CO2 solubility and speciation in intermediate (andesitic) melts: the role of H2O and composition

We determined total CO2 solubilities in andesite melts with a range of compositions. Melts were equilibrated with excess C-O(-H) fluid at 1 GPa and 1300 C and then quenched to glasses. Samples were analyzed using an electron microprobe for major elements, ion microprobe for C-O-H volatiles, and Fourier transform infrared spectroscopy for molecular H2O, OH(-), molecular CO2, and CO3(2-). CO2 solubility was determined in hydrous andesite glasses and we found that H2O content has a strong influence on C-O speciation and total CO2 solubility. The findings have implications for CO2 degassing. If substantial H2O is present, total CO2 solubility is higher and CO2 will degas at relatively shallow levels compared to a drier melt. Total CO2 solubility was also examined in andesitic glasses with additional Ca, K, or Mg and low H2O contents ( < 1 wt pct). We found that total CO2 solubility is negatively correlated with (Si + Al) cation mole fraction and positively correlated with cations with large Gibbs free energy of decarbonation or high charge-to-radius ratios (e.g., Ca). (Author)