isotopes is about 8‰ and in H-isotopes about 90‰.

T (°C)

Fig. 3: Isotope fractionation of hydrogen and oxygen between water and water vapor(after GAT 1981)

A. OCEAN WATERPer definition the mean isotopic composition of H and O in ocean water has a value of 0‰. The isotope variation of oxygen in ocean water is shown in Fig. 4 in relation to thesalinity. As can be seen, the total isotopic variation of about 2 ‰ is rather small andshow generally a good correlation between the oxygen isotopes and the salinity, the socalled North Atlantic Mixture line.

Fig. 4: δ18O of ocean water in relation to the salinity (from Magaritz and Gat, 1981)

The strong correlation between the isotopic composition of oxygen (hydrogen issimilar) and salinity shows two exceptions:- Isotopically heavy waters occur in areas with very high evaporation (evaporationis higher than river inflow) as for example the Persian Gulf, the Red Sea or theMediterranean Sea.- Isotopically light waters occur either in areas, where large rivers are entering theSea (Amazon, Lena), or at areas where large ice masses are melting (Greenland,Antarctic Ice shelfs).

B. EVAPORATION AND PRECIPITATIONAs shown above, during the evaporation of ocean water the light isotopes (H, 16O) arealways enriched in the vapor phase. This process is illustrated in Fig. 5 for the O isotopefractionation (the process is similar for H). The process starts with ocean water havingan isotopic composition of 0 ‰. During evaporation the lighter isotope 16O is enrichedin the vapor and transported in clouds towards the continent. When the vaporcondensates the precipitating water (rain, snow) is heavier than the vapor whichbecomes isotopically lighter than the ocean water. Due to the extraction of heavy water,the remaining cloud becomes more negative. When the vapor in the cloud condensates asecond time, the precipitation is lighter than the first one and the remaining vaporbecomes even lighter. This process continues until all vapor is condensed and the clouddisappears. The principal result of this process is that precipitation becomes isotopicallylighter the larger the distance is from the coast or the further the cloud has moved into acontinent.

This process is called “the continental effect”

Fig. 5: Schematic fractionation in the atmospheric water cycle (Siegenthaler, 1979)

As mentioned above, condensation and precipitation leads to an enrichment of theheavier isotope in the precipitation. The basic principles of this fractionation are shownin Fig. 6. Two important principles can be derived from this figure. The first is the factthat the fractionation from the beginning of the condensation can be described as a“Raleigh-fractionation” process which leads to extreme negative isotope values of theprecipitation far inland of a continent. The second is that this fractionation is

temperature dependent, the colder the temperature the higher is the fractionationbetween vapor and condensate.

Fig. 6: Raleigh-fractionation between vapor and condensate within a cloud (afterDansgaard, 1964).

From both processes, i.e. evaporation and condensation we could see, that thetemperature plays an important role in the isotopic composition of the precipitation.This effect becomes obvious, if we look on the O isotopic composition of precipitationat coastal stations compared to the mean annual air temperature that is shown in Fig. 7.It is obviously that at stations with rather high air temperature like Barbados theprecipitation is only slightly negative, whereas stations in middle Europe like Dublin orCopenhagen show values of about – 10 ‰. More negative isotope values are commonin Polar regions ranging from – 15 to – 35 ‰. The extreme is from the South Pole witha δ18O of – 50 ‰. Here, however, the continental effect plays an important role inaddition to the temperature.

The systematic variation of the isotopes with temperature is called the “temperatureeffect”.

Fig. 7: Relation between the mean annual air temperature and the oxygen isotope valuesin precipitation from coastal stations (Dansgaard, 1964).

The temperature effect in precipitation can also be observed when looking on theisotope variation of precipitation with respect to the mean monthly temperature asshown in Fig. 8 for the city of Vienna.

The variation of temperature over 10 years which is shown in the upper graphcorrespond nicely with the isotopic of H and O shown below, i.e. summer and winterprecipitation can be distinguished with the help of stable isotopes.

Fig. 8: Seasonal variability of oxygen and hydrogen isotope ratios in precipitation withrespect to the mean monthly temperatures of Vienna (Yurtsever and Gat, 1981).

In addition to the continental effect and the temperature effect, a so called “altitudeeffect” has been defined. This effect describes the dependence of the isotopecomposition of precipitation in high altitude recharge areas of mountains. The higherthe recharge altitude, the lighter (negative) is the isotope ratio of H and O.This effect is nicely shown in Fig. 9 in which the isotope values of precipitation in theEuropean Alpes are shown for different altitudes.

Fig. 9: Oxygen isotope values of precipitation in the Alpes (Pearson et al., 1991)

The meteoric water line (MWL)Usually isotopic fractionation during evaporation, condensation and precipitation isproportional for hydrogen and oxygen. The relation between both isotope fractionationsis described by the global meteoric water line which is defined by

δD = 8δ18O +10

Deviations from the meteoric water line are due to special local evaporation or mixingphenomena and are described by the so called d-excess, the Deuterium excess.

C. RIVERS, LAKES, GROUNDWATER

The isotope composition of lake, river and ground waters is mainly caused by theisotope composition of the precipitation. The composition of the precipitation may be

altered by processes such as evaporation, mixing, exchange with minerals and others.These resulting trends can be seen in Fig. 10.

Fig. 10: H and O isotope ratios in precipitation and deviation from the meteoric waterline by evaporation and exchange with minerals (Fontes, 1980).

Large lakes very often show the isotope composition of the mean annual precipitation ofthe region. Smaller lakes are more sensitive to inter annual variations and often reflectthe summer-winter variation of the precipitation. Lakes in areas with strongerevaporation and smaller amounts of summer precipitation are shifted isotopically toheavy compositions due to the preferred evaporation of the light isotopes.The local precipitation is also reflected in the isotope composition of rivers that may bechanged by mixing with rivers and/or groundwater.The isotope composition of groundwater is controlled by the isotope composition ofinfiltrating precipitation, river and lake water. Depending on the residence time of thegroundwater, a mixture with other waters (for example older river bank filtrate) or thereaction with minerals may significantly alter the isotope composition. Deeperpenetrating groundwater may be heated due to geothermal heat flow or local heatanomalies (volcanoes, rift zones, etc.) and react more intensively with minerals of therocks or sediments. These processes may significantly change the isotopic compositionof these geothermal waters.

5. Isotopes in geothermal systems

When we talk about geothermal waters, we talk almost exclusively about meteoricwaters, i.e. surface waters that penetrated into the underground and were warmed up byan underground heat source. This is schematically shown in Fig, 11. In this example theheat source is a near surface magma chamber. Meteoric waters are entering the groundand are heated to a certain extend depending on the depth and the distance to the heatsource. They react with the sediments they are penetrating and leach minerals fromthem. If they are coming into a closer contact with the volcano, they can mix withmagmatic fluids and gases. Without the direct presence of volcanoes, geothermal watersare formed by deeper penetrating meteoric waters in regions with higher geothermalheat flow as, for example, rift zones or formerly active areas. Depending on theseprocesses the composition and temperature of the waters reaching the surface as thermalsprings are defined.

Fig. 11: Scheme of the formation of geothermal waters using a volcano as heat source

Based on the assumption that geothermal waters are of meteoric origin, we should findthe typical signals of meteoric waters also in the geothermal waters. In the followingchapter, we will see wether this is the case and will learn what kind of processes canchange the initial isotope composition of waters.

For the formation of geothermal waters, apart from meteoric waters only one heatsource is needed. Geothermal waters occur worldwide either related to subduction zones