Changes in flow strength induced by the decomposition of polycrystalline albite

Phase transformations of minerals have an important role on the rheology of earth's crust and mantle. Flow properties and the dominant mechanism of deformation are possibly affected through changes of the crystal structure, grain size and polycrystalline texture during the transformation. In this study, we have carried out in-situ X-ray diffraction experiments on the high-pressure decomposition reaction from albite, one end member of plagioclase, into jadeite and quartz under uniaxial differential stress. High-pressure deformation experiments were conducted using multi-anvil type deformation apparatus D-CAP 700 installed at the NE-7 beamline of PF-AR, KEK. The synthesized polycrystalline albite was annealed at the P-T conditions near the equilibrium phase boundary of albite decomposition. Then, the sample was deformed with the constant strain rates of 0.3-6.1×10-5/s at 1-4 GPa and 673-1073 K. Confining pressure increased during the deformation. The plastic deformation and high-pressure transformation processes were simultaneously observed by time-resolved two-dimensional X-ray diffraction (2DXRD) measurements using monochromatic X-ray (energy 50 keV) and imaging plate (IP). 2DXRD patterns were used to obtain the transformed fraction and the differential stress of the sample that was estimated from the distortion of the Debye ring (azimuthal angle-dependence of d-values) on IP. Plastic strain of the sample was measured from the X-ray radiography image. Deformation mechanism of polycrystalline albite was investigated from the results of 7 samples deformed at about 2.5 GPa and 873-1073K. Constant flow stress of albite was obtained after 5 percent axial strain. The stress exponent n was estimated to be 2.3 from the relationships of the steady-state flow stress and strain rate. The n-value and the elongated shape of albite grains suggest that the dominant deformation mechanism is dislocation creep. The six samples decomposed into jadeite and quartz during deformation experiments at 873-1073K. The decomposition started when overpressure from the phase boundary reached 0.4-1.4 GPa. FE-SEM observations of recovered samples revealed that the high-pressure phases nucleate at grain boundaries of parental albite and form colonies having a eutectoid structure. The eutectoid colonies are connected each other after about 50 % transformation. These changes in transformation textures affect the flow strength of each phase significantly. The flow stress of albite did not reach constant but increased with the transformed fraction, suggesting that strain hardening occurred in the parental phase during the transformation. On the other hand, the flow stress of newly formed jadeite rapidly dropped by one order of magnitude when the transformed fraction reached about 80 %. We will discuss about the relationship between the flow strength and the microstructural changes due to the transformation.