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New experimental data are presented that allows the biotite-magnetite-sanidine equilibrium to be used for estimating water fugacity (f H₂O ) in hydrous phonolitic magmas. It is also demonstrated that the partly ionic model gives the best estimate for the annite activity (a annite ). Crystallization experiments were carried out on a representative sample of peralkaline, phonolitic obsidian of Montaña Blanca (MB) pumice deposit, Tenerife, Canary Islands. Experiments were performed from 720.810 °C and 50.250 MPa. Redox conditions were varied between NNO (nickel + nickel oxide) + 1 (±0.2) and FMQ (fayalite + magnetite + quartz). The majority of the experiments were done under H₂O saturation conditions (P water  = P total ). Several experiments were done using a mixed H 2 O-CO 2  fluid phase whereas in other experiments 10 or 20 wt% powdered alkali feldspar was added to the starting material. The pre-eruptive f H₂O  of the Montaña Blanca magma is estimated at 676 ± 200 bars. The pre-eruptive f H₂O  for the Fish Canyon tuff (753.2978 bars) and Bishop tuff rhyolite (1065.2440 bars) were also calculated, as well as f H₂O  for metamorphic biotite from Au Sable Forks (≈ 130 bars). The results of this study suggest that this geohygrometer can be used in any magmatic system in which biotite-magnetite-sanidine is a stable assemblage.

New experimental data on biotite + magnetite + sanidine saturated phonolitic melts and application to the estimation of magmatic water fugacity