Solubility and speciation of C–O–H fluids in andesitic melt at T = 1100–1300 °C and P = 200 and 500 MPa

The solubility and speciation of C–O–H fluids in andesitic melts were experimentally investigated at 1100–1300 °C and 200, 500 MPa. Solubilities of H2O and CO2 increase with pressure and the maximum solubility values are 5.5 and 10.0 wt.% H2O and 2200 and 4500 ppm CO2 at 200 and 500 MPa, respectively. The effect of temperature on the solubility of C–O–H fluids is not resolved from the obtained data. The solubility values for CO2-rich fluid in andesitic melts at 200 and 500 MPa are similar to the data for tholeiitic, dacitic and rhyolitic melts having compositions falling along an inverse linear trend on the (Ca + Mg + Fe) cation fraction vs. (Na + K) / (Si + Al) ratio diagram. This indicates that for these melts the decrease in the proportion of reactive oxygens (or network-modifiers), which are responsible for the incorporation of carbonate species, can be compensated in a certain degree by an increase in ionic porosity of polymerized melts, favoring the incorporation of molecular CO2.The speciation of water and carbon in quenched glasses depends strongly on total water content (CH2Otot). The CH2Omol / CH2Otot ratio increases from about 0.1 to 0.7 with CH2Otot varying from 0.8 to 8 wt.%. The proportion of CO2mol decreases nonlinearly from 0.033 to almost 0 in the same range of CH2Otot. The observed change in speciation can be attributed to the effect of water on the glass transition temperature, i.e. to the conditions at which the speciation can be frozen in. The log-form of reaction constants K1 and K2 for water and carbon speciation in glasses show well-defined Arrhenian dependences on reciprocal fictive temperature of the glass, consistent with the assumption that water and carbon species are frozen in at the glass transition.