High-temperature magnetic fabric development from plastically deformed magnetite in experimental shear zones

Magnetic fabric development has been studied in synthetic magnetite-silicate aggregates in a series of high-temperature simple shear deformation experiments. Samples composed of magnetite grains with a nominal size of 20-40 μm dispersed at 3 wt % in a matrix of plagioclase were deformed between 1000 and 1200°C with a confining pressure of 300 MPa, shear stresses in the range 10-130 MPa, and shear strains up to γ= 3. We determined that both magnetite and plagioclase were deformed plastically at these conditions. An oblate magnetic fabric ellipsoid develops subparallel to the shear plane and the degree of AMS rapidly increases with strain up to a value of 2.5. Theoretical strain response models of magnetic fabric development were applied to the data to obtain estimated magnetite strains. The shape-preferred orientation of magnetite grains after deformation, determined from reflected light image analysis, was used to calculate independent magnetite strain estimates. These results were then compared with strains estimated from theoretical magnetic anisotropy development. Both strain estimation methods indicate strain partitioning between magnetite and the plagioclase matrix and the results are used to determine approximate viscosity ratios between the two phases at the experimental conditions.