Water-induced fabric transitions in olivine

The interpretation of seismic anisotropy in Earth's upper mantle has tradit ionally been based on the fabrics (lattice-preferred orientation) of relati vely water-poor olivine. Here we show that when a large amount of water is added to olivine, the relation between flow geometry and seismic anisotropy undergoes marked changes. Some of the puzzling observations of seismic ani sotropy in the upper mantle, including the anomalous anisotropy in the cent ral Pacific and the complicated anisotropy in subduction zones, can be attr ibuted to the enrichment of water in these regions.