Fe-Mn interdiffusion in aluminosilicate garnets

Precise determination of cation diffusivity in garnet can provide critical information for quantitatively understanding the timescales and thermodynamics of various geological processes, but very few studies have been performed for Fe-Mn interdiffusion. In this study, Fe-Mn interdiffusion rates in natural single crystals of Mn-bearing garnet with 750 ppm H₂O are determined at 6 GPa and 1273-1573 K in a Kawai-type multi-anvil apparatus. Diffusion profiles were acquired by electron microprobe and fitted using Boltzmann-Matano equation. The experimental results show that the Fe-Mn interdiffusion coefficient (D_Fe-Mn) slightly decreases with increasing X_Fe. The experimentally determined D_Fe-Mn in Mn-bearing garnet can be fitted by the Arrhenius equation: D_Fe-Mn(m²/s)=D₀X_Feⁿexp(-E^*/RT), where E^*=(1-X_Fe)E^*_Mn+X_FeE^*_Fe, D₀ = 8.06_-6.04^+9.87×10^-9 m²/s,E^*_Mn = 248 ±27 KJ/mol,E^*_Fe = 226 ±59 KJ/mol, n = -1.36 ±0.51. The comparing the present results with previous experimental data suggest that water can greatly enhance the D_Fe-Mn in garnet. Our results indicate that the time required for homogenization of the compositional zoning of a garnet is much shorter than previously thought.