EXPERIMENTAL-STUDY OF CHLORINE BEHAVIOR IN HYDROUS SILICIC MELTS

Chlorine solubility in silicate melts has been investigated at 830-850 +/- 5-degrees-C and at pressures ranging from 50 to 200 MPa, using both natural (pantellerite, rhyolite, phonolite) and synthetic (SiO2-Al2O3-K2O-Na2O) compositions and a stated H2O-NaCl-KCl fluid phase near 4 molal. At 100 MPa, Cl contents in pantelleritic melts reach a solubility plateau at initial aqueous phase molality near 2. This plateau coincides with a large immiscibility gap between aqueous and chloride-rich fluids. With the coexisting Cl-saturated aqueous phase, Cl ranges from 2720 +/- 120 ppm in rhyolite to 8960 +/- 85 ppm in pantellerite and reaches 6270 +/- 170 ppm in phonolite, at 100 MPa. Between 50 and 200 MPa, the Cl content in pantelleritic melt decreases from 9640 +/- 200 ppm to 5040 +/- 150 ppm. Although Cl solubility increases with increasing FeO* in high SiO2 melts, it is mainly controlled by the Al/Si and (Na + K)/Al molar ratios of the melt with a minimum at Na + K/Al = 1 in a series of synthetic rhyolitic to pantelleritic melts. The experimental results suggest that chlorine occurs as alkali-chloride complexes in high SiO2 melts. They also indicate that Cl is concentrated in the aqueous fluids in equilibrium with SiO2-rich melts, the exact value of D depending on melt composition and melt chlorine concentration. Volcanic degassing will create chlorine-rich hydrothermal fluids and decrease chlorine melt content.