The effects of small amounts of H₂O, CO₂ and Na₂O on the partial melting of spinel Lherzolite in the system CaO-MgO-Al₂O₃-SiO₂ ± H₂O ± CO₂ ± Na₂O at 1·1GPa

The effects of small amounts of H₂O (<4wt % in the melt) on the multiply saturated partial melting of spinel lherzolite in the system CaO-MgO-Al₂O₃-SiO₂ ± Na₂O ± CO₂ have been determined at 1·1GPa in the piston-cylinder apparatus. Electron microprobe analysis and Fourier transform infrared spectroscopy were used to analyse the experimental products. The effects of H₂O are to decrease the melting temperature by 45°C per wt % H₂O in the melt, to increase the Al₂O₃ of the melts, decrease MgO and CaO, and leave SiO₂ approximately constant, with melts changing from olivine- to quartz-normative. The effects of CO₂ are insignificant at zero H₂O, but become noticeable as H₂O increases, tending to counteract the H₂O. The interaction between H₂O and CO₂ causes the solubility of CO₂ at vapour saturation to increase with increasing H₂O, for small amounts of H₂O. Neglect of the influence of CO₂ in some previous studies on the hydrous partial melting of natural peridotite may explain apparent inconsistencies between the results. The effect of small amounts of H₂O on multiply saturated melt compositions at 1·1GPa is similar to that of K₂O, i.e. increasing H₂O or K₂O leads to quartz-normative compositions, but increasing Na₂O produces an almost opposite trend, towards nepheline-normative compositions.