Lattice-preferred orientation, water content and seismic anisotropy of olivine: Implications for the lithosphere-asthenosphere boundary of continents
|Category||Tectonic & Seismotectonic|
|Location||International Geological Congress,oslo 2008|
|Holding Date||15 September 2008|
The lattice-preferred orientation (LPO) of olivine under different thermal-mechanical conditions is critical to understand deformation and seismic anisotropy of the upper mantle. Most naturally deformed peridotites develop (010) olivine fabric, which is characterized by the  axis parallel to the lineation and the (010) plane parallel to the foliation. As a result the fastest P-wave velocity of peridotites is parallel to the lineation and the maximum S-wave splitting parallel to the foliation but normal to the lineation. This relationship has been widely used to trace the mantle flow from seismic anisotropy observations. However, some garnet peridotites from the ultrahigh-pressure (UHP) metamorphic terranes (e.g., Alpe Arami and Cima di Gagnone in the Central Alps, the Norwegian Caledonides, the Sulu terrane), which experienced UHP metamorphism at P =3-7 GPa and T = 750-950 °C, display (100) olivine fabric and have the fastest P-wave velocity and the minimum shear wave splitting normal to the foliation. Recent deformation experiments and theoretical calculations indicate that UHP and/or low temperature can promote a fabric transition in olivine from (010) to (100). The Lehmann discontinuity, which marks a rapid decrease of anisotropy at depths of 200-250 km beneath continents, can be interpreted by the pressure-induced activation of olivine (100) slip system. Because the  glide is easier than  glide at UHP conditions, the Lehmann discontinuity may be associated with a rheological boundary between lithosphere and asthenosphere, especially under continental cratons. In continental subduction zones under UHP, low temperature and low fluid activity (e.g., the Sulu terrane), the transition of olivine LPO from (010) to (100) would occur at shallower depths (probably between 120~220 km) and lead to the fastest P-wave velocity normal to the subduction direction. On the other hand, Jung and Karato (2001) observed a selective enhanced  slip along (100) and (010) planes in water-saturated olivine at 1.0-2.2 GPa and 1200-1300 °C. Statistical analysis of water contents in 170 natural olivine samples reveals a wide variation of 0120~170 ×10-6 H2O. Olivine from peridotite xenoliths in basalts contains relatively low water content, while all water-rich olivine crystals are from garnet peridotite xenoliths in craton kimberlites. Therefore besides the heterogeneous water distribution in the upper mantle, hydrogen diffusion during transport of peridotites can significantly affect the water concentration in olivine. Taking account of the predominant (010) fabric in peridotite xenoliths, the presence of high water content is more likely to increase grain boundary mobility than to change the dominant slip systems of olivine. Nevertheless high water concentration will facilitate partial melting of the upper mantle and produce a shallower lithosphere-asthenosphere boundary.