Abstract
The solubility of H2O in a series of 13 melts based on a haplogranitic composition (HPG8) have been determined for the conditions of 500–5000 bar pressure and 800–1000 °C. The compositions represent the additions of individual components (Cs2O, Rb2O, K2O, Na2O, Li2O, and Al2O3) to HPG8 (in wt% 78.6 SiO2, 12.5 Al2O3, 4.6 Na2O, 4.2 K2O; see Knoche et al. 1995) to generate peralkaline and peraluminous compositions, respectively. The H2O-saturated melts were generated by hydrothermal fusion of dry glassy starting materials in an internally heated pressure vessel. The quenched products of the hydration experiments were analyzed by Karl-Fischer titration for bulk H2O contents.
H2O solubility increases with added excess alkali oxide. Compared on a weight percent basis, the solubility of H2O for a given degree of peralkalinity increases 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 of H2O are the same, whereas the effect of Li2O is somewhat lower. This contrasts with the relatively high solubility of H2O in LiAlSi3O8 melt 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 compositions, the solubility of H2O decreases with the addition of 2 wt% Al2O3 to the HPG8 composition, then increases strongly with the further addition 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 melt composition. The solubility of H2O in the melt with 5 wt% excess Al2O3 is significantly larger than that in HPG8. If the peraluminous composition with the minimum solubility of H2O is taken as a basis for estimating the influence of nonbridging O atoms associated with “excess” alkalies or Al in the melt structure then the effects of excess Al and alkalies (Cs, Rb, K, Na) are comparable on the basis of the number of nonbridging O atoms added to the slightly peraluminous base.
The addition of Na2O up to levels of peralkalinity commonly encountered in glassy magmatic eruptive rocks of peralkaline volcanic provinces leads to an almost doubling of the low pressure (500 bar) solubility of H2O in the melt. Evidence for an H2O-rich history of glassy peralkaline obsidians from melt inclusions or from stable isotopes may indicate much shallower depths of saturation than previously thought possible.
