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Kolonin G.R., Shironosova G.P. Thermody-
namic modeling of possible reasons of REE
fractionation with participation of high tem-
perature fluids of complicated composition.
Institute of Mineralogy and Petrography of SB RAS (IMP SB RAS), key words [thermodynamic modeling solubility REE-fluorite fluid The data base of the stability constants of the complex forms of REE in the solutions of complicated composition at temperatures up to 500°C and pressures of up to 2 kbar[1] has been adapted on the basis of the parameters of HKF equation for simple and complex REE ions [2,3]. An extended thermodynamic modeling of possible behavior of REE elements during the evolution of the parameters ofcomplex fluoride-chloride-carbonate fluids, saturated with Fig. 2. The influence of acidity-alkalinity of a fluid on
REE-containing fluorite, has been conducted on the basis relationships between both concentrations of Eu2+, Eu3+
of this data base [4], using the software package “Hch” and total concentrations of Eu(II), Eu(III) at T=350°C,
[5]. The peculiarities of complexing of trivalent and biva- P=1000 bar and 1,0m NaCl, 0,1m HF, 0,5m H
lent Eu, including the influence of acidity-alkalinity on the ratio of total concentrations of Eu(III) and Eu(II), have Reasoning from the degree of solubility of 11 REE + Y model fluorides tested in the presence of fluorite, which Specifically, it has been shown that during the evolu- provides high (about 10-2 m) total HF0+F- contents, the tion of fluid compositions typical of rare-metal fluorite- following three groups of REE are distinguished: 1) mod- containing deposits of hydrothermal genesis [7], two REE erately soluble (10-5 – 10-6m) La, Nd, Pr, Ho and Lu; 2) groups, which differ in the specificity of the complex highly soluble (more than 10-4 m) Sm, Eu and Yb; 3) the forms predominant in the fluid, can be recognized. In re- most low soluble (about 10-7 m) Tb and Y. Fig.1 demon- gard to the LREE (La, Ce, Nd and Pr), the LnF++ and strates the expected differences in the solubility of fto- 2 (for reduced temperatures) fluoride complexes rides, obtained on the basis of thermodynamic calcula- would be expected to dominate, if positively charged or tions. It shows an order of possible leaching or precipita- neutral hydroxocomplexes are insignificant. As for MREE tion of REE together with fluorite in relation to the value (Sm, Eu, Tb) and HREE (Ho, Yb and Lu), the anion or of the fluid/solid phase. These results support the known neutral hydrocomplexes with the subordinate concentra- evaluations of possible REE fractionation during the ac- tion of fluid on the minerals and rocks only in the case, when this ratio is higher than 10-2 – 10-3 [8].
The abnormal possibility of participation of a reduced Eu2+ forms in the processes of fractionation of the whole REE group is thought to be a reason, which causes Eumaxima and minima in the spectra of REE distribution in various minerals and rocks. The influence of temperature and pH solution as well as the presence of various com- PrF3 s-d
NdF3 s-d

plex-forming ligands (with sulfate sulfur as an example) on the conditions, which give rise to bivalent Eu has been thermodynamically analyzed in [9, 10]. Finally, [8] gener- ally proves that the role of the complex formation of all REE and Eu, in particular, in the processes of their frac- tionation at the interactions of water/rock type needs to be Fig. 2 presents the influence of pH of the high tem- Fluorite
perature fluid under the effect of hematite-magnetite buffer both on Eu3+/Eu2+, Eu(III)/Eu(II) and on more gen-eral red/ox characteristics: Eh and CO2/CH4 ratio. Ac- Fig. 1. The change of mole quantities in the fluoride
cording to the commonly accepted expectations, practi- solid phase of 1 g-mole CaF2 + n10-4 g-mole RF3
cally complete reduction of aquaion of trivalent Eu to the (n=11) composition in relation to the quantity of the
bivalent one (see two low dashed lines), occurs at the ele- reacted model solution of 4,0m NaCl + 1,5m H2CO3 +
vated temperatures even in the presence of hematite. In 0,001m HCl composition at 500°C and 2000 bar
this case their ratio decreases by more than 6 orders ofmagnitude as pH increases (according to the third dashedline, lower than 10-8). Based on the complex formationthree solid lines of total Eu(III) and Eu(II) concentrations and their ratios really reflect the situation. These threelines begin to rise quickly at pH>6, reaching positive val-ues in the alkaline conditions. This fact can be explainedby higher stability of Eu(III) hydroxocomplexes as com-pared to Eu(II) chlorocomplexes, which are dominant inthe acid conditions.
The work is carried out under the support of the RFBR grant NN 01-05-65255 and Scientific program“Universities of Russia –Fundamental Investigations”,grant
1. Kolonin G.R., Morgunov K.G., Shironosova G.P.
(2001) // Geologiya i geofizika, t. 42, N6, pp.881-890(in Russian).
2. Shock E.L., Sassani D.C., Willis M., Sverjensky D.A.
(1997) // Geochim. et Cosmochim. Acta, V.61, pp.907-950.
3. Haas J.R., Shock E.L., Sassani D.S. (1995) // Geochim.
et Cosmochim. Acta., V.59, pp.4329-4350.
4. Kolonin G.R., Shironosova G.P. (2000) // Proceedings of Joint ISHR and ICSTR (Kochi, Japan), pp.35-40.
5. Shvarov, Yu.V. (1999) // Geokhimya, N6, pp.646-652 6. Kolonin G.R., Shironosova G.P. (2001) // Water Rock Interaction, V.1. Rosa Cidy, editor. (Proceedings ofWRI-10 Int. Symp., Italy) pp.287-290.
7. Shironosova G.P., Kolonin G.R., Sushchevskaya T.M.
(2001) // Geochemistry International, Suppl. issue,V.2, p.250-255.
8. Bau M. (1991) // Chem. Geol., V.93, pp.219-230.
9. Sverjensky D.A. (1984) // Earth Planet. Sci. Lett., 10. Wood S.A. (1990) // Chem. Geol., V.88, pp.99-125


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