Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

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Fractal Analysis of Magnetite Grains - Implications for Interpreting Deformation Mechanism


Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur – 721302, India
     

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In the present study, the grain size (d) and shape of 225 magnetite grains, that crystallized at T>600°C in a syntectonic granite (Godhra Granite, India) are evaluated and implications of data to decipher deformation mechanism of magnetite are discussed. Fractal (ruler) dimension (D) analysis of magnetite grains is performed and it is demonstrated that they show fractal behaviour. Smaller magnetite grains tend to be more serrated than the larger ones, which is manifested in the higher fractal (ruler) dimension (D) of the former. Assuming a natural strain rate ranging between 10-10 s-1 and 10-14 s-1 , the grain size data fall dominantly in the dislocation creep field of the existing deformation mechanism map of magnetite for 630°C. However, SEM-EBSD studies reveal that subgrains are absent in the magnetite grains and they did not undergo dislocation creep. Thus it is inferred that the shape of magnetite grains was not controlled by dislocation creep. It is concluded that the higher serration and increased fractal dimension of finer magnetite grains implies the importance of diffusion creep as an important deformation mechanism at high-T for magnetite in polymineralic rocks.

Keywords

Magnetite, Granite, Deformation Mechanism, Dislocation Creep, Diffusion Creep, Fractals.
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  • Fractal Analysis of Magnetite Grains - Implications for Interpreting Deformation Mechanism

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Authors

Manish A. Mamtani
Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur – 721302, India

Abstract


In the present study, the grain size (d) and shape of 225 magnetite grains, that crystallized at T>600°C in a syntectonic granite (Godhra Granite, India) are evaluated and implications of data to decipher deformation mechanism of magnetite are discussed. Fractal (ruler) dimension (D) analysis of magnetite grains is performed and it is demonstrated that they show fractal behaviour. Smaller magnetite grains tend to be more serrated than the larger ones, which is manifested in the higher fractal (ruler) dimension (D) of the former. Assuming a natural strain rate ranging between 10-10 s-1 and 10-14 s-1 , the grain size data fall dominantly in the dislocation creep field of the existing deformation mechanism map of magnetite for 630°C. However, SEM-EBSD studies reveal that subgrains are absent in the magnetite grains and they did not undergo dislocation creep. Thus it is inferred that the shape of magnetite grains was not controlled by dislocation creep. It is concluded that the higher serration and increased fractal dimension of finer magnetite grains implies the importance of diffusion creep as an important deformation mechanism at high-T for magnetite in polymineralic rocks.

Keywords


Magnetite, Granite, Deformation Mechanism, Dislocation Creep, Diffusion Creep, Fractals.

References