Development of A-type olivine fabric in water-rich deep upper mantle

作者：Ohuchi Tomohiro^*; Irifune Tetsuo

来源：Earth and Planetary Science Letters, 2013, 362: 20-30.

DOI：10.1016/j.epsl.2012.11.029

• olivine
• crystallographic preferred orientation
• water
• deep upper mantle
• seismic anisotropy

Water controls the activity of slip systems in olivine resulting in various types of olivine crystallographic preferred orientation (i.e., fabric) in mantle rocks. The A-type olivine fabric is the most commonly observed olivine fabric in natural peridotites. Development of A-type olivine fabric (developed by the (010)(100] slip system) is known to be limited to the water-poor conditions of the shallow upper mantle (%26lt;200 km depth). We have performed simple-shear deformation experiments of olivine at 7.2-11.1 GPa and 1400-1770 K. Here we show that A-type olivine fabric was developed under water-rich conditions (%26gt; 2130 ppm H/Si in olivine), while B-type fabric (by the (010)[001] slip system) was observed under moderately wet conditions (750-2130 ppm H/Si). Developments of C-type (by the (100)[001] slip system) fabric was limited to water-poor conditions (%26lt; 220 ppm H/Si). We found that monotonic decrease in the seismic anisotropy V-SH/V-SV (the ratio of horizontally and vertically polarized shear waves) with depth in the global one-dimensional models is well explained by the olivine fabrics developed in the horizontal flow of a water-poor mantle. Only A-type olivine fabric can explain the vertical mantle flow which associates the seismic anisotropy of V-SH/V-SV %26lt; 1 in the deep upper mantle (%26gt; 200 km depth). A strong anomaly of V-SH/V-SV %26lt; 1 observed in the deep upper mantle beneath the East Pacific Rise is well explained by the distribution of water-rich regions (in which A-type olivine fabric is dominantly developed) in the deep upper mantle and upwelling mantle flows.

• 出版日期2013-1-15