Transformation experiment on sodium feldspar using deformation-Cubic Anvil, D-CAP 700, with synchrotron X rays

The gabbro-eclogite transformation has an important role on the dynamics of subducting oceanic crust, because this transformation involves large density increase and probably changes the rheological property. As plastic deformation and high-pressure transformation are influenced each other, it is important to understand the coupling of these processes. Decomposition of plagioclase is one of the principal reactions in the gabbro-eclogite 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 of albite were conducted using multi-anvil type deformation apparatus (D-CAP 700) installed at the 14C2 beamline of Photon Factory, KEK. 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 (50 keV) and imaging plate. 2DXRD patterns are used to obtain the transformed fraction and lattice strain of the sample. Plastic strain of the sample was measured from the X-ray radiography images. Synthesized polycrystalline albite with grain size of about ~20micron was uniaxialy deformed at 1-4 GPa, 673-973 K, and constant strain rate of 0.6-3.3×10^-5/s. The maximum axial strain of sample was amounted to about 30% and differential stress calculated from diffraction planes of albite was 0.6-1.7GPa. The water content of recovered samples was about 0.01-0.13 weight percent H2O. Flow strengths of albite obtained in the present study were much lower than those expected from the previous study in the dislocation creep regime at 0.3 GPa and higher temperature conditions (Offerhaus et al., 2001). This implies that the Peierls mechanism (low-temperature plasticity) is a dominant deformation mechanism in albite under experimental conditions of the present study. High-pressure transformation of albite occurred above 873 K during plastic deformation. SEM observations of reacted samples revealed that reaction products consisting of sub-micron fine grains of jadeite and quartz preferentially grow perpendicular to the direction of the first principal stress. Nucleation of high-pressure phases occurred at grain boundaries of albite without forming reaction rims, which suggests that the nucleation rate is not so high. Transformation rates observed in the present deformation experiments are much slower than those in the previous experiments containing higher water contents of approximately 0.5 weight percent H2O without deformation (Kubo et al., 2006).