Grain-growth kinetics in wadsleyite was investigated using a multianvil high-pressure apparatus. Fine-grained wadsleyite aggregates gates 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) = kt where it 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(O2): oxygen fugacity in Pa, C-OH: water content in H/10(6)Si) as:
k=A'(D)f(O2)'(D) exp (-H-D*/RT) + A'wf'(w)(O2) C-OH(q) exp (-H-w*/RT)
with A'(-4.9 +/- 6.1(-8.0 +/- 7.4))(D) (m(n) s(-1) Pa-rD), r(D) = 0.12 +/- 0.11 (0.20 +/- 0.14), h(D)* = 410 +/- 230(500 +/- 270) U/mol, A'(w)= 10(-18.2 +/- 1.4(-24.0 +/- 1.7)) (m(n) s(-1) Pa-rw), 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 it = 3). Both water and oxygen fugacities significantly enhance grain-growth kinetics. The large value of the parameter describing the water fugacity dependence, q similar to 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 (similar to 1 My) under "dry" (< 200 H/10(6)Si) conditions when the temperature is lower than 1500 K, whereas when a large amount of water (> 100,000 H/10(6)Si) is present, a small grain-size (< 1 mm) can only be maintained for a significant time at low temperatures (< 600 K). (c) 2005 Elsevier B.V All rights reserved.