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

New experimental data are presented that allows the biotitemagnetitesanidine equilibrium to be used for estimating water fugacity (H2O) in hydrous phonolitic magmas. It is also demonstrated that the partly ionic model gives the best estimate for the annite activity (aannite). 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 720810 °C and 50250 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 (Pwater = Ptotal). Several experiments were done using a mixed H2O-CO2 ß uid phase whereas in other experiments 10 or 20 wt% powdered alkali feldspar was added to the starting material. The pre-eruptive H2O of the Montaña Blanca magma is estimated at 676 ± 200 bars. The pre-eruptive H2O for the Fish Canyon tuff (7532978 bars) and Bishop tuff rhyolite (10652440 bars) were also calculated, as well as 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 biotitemagnetitesanidine is a stable assemblage. Keywords: Crystallization experiments, geohygrometer, phonolitic magmas, water fugacit