The influence of fluid unmixing on cation exchange between plagioclases and aqueous chloride solutions at 700° C, 1 kbar
✍ Scribed by Saint Clair Dujon; Martine Lagache
- Publisher
- Springer
- Year
- 1986
- Tongue
- English
- Weight
- 701 KB
- Volume
- 92
- Category
- Article
- ISSN
- 0010-7999
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✦ Synopsis
Experimental exchanges between plagioclases (synthesized from gels) and aqueous solutions (0.5N 8N) were carried out according to the reaction 2 NaA1Si308 + CaC12 CaA12Si208 + 4 SiO2 + 2NaC1. Distribution coefficients defined by XA. (XN.cl) 2 KD = (XAb)2 X'CaC12
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 Ko if the fluid is a single phase. In this study, variation of Ko with the concentration of the solutions is interpreted as the result of fluid unmixing to vapour and brine phases. This implies boiling of CaC12-NaC1-H20 fluids analogous to that known for the system NaC1-H20. 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. vapour (gas) and a concentrated liquid phase (brine). The geological consequences of such unmixing are of the utmost importance (Denis et al. 1980;Lagache and Weisbrod 1977;Leleu et al. 1976).
One of the present authors (Lagache 1980; Lagache and Weisbrod 1977) has shown by an indirect method that unmixing in the NaC1-KC1-H20 system appeared enlarged with respect to the NaC1-H20 system. These experiments were designed to measure the composition of an aqueous solution in equilibrium with two alkali feldspars, lying on the feldspar solvus, which buffer the composition of the single phase fluid (invariant system).
It could be assumed that a mixed NaC1-CaC12-H20 solution would undergo similar unmixing given the same temperature and pressure conditions, but we do not know to what extent. In this paper, we attempt to prove this experimentally by examining the equilibrium between a synthetic plagioclase and a sodi-calcic chloride solution with different salt concentrations.
Experiments on exchanges between plagioclase and chloride solution have already been reported and commented on by Orville (1972), Kotel'Nikov et al. (1981) and D~on and Lagache (1984), emphasis being placed on the properties of the solid phases. These experiments were performed between 400 ~ C and 800 ~ C, 1 and 3 kbar. The conditions of fluid unmixing were either avoided or ignored althogether (Kotel'Nikov et al. 1981).