The seismological signature of temperature and grain size variations in the upper mantle

The shear modulus and attenuation of pure and therefore genuinely melt-free polycrystalline aggregates of Fo₉₀ olivine have recently been measured over a wide range of mean grain size at upper mantle temperatures and seismic frequencies [1,2] [B.H. Tan, I. Jackson, J.D. Fitz Gerald, High-temperature viscoelasticity of fine-grained polycrystalline olivine, Phys. Chem. Miner. 28 (2001) 641-664; I. Jackson, J.D. Fitz Gerald, U.H. Faul, B.H. Tan, Grain-size sensitive seismic wave attenuation in polycrystalline olivine, J. Geophys. Res. 107 (B12 2360) (2002) doi:10.1029/2001JB001225]. Here for the first time we fit the experimental shear modulus and attenuation data to a common model that provides an internally consistent description of the observed variations with frequency, temperature and grain size. This model is used to perform indicative calculations of shear wave speed (V _s) and attenuation (Q) along geotherms representative of both oceanic and continental settings, intended to highlight the sensitivity of V_s and Q to variations of temperature and grain size. Comparison of the results of these calculations with seismological models suggests the following: (1) The low velocity zone (LVZ), commonly observed below ocean basins, and its tendency to become less pronounced and deeper with increasing lithospheric age, can be explained by solid state mechanisms without the presence of melt or fluids. (2) The relatively large velocity variations in the upper mantle seen in global tomographic models can be explained by reasonable temperature differences within continents and between continents and oceans. (3) The velocity increase below ∼200 km seen in most seismological models may indicate an increase in grain size from order of mm in the shallow upper mantle to ∼cm above the transition zone.