Small effect of water incorporation on dislocation mobility in olivine: Negligible creep enhancement and water-induced fabric transition in the asthenosphere

To constrain the effect of water on upper mantle dynamics, we measured the annihilation rate coefficients (k) of [100](010) and [001](100) dislocations in olivine, referred to as a-dislocations and c-dislocations, respectively, as a function of water content. Natural olivine single crystals were doped with 5-800 wt. ppm water and sheared in the [100] or [001] directions along the (010) and (100) planes to produce a- and c-dislocations, respectively, and then annealed under quasi-hydrostatic conditions at a constant pressure and temperature of 5 GPa and 1473 K. The obtained annihilation rate coefficients are fitted to a power-law equation, yielding astonishingly small water-content exponents of 0.0 +/-& nbsp;0.1 and 0.2 +/- 0.2 for the a- and c-dislocations, respectively. The overall effect of water on dislocation mobility is therefore small because these two slip systems are considered to be the least and most sensitive to water, respectively. These results imply that water incorporation does not effectively increase the dislocation creep rate and that a water-induced fabric transition is unlikely. The effects of water on asthenospheric dynamics may thus be limited, and the lateral seismic anisotropy changes observed in the asthenosphere may solely reflect changes of mantle flow geometry. (C)& nbsp;2021 The Author(s). Published by Elsevier B.V.