### Abstract

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 H_{2}O on discontinuity properties, it appears that 410 km discontinuity reflectivity is much more sensitive to varying H_{2}O concentration than to temperature, suggesting that discontinuity reflectivity variations reflect changes in mantle chemistry.

Original language | English |
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Journal | Geophysical Journal International |

Volume | 126 |

Issue number | 3 |

Publication status | Published - Sep 1996 |

### Keywords

- Mantle discontinuities
- Phase transitions
- Seismology

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## Cite this

*Geophysical Journal International*,

*126*(3).