The distribution of H2O-CO2 between cordierite and granitic melt under fluid-saturated conditions at 5 kbar and 900 degrees C

Experiments to determine the distribution of H2O and CO2 between synthetic peraluminous granitic melt and natural Fe-Mg cordierite have been conducted at 5 kbar and 900 degreesC under fluid-saturated conditions. Up to seven c harges, saturated with a range of H2O-CO2 fluid compositions, were run simu ltaneously. The H2O and CO2 contents of the cordierites and the H2O content s of the melts were determined using secondary ion mass spectrometry. As th e CO2 content of the fluid increases, the H2O content of both the cordierit e and the melt decrease in a similar manner to fluid-undersaturated experim ents in the H2O only system. The cordierite H2O contents in the experiments range from 0.5 to 1.7 wt% H2O. The distribution coefficients (Dw=wt% H2O(m elt)/wt% H2O (cordierite)) are in the range 3.5-5.9 and agree with the mode l of Harley and Carrington for 5 kbar and 900 degreesC. The C02 content of the cordierite increases from 0 to 1 Wt% CO2 in the Most CO2-rich experimen t as the cordierite and melt H2O contents decrease. Our H2O-CO2 cordierite data allow us to model and predict the maximum C02 content in cordierite to be 0.16 +/- 0.01 molecules per formula unit (i.e. 1.16 wt% CO2) at 5 kbar and 900 degreesC. This value, and the channel C02 contents of cordierite in equilibrium with H2O-CO2 fluids at high XCO2, are significantly lower than predicted using previous models. On the basis of these experiments cordier ites that are saturated in H2O + C02 and have channel XCO2>0.25 require aCO (2) of over 0.75.