Grain-growth kinetics in wadsleyite: Effects of chemical environment

Details Grain-growth kinetics in wadsleyite: Effects of chemical environment Grain-growth kinetics in wadsleyite: Effects of chemical environment

Jan 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006pepi..154...30n&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 154, Issue 1, p. 30-43.

Physics

22

Scientific paper

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-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 (f: oxygen fugacity in Pa, COH: water content in H/106Si) as: k=ADfOrexp-HD*RT+AWfOrCOHqexp-HW*RT with AD=10(msPa), rD = 0.12 ± 0.11(0.20 ± 0.14), HD* = 410 ± 230(500 ± 270) kJ/mol, AW=10(msPa), 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).

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