Lattice Preferred Orientation of Enstatite and Implications for Seismic Anisotropy
Abstract
Seismic anisotropy in the Earth’s upper mantle is widely observed and considered to be mainly caused by the lattice-preferred orientation (LPO) of both olivine and enstatite. Even though there are many studies on the LPO of olivine, studies on the LPO of enstatite are very limited. LPO and seismic anisotropy of orthopyroxene (enstatite) in mantle xenoliths from Spitsbergen, Svalbard, near the Arctic, were studied. LPO of enstatite was determined using electron backscattered diffraction (EBSD) with the HKL Channel 5 software. Water content was measured using the Nicolet 6700 FTIR with continuum IR microscope. We found four different types of LPOs of enstatite and defined them as type-AB, -AC, -BC, and -ABC. Type-AB LPO of enstatite is defined as (100) plane aligned sub-parallel to foliation and [010] axis aligned sub-parallel to lineation. Type-AC LPO is defined as (100) plane aligned sub-parallel to foliation and [001] axis aligned sub-parallel to lineation. Type-BC LPO is defined as (010) plane aligned sub-parallel to foliation and [001] axis aligned sub-parallel to lineation. Type-ABC LPO is defined as both (100) and (010) planes aligned sub-parallel to foliation and [001] axis aligned sub-parallel to lineation. We report for the first time the type-AB, -BC, and -ABC LPO of enstatite. FTIR study showed that type-AC LPO was observed mostly in the samples of enstatite with low water content. It is found that the strength of the LPO of enstatite decreases with increasing water content and has a correlation with the strength of the LPO of olivine: the stronger the LPO of enstatite, the stronger the LPO of olivine. Four different types of LPOs of enstatite result in different seismic anisotropies of enstatite. Our results suggest that seismic anisotropy of enstatite is more complex than previously thought and needs to be reconsidered for the interpretation of the observed seismic anisotropy in nature.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMMR41A1963J
- Keywords:
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- 3621 MINERALOGY AND PETROLOGY / Mantle processes;
- 3902 MINERAL PHYSICS / Creep and deformation;
- 7208 SEISMOLOGY / Mantle;
- 8030 STRUCTURAL GEOLOGY / Microstructures