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Experimentally deformed hornblende displays a transition to dislocation‐mediated deformation approaching 800°C at a pressure of 1 GPa; Dislocation‐mediated deformation is accompanied by a transition in intragrain‐misorientation axis from [001] to [010]; At 800°C, fractures nucleate from dislocation structures
Abstract:
Hornblende amphibole is difficult to deform plastically in experiments due to its anisotropic nature and breakdown at relatively low temperatures (∼850°C). The lack of experimental analysis of hornblende plasticity hampers interpreting the deformation mechanisms of natural samples, which remain unresolved and debated. Here, we used strongly textured amphibolite, oriented for the activation of hornblende's cleavage and/or easy slip system, to investigate the interplay of brittle and plastic deformation mechanisms. Samples with the lineation oriented at 30° to the loading direction were deformed at a confining pressure of 1 GPa, strain rates of 10-5 to 10-4 s-1, and temperatures of 400, 600, and 800°C. Deformed samples exhibit marked tilting of significant subvolumes manifested as kink bands. On the grain scale, deformation is accommodated by fracturing and dislocation mechanisms. A significant decrease in sample strength with temperature is accompanied by an increase in intragrain misorientations due to an increase in dislocation activity. The dominant orientation of the intragrain misorientation axis shifts from [001] at 400°C to [010] at 800°C. Nano‐scale analysis revealed that at 800°C, intragrain misorientation occurs through a sequence in which dislocation structures develop first and then act as sites for fracture nucleation. The observed intragrain misorientation is corroborated by an example from the Javanahalli schist belt (India). We conclude that the experimentally observed transition in the dominant intragrain misorientation axis accompanying the transition from fracture to dislocation‐mediated deformation can be used to interpret conditions experienced by naturally deformed samples.
