Journal of Earth Science, Vol. 21, No. 5, p. 541–554, October 2010 Printed in China DOI: 10.1007/s12583-010-0113-1
ISSN 1674-487X
Dislocation Creep Accommodated by Grain Boundary Sliding in Dunite Zhongyan Wang Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA Yonghong Zhao School of Earth and Space Sciences, Peking University, Beijing 100871, China; Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, USA David L Kohlstedt* Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, USA ABSTRACT: To investigate the role of grain boundary sliding during dislocation creep of dunite, a series of deformation experiments were carried out under anhydrous conditions on fine-grained (~15 μm) samples synthesized from powdered San Carlos olivine and powdered San Carlos olivine+1.5 vol.% MORB. Triaxial compressive creep tests were conducted at a temperature of 1 473 K and confining pressures of 200 and 400 MPa using a high-resolution, gas-medium deformation apparatus. Each sample was deformed at several levels of differential stress between 100 and 250 MPa to yield strain rates in the range of 10-6 to 10-4 s-1. Under these conditions, the dominant creep mechanism involves the motion of dislocations, largely on the easy slip system (010)[100], accommodated by grain boundary sliding (gbs). This grain size-sensitive creep regime is characterized by a stress exponent of n=3.4±0.2 and a grain size exponent of p=2.0±0.2. The activation volume for this gbs-accommodated dislocation creep regime is V*=(26±3)×10-6 m2·mol-1. Comparison of our flow law for gbs-accommodated dislocation creep with those for diffusion creep and for dislocation creep reveals that the present flow law is important for the flow of mantle rocks with grain sizes of 20 MPa. Hence, gbs-accommodated dislocation creep is likely to be an important deformation mechanism in deep-rooted, highly localized shear zones in the lithospheric upper mantle. KEY WORDS: grain boundary sliding, creep, olivine, flow law.
This study was supported by the National Science Foundation of USA (No. EAR-0910687), and the National Natural Science Foundation of China (No. 40874043). *Corresponding author:
[email protected] © China University of Geosciences and Springer-Verlag Berlin Heidelberg 2010 Manuscript received April 4, 2010. Manuscript accepted May 20, 2010.
INTRODUCTION Grain boundary sliding (gbs) is an important mechanism of deformation in many rocks deforming by dislocation creep processes (e.g., Precigout et al., 2007; Warren and Hirth, 2006; Drury, 2005; Goldsby and Kohlstedt, 2001; Fliervoet et al., 1997; Rutter et al., 1994; Gilotti and Hull, 1990; Schmid et al., 1987, 1977; Behrmann, 1985; Zeuch, 1984; Etheridge and Wilkie, 1979; Boullier and Gueguen, 1975). For olivine, this point was emphasized by Hirth and
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Kohlstedt (1995b) who observed that the strength of olivine-rich aggregates increased with increasing grain size in deformation experiments carried out in a creep regime in which the stress exponent, n, was ~3.5. Based on an analysis of creep results from a number of laboratory studies, these researchers subsequently proposed that strain rate, ε , is proportional to one over grain size, d, squared, ε ∝ 1/ d 2 (Hirth and Kohlstedt, 2003). These results are broadly consistent with models for gbs-accommodated dislocation creep in which n σ ε = A1 (T , P, fO , f H O , aox , φ ,...) p d 2 2
(1)
with both n and p>1 (Langdon, 1994, 1970; Gifkins, 1976; Mukherjee, 1971). In equation (1) the material-dependent parameter, A1 is a function of temperature T, pressure P, oxygen fugacity fO2, water fugacity fH2O, component oxide activities aox, melt fraction φ, and other appropriate thermodynamic and structural parameters. In the earth sciences, possibly the most complete study of the importance of grain boundary sliding during deformation dominated by dislocation processes was carried out on ice (Goldsby, 2006; Goldsby and Kohlstedt, 2001). Based on the results of an extensive set of experiments performed over wide ranges in stress, strain rate and grain size, these authors proposed that the strain rate of deforming polycrystalline ice can be described by contributions from diffusional creep, εdiff , grain boundary sliding, εgbs , and dislocation creep, εdisl , all operating in parallel with one another and with grain boundary sliding operating in series with dislocation activity on the weakest slip system (i.e., basal slip in ice) with strain rate εeasy −1
ε = εdiff
⎛ 1 1 ⎞ +⎜ + ⎟⎟ + εdisl ⎜ ε ⎝ easy εgbs ⎠
(2)
As pointed out by Raj and Ashby (1971), diffusion creep is necessarily accompanied by grain boundary sliding and might appropriately be called diffusionaccommodated grain boundary sliding, with lattice or grain boundary diffusion (the accommodation process) controlling the rate of grain boundary sliding. In the dislocation creep field, all of the strain is obtained by dislocation glide and climb. In the regime involving sliding on grain boundaries combined with dislocation
movement on the easy slip system, the rate of deformation is controlled by the slower of the two processes. Laboratory deformation experiments of ice samples identified all of the regimes indicated by equation (2) except for diffusion creep. The dislocation creep regime was characterized by ε ∝ σ 4.0 /d 0 , the easy (basal) slip-accommodated gbs regime was described by ε ∝ σ 2.4 /d 0 , and the gbs-accommodated easy slip regime was represented by ε ∝ σ 1.8 /d 1.4 . To further explore deformation in the grain boundary sliding-easy slip regime in dunite, we present a new set of creep results on olivine aggregates, some with and others without a small amount of MORB (mid-ocean ridge basalt), all deformed under anhydrous conditions. These data are analyzed to determine the values n and p in the gbs-accommodated easy slip creep regime. The results are compared to published flow laws for deformation of olivine-rich aggregates in the gbs-accommodated easy slip creep regime. Finally, implications for deformation of the earth’s upper mantle are discussed. EXPERIMENTAL PROCEDURES Sample Fabrication Optically clear crystals of San Carlos olivine, (Fe0.1Mg0.9)2SiO4, were hand picked on a light box to avoid inclusions and other visible impurities. These olivine crystals were ground into a coarse powder using a vibrating stainless steel shatterbox. A finer powder was then produced by pulverization in a fluid energy mill in which two streams of the coarse powder, accelerated by compressed air, collide with each other. A still finer powder was created by recycling the ground particles back through the mill. Particle size was determined with a laser particle-size analyzer. The powder used for hot-pressing samples had an average particle size of ~2 μm with a maximum particle size of
