Water content, fabric development and viscosity of natural spinel peridotites

Spinel peridotites from Kilborne Hole, representing samples of the mantle beneath the western United States have relatively high concentrations of water in olivine, 40-50 ppm. These values are close to the water saturation values in the uppermost mantle (30-50 km) where these rocks resided before being entrained as xenoliths. Lattice preferred orientation analysis of the olivine grains in these samples show that these peridotites have a strong fabric, which would correspond to a P-wave anisotropy of ~10 percent. Our results performed on natural samples confirm experimental predictions that high water contents in olivine determine coarse textures and mechanically weak peridotites. The viscosity of the mantle deduced from our data is about 10^(17) Pa's. If this number is representative for the mantle beneath Kilborne Hole, it suggests that even the shallowest subcrustal domains which make up the "lithosphere", according to thermometric and isotopic determinations on xenoliths, as well as various geophysical interpretations, is capable of flowing because of high water contents.