Multicomponent cation diffusion in aluminosilicate garnets: Theory, experiments and applications.

Abstract

Results from experiments in this work have been combined with existing data to yield tracer diffusivities of divalent cations in natural multicomponent garnets at 10-40 kb, 1100 - 1475°C. For the garnet compositions studied in this work, the activation energy for tracer diffusion of Mn < Fe ≃ Mg, while activation volumes increase in the order Mg < Fe < Mn. These data may be used with theoretical models to calculate the full multicomponent diffusion coefficient matrix as a function of pressure, temperature, composition and oxygen fugacity. An analytical model has been developed to describe the relaxation of compositional zoning in metapelitic garnets during metamorphism. It is found that a significant amount of relaxation occurs during heating and the composition at the core of a garnet crystal is disturbed very early in the process. The extent of relaxation depends on a number of factors which include the grain size, the initial shape of the compositional profile, details of the thermal history and the textural mode of occurrence of the garnet. Complications such as oscillations in a compositional profile may be explained by the mathematical form of the equations describing the process. Graphical representation of the results of this model have been provided that allow quick and easy determination of the extent of relaxation or time scale of metamorphic processes. Calculations using the diffusion data indicate that Fe-Mg exchange geothennometry and Sm-Nd geochronology involving garnets may not yield the peak metamorphic conditions for certain geologically realistic situations. Model calculations on natural assemblages suggest that the diffusion data may be used to yield infonnation on metamorphic processes ranging from time scales to crystal growth rates during metamorphism. Veracity of P-T paths calculated from garnet zoning may also be checked using the diffusion data. Illustrative examples of such calculations have been provided.