Equilibration scales in silicic to intermediate magmas - Implications for experimental studies

Experimental phase equilibrium studies are increasingly being used for the determination of intensive variables ( P, T, fH(2)O, fO(2)) in silicic to intermediate magmas. In contrast, silicic igneous bodies are now perceived as open, periodically recharged, systems involving only limited chemical equilibration. Thus, the use of laboratory-determined crystal-liquid equilibrium data needs clarification. Here we review the field, petrological and geochemical evidence concerning states and scales of chemical equilibrium in silicic magma bodies. It is concluded that total chemical equilibrium is generally not the rule. However, a subsystem in local equilibrium ( the reactive magma) can be identified. Equilibration scales in silicic magmas are rate-limited either by diffusive flux in crystals (DICL regime) or by diffusive flux in the melt ( MD regime). The recognition that equilibrium in magmas is limited to a reactive subsystem requires phase equilibrium studies to be chemically scaled. Experiments, either of total or partial equilibrium type, should aim at a close reproduction of equilibrium states specific to natural systems. The laboratory reconstruction of the natural equilibrium states guarantees a precise determination of the pre-eruptive parameters and a reliable application of the experimental data to active volcanic systems.