An experimental study of Ca-(Fe,Mg) interdiffusion in silicate garnets

Ca-(Fe,Mg) interdiffusion experiments between natural single crystals of gr ossular (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 degrees C and 30 kbar, and pressures of 15.0- 32.5 kbar at 1000 degrees C. Samples were buffered by Fe/FeO in most cases. Diffusion profiles were determined by electron microprobe. Across the expe rimental couples the interdiffusion coefficients ((D) over tilde) were almo st independent of composition. The diffusion rates in an unbuffered sample were significantly faster than in buffered samples. The temperature depende nce of the (D) over tilde (Ca-Fe,Mg) interdiffusion coefficients may be des cribed by (D) over tilde = 1.22(-1.00)(+6.44) x 10(-6) exp(-270.4 +/- 19.3 kJ/RT) m(2 )s(-1) at 30 kbar and 900-1100 degrees C, This activation energy is marginally hig her than previous experimental studies involving Ca-free garnets; the inter diffusion coefficients are higher than previous studies for Fe-Mg and Fe-h in exchange in garnet. The pressure dependence of (D) over tilde (Ca-Fe,Mg) at 1000 degrees C yielded an activation volume of 11.2 cm(3) mol(-1), whic h is higher than previous results from studies involving garnet and olivine . Comparison with simulation studies suggests a vacancy mechanism for dival ent ion migration in garnet, with extrinsic processes being dominant up to very high temperatures.