Coherent Lamellar Growth of Akimotoite within Enstatite Host at High Pressure and Temperature
Abstract
To constrain models of the rheology and dynamic behavior of subducting oceanic lithosphere within the mantle transition zone, it is important to understand the mechanisms by which the major mineralogical components of the slab transform to their high-pressure polymorphs. Despite being the second most abundant component of the upper mantle, the mechanisms of polymorphic phase transformations in pyroxene remain poorly understood. In subducting oceanic lithosphere, which is colder than the surrounding mantle, akimotoite would be stable relative to majorite and the polymorphic reaction to akimotoite would be kinetically favored over disproportionation reactions to ringwoodite or wadsleyite plus stishovite. There are two transformation mechanisms that have been observed in experiments studying polymorphic transformation of mantle silicates. Heterogeneous nucleation and interface controlled growth is the most commonly observed mechanism, with product phases typically growing as incoherent, granular rims. A second, shear-induced mechanism is characterized by coherent lamellar growth of high-pressure phases on stacking faults within the host grain. Coherent lamellar intergrowth has been observed experimentally in (Mg,Fe)2SiO4 olivine (Kerschhofer 1996, 1998) and within shocked chondritic meteorites for both olivine (Chen et al., 2004) and enstatite (Tomioka and Fujino 1997, 1999). Tomioka (2007) modeled a shear-induced mechanism for the enstatite-akimotoite transition, but this mechanism has not been observed experimentally. We are investigating transformation reactions and kinetics of natural Mg0.9Fe0.1SiO3 enstatite. Within a sample transformed at 20GPa and 1400°C, we observe akimotoite lamellae within the enstatite host. Using transmission electron microscopy, we confirm the coherent, topotaxial relationship between the akimotoite and enstatite as modeled in Tomioka (2007). These findings suggest that shear-induced intracrystalline transformation of enstatite to akimotoite may be important in subducting oceanic lithosphere.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMDI21B2270L
- Keywords:
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- 3612 MINERALOGY AND PETROLOGY Reactions and phase equilibria;
- 3630 MINERALOGY AND PETROLOGY Experimental mineralogy and petrology;
- 3924 MINERAL PHYSICS High-pressure behavior;
- 3621 MINERALOGY AND PETROLOGY Mantle processes