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[R12-P-5] Extremely slow grain growth of albite feldspar
Keywords:Albite, Grain Growth, Crustal Rheology, Plagioclase
Feldspar is one of the main constituent minerals of the Earth’s crust, and the evolution of grain size is one of the keys in the rheology of the crust. Therefore, grain growth of albite was experimentally examined at temperatures of 1030, 1050 and 1065 ºC, at the atmospheric pressure, and under anhydrous conditions. Previous results of anorthite grain growth (Dresen et al., 1996) were also examined to compare with the results in this study. Grain growth of albite follows the grain growth law. Grain growth exponent of 2.6 was assumed. The least squares regression of the results of albite yields the activation energy of Q=573.3±27.2 kJ/mol and the pre-exponent of the rate constant k0=100.53±1.08 m2.6s-1. Previous experimental results of anorthite grain growth yield the activation energy of Q=191.8±22.6 kJ/mol and the pre-exponent of k0=10-10.3±0.79 m2.6s-1. The obtained grain growth of albite is four or five orders of magnitude slower than that of anorthite. Once fine-grained feldspar is formed, these grains do not grow even after 1 million year even at the temperature of 700 ºC according to our anhydrous grain growth law. The extremely slow grain growth of albite found in this study can be explained by loading of grain boundary migration by a dragging force exerted by solute impurity atoms. To apply our results to the natural conditions, the grain growth law under anhydrous conditions was estimated based on several previous studies. Even under hydrous conditions, grain growth is very slow in the middle to upper crust. It can be considered that this slow grain growth affects the mechanical behaviour of crustal rocks and results in the maintenance of shear zones for long period in the crust.
Dresen, G., Wang, G. and Bai, Q.: Tectonophysics, 258, 251–262.
Dresen, G., Wang, G. and Bai, Q.: Tectonophysics, 258, 251–262.