The effect of dissolved magnesium on creep of calcite II: transition from diffusion creep to dislocation creep |
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| Authors: | Lili Xu Jörg Renner Marco Herwegh Brian Evans |
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| Institution: | (1) Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Mass. Avenue, Cambridge, MA 02139, USA;(2) Ruhr-University Bochum, 44780 Bochum, Germany;(3) Institute for Geology, University Bern, 3012 Bern, Switzerland |
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| Abstract: | We extended a previous study on the influence of Mg solute impurity on diffusion creep in calcite to include deformation under
a broader range of stress conditions and over a wider range of Mg contents. Synthetic marbles were produced by hot isostatic
pressing (HIP) mixtures of calcite and dolomite powders for different intervals (2–30 h) at 850°C and 300 MPa confining pressure.
The HIP treatment resulted in high-magnesian calcite aggregates with Mg content ranging from 0.5 to 17 mol%. Both back-scattered
electron images and chemical analysis suggested that the dolomite phase was completely dissolved, and that Mg distribution
was homogeneous throughout the samples at the scale of about two micrometers. The grain size after HIP varied from 8 to 31 μm,
increased with time at temperature, and decreased with increasing Mg content (>3.0 mol%). Grain size and time were consistent
with a normal grain growth equation, with exponents from 2.4 to 4.7, for samples containing 0.5–17.0 mol% Mg, respectively.
We deformed samples after HIP at the same confining pressure with differential stresses between 20 and 200 MPa using either
constant strain rate or stepping intervals of loading at constant stresses in a Paterson gas-medium deformation apparatus.
The deformation tests took place at between 700 and 800°C and at strain rates between 10−6 and 10−3 s−1. After deformation to strains of about 25%, a bimodal distribution of large protoblasts and small recrystallized neoblasts
coexisted in some samples loaded at higher stresses. The deformation data indicated a transition in mechanism from diffusion
creep to dislocation creep. At stresses below 40 MPa, the strength was directly proportional to grain size and decreased with
increasing Mg content due to the reductions in grain size. At about 40 MPa, the sensitivity of log strain rate to log stress,
(n), became greater than 1 and eventually exceeded 3 for stresses above 80 MPa. At a given strain rate and temperature, the
stress at which that transition occurred was larger for samples with higher Mg content and smaller grain size. At given strain
rates, constant temperature, and fixed grain size, the strength of calcite in the dislocation creep regime increased with
solute content, while the strength in the diffusion creep regime was independent of Mg content. The results suggest that chemical
composition will be an important element to consider when solid substitution can occur during natural deformation. |
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| Keywords: | Rheology Carbonates Constitutive laws Solute strengthening Normal grain growth |
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