Crystallographic preferred orientation of olivine in the Earth’s deep upper mantle

• Olivine fabrics under Earth’s deep upper mantle conditions were investigated.
• Formation of the C-type and C-type-like olivine fabrics were observed.
• The positive-negative transition of VSH–VSV is strongly affected by water.
• Water content in the whole upper mantle is suggested to be less than 750 ppm H/Si.

Simple-shear deformation experiments of olivine were conducted at pressures of 11.1–12.7 GPa and temperatures of 1670–1770 K. C-type olivine fabric (developed by the (1 0 0) [001] slip system) and similar fabrics were dominantly developed in a wide range of water content (COH = 49–19,242 ppm H/Si in olivine). Because the seismic fast a-axes are perpendicular to the shear plane only for C-type fabric, a positive–negative transition of VSH–VSV (difference in velocity between horizontally and vertically polarized shear waves) inevitably occurs when C-type fabric is developed by a pressure-induced fabric transition. The positive–negative transition of VSH–VSV at ∼200 km depth reported in global one-dimensional models is known to be well explained by the horizontal flow of olivine with a low water content (<750 ppm H/Si in olivine). Our results demonstrate that the presence of a moderate or high water content in olivine (⩾750 ppm H/Si) results in a drastic increase of the boundary depth for the VSH–VSV positive–negative transition (300–370 km depth), namely, “wet” olivine flows cannot account for global one-dimensional models, suggesting that the whole upper mantle is mostly occupied by low-water content regions and the distribution of moderate- or high-water content regions is limited to a small part of the upper mantle.