CO2–H2O solubility in K-rich phonolitic and leucititic melts

Abstract

The solubility of CO2 and H2O in phonolitic and leucititic melts from Vesuvius and Colli Albani was investigated experimentally at 1250 °C and pressures between 50 and 300 MPa as a function of CO2–H2O fluid composition. Quenched glasses were analyzed for their volatile contents by thermogravimetry, carbon–sulfur analysis, and Fourier transform infrared (FTIR) spectroscopy, which enabled the determination of the absorption coefficients of the H2O- and CO2-related IR bands at 5200 cm−1 (H2O molecules), 4500 cm−1 (OH groups), and the carbonate doublet at 1510 and 1430 cm−1. No molecular CO2 was detected in our phonolitic and leucititic glasses. Leucititic glasses with elevated CO2 concentrations (approaching total absorption in transmission FTIR measurements) were also analyzed quantitatively by micro-ATR (attenuated total reflection) IR spectroscopy. While the water solubility in both melts is quite similar for pure H2O as well as for mixed CO2–H2O fluids (at given $$ f_{{{\text{H}}_{ 2} {\text{O}}}} $$ ), the CO2 solubility depends strongly on melt composition. In the range of 100–300 MPa, the solubility of pure CO2 increases from 580 to 1800 ppm in the phonolite melt and from 2950 to 8460 ppm in the leucitite melt. For the leucitite melt, we observe a single power law trend of CO2 solubility as function of $$ f_{{{\text{CO}}_{ 2} }} $$ , regardless if the melt was equilibrated with pure CO2 or mixed CO2–H2O fluids, indicating that water acts as diluent of the fluid phase. However, for the phonolite melt, we observe for mixed CO2–H2O samples a positive solubility deviation from the power law trend defined by the data for pure CO2 solubility. This effect seems to increase with increasing water content and pressure. Our interpretation is that this enhanced CO2 solubility is caused by melt depolymerization induced by water and is more apparent in the relatively polymerized phonolitic melt compared to the relatively depolymerized leucititic melt.