AN EXPERIMENTAL STUDY OF CA-(FE,MG) INTERDIFFUSION IN SILICATE GARNETS

Abstract

Ca-(Fe,Mg) interdiffusion experiments between natural single crystals of grossular (Ca2.74Mg0.15 Fe0.23Al1.76Cr0.04Si3.05O12) and almandine (Ca0.21Mg0.40 Fe2.23Mn0.13Al2.00Cr0.08Si2.99O12 or Ca0.43Mg0.36Fe2.11 Al1.95Si3.04O12), were undertaken at 900-1100 °C and 30 kbar, and pressures of 15.0-32.5 kbar at 1000 °C. Samples were buffered by Fe/FeO in most cases. Diffusion profiles were determined by electron microprobe. Across the experimental couples the interdiffusion coefficients (D˜) were almost independent of composition. The diffusion rates in an unbuffered sample were significantly faster than in buffered samples. The temperature dependence of the D˜ (Ca-Fe,Mg) interdiffusion coefficients may be described by<formula form="DISPLAY"><![IGNORE[$$\tilde D=1.22^{+6.44}_{-1.00}\times 10^{-6} \exp (-270.4 \pm 19.3\,{\hbox{kJ/R}}T) \, \hbox{m}^2 \hbox{s}^{-1}\fleqno{}$$]]></formula> at 30 kbar and 900-1100°C. This activation energy is marginally higher than previous experimental studies involving Ca-free garnets; the interdiffusion coefficients are higher than previous studies for Fe-Mg and Fe-Mn exchange in garnet. The pressure dependence of D˜ (Ca-Fe,Mg) at 1000 °C yielded an activation volume of 11.2 cm3 mol-1, which is higher than previous results from studies involving garnet and olivine. Comparison with simulation studies suggests a vacancy mechanism for divalent ion migration in garnet, with extrinsic processes being dominant up to very high temperatures.