An experimental study of H solubility in feldspars: Effect of composition, oxygen fugacity, temperature and pressure and implications for crustal processes

A series of experiments, with run durations of ∼100-240 h, have been carried out on H solubility in natural labradorite, albite and anorthoclase at 2-10 kbar and 700-850 °C, in equilibrium with different oxygen buffer materials: Fe-FeO, Ni-NiO, Re-ReO2 and Fe2O3-Fe3O4. Attainment of equilibrium water contents were examined by conducting experiments with different run-durations and by performing profile analysis along crystal core-to-rim paths. The annealed products generally show typical OH-related infrared absorption bands in the range 3700-2600 cm−1: the spectra of the labradorite and anorthoclase are dominated by broad bands at ∼3300-3100 cm−1, while the spectra of the albite consist of broad bands at ∼3100-3000 cm−1 and sharp bands at ∼3600-3400 cm−1. The measured H solubility, ranging from ∼80 to 285 ppm H2O, differs greatly between the feldspars with different chemical compositions, depends strongly on oxygen fugacity, and increases with increasing temperature and pressure. The H solubility is significantly higher, e.g., by a factor of ∼3 or even more, at very reducing conditions than at oxidizing conditions. The high H solubility of feldspar at reduced conditions indicates that, at least locally, the Earth's ancient deep continental crust was probably more hydrous than the modern one. Crustal dehydration can be caused by a change of oxygen fugacity and a corresponding change of water solubility in the constituent feldspar under isothermal conditions, without the requirement of high temperature and/or additional heat sources.