Introduction

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Introduction - major issues, extreme models: single- and double-layer convection, seismic and geochemical heterogeneities. Introduction to tools: seismology, geodynamics, mineralogy. 1D mantle structure and mineralogy: seismic discontinuities, the D zone.

We began with a historical introduction detailing the main stages in the construction of the average model of the Earth, a spherically symmetrical "onion skin" model, with its crust and mantle solid and made up of silicates, its liquid core (Oldham, 1906) formed of iron alloyed with 10% light elements and its solid seed, formed of purer iron. This average model, built by combining information from seismological measurements with material physics experiments at high pressure and temperature, provides a static image of the Earth's interior. However, the phenomena observed on the surface - earthquakes and volcanic eruptions - indicate that the Earth's interior is not static, and today we readily accept that the Earth's mantle is animated by convection movements, serving to evacuate primordial heat (cooling of the Earth during geological time) and that produced by radioactive elements. These convection currents are the driving force behind plate tectonics, but we still don't know their morphology in detail.

We recalled the main seismological tools (measurement of propagation times and amplitudes of P- and S-type volume waves, surface waves, frequencies of natural vibrations), how discontinuities in the mantle are detected seismically, in particular discontinuities in the upper mantle at depths of 400 km, 520 km and 660 km, linked to phase changes in olivine, which makes up over 50% of the mantle's composition.

We presented the main tools for studying terrestrial materials at mantle pressures and temperatures, experimental tools such as the laser-heated diamond cell, or the multi-enclume cell. Before turning to the structure of the deep mantle, we presented a summary of recent knowledge of the mantle's "transition zone", where the main phase changes take place. We introduced the pyrolitic model of mantle composition, based on observations of volcanic lavas erupted in oceanic ripples. We mentioned the influence of water on these phase transitions, bearing in mind that the amount of water contained in the mantle is not precisely known.

We then turned to the composition of the lower mantle, which consists mainly of magnesium-iron perovskite (Mg, Fe)_SiO3, with around 20% oxides (feropericlase (Fe, Mg)O). We mentioned the 2004 discovery of the perovskite → post-perovskite phase transition in magnetic perovskite, which is thought to occur at pressures and temperatures corresponding to a mantle depth of around 200 km above the core-mantle boundary. At this depth, a seismic discontinuity has been observed in several parts of the world. We'll come back to this later.

We have described our knowledge of the average temperature profile in the Earth, known as the "geotherm", constrained by 1) near-surface measurements,
2) the Clapeyron slopes of upper mantle phase transitions, and the outer core/seed boundary, and
3) the adiabaticity assumption associated with the presence of convection in most of the mantle (except boundary layers where heat transfer is by conduction, near the surface and the core-mantle boundary), and in the fluid outer core.

Considerable uncertainties remain concerning the temperature jump at the mantle-core boundary (MCC) and the melting temperature of iron at pressures corresponding to the seed boundary.

Finally, we reviewed our knowledge of rheology in the mantle and the mean viscosity profile, constrained by gravity measurements and temporal variations in altitude in some continental regions, due to postglacial rebound. Significant viscosity variations are present due to the influence of temperature and other physical parameters on viscosity. Uncertainties about the viscosity profile in the mantle are significant, but it is known that viscosity increases with depth, with a difference of the order of a factor of 30 between the upper mantle (the first 660 km of depth) and the lower mantle.