Volatile elements (e.g. H, C, S) have a fundamental role in planetary evolution. But how and when budgets of volatiles were set in planets and the mechanism of volatile depletion in planetary bodies remains poorly understood and represents a fundamental obstacle in understanding the chemical processing of terrestrial planets. Two main theories exist. Either Earth accreted ‘dry’, with Earth’s building blocks completely devoid of volatile elements. Then, the Earth’s complement of volatile elements was only established later, once the Earth was differentiated into a core and mantle, by the addition of a late veneer. Or, the Earth accreted ‘wet’ where volatiles where present during the main stages of accretion and differentiation of the Earth. The imprint of core formation on the geochemistry of siderophile and volatile elements of the present mantle can discriminate between these two competing scenarios. We will use core formation experiments and the geochemical signatures from metal-silicate equilibration of three siderophile and volatile elements sulfur, selenium, and tellurium. An original and complementary multi-techniques approach combining experiments at high pressure and high temperature, and high-resolution analyses on quenched samples will be developed to obtain new constraints on the origin of volatiles elements on Earth. The objectives of our research program will be divided into three main targets: (1) Determining the S, Se, and Te metal-silicate partitioning at the direct pressure and temperature conditions of core formation in a deep magma ocean. These results will be used to test whether the abundances of these elements can be predicted by current models of Earth differentiation involving metal-silicate equilibrium. We will consequently evaluate if the addition of a given type of meteorite component following initial core formation can raise mantle abundances of S, Se, and Te to their current level. (2) Determining the sulfur isotopic fractionation between metal and silicate at high pressure and high temperature. The results will prove whether core–mantle differentiation generated the recently observed non-chondritic 34S/32S ratio of the silicate Earth. This will provide new independent constraints for the budget of sulfur in the core and the volatile accretion history of the Earth. (3) Determining the speciation of sulfur in (Fe, S) alloys at HP-HT for varying sulfur contents. The results will provide a mechanism to drive sulfur isotopic fractionation to the predicted higher 34S/32S ratio in the core than that of chondrites. Experimental methods developed will place this project at the frontier in between experimental petrology, stable isotopes geochemistry and mineralogy, as within the new scientific objectives of the Institut de Physique du Globe de Paris (IPGP).

The Moho and lithosphere-asthenosphere boundary (LAB) are two of the most important boundaries in the Earth. The Moho is the greatest manifestation of the chemically differentiated Earth, separating light rocks in the crust from dense rocks in the mantle whereas the LAB is fundamental for plate tectonics, defining the boundary between the floating rigid lithosphere and the weak, deformable asthenosphere. They are both first born at mid-ocean ridges, where two-thirds of the Earth’s crust and lithosphere are formed, and then evolve with the age, but what happens during the early part of their lives remains an enigma. Together, they serve as the location where coupling and exchange take place between the shallow and deep mantle, and therefore, it is fundamental to understand their formation and evolution during the early stages of their existence. Here, I propose to acquire exceptional seismic data using the most advanced technology available in industry, >2500 Ocean Bottom Nodes, and subject them to the advanced analysis techniques of full waveform inversion, to quantify the nature of the Moho and the LAB during the first 5 Ma of their lives at the fast-spreading East Pacific Rise. This new technology will allow us to record multi-component seismic reflection and refraction data continuously up to 400 km offset at ~200 m interval, thus opening up a new frontier of research. These new seismic data will provide quantitative P-wave and S-wave velocities of the whole crust, the Moho, the upper mantle and the LAB at unprecedented resolution, which combined with petrological studies will allow us to develop a comprehensive geodynamical model for mantle melting, melt migration, crustal accretion, and lithospheric evolution. The quantitative imaging of structures down to 20-30 km depth on a few hundred metre scale would be a revolution, develop synergy between academic and industrial research, and open up new horizons for deep seismic imaging for upper mantle studies.

Understanding the formation and early evolution of terrestrial planets is one of the most important goals in sciences. The objectives of the proposal, METAL (Making tErresTriA pLanets), are to study the accretion and differentiation processes that have shaped the present composition of the Earth, Moon, Mars and differentiated asteroids including understanding the origin, and timing of delivery of their volatile and siderophile elements. To reach this goal we have identified the best-suited isotopic tools, which are sensitive to the different physico-chemical processes acting at different stages of planetary formation. This work will involve: 1) Development and use of new cutting-edge stable isotope systems for moderately volatile elements (e.g. In, Sb, Sn) in terrestrial, lunar and meteoritic materials, in order to constrain the origin of solar system’s volatile element depletion. 2) Quantifying experimentally the isotopic effects during metal/silicate partitioning and evaporation in all conditions relevant to planetary accretion and differentiation. 3) Building a physical model of volatile loss. 4) Studying the timing, proportions, fate and nature of the material that accreted to Earth and Mars after core formation (i.e. the late-veneer) by using a new method based on the stable isotopes of a highly siderophile element, Pt. This high-risk high-rewards approach seeks to link innovative novel isotopic systems, experiments under extreme conditions, and dynamical modelling, to solve long-standing major scientific questions related to the formation and evolution of the terrestrial planets.