WATER SOLUBILITY AND D H FRACTIONATION IN THE SYSTEM BASALTIC ANDESITE-H2O AT 1250-DEGREES-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 degrees C. The satur ation curve obey an empirical relationship very close to the square ro ot 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 fuga city and water content on crystallisation in mafic magmas. J. Petrol., 5, 21-39]. The best fit is obtained by a power law H2O wt.% = aP(n) ( 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 (Delta Dv-m) has been mea sured at 0.5, 2 and 3 kb. Delta Dv-m decreases with pressure from 32 t o 20 +/- 2%. This variation is related to the increase in molecular wa ter proportions relative to hydroxyl group when water solubility incre ases. The variation of the D/H ratios in water dissolved in such melts thus appears as a possible indirect measurement of the original OH/H2 O ratio of total dissolved water, not subject to variation during quen ching as the later is. This correlation shows that the Delta Dv-m isot opic fractionation at very high water vapor pressure tend toward a lim it of 16 +/- 3% for basic magmas. (C) 1998 Elsevier Science B.V. All n ights reserved.