The Effect of CO2 In S-TYPE Magmas: Its Effect on Fluid Saturation

Metamorphism of the source region of S-type granites is likely to result in graphite formation, and the Fe2O3 component present in the source rocks will react with graphite during partial melting to form CO2, which is soluble in small amounts in silicic melts. The much lower solubility of CO2 compared to H2O in silicic melts results in exsolution of COH fluids at greater depths than would occur in the absence of CO2 and result in loss of H2O from the melt to evolving fluid. The early fluid exsolution from the melt will result in lower dissolved H2O contents and higher crystallization temperatures. Because of the low CO2 solubility, its effect on fluid phase formation is large, even if CO2 is present in small amounts (as limited by the Fe3+ content of the source region). The consumption of Fe3+ and subsequent loss of CO2 in this process contributes to the reduced nature of S-type granites.