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

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 (<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 (>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 (<220 ppm H/Si). We found that monotonic decrease in the seismic anisotropy VSH/VSV (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 VSH/VSV<1 in the deep upper mantle (>200 km depth). A strong anomaly of VSH/VSV<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.