Water solubility and D/H fractionation in the system basaltic andesite–H2O at 1250°C and between 0.5 and 3 kbars

Water solubilities in melt of basaltic andesite composition have been measured in the range 500 to 3000 bars, 1200–1250°C. The saturation curve obey an empirical relationship very close to the square root of pressure law of Hamilton et al. [Hamilton, D.L., Burnham, C.W., Osborn, E.F., 1964. The solubility of water and effects of oxygen fugacity and water content on crystallisation in mafic magmas. J. Petrol., 5, 21–39]. The best fit is obtained by a power law H2O wt.%=aPn (P in b), where a=0.05±0.01 and n=0.63; the departure from n=0.5 is mainly due to the increasing importance of dissolved molecular water at high pressures. Hydrogen isotopic fractionation between water vapor and total water dissolved in the melt (ΔDv–m) has been measured at 0.5, 2 and 3 kb. ΔDv–m decreases with pressure from 32 to 20±2‰. This variation is related to the increase in molecular water proportions relative to hydroxyl group when water solubility increases. The variation of the D/H ratios in water dissolved in such melts thus appears as a possible indirect measurement of the original OH/H2O ratio of total dissolved water, not subject to variation during quenching as the later is. This correlation shows that the ΔDv–m isotopic fractionation at very high water vapor pressure tend toward a limit of 16±3‰ for basic magmas.