Water solubility in phonolite melts: Influence of melt composition and temperature

摘要

The water solubility in various phonolite melts was determined experimentally at pressures between 50 and 395 MPa and at temperatures between 850 and 1200 degrees C. The aim of this study was to elucidate to which extent chemical and physical parameters such as relative amounts of alkali and alkaline earth elements or temperature affect the water solubility. For this purpose a total of 10 different phonolite glasses were prepared. For a series of four glasses, the Na/(Na+ K) ratio was varied from 0.2 to 0.8, for another series of four glasses, the (Na+K)/(Ca+Mg) ratio was varied from 2.8 to 10.1, while keeping Na/(Na+K) constant at 0.4. For these two series water solubility experiments were performed at 1200 degrees C and pressures between 50 and 395 MPa in an internally heated pressure vessel. The data show that both chemical variables have rather small effects on the water Solubility. For instance, at 2 kbar and 1200 degrees C an increase in Na/(Na+ K) ratio from 0.2 to 0.8 leads to an increase in water solubility of about 0.4 wt.% (5.58 to 5.94 wt.%), while increasing the (Na+K)/(Ca+Mg) ratio from 2.8 to 10.1 leads to an increase of about 0.2 wt.% water. The effect of temperature was investigated for two phonolite compositions, representative of Montana Blanca, Tenerife, Spain and the Laacher See volcanic complex, East Eifel, Germany at pressures of 50, 100, 150, 200, 250 and 300 MPa, and temperatures between 850 and 1200 degrees C. At these conditions both melts exhibit negative temperature dependence ranging from about -0.1 to -0.24 wt.% water per 100 degrees C. The strongest temperature dependence (most negative values) occurs at 150 to 200 MPa and seems to approach zero slightly above 300 MPa.