Diffusion-controlled growth of albite and pyroxene reaction rims |
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| Authors: | M Liu J C Peterson Richard A Yund |
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| Institution: | (1) Department of Geological Sciences, Brown University, Providence, RI 02912, USA, US |
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| Abstract: | The growth rates of albite and pyroxene (enstatite + diopside + spinel) reaction rims were measured at 1000°C and ˜700 MPa
and found to be parabolic indicating diffusion-controlled growth. The parabolic rate constants for the pyroxene (+ spinel)
rims in samples with 0.5 wt% H2O added or initially vacuum dried at 25°C and 250°C are 1.68 ± 0.09, 0.54 ± 0.05 and 0.25 ± 0.06 μm2/h, respectively. The values for albite rim growth in samples initially dried at 60°C and with 0.1 wt% H2O added are 0.25 ± 0.04 and 0.33 ± 0.03 μm2/h, respectively. The latter values were used to derive the product of the grain boundary diffusion coefficient D′A, where A = SiO2, NaAlO2, or NaAlSi−1, and the grain boundary thickness δ in albite. The calculated D′SIO2δ in the albite aggregate for the situations of two different water contents are about 9.9 × 10−23 and 1.4 × 10−22 m3 s−1, respectively. Both the rate constants and the calculated D′Aδ demonstrate that the effect of water content on the grain boundary diffusion rate in monomineralic albite and polymineralic
pyroxene (+ spinel) aggregates is small, consistent with recent studies of monomineralic enstatite and forsterite rims.
Received: 1 July 1995 / Accepted: 1 August 1996 |
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