Combined Rb-Sr and U-Th evidence for protracted pre-eruptive magma formation of rhyolites.

The dynamics of crustal magma reservoirs are controlled by chemical differentiation of the magma in response to numerous processes. It is generally considered that crystal-liquid separation is dominant. Fluid dynamic models increasingly envisage magma chambers as open, dynamic multicomponent systems subject to the interplay of melts and crystal mushes. Boundary-layer crystallization is thought to occur along the colder walls of a magma chamber where the positive buoyancy of residual interstitial melt causes ascent of differentiated liquids through the crystal mush and separation from the less evolved bulk magma volume. Low-density melts fractionated in such a way can accumulate near the top of a magma reservoir and reside there, if isolated from convection or prevented from crystal l izing. Theoretical models often require parametric assumptions or simplifications, such that they essentially describe stationary processes and therefore lack viable predictions for the temporal evolution of magmatic systems. In contrast, analyses of radio-