410 km discontinuity sharpness and the form of the olivine α-β phase diagram: Resolution of apparent seismic contradictions

Estimates of the thickness of the 410 km seismic discontinuity, believed to be due to the α-olivine→-β-modined-spinel transformation in olivine, are as low as 4 km based on discontinuity reflectivity. The seismically estimated thickness is, however, biased to values narrow than the true transformation interval if linear interpolation of properties is used lor modelling. A 5 km linear velocity gradient yields an average reflection coefficient identical to that of a 10 km transition interval based on olivine phase-diagram features. Moreover, alternative forms of the phase diagram, equally consistent with experimentally determined iron magnesium partitioning, on yield true transition intervals narrow as 4 km. This reconciles a discrepancy between phase equilibrium and seismic measures of discontinuity thickness in two ways: (1) seismic thickness estimates are too narrow; and (2) narrow transition intervals are permissible given existing phase-equilibrium constraints. Incorporating recent results on the influence of H₂O on discontinuity properties, it appears that 410 km discontinuity reflectivity is much more sensitive to varying H₂O concentration than to temperature, suggesting that discontinuity reflectivity variations reflect changes in mantle chemistry.