The solubility of H ₂ O in peralkaline and peraluminous granitic melts

Universitaet Bayreuth, Bayerisches Geoinstitut, Bayreuth, Federal Republic of Germany
CRSCM-CNRS, France
Universitaet Hannover, Federal Republic of Germany

The solubility of H ₂ O 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 degrees C. The compositions represent the additions of individual components (Cs ₂ O, Rb ₂ O, K ₂ O, Na ₂ O, Li ₂ O, and Al ₂ O ₃ ) to HPG8 (in wt% 78.6 SiO ₂ , 12.5 Al ₂ O ₃ , 4.6 Na ₂ O, 4.2 K ₂ O; see Knoche et al. 1995) to generate peralkaline and peraluminous compositions, respectively. The H ₂ O-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 H ₂ O contents. H ₂ O solubility increases with added excess alkali oxide. Compared on a weight percent basis, the solubility of H ₂ O for a given degree of peralkalinity increases in the order Cs, Rb2 O are the same, whereas the effect of Li ₂ O is somewhat lower. This contrasts with the relatively high solubility of H ₂ O in LiAlSi ₃ O ₈ melt relative to albite and orthoclase melts and implies that excess Li ₂ O 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 H ₂ O decreases with the addition of 2 wt% Al ₂ O ₃ to the HPG8 composition, then increases strongly with the further addition of excess Al ₂ O ₃ , 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 H ₂ O in the melt with 5 wt% excess Al ₂ O ₃ is significantly larger than that in HPG8. If the peraluminous composition with the minimum solubility of H ₂ O 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 Na ₂ O 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 H ₂ O in the melt. Evidence for an H ₂ O-rich history of glassy peralkaline obsidians from melt inclusions or from stable isotopes may indicate much shallower depths of saturation than previously thought possible.

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