CALCIUM AND SODIUM EXCHANGE DURING HYDROTHERMAL ALTERATION OF CALCIC PLAGIOCLASE AT 400-DEGREES-C AND 400 BARS

A series of experiments were conducted to measure the effect of plagio clase composition on the composition of coexisting Cl-dominated hydrot hermal fluids. Assemblages containing quartz and natural plagioclase w ith a composition of either An60 or An82 were reacted with Na-Ca-Cl fl uids at 400-degrees-C and, 400 bars using flexible cell hydrothermal f acilities. Plagioclase remained homogeneous during these experiments d espite the fact that homogeneous solid solutions with intermediate com positions are likely metastable with respect to mechanical mixtures at these conditions. It was found that Ca and Na exchange between fluids and minerals appears to be reversible, and highly dependent on plagio clase composition but m(Ca++)/m2Na+ ratios remained constant with incr easing Cl for a given plagioclase composition. In fact, a solid soluti on model which approaches ideal behavior appears to account best for t he chemistry of fluids coexisting with An60 and An82 plagioclase. Thes e data may have a direct bearing on hydrothermal alteration processes responsible for the chemistry of hot spring fluids at mid-ocean ridges , which appear to have equilibrated with metastable plagioclase solid solutions of composition An60 to An70.