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Oxygen: a major player in the formation of the Earth

By conducting experiments at high pressure and high temperature, IMPMC researchers (CNRS, Pierre and Marie Curie University IRD), the Institute of Earth Physics of Paris (CNRS, Paris Diderot) and the Lawrence Livermore National Laboratory (USA) solve the paradox of some current models for the formation of the Earth and the differentiation of the nucleus. They show that the accretion of the Earth from the most common meteorites (ordinary chondrites, carbonaceous chondrites) leads during the core formation by migration of the metal in the early Earth, in the abundance of Cr and V in residual silicate mantle compatible with the observed abundance.

The differentiation of the Earth into a silicate mantle and a metallic core is the most important chemical fractionation and mass transfer episode in its history. It happened during its accretion from chondrite meteorites, about 4.5 billion years ago. It is estimated that during the accretion, high-energy impacts generated a magma ocean from which heavier metal separated from silicates by density difference and migrated towards the center of the planet to form the core, the residue forming the silicate mantle.

The composition of the Earth's core remains a major open question of geosciences. If we know that the core is composed mainly of iron and some nickel, the identity of the light elements (about 10% of its composition to meet seismic observables) is still unknown. Oxygen, such as silicon or sulphur is one of the most favorable candidates to enter the core composition.

During the differentiation of the Earth mantle and core, chemical elements are distributed according to their affinity between metal and silicate based on extreme conditions of pressure and temperature in the deep magma ocean. For example, iron or nickel are concentrated in the core and depleted in silicate phases of the residual mantle. The composition of mantle rocks still bears the chemical fingerprint of the formation of the core, which allows us to assess the conditions of accretion and differentiation of the Earth.

To explain the abundances of the mantle of a number of elements such as nickel and cobalt just take into account the formation of the core pressures (50 Gigapascals, 500 000 atm) and temperatures (3500 ° C) of a magma ocean about 1500 km depth. However, to explain the abundances of chromium (Cr) and vanadium (V) of the mantle, it is necessary to involve the greater or lesser oxygen content of the materials from which the Earth formed, for example: chondrites.

So far, the experiences of distribution of these elements (Cr and V) between metal and silicate at high pressure and high temperature led to a model of Earth formed mainly from relatively oxygen poor materials. Yet most primitive meteorites (chondrites) are more oxidized than those required by the accretion model materials.

To understand this paradox, the authors of this publication have conducted experiments at high pressure and high temperature in the diamond anvil cell and laser heating. This technique is used to measure the distribution of chemical elements between metal (core) and silicate (mantle) to direct conditions of pressure and temperature of the base of the magma ocean: about 50 GPa and 3500 ° C.

They were able to observe the presence of significant amounts of oxygen in the metal, which had not been highlighted in previous studies to lower pressure and lower temperature. In addition, the presence of oxygen in the metal significantly affects the distribution of chromium and vanadium between metal and silicate, thus produce the abundances of these elements in the same as those known in the silicate mantle. Forming an oxygen-rich core can explain the contents of vanadium and chromium in terrestrial mantle.

In conclusion, the results of this study show that a core rich in oxygen generates from forming the earth with materials whose oxygen content is similar to that of the most common meteorites (ordinary chondrites, carbonaceous chondrites). This accretion model allows both to obtain adequate levels of V and Cr mantle, with oxygen as the dominant light element composition of the core, and avoid the use of poor materials in oxygen, poorly sampled from primitive meteorites in the solar system to form the Earth.

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