Crystal-Liquid Equilibria In Volatile-Bearing Systems: The Legacy of Tuttle and Bowen (1958)

The Tuttle and Bowen 1958 paper marked the beginning of a new period in experimental petrology. Quartz and alkali feldspar-liquid equilibria were investigated systematically at high pressures in presence of an hydrous vapour phase. The study combined the use of the at the time newly designed cold-seal pressure vessel with theoretical and practical background inherited from simple silicate systems at 1 atm. Yet, the compositions investigated, although mostly synthetic, were close analogs of natural granites and rhyolites. Thus, the 1958 paper had a major impact on granite petrology. From an experimental point of view, the study provided a basis from which subsequent phase equilibria have been undertaken. In the haplogranite system, efforts have concentrated on the determination of vapour-present crystal-liquid equilibria with volatiles (F, B, Li, P) added to H2O. The realization that silicic magmas are probably never H2O-saturated prompted the effect of isobaric changes of the melt H2O content on phase equilibria to be investigated. Studies in simple systems have evolved towards the determination of the effects of volatiles on the structure and physical properties of glasses and melts, and towards the modelling of crystal-liquid equilibria. Yet, it is worth pointing out that experimental data are still scarce for the peralkaline haplogranite and for the haplogranodiorite systems. For natural felsic magma compositions, experimental phase equilibria have been used to constrain crystallization sequences and intensive variables, and to test the equilibrium state of silicic magma bodies. The development of techniques of control of fO2 have extended the determination of crystal-liquid equilibria to phases such as Fe-Ti oxides and Fe-Mg silicates. The field is now evolving toward systematic investigations of the effects of H2O, CO2 and halogens on phase equilibria for intermediate to mafic compositions such as andesites and basalts.