Grain-growth kinetics in wadsleyite: Effects of chemical environment

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, Ln−L0n=kt where n is a constant, L the grain-size at time t, L0 the grain-size at time t = 0 and k is a rate constant that depends on temperature T   and chemical environments (fO2fO2: oxygen fugacity in Pa, COH: water content in H/106Si) as:k=A′DfO2rD exp−HD′*RT+A′WfO2rWCOHq exp−HW′*RTwith A′D=10−4.9±6.1(−8.0±7.4) (mn s−1 Pa−rD)A′D=10−4.9±6.1(−8.0±7.4) (mn s−1 Pa−rD), rD = 0.12 ± 0.11(0.20 ± 0.14), HD′* = 410 ± 230(500 ± 270) kJ/mol, A′W=10−18.2±1.4(−24.0±1.7) (mn s−1 Pa−rW)A′W=10−18.2±1.4(−24.0±1.7) (mn s−1 Pa−rW), rW = 0.14 ± 0.05(0.22 ± 0.06), q = 1.7 ± 0.3(2.2 ± 0.3) and HW′* = 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/106Si) conditions when the temperature is lower than 1500 K, whereas when a large amount of water (>100,000 H/106Si) is present, a small grain-size (<1 mm) can only be maintained for a significant time at low temperatures (<600 K).