The phase stability and elastic properties of iron and iron alloys at inner core conditions

!--ANNULE--! Résumé: There are many unresolved problems concerning our understanding of the Earth's inner core; even fundamental properties, such as its internal structure and exact composition, are poorly known. Although it is well established that the inner core is made of iron with some alloying element(s), the structural state of the iron and the nature of the light element(s) involved remain controversial. Furthermore, seismically observed P-waves show the inner core to be anisotropic and layered, but the origins of this are not understood; seismically observed S-waves add to the complexity as they have unexpectedly low velocities. Seismic interpretation is hampered by the lack of knowledge of the physical properties of core phases at core conditions. Moreover, the resolution of seismic data are hampered by the need to de-convolve inner core observations from seismic structure elsewhere in the Earth; this is particularly relevant in the case of shear waves where detection is far from straightforward. If we had sufficiently well constrained seismological data, together with accurate high-pressure, high-temperature elastic properties of the candidate materials, we could, in principle, fully determine the structure and composition of the inner core - an essential pre-requisite to understanding its evolution. Unfortunately, the extreme conditions of pressure and temperature required make results from laboratory experiments unavoidably inconclusive. However, computer simulations of materials at inner core conditions are now achievable. In this talk, therefore, I will describe how /ab initio/ molecular dynamics simulations have been used to determine the stable phase(s) of iron in the Earth's core and to calculate the elasticity of iron and iron alloys at core conditions. The calculated shear wave velocities are significantly higher than those inferred from seismology. If the seismological observations are robust, then a possible explanation for this discrepancy is if the inner core contains >8% of trapped liquid inclusions. -------------------------------------------------------------------------------- Séminaires Minéralogie --------------------------------------------------------------------------------