When the Earth mantle finds its core
On top of the core of the Earth, constituted of liquid iron, lies the solid mantle, which is made up essentially of magnesium oxides, iron and silicon. The border between the core and the mantle, located at 2900 km under our feet, is highly intriguing to geophysicists. With a pressure of around 1.4 million times the atmospheric pressure and a temperature of more than 4000 kelvins, this zone is the home to chemical reactions and changes in states of matter still unknown. The seismologists who have studied this subject have acknowledged an abrupt reduction of the speed of the seismic waves, which sometimes reach 30% when getting close to this border. This fact has led scientists to formulate the hypothesis, for the last 15 years, of the partial melting of the Earth mantle at the level of this mantle-core border. Today it has been confirmed.
In order to access the depths of the Earth, scientists have not only seismological images but also a precious experimental technique: diamond anvil cells, coupled with a heating layer. This instrument allows to re-create the same pressure and temperature condition than those in the interior of the Earth, on samples of a few microns. This is the technique used by the researchers of the Institut de minéralogie et de physique des milieux condensés on natural samples that are representatives of the Earth mantle and that have been put under pressures of more than 140 gigapascals (or 1.4 million times the atmospheric pressure), and temperatures of more than 5000 kelvins.
Scanning electron microscopy image of a “mantle” sample after transformation, stuck on a copper grille and thinned down by a focused ion beam (FIB). It allows to detect the different synthesized minerals and liquids during these experiments: a matrix, consisting of a phase of a perovskite structure ((Mg,Fe)SiO3), - the most abundant mineral in the Earth because it is the most stable in the inferior mantle), is shown in light grey. The veins and liquid pockets enriched in iron and calcium are visible (in dark grey). Scale of the horizontal bar is 2 micrometres. Credits: G.Fiquet.
A new approach to this study has been the use of the X-ray diffraction technique at the high-pressure beamline ID27. This has allowed the scientists to determine what mineral phases melt first, and they have also established, without extrapolation, fusion curves of the deep Earth mantle, i.e. the characterization of the passage from a solid state to a partially liquid state. Their observations show that the partial fusion of the mantle is possible when the temperature approaches 4200 kelvins. These experiments also prove that the liquid produced during this partial fusion is dense and that it can hold multiple chemical elements, among which are important markers of the dynamics of the Earth mantle. These studies will allow geophysicists and geochemists to achieve a deeper knowledge of the mechanisms of differentiation of the Earth and the history of its formation, which started around 4.5 billion years ago.
Fiquet, G., et al, Science 17 September 2010: 1516-1518.