The Kinetics of the Reaction Perovskite Plus Ferropericlase to Ringwoodite, and the Effect of Al on The Grain Size of the Transition Zone
Abstract
The grain size of the mantle is an important, but poorly constrained, parameter; grain size can control physical and chemical properties such as viscosity, reaction rates, anelasticity, diffusion length-scales and trace element inventories. The grain size can be reset by phase transitions such as the disproportionation of spinel Mg2SiO4 (Rw) to perovskite (Pv) and ferropericlase (Fp) but other transitions are thought to preserve grain size. While the kinetics of the forward reaction Rw = Pv + Fp has been previously studied experimentally we present here a study of the reverse reaction, which must involve chemical diffusion on the grain scale. During mantle upwelling through 670 km depth, perovskite will transform to majorite garnet (Mj) and the metastable Mj+Mw assemblage then react to produce ringwoodite. The kinetics of this reaction is controlled by diffusion through the ringwoodite layer and the original grain size could be preserved as strong topotaxy is observed between the ferropericlase and rignwoodite overgrowth. At 1873 K reaction would be complete in an assemblage with an initial grain size of 10 cm in 6000 years. The kinetics of the reaction do not vary with iron content, however the presence of aluminium in the starting majorite causes a double-diffusive instability. In this case, the resultant texture is a symplectite of ringwoodite and Al-rich majorite with a grain size of around 2 microns. This fine-grained intergrowth might cause much of the mantle at 670 km depth to be weak compared to regions immediately above and below it, allowing some viscous decoupling between the upper and lower mantle. Due to the fast kinetics of the reaction, it seems unlikely that the metastable assemblage would be visible seismically, except in regions of unusually fast upwelling and large grainsize; it might therefore be possible to use the metastable assemblage to prospect for deep-mantle plumes (see Lessing et al, this meeting).Majorite-MgO diffusion couple showing ringwoodite layer next to MgO and rw-mj symplectite growing into the majorite
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMDI23B..03D
- Keywords:
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- 3612 MINERALOGY AND PETROLOGY / Reactions and phase equilibria;
- 3630 MINERALOGY AND PETROLOGY / Experimental mineralogy and petrology;
- 5199 PHYSICAL PROPERTIES OF ROCKS / General or miscellaneous