THE SOLUBILITY OF H2O IN PERALKALINE AND PERALUMINOUS GRANITIC MELTS

The solubility of H2O in a series of 13 melts based on a haplogranitic composition (HPG8) have been determined for the conditions of 500-500 0 bar pressure and 800-1000 degrees C. The compositions represent the additions of individual components (Cs2O, Rb2O, K2O, Na2O, Li2O, and A l2O3) to HPG8 (in wt% 78.6 SiO2, 12.5 Al2O3, 4.6 Na2O, 3.2 K2O; see Kn oche et al. 1995) to generate peralkaline and peraluminous composition s, respectively. The H2O-saturated melts were generated by hydrotherma l fusion of dry glassy starting materials in an internally heated pres sure vessel. The quenched products of the hydration experiments were a nalyzed by Karl-Fischer titration for bulk H2O contents. H2O solubilit y increases with added excess alkali oxide. Compared on a weight perce nt basis, the solubility of H2O for a given degree of peralkalinity in creases in the order Cs, Rb <K < Na < Li. On a molar equivalent basis the effects of excess Cs, Rb, K, and Na on increasing the solubility o f H2O are the same, whereas the effect of Li2O is somewhat lower. This contrasts with the relatively high solubility of H2O in LiAlSi3O8 mel t relative to albite and orthoclase melts and implies that excess Li2O is not contributing to the content of nonbridging O atoms in the melt as efficiently as the other alkalies. For the peraluminous compositio ns, the solubility of H2O decreases with the addition of 2 wt% Al2O3 t o the HPG8 composition, then increases strongly with the further addit ion of excess Al2O3, so that a solubility minimum exists not at the 1: 1 alkali-Al ratio of melt composition but at a slightly peraluminous m elt composition. The solubility of H2O in the melt with 5 wt% excess A l2O3 is significantly larger than that in HPG8. If the peraluminous co mposition with the minimum solubility of H2O is taken as a basis for e stimating the influence of nonbridging O atoms associated with ''exces s'' alkalies or Al in the melt structure then the effects of excess Al and alkalies (Cs, Rb, EC., Na) are comparable on the basis of the num ber of nonbridging O atoms added to the slightly peraluminous base. Th e addition of Na2O up to levels of peralkalinity commonly encountered in glassy magmatic eruptive rocks of peralkaline volcanic provinces le ads to an almost doubling of the low pressure (500 bar) solubility of H2O in the melt. Evidence for an H2O-rich history of glassy peralkalin e obsidians from melt inclusions or from stable isotopes may indicate much shallower depths of saturation than previously thought possible.