HR: 08:25h
AN: T41B-02 INVITED     [PDF]
TI: The Effect of Magnesium on Diffusion in Calcite Under Static and Deformational Conditions
AU: * Herwegh, M
EM: herwegh@geo.unibe.ch
AF: Institute of Geological Sciences, University of Bern Baltzerstr. 1, Bern, 3012 Switzerland
AU: Evans, B
EM: brievans@mit.edu
AF: EAPS, MIT Massachusetts Av. 77, Cambridge, MA 02139 United States
AU: Xiao, X
EM: xhxaio@mit.edu
AF: EAPS, MIT Massachusetts Av. 77, Cambridge, MA 02139 United States
AB: In order to test the influence of solute impurities on diffusion creep and static grain growth, synthetic calcite aggregates with variable Mg contents were tested. The synthetic marbles were produced by HIPing different mixtures of dry calcite and dolomite powders at 1123K for 2 hours. During the HIP, chemically induced grain boundary migration occurred via a combination of diffusion processes and chemically induced grain boundary migration. After HIP, creep tests were performed at 300 MPa and temperatures in the range of 923-1123K. Creep experiments on aggregates with small grain sizes show that the strength of calcite aggregates correlates inversely with dissolved Mg content at flow stresses below 40 MPa. However, under static conditions, grain growth rates decrease with increasing Mg content. Thus, the inverse correlation between strength and Mg content appears to arise from suppression of grain size with increasing Mg content, i. e., the effect of Mg on strength is indirect through an extrinsic control of calcite grain size, rather than an extrinsic control on grain boundary diffusion rate during deformation. Creep tests in the diffusion creep field for samples with identical grain size, but with variable Mg content, revealed no dependence of strength on the Mg content. The creep tests seem to indicate that the rate controlling step during self-diffusion is not affected by changes in the Mg concentration. It is possible that solid solutes are segregated to the calcite grain boundaries and that impurity drag affects the boundary mobility, but not the self-diffusion rate.
DE: 3660 Metamorphic petrology
DE: 3902 Creep and deformation
DE: 5112 Microstructure
DE: 5120 Plasticity, diffusion, and creep
DE: 8020 Mechanics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting