NDLI: Rates of grain boundary diffusion through enstatite and forsterite reaction rims

Content Provider	Springer Nature Link
Author	Yund, Richard A.
Copyright Year	1996
Abstract	The growth rates of enstatite rims produced by reaction of Fo92 and SiO2 were determined at 250–1500 MPa and 900–1100°C for a wide range of water contents. Growth rates were also determined for forsterite rims between MgO and Mg2Si2O6 and between MgO and SiO2. Rim growth rates are parabolic indicating diffusion-controlled growth of the polycrystalline rims which are composed of ˜ 2 μm diameter grains. Rim growth rates were used to calculate the product of the grain boundary diffusion coefficient (D'A) times the effective grain boundary thickness (δ) assuming in turn that MgO, SiO2, and Mg2Si−1 are the diffusing components (coupled diffusion of a cation and oxygen or interdiffusion of Mg and Si). The values for D'MgOδ, D' , and D' for enstatite at 1000°C and 700 MPa confining pressure with about 0.1 wt % water are about five times larger than the corresponding D'Aδ values for samples initially vacuum dried at 250°C. Most of the increase in D'Aδ occurs with the first 0.1 wt % water. The activation energy for diffusion through the enstatite rims (1100–950°C) is 162 ± 30 kJ/mole. The diffusion rate through enstatite rims is essentially unchanged for confining pressures from 210–1400 MPa, but the nucleation rate is greatly reduced at low confining pressure (for ≤ 1.0 wt % water present) and limits the conditions at which rim growth can be measured. The corresponding values for D'Aδ through forsterite rims are essentially identical for the two forsterite-producing reactions when 0.1 wt % water is added and similar to the D'Aδ values for enstatite at the same conditions. The D'Aδ values for forsterite are ˜ 28 times larger for samples starting with 0.1 wt % water compared to samples that were first vacuum dried. Thus water enhances these grain boundary diffusion rates by a factor of 5–30 depending on the mineralogy, but the total range in D'Aδ is only slightly more than an order of magnitude for as wide a range of water contents as expected for most crustal conditions.
Starting Page	224
Ending Page	236
Page Count	13
File Format	PDF
ISSN	00107999
Journal	Contributions to Mineralogy and Petrology
Volume Number	126
Issue Number	3
e-ISSN	14320967
Language	English
Publisher	Springer-Verlag
Publisher Date	1997-01-23
Publisher Place	Berlin/Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Content Type	Text
Resource Type	Article
Subject	Geochemistry and Petrology Geophysics

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