Lattice preferred orientation development in magnesite aggregates
Abstract
Recent deformation experiments on magnesite aggregates have evaluated the flow laws for deformation mechanisms of magnesite, a common peridotite alteration product. Flow law parameters for magnesite are nearly identical to those of calcite deforming by the same deformation mechanisms, reflecting their common crystal symmetries and structures. In contrast, the flow law parameters for dolomite, which has different crystal symmetry than calcite and dolomite, deformed by the same mechanisms are very different than the Mg and Ca carbonate end members.
In order to determine if the lattice preferred orientations (LPO) of magnesite aggregates deformed by low temperature plasticity (LTP) mechanisms (kinking and dislocation glide), diffusion creep, or dislocation creep are similar to those of calcite and dolomite, we have analyzed a suite of deformation experiments using electron backscatter diffraction (EBSD). These triaxial experiments were performed on two natural magnesite aggregates (d 3 and 80 μm) over a wide range of pressures (Peff = 0.76 - 7.5 GPa), a strain-rate of 10-5/s and temperatures (T) = 500, 750, and 900°C, using the D-DIA at Beamline 6-BMB of the Advanced Photon source at Argonne National Lab or the Griggs apparatus at the University of Akron. Optical and scanning electron microscope-scale microstructures in these sets of samples deformed at T = 500, 750, and 900°C are consistent with deformation by LTP mechanisms, diffusion creep and dislocation creep, respectively. EBSD analyses of the magnesite starting materials indicate that both have a random LPO. EBSD analyses of the 500oC samples indicate that c-axes form a strong point maximum subparallel to the compression direction and a-axes form a girdle. EBSD analyses of the 750oC samples indicate that c-axes form a diffuse point maximum 35 degrees to the compression direction and a-axes form a diffuse girdle. EBSD analyses of the 900oC samples indicate that c-axes in coarse grains form a diffuse point maximum parallel to the compression direction, whereas recrystallized grains form a strong point maximum 40 degrees from the compression direction. These magnesite LPOs are generally consistent with those observed in calcite, but are considerably stronger than those observed in dolomite deformed by the same deformation mechanisms.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T21D0224H
- Keywords:
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- 3902 Creep and deformation;
- MINERAL PHYSICSDE: 3621 Mantle processes;
- MINERALOGY AND PETROLOGYDE: 8120 Dynamics of lithosphere and mantle: general;
- TECTONOPHYSICS