Contraintes experimentales sur la composition et la formation du noyau

Earth’s core formed as a results of a major chemical differentiation event; the melting of accretionary building blocks (meteorites, planetesimals, protoplanets) leads to a separation of the metal from the silicate, ensued by a gravitationally-driven segregation of a dense metal-rich core at the centre of the planet, with the lighter buoyant silicates remaining on top to form the mantle and crust. The bulk composition of the core depends on the path and conditions (pressure, temperature, redox) at which core formation took place; the process also leaves an imprint on the residual bulk silicate Earth, a record that is observable in present-day mantle rocks. Constraining experimental and theoretical data with geophysical (core density and velocity profiles) and geochemical observations (siderophile trace-element concentration in the upper mantle) provides a robust way to estimate the present day composition of the core, as well as the conditions under which it formed. We will present a set of results obtained from a variety of techniques: (i) extreme condition experimental geochemistry through the study of metal-silicate partitioning in the laser-heated diamond anvil cell, (ii) first principles calculations with the study of outer-core density and seismic velocity, and (iii) thermodynamical modelling with the study of trace-element interaction parameters, and (iv) mineral physics data through the study of inner-core elasticity. We will combine and interpret these results and propose a compositional model consistent with the observations, and formulate plausible scenarios for core formation.