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[1] Variations in the isotopic content (18 O/ 16 O and D/H ratios) of water in the natural environment provide a valuable tracer of the present-day global hydrologic cycle and a record of the climate over at least 400,000 years that is preserved in glacial ice. The interpretation of observed isotopic ratios in water vapor, rain, snow, and ice depends on our understanding of the processes (mainly phase changes) that produce isotopic fractionation. Whereas equilibrium isotopic fractionation is well understood, kinetic effects, or diffusion-controlled fractionation, has a limited experimental foundation. Kinetic effects are significant during evaporation into unsaturated air and during condensation to form ice from vapor. Kinetic effects are also thought to control the deuterium excess (d = dD À 8d 18 O) of precipitation. We describe experiments to observe kinetic effects associated with evaporation. Analysis of our own and previous experiments shows that surface cooling of the liquid is a crucial variable affecting fractionation from evaporating water that has not been properly considered before.

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The relatively low rates of magma production in island arcs and continental extensional settings require that the volume of silicic magma involved in large catastrophic caldera-forming (CCF) eruptions must accumulate over periods of 10 5 to 10 6 years. We address the question of why buoyant and otherwise eruptible highsilica magma should accumulate for long times in shallow chambers rather than erupt more continuously as magma is supplied from greater depths. Our hypothesis is that the viscoelastic behavior of magma chamber wall rocks may prevent an accumulation of overpressure sufficient to generate rhyolite dikes that can propagate to the surface and cause an eruption. The critical overpressure required for eruption is based on the model of Rubin (1995a). An approximate analytical model is used to evaluate the controls on magma overpressure for a continuously or episodically replenished spherical magma chamber contained in wall rocks with a Maxwell viscoelastic rheology. The governing parameters are the long-term magma supply, the magma chamber volume, and the effective viscosity of the wall rocks. The long-term magma supply, a parameter that is not typically incorporated into dike formation models, can be constrained from observations and melt generation models. For effective wall-rock viscosities in the range 10 18 to 10 20 Pa s -1 , dynamical regimes are identified that lead to the suppression of dikes capable of propagating to the surface. Frequent small eruptions that relieve magma chamber overpressure are favored when the chamber volume is small relative to the magma supply and when the wall rocks are cool. Magma storage, leading to conditions suitable for a CCF eruption, is favored for larger magma chambers (>10 2 km 3 ) with warm wall rocks that have a low effective viscosity. Magma storage is further enhanced by regional tectonic extension, high magma crystal contents, and if the effective wall-rock viscosity is lowered by microfracturing, fluid infiltration, or metamorphic reactions. The longterm magma supply rate and chamber volume are important controls on eruption frequency for all magma chamber sizes. The model can explain certain aspects of the frequency, volume, and spatial distribution of smallvolume silicic eruptions in caldera systems, and helps account for the large size of granitic plutons, their association with extensional settings and high thermal gradients, and the fact that they usually post-date associated volcanic deposits.

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