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Published August 2007 | metadata_only
Journal Article

Calcium diffusivity in alumino-silicate garnets: an experimental and ATEM study


Concentration gradients in calcium are common in metamorphic or magmatic garnets and can be used to determine the timescales of geological processes. However, the kinetics of Ca diffusion in garnet is poorly constrained and experimental studies have to date yielded widely varying diffusion coefficients. In this paper, we describe a new method for generating diffusion profiles in garnet. We incorporated polished and compositionally homogeneous garnet seeds in a finely ground powder of clinopyroxene and garnet. During the experiments (1.3 GPa, 1,050–1,250°C, and ƒO_2 ≤ the graphite-O_2 buffer), the mineral powder partially melted, recrystallized, and formed a 10–50 μm wide overgrowth zone of compositionally distinct garnet around the seeds. Long duration experiments generated measurable relaxation profiles at these seed/overgrowth interfaces. We performed analytical transmission electron microscope traverses across the interfaces in each experiment. Thirteen usable compositional profiles were obtained with characteristic distances of diffusion ranging from 300 to 1,000 nm. From these profiles, Ca–(Fe, Mg) interdiffusion coefficients were retrieved using an analytical solution for the diffusion equation and the data were cast in an Arrhenius relation. Linear regression of the data yields an activation energy Q_(Ca–(Fe, Mg)) equal to 188 ± 48 kJ mol^(−1) and a frequency factor D_0 equal to 6.6 × 10^(−14) m^2 s^(−1). Within the compositional range studied, the composition of garnet has no major effect on the Ca–(Fe, Mg) interdiffusion coefficient. The very slow diffusion rate of Ca is in agreement with natural observations indicating that Ca diffuses more slowly than Fe and Mg. The Ca diffusion coefficients derived from this study are not model-dependent and can be used to determine the durations of geological events from Ca relaxation profiles in natural garnets.

Additional Information

© 2007 Springer-Verlag. Received: 20 July 2006; Accepted: 25 January 2007; Published online: 27 February 2007. This work was supported by Centre National de la Recherche Scientifique—Institut National des Sciences de l'Univers through grants DyETI 2002 and DyETI 2005 to D.Vielzeuf. We thank J.-L. Devidal and F. Faure for their assistance with the electron microprobe and SEM at LMV, and C. Vanni, W. Saikali, and C. Dominici for their help with the FIB at CP2M. The preparation of the polished garnet seeds benefited from techniques developed by J. L. Paquette (LMV). We performed the ATEM analyses at the French Earth Science TEM facility (Marseille and Lille). We thank A. Addad for his supervision during the analytical sessions at Lille, and P. Cordier for the LACBED images published in this paper. K. Benzerara is gratefully acknowledged for performing a preliminary series of Ca, Fe, and Mg mapping of a FIB foil (Dif1) by Scanning Transmission X-ray Microscopy on a Synchrotron source at Berkeley. A portion of this work was done while D.V. was at Caltech for a year as part of a CNRS/Caltech exchange; E. M. Stolper is gratefully acknowledged for his financial support. A.L. Perchuk acknowledges Université Blaise Pascal for providing a 4-month Visiting Professor position at LMV—Clermont-Ferrand, and the Russian Foundation for Basic Research for financial support (project 06-05-65204). This paper benefited from suggestions by B. Devouard, discussions with P. Asimow, and formal reviews by C.S. Schwandt and S. Chakraborty.

Additional details

August 19, 2023
August 19, 2023