HIPEG-IGCC · High-pressure experimental geochemistry; reactivity of inert gases and the composition of planetary cores

Final Activity Report Summary - HIPEG-IGCC (High-pressure experimental geochemistry; reactivity of inert gases and the composition of planetary cores)

The objectives of this project were to investigate geochemical problems concerning the distribution of chemical elements deep within planets by means of high-pressure experimental physics and chemistry techniques. The studies had two foci: 1) the reactivity of inert gases (especially xenon) with planetary materials, 2) the composition of the core of terrestrial planets. Xenon (Xe), the heaviest gas we studied, was observed to react with terrestrial silicates at high pressures and temperatures. The first achievement of this project has been the demonstration of the reactivity of xenon by the synthesis of Xe-compounds, a major goal in fundamental chemistry. However, this fascinating chemistry does not seem to extend to lighter rare gases in the conditions of the deep Earth. Xe chemistry has important impacts on our knowledge of Earth formation, and in particular on the formation of its atmosphere. Geologists believe that planet Earth was struck by an asteroid the size of Mars about 4.5 billion years ago. The impact melted most of the planet's rocks, creating the so-called magma ocean and affecting many aspects of subsequent rock chemistry (petrochemistry) and mineralogy. During the magma ocean stage, the primary atmosphere was partially dissolved in the melt and Xe got trapped in the silicates at depth, its heavy isotopes getting trapped preferentially. As the primary atmosphere was later blown off, the gases released by the mantle subsequently and forming our current atmosphere are therefore depleted in Xe and in its light isotopes. The Earth's core is divided into two zones: the outer core, which is liquid, and the inner core, which is solid. The main constituent of the core is iron, which is crystallised in the inner core. However, iron is not alone in the core. Seismology shows that both parts of the core are too light to have only pure, dense iron and that at the same time, there is a big difference in density between the inner core and the outer core, so there needs to be a rejection of lights elements (such as sulphur, silicon or oxygen) at the inner core boundary. During the course of this project, we observed that at extreme pressures the presence of sulphur commonly acts to reduce melting temperatures for iron. However, although a liquid with a planetary-plausible sulphur content, crystallizes Fe first at low pressures, this ceases to be true at high pressures where Fe3S crystallises first. This implies that sulphur gets trapped in the solid phase and contradicts the geophysical requirements for the rejection of light elements into the liquid outer core. We therefore demonstrate that sulphur cannot be a major light element in the core, leaving silicon and oxygen as the main candidates. The second major achievement of this part of the project concerns models of core formation; that is how we can derive the pressure and temperature conditions prevalent for the segregation of molten Fe-rich metal in the magma ocean from silicate melt. A cornerstone of these models has been how the elements nickel (Ni) and cobalt(Co) are distributed or divided between liquid metal and silicate melt at high pressures. We have shown that Ni and Co distribution depends on how both liquids contract under pressure, and that an abrupt change in the contraction of liquid Fe strongly affects Ni/Co distribution. This implies that Ni/Co cannot effectively be used to derive core formation conditions.

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