Compositional and pressure effects on the solubility of H2O and CO2 in mafic melts

Abstract

The effect of the anhydrous composition on the solubilities of H2O and CO2 in mafic melts varying from MORB to nephelinite was investigated experimentally between 50 and 500MPa at 1200 to 1250°C. In all compositions, CO2 is only present as carbonate species in the quenched glasses. The concentrations of dissolved H2O and CO2 have been analyzed by KFT (Karl–Fischer titration) and FTIR (Fourier-transform infrared spectroscopy). The Mid-Infrared (MIR) absorption coefficients for the H2O band at 3500cm−1 are identical within error for all investigated melt compositions and equal to 59.2±4L/(mol∗cm). The absorption coefficients for the carbonate bands vary in the range 306±32 to 360±24L/(mol∗cm) for the 1430cm−1 band and in the range 349±25 to 394±27L/(mol∗cm) for the 1520cm−1 band. However, a simple correlation with the melt composition could not be determined.Water solubility in mafic to intermediate melts increases slightly with the total alkali content and the effect of composition is more pronounced at higher pressures. At 500MPa, the solubility of H2O in melts coexisting with nearly pure H2O fluids varies from 8.8 to 9.5wt.% H2O. A strong effect of melt composition on the solubility of CO2 is observed at all investigated pressures. For instance, at 500MPa, mafic melts coexisting with nearly pure CO2 fluids can dissolve from around 0.32 to more than 1.30wt.% CO2 as melt composition changes from tholeiite to nephelinite. The compositional effect on the solubility of CO2 in melts coexisting with pure CO2 fluid is best described by non-linear (exponential) correlations with compositional parameters such as the parameter Π proposed by Dixon (1997; American Mineralogist, 82: 368–378) or structural parameters (e.g., nonbridging oxygen per tetrahedrally coordinated cation). The obtained relationships are used to propose empirically derived equations of the form ln(CO2)=1.150·lnP+6.71·Π*−1.345, where CO2 is the solubility of CO2 in silicate melts in wt.% (at 1200 to 1250°C), P is pressure in MPa and Π* is a compositional parameter (Π*=Ca2++0.8K++0.7Na++0.4Mg2++0.4Fe2+)/(Si4++Al3+) with cations expressed as cation fractions in melt.