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The effects of grain size on the strength of magnesite aggregates deforming by low temperature plasticity and diffusion creep

McDaniel, Caleb Alan

Abstract Details

2018, Master of Science, University of Akron, Geology.
Magnesite is a common mineral found in altered peridotites and is much weaker than olivine, which may cause strain localization and deep focus earthquakes (Holyoke et al., 2014). Holyoke et al. (2014) determined the temperature and strain rate dependence of the strength of magnesite deforming by low temperature plasticity and diffusion creep mechanisms, but did not determine the grain size dependence of the strength of magnesite deforming by these mechanisms. In order to determine whether the strength of magnesite depends on grain size (d), we performed experiments on magnesite aggregates with d = 3, 6, 16 and 80 µm for constant values of strain rate = 10-5/s at temperature (T) = 500 - 700 °C, and pressures (P) of 0.2, 0.9, 2.4, 5.0 and 8.0 GPa, in the Griggs Apparatus and Deformation-DIA (D-DIA). At low temperatures microstructures were consistent with low temperature plasticity, which include extensive mechanical twinning. Strength of magnesite aggregates deformed at T = 500 °C and P = 0.9 GPa, σ decreases from 2.40 to 1.60 GPa as d increases from 3 to 80 µm. At P = 2.4 GPa, σ decreases from 1.72 to1.60 GPa as d increases over the same range. At P = 5.0 GPa, σ decreases from 2.73 to 2.69 GPa as d increases from 3 to 16 µm. At P = 8.0 GPa, strengths are the same at all grain sizes. As pressure increases the grain size dependence of σ decreases. The grain size sensitivity exponent mLTP decreases from mLTP = 0.25 (at P = 0.9 GPa) to mLTP = 0.08 (P = 2.4 GPa) to mLTP = 0.008 (P = 5.0 GPa) and mLTP = 0 (P = 8.0 GPa). These results demonstrate that the grain size sensitivity of low temperature plasticity deformation of magnesite is a function of pressure, and by inference, which may also be the case for low temperature plasticity of calcite. These results indicate that along much of the P-T path of subducting slabs, grain size will not significantly affect magnesite aggregate strength if deforming by low temperature plasticity. At higher temperatures (T = 700 °C) and low pressures (P = 300 MPa) the microstructures present are rounded grains and pore spaces around the grain boundaries which is consistent with diffusion creep deformation mechanism. When magnesite is deforming by diffusion creep the grain size sensitivity md is 1.0. The evolution of microstructures has a significant effect on the strength of the magnesite. Increased time at high temperatures causes an evolution from angular to rounded grains which caused the strength to decrease. When extrapolated to natural conditions, the dominant deformation mechanism occurring in the subducting slab is diffusion creep. Furthermore, the viscosity of magnesite increases as the grain size increases, which could potentially cause strain localization in fine-grained zones of magnesite leading to the formation of deep focus earthquakes.
Caleb Holyoke, III (Advisor)
LaVerne Friberg (Committee Member)
John Senko (Committee Member)
69 p.

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Citations

  • McDaniel, C. A. (2018). The effects of grain size on the strength of magnesite aggregates deforming by low temperature plasticity and diffusion creep [Master's thesis, University of Akron]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=akron1526571269872349

    APA Style (7th edition)

  • McDaniel, Caleb. The effects of grain size on the strength of magnesite aggregates deforming by low temperature plasticity and diffusion creep. 2018. University of Akron, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=akron1526571269872349.

    MLA Style (8th edition)

  • McDaniel, Caleb. "The effects of grain size on the strength of magnesite aggregates deforming by low temperature plasticity and diffusion creep." Master's thesis, University of Akron, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=akron1526571269872349

    Chicago Manual of Style (17th edition)