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 viscous shearing imposed by a flowing host magma acting to deform and disperse the enclave. We model the time required to grow a rigid chilled margin and 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 the thermal and deformation timescales Pe=σapp3a2/(κμsσr2) and the dimensionless rigidification temperature θ=(Tr−Ts)/(Tm−Ts) where σapp is the applied shear stress, a is the enclave radius, κ is the thermal diffusivity, μs is the viscosity of the host, σr is the strength of the chilled rind, Tr is the temperature at which the enclave attains strength σr and Tm and Ts are the initial temperatures of the mafic enclave and silicic host. According to the model, small values of Pe and large values of θ promote rapid rigidification. The model is verified by laboratory experiments on the flow and freezing of polyglycol wax droplets in cold water. Geological observations show that correlations between enclave size, degree of deformation and local shear stress match the modelʹs predictions. Studies of enclave size and shape as functions of eruption rate and position within lava flows and minor intrusions offer a new technique for studying magma flow processes during eruptions.