Grain-growth kinetics in wadsleyite: Effects of chemical environment

Abstract

Grain-growth kinetics in wadsleyite was investigated using a multianvil high-pressure apparatus. Fine-grained wadsleyite aggregates were synthesized by isostatic hot-pressing and were subsequently annealed under high pressure and temperature in a controlled chemical environment. Wadsleyite samples show normal grain-growth characterized by a log-normal grain-size distribution following the relation, L n − L 0 n = k t where n is a constant, L the grain-size at time t, L 0 the grain-size at time t = 0 and k is a rate constant that depends on temperature T and chemical environments ( f O 2 : oxygen fugacity in Pa, C OH: water content in H/10 6Si) as: k = A ′ D f O 2 r D exp − H D ′ * R T + A ′ W f O 2 r W C OH q exp − H W ′ * R T with A ′ D = 10 − 4.9 ± 6.1 ( − 8.0 ± 7.4 ) ( m n s − 1 P a − r D ) , r D = 0.12 ± 0.11(0.20 ± 0.14), H D ′ * = 410 ± 230(500 ± 270) kJ/mol, A ′ W = 10 − 18.2 ± 1.4 ( − 24.0 ± 1.7 ) ( m n s − 1 P a − r W ) , r W = 0.14 ± 0.05(0.22 ± 0.06), q = 1.7 ± 0.3(2.2 ± 0.3) and H W ′ * = 120 ± 60(160 ± 70) kJ/mol with assumed value of n = 2(3) (values in parentheses denote parameters for n = 3). Both water and oxygen fugacities significantly enhance grain-growth kinetics. The large value of the parameter describing the water fugacity dependence, q ∼ 1.5–2.5, cannot be explained solely by a simple model in which grain-growth is controlled by diffusion of atoms (defects) across the grain-boundaries The interaction of grain-boundaries with charged defects or the density of hydrated ledges may be important factors that control the grain-growth kinetics of wadsleyite. When compared at similar thermo-chemical conditions, grain-growth of wadsleyite is found to be more sluggish than grain-growth of olivine. The present results show that a small wadsleyite grain-size (<1 mm) in subducting slabs can be maintained for a significant geological time (∼1 My) under “dry” (<200 H/10 6Si) conditions when the temperature is lower than 1500 K, whereas when a large amount of water (>100,000 H/10 6Si) is present, a small grain-size (<1 mm) can only be maintained for a significant time at low temperatures (<600 K).