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Water solubility in haplobasaltic melts

Institut für Mineralogie, Universität Hannover, Welfengarten 1, D-30167 Hannover, Germany

^* Corresponding author. Present address: Otto-Schott-Institut für Glaschemie, Universität Jena, Fraunhoferstr. 6, D-07743 Jena, Germany. E-mail: darja.benne{at}uni-jena.de

The solubility of water in melts of the haplobasaltic system (N_aAlSi₃O_g-CaAl₂Si₂O₈-CaMgSi₂O₆, Ab-An-Di) was investigated at pressures of 50, 200 and 500 MPa and at temperatures between 1200°C and 1420°C using an internally heated gas pressure vessel. Compositions close to the binary joins Ab₁₀₀-An₁₀₀ and Ab₅₀Di₅₀-An₅₀Di₅₀ were studied. The water content of the glasses was analysed using Karl-Fischer-titration and near-infrared (NIR) spectroscopy. Linear molar absorption coefficients for the NIR combination bands of OH groups at 4500 cm^–1 (_OH) and molecular H₂O at 5200 cm^–1 (_H2O) were determined for Ab₅₀An₅₀, An₅₆Di₄₄, and Ab₅₄Di₄₆ compositions. Concentration of OH groups is found to be significantly higher in Ab₅₀An₅₀ glasses than in other glasses quenched at similar rate. This is attributed to the depolymerisation of the network structure enabling the formation of stable Ca-complexes in the glasses.

An increase in pressure from 50 to 200 or from 200 to 500 MPa approximately doubles the water solubility in haplobasaltic melts. Depending on compositions 2.0–3.0 wt% water can be dissolved at 50 MPa, 3.7–6.1 wt% at 200 MPa and 7.6–12.4 wt% at 500 MPa. In general, the lowest H₂O solubility was found in the An₅₆Di₄₄ melt. At 50 MPa, the variation of water solubility with composition appears to be complex but the precision of the data does not allow to extract reliably compositional trends. In general, data at 200 and 500 MPa show a strong decrease in H₂O solubility with increasing Di content, e.g. from 5.99 wt% (Ab₁₀₀) to 4.94 wt% (Ab₅₄Di₄₆) and from 5.81 wt% (Ab₅₀An₅₀) to 5.21 wt% (Ab₂₇An₂₉Di₄₄) at 1200°C/200 MPa. The effect of Ab/An ratio on water solubility appears to be small at 200 MPa. In contrast, at 500 MPa the water solubility decreases strongly with increasing An content of the melt.

Key-words: water solubility, haplobasaltic melts, Kari-Fischer titration, IR spectroscopy, H₂O speciation.