log (mEu or mi /mj ), pH, Eh
Kolonin G.R., Shironosova G.P. Thermodynamic modeling of possible reasons of REE fractionation with participation of high temperature fluids of complicated composition.
Institute of Mineralogy and Petrography of SB RAS (IMP SB RAS), Novosibirsk, email@example.com key words [thermodynamic modeling solubility REE-fluorite fluid fractionation Eu-anomalies]
8 4 0-4 -8 -12 -16 -20 -24 0,1 0,01 0,01 0,1 0,2 0,5 1
pH Eh CO2/CH4 Eu++ Eu+++ Eu+++/Eu++ Eu(II) Eu(III) Eu(III)/Eu(II)
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  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 of complex fluoride-chloride-carbonate fluids, saturated with REE-containing fluorite, has been conducted on the basis of this data base , using the software package “Hch” . The peculiarities of complexing of trivalent and bivalent Eu, including the influence ofacidity-alkalinity on the ratio of total concentrations of Eu(III) and Eu(II), have been also discussed . Specifically, it has been shown that during the evolution of fluid compositions typical of rare-metal fluoritecontaining deposits of hydrothermal genesis , two REE groups, which differ in the specificity of the complex forms predominant in the fluid, can be recognized. In regard to the LREE (La, Ce,Nd and Pr), the LnF++ and LnF2+ (for reduced temperatures) fluoride complexes would be expected to dominate, if positively charged or neutral hydroxocomplexes are insignificant. As for MREE (Sm, Eu, Tb) and HREE (Ho, Yb and Lu), the anion or neutral hydrocomplexes with the subordinate concentrations of LnF++ and LnF2+ fluoride complexes are bound to dominate.
Fig.2. The influence of acidity-alkalinity of a fluid on relationships between both concentrations of Eu2+, Eu3+ and total concentrations of Eu(II), Eu(III) at T=350°C, P=1000 bar and 1,0m NaCl, 0,1m HF, 0,5m H2CO3. Reasoning from the degree of solubility of 11 REE + Y model fluorides tested in the presence of fluorite, which provides high (about 10-2 m) total HF0+F- contents, the following threegroups of REE are distinguished: 1) moderately soluble (10-5 – 10-6m) La, Nd, Pr, Ho and Lu; 2) highly soluble (more than 10-4 m) Sm, Eu and Yb; 3) the most low soluble (about 10-7 m) Tb and Y. Fig.1 demonstrates the expected differences in the solubility of ftorides, obtained on the basis of thermodynamic calculations. It shows an order of possible leaching or precipitation of REE together withfluorite in relation to the value of the fluid/solid phase. These results support the known evaluations of possible REE fractionation during the action of fluid on the minerals and rocks only in the case, when this ratio is higher than 10-2 – 10-3 . 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 Eu maxima 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 complex-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,  generally proves that the role of the complexformation of all REE and Eu, in particular, in the processes of their fractionation at the interactions of water/rock type needs to be studied. Fig. 2 presents the influence of pH of the high temperature fluid under the effect of hematite-magnetite buffer both on Eu3+/Eu2+, Eu(III)/Eu(II) and on more general red/ox characteristics: Eh and CO2/CH4 ratio. According to the commonly accepted expectations,...
Lire le document complet
Veuillez vous inscrire pour avoir accès au document.