THE DISTRIBUTION OF H2O-CO2 BETWEEN CORDIERITE AND GRANITIC MELT UNDER FLUID-SATURATED CONDITIONS AT 5 KBAR AND 900 °C

Abstract

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 C under fluid-saturated conditions. Up to seven charges, saturated with a range of H2O-CO2 fluid compositions, were run simultaneously. The H2O and CO2 contents of the cordierites and the H2O contents of the melts were determined using secondary ion mass spectrometry. As the CO2 content of the fluid increases, the H2O content of both the cordierite and the melt decrease in a similar manner to fluid-undersaturated experiments 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(melt)/wt% H2O(cordierite)) are in the range 3.5-5.9 and agree with the model of Harley and Carrington for 5 kbar and 900 C. The CO2 content of the cordierite increases from 0 to 1 wt% CO2 in the most CO2-rich experiment as the cordierite and melt H2O contents decrease. Our H2O-CO2 cordierite data allow us to model and predict the maximum CO2 content in cordierite to be 0.16±0.01 molecules per formula unit (i.e. 1.16 wt% CO2) at 5 kbar and 900 C. This value, and the channel CO2 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 cordierites that are saturated in H2O + CO2 and have channel XCO2>0.25 require aCO2 of over 0.75.