An experimental study of Ca-(Fe,Mg) interdiffusion in silicate garnets |
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Authors: | R Freer A Edwards |
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Institution: | (1) Materials Science Centre, University of Manchester/UMIST, Manchester M1 7HS, UK; Fax: 0161-200-3586, E-mail: Robert.Freer@umist.ac.uk, GB |
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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
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.
Received: 15 December 1996 / Accepted: 3 November 1998 |
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