On the deformation and freezing of enclaves during magma mixing

Mixing of an enclave of hot mafic magma into cooler silicic magma involves the competing effects of heat transfer acting to rigidify the enclave and v iscous shearing imposed by a flowing host magma acting to deform and disper se the enclave. We model the time required to grow a rigid chilled margin a nd compare this with the time required to deform the initially hot enclave. Whether an enclave will deform before it freezes depends on the ratio of t he thermal and deformation timescales Pe = sigma(app)(3)a(2)/(kappa mu(s)si gma(r)(2)) and the dimensionless rigidification temperature theta = (T-r - T-s)/(T-m - T-s) where sigma(app) is the applied shear stress, n is the enc lave radius, kappa is the thermal diffusivity, mu(s) is the viscosity of th e host, sigma(r) is the strength of the chilled rind, T-r is the temperatur e at which the enclave attains strength sigma(r) and T-m and T-s are the in itial temperatures of the mafic enclave and silicic host. According to the model, small values of Pe and large values of a promote rapid rigidificatio n. The model is verified by laboratory experiments on the flow and freezing of polyglycol wax droplets in cold water. Geological observations show tha t correlations between enclave size, degree of deformation and local shear stress match the model's predictions. Studies of enclave size and shape as functions of eruption rare and position within lava flows and minor intrusi ons offer a new technique for studying magma flow processes during eruption s. (C) 2000 Elsevier Science B.V. All rights reserved.