The influence of fluid unmixing on cation exchange between plagioclases and aqueous chloride solutions at 700° C, 1 kbar |
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| Authors: | Saint Clair Dujon Martine Lagache |
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| Institution: | 1. Laboratoire de Géologie, ER 224 CNRS, Ecole Normale Supérieure, 46 rue d'Ulm, 75005, Paris, France
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| Abstract: | Experimental exchanges between plagioclases (synthesized from gels) and aqueous solutions (0.5N–8N) were carried out according to the reaction $$\begin{gathered} 2NaA1Si_3 O_8 + CaC1_2 \hfill \\ \leftrightarrow CaA1_2 Si_2 O_8 + 4SiO_2 + 2NaC1. \hfill \\ \end{gathered}$$ Distribution coefficients defined by $$K_D = \frac{{X_{An} }}{{(X_{Ab} )^2 }}\frac{{(X_{NaC1} )^2 }}{{X_{CaC1_2 } }}$$ were determined at 700° C and 1 kbar. From previous studies it is known that variations in the concentration of the aqueous solutions have no influence upon K D if the fluid is a single phase. In this study, variation of K D with the concentration of the solutions is interpreted as the result of fluid unmixing to vapour and brine phases. This implies boiling of CaCl2-NaCl-H2O fluids analogous to that known for the system NaCl-H2O. Experimental data permit calculation of the compositions of vapours and estimation of those of the brines for fluids in which Ca/Na<0.5. Boiling has an effect upon the exchange between feldspars and solutions (metasomatism) and must be considered when determining the activity coefficients. |
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