Effects of melt depletion on the density and seismic velocity of garnet and spinel lherzolite

[ 1] We examine the effects of melt depletion on density and anharmonic V-P and V-S on the basis of the melting relations of fertile peridotite between 1 and 7 GPa. We incorporate the effects of variable mineral mode and composition into a parameterization of mantle residuum density and velocity, using a new compilation of mantle mineral physics results. The effect of melt depletion on mantle density changes with pressure, and at 20% melt removal, residue density changes are - 0.42%, - 0.46%, - 0.90%, - 1.14%, - 0.95%, - 0.66%, and - 0.57%, for pressures of 1, 3, 3.5, 4, 6, and 7 GPa, respectively. We note that at adiabatic temperatures, realistic composition upper mantle has a higher thermal expansivity than olivine, ranging from alpha = 4.91 to 3.47 x 10(-5) K-1 between 1 and 7 GPa. This implies that 1% melt depletion is equivalent in density effect as a 3 - 15 degrees increase in temperature, depending on pressure. Under Archean cratons, where cold melt-depleted mantle generally has been considered to have the same density as fertile adiabatic mantle (i.e., is isopycnic), we find subcratonic mantle formed above similar to 110 km is negatively buoyant with respect to adiabatic mantle. This suggests that vertical transport of residues initially formed above 110 km may play a role in the stabilization of subcratonic mantle. Regarding V-P and V-S, melt depletion has almost no effect, except for a small - 0.5% change in V-P at 20% melting of spinel peridotite. The major element effects of melt depletion are thus insufficient to produce the high mantle velocities imaged beneath cratons or to cause significant velocity variations.