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Solute association in hydrothermal fluid systems
164 STABLE
ISOTOPE
GEOCHEMISTRY
ROCKS AT L A R D E R E L L O
OF THE M E T A M O R P H I C
GEOTHERMAL
FIELD:
PRELIMINA-
RY RESULTS. E. PETRUCCI,
B. 1 ~ I ,
(Dott. R i c e r c a
S.M.F.
Dip. S C i e n z e
O} S U R F A C E
K.~ev~k. /G~cal Survey, Prague, Czechoslovakia/
SHEPPARD. della Terra Univ.
T H E TE~[PORARY A L K A L I Z A [ I O N
di
Rome "LB Sapienza").
A petrographic and stable isotope study has been carried out on samples of m e t a m o r p h i c rocks from the deep boreholes S. Giovanni and Monteverdi 2 drilled by the National Board of Electricity (RNEL) in the geothermal area of Monteverdi garittimo, to the west of Larderello. The rocks studied belong to the autochthonous basement of Southern Tuscany (Filladi inferiori and Micascisti Units, which in this area occur only at some hundreds meters depth). The whole rock ~ 180 values calculated for the Micascisti Unit vary from +3 to +7~; higher values were generally obtained for the Filladi inferiori Unit. The whole rock 6 D values are quite homogeneous; no clear-cut difference is noticeable between the two Units. These data suggest that both Units isotopically exchanged with a geothermal fluid of p r o b a b l e m e t e o r i c origin. H and 0 isotopic logs are presented relsting measured values with litbolo~v end mineralogical associations in order to characterize, in each well, the levels showing the largest interaction with the fluids.
The s t u d y is b a s e d on a c o m p a r i s o n a 130 s a m p l e s of s u r f a c e w a t e r s f r o m the 0 r l i c k 4 m o u n t a i n s in C z e o h o s l o v a l i a eolected in 1976 y e a r and in 1986 y e a r a ~ a i n . In t h i s t i m e a r e a w a s a f f l i c t e d by an a c i d rain. In p a r t of a r e a is o b s e r v e d alkalization. O n s b a s e a l a b o r a t o r y sim u l s t J o n of this p h e n o m e n o n a n d e v a l u a tion of a pH-a!kalinity relationship the one is e x p l a i n by a i n f l u e n c e of k i n e t i c c h a n g e s in w e a t h e r i n Z r o c k s .
SOLUTE ASSOCIATION IN H Y D R ~
FLUID S Y ~
SOLUTE THERMODYNAMIC PROPERTIES AT HIGH TEMPERATURE AND PRESSURE MODELED ON SOLVENT DIELECTRIC PROPERTIES.
T.M. SEWARD (Institut fiiz Mineralogie und Petrographie, Eq~-Zentrum, CH-8092 Ziirich, Schweiz)
J. SCHOTT, J.V. WALTHER and J.L. DANDURAND (Laboratoire de Min~ralogie et Cristallographie, Universit~ Paul Sabatier, 31062 Toulouse, France).
An understanding of mass transport and fluidmineral equilibria in hydrothermal syst6rns is premised on our knowledge of the chemistry of metal cx~nplex and ion pair equilibria at elevated te~poratures and pressures. Association equilibria are enhanced in high temperature aqueous solution,~ because of changes in ion-solvent interaction associated with expanding water structure. Ion pairing of so-called 'strong' electrolytes has been studied at T~350°C and pressures to 5 }char, however, there are few studies of ion association in the intermediate tes~perature from 25 to 300°C. Recent experimental studies desonstrate the continuum of change from barely detectable ion association at ambient temperatures • . o to appreciable fun pairing at T>250 C. For example, the ctrnulative equilibrit~n formation constant for CaCI o ex~ends from log K2= +0.24 at 150-C to +4.70 at2360vC. The existance of weak ion pairs in strong electrolyte solutions at low ter0peratares (i.e. at 25°C) has been regarded with scepticism by some workers because their presence may only he indirectly inferred. However, first row transition metal chloride complexes are also weak but may be ~tudied sp~ctrophetometrically. New data on NiCI and F e C I formation indicate stabilities similar to the HCI O ion pair. For example lo ~ K 1 for the formation of NiCI + and HCI O at . . . . . . . . . . . . . . o
Fluids in the earth's crust, in addition to the solvent, H20, often contain high concentrations of other volatiles (e.g.C02, CH4, N2) and salts (e.g. NaC1, KC1, CaCI2, MgC12, CaS04). Knowledge of the thermodynamic behavior of aqueous species in these fluids is of prime importance for understanding mineral transformations in metasomatic and metamorphic processes. Here we adopt an approach based on the dielectric constant of the solvent. The main feature on this model is to account for the non electrostatic contributions to excess free energy of aqueous species with aqueous species-solvent (Born) and short range (Br~nsted) interactions. This approach has been applied to analyse quartz solubility in H20-C02, H20-Ar, H20-KC1 and H20-NaCI mixtures at high T and P. We showed that changes in the Gibbs free energy of aqueous silica can be simply deduced from changes in the dielectric constant of the fluid mixture with an uncharged monomer model for aqueous silica. Similar approach works for other neutral species (AI(OH)3, H2S) and charged species (Na+, Ca++, OH-,...). As the dielectric constant for the mixture of H20 with other volatiles or salts can be predicted to a good accuracy to high temperatures and pressures, our approach allows excess free energy and speciation calculations in a wide variety of crustal solutions.
log KI(NiCl )= +1.72 and log KI(HCI°)= +1.51. Th~ inqportance of such data in geochemical modelling of hydrothermal mass transport and deposition will be discn/ssed.
ISOTOPE
GEOCHEMISTRY
ROCKS AT L A R D E R E L L O
OF THE M E T A M O R P H I C
GEOTHERMAL
FIELD:
PRELIMINA-
RY RESULTS. E. PETRUCCI,
B. 1 ~ I ,
(Dott. R i c e r c a
S.M.F.
Dip. S C i e n z e
O} S U R F A C E
K.~ev~k. /G~cal Survey, Prague, Czechoslovakia/
SHEPPARD. della Terra Univ.
T H E TE~[PORARY A L K A L I Z A [ I O N
di
Rome "LB Sapienza").
A petrographic and stable isotope study has been carried out on samples of m e t a m o r p h i c rocks from the deep boreholes S. Giovanni and Monteverdi 2 drilled by the National Board of Electricity (RNEL) in the geothermal area of Monteverdi garittimo, to the west of Larderello. The rocks studied belong to the autochthonous basement of Southern Tuscany (Filladi inferiori and Micascisti Units, which in this area occur only at some hundreds meters depth). The whole rock ~ 180 values calculated for the Micascisti Unit vary from +3 to +7~; higher values were generally obtained for the Filladi inferiori Unit. The whole rock 6 D values are quite homogeneous; no clear-cut difference is noticeable between the two Units. These data suggest that both Units isotopically exchanged with a geothermal fluid of p r o b a b l e m e t e o r i c origin. H and 0 isotopic logs are presented relsting measured values with litbolo~v end mineralogical associations in order to characterize, in each well, the levels showing the largest interaction with the fluids.
The s t u d y is b a s e d on a c o m p a r i s o n a 130 s a m p l e s of s u r f a c e w a t e r s f r o m the 0 r l i c k 4 m o u n t a i n s in C z e o h o s l o v a l i a eolected in 1976 y e a r and in 1986 y e a r a ~ a i n . In t h i s t i m e a r e a w a s a f f l i c t e d by an a c i d rain. In p a r t of a r e a is o b s e r v e d alkalization. O n s b a s e a l a b o r a t o r y sim u l s t J o n of this p h e n o m e n o n a n d e v a l u a tion of a pH-a!kalinity relationship the one is e x p l a i n by a i n f l u e n c e of k i n e t i c c h a n g e s in w e a t h e r i n Z r o c k s .
SOLUTE ASSOCIATION IN H Y D R ~
FLUID S Y ~
SOLUTE THERMODYNAMIC PROPERTIES AT HIGH TEMPERATURE AND PRESSURE MODELED ON SOLVENT DIELECTRIC PROPERTIES.
T.M. SEWARD (Institut fiiz Mineralogie und Petrographie, Eq~-Zentrum, CH-8092 Ziirich, Schweiz)
J. SCHOTT, J.V. WALTHER and J.L. DANDURAND (Laboratoire de Min~ralogie et Cristallographie, Universit~ Paul Sabatier, 31062 Toulouse, France).
An understanding of mass transport and fluidmineral equilibria in hydrothermal syst6rns is premised on our knowledge of the chemistry of metal cx~nplex and ion pair equilibria at elevated te~poratures and pressures. Association equilibria are enhanced in high temperature aqueous solution,~ because of changes in ion-solvent interaction associated with expanding water structure. Ion pairing of so-called 'strong' electrolytes has been studied at T~350°C and pressures to 5 }char, however, there are few studies of ion association in the intermediate tes~perature from 25 to 300°C. Recent experimental studies desonstrate the continuum of change from barely detectable ion association at ambient temperatures • . o to appreciable fun pairing at T>250 C. For example, the ctrnulative equilibrit~n formation constant for CaCI o ex~ends from log K2= +0.24 at 150-C to +4.70 at2360vC. The existance of weak ion pairs in strong electrolyte solutions at low ter0peratares (i.e. at 25°C) has been regarded with scepticism by some workers because their presence may only he indirectly inferred. However, first row transition metal chloride complexes are also weak but may be ~tudied sp~ctrophetometrically. New data on NiCI and F e C I formation indicate stabilities similar to the HCI O ion pair. For example lo ~ K 1 for the formation of NiCI + and HCI O at . . . . . . . . . . . . . . o
Fluids in the earth's crust, in addition to the solvent, H20, often contain high concentrations of other volatiles (e.g.C02, CH4, N2) and salts (e.g. NaC1, KC1, CaCI2, MgC12, CaS04). Knowledge of the thermodynamic behavior of aqueous species in these fluids is of prime importance for understanding mineral transformations in metasomatic and metamorphic processes. Here we adopt an approach based on the dielectric constant of the solvent. The main feature on this model is to account for the non electrostatic contributions to excess free energy of aqueous species with aqueous species-solvent (Born) and short range (Br~nsted) interactions. This approach has been applied to analyse quartz solubility in H20-C02, H20-Ar, H20-KC1 and H20-NaCI mixtures at high T and P. We showed that changes in the Gibbs free energy of aqueous silica can be simply deduced from changes in the dielectric constant of the fluid mixture with an uncharged monomer model for aqueous silica. Similar approach works for other neutral species (AI(OH)3, H2S) and charged species (Na+, Ca++, OH-,...). As the dielectric constant for the mixture of H20 with other volatiles or salts can be predicted to a good accuracy to high temperatures and pressures, our approach allows excess free energy and speciation calculations in a wide variety of crustal solutions.
log KI(NiCl )= +1.72 and log KI(HCI°)= +1.51. Th~ inqportance of such data in geochemical modelling of hydrothermal mass transport and deposition will be discn/ssed.