Planet differentiation: an integrated Experimental and numerical Modeling of Germanium isotopic fractionation – PlanetGEM

How have planets in our Solar System formed and evolved to their present state? In the turbulent environment of the early Solar System, complex and successive stages of accretion-collision between asteroidal bodies resulted in the formation of molten silicate, leading to the segregation of metallic cores surrounded by silicate mantles under reduced conditions. The distribution of the siderophile (iron-loving) and volatile elements between these reservoirs points to a strong fractionation under the extreme conditions of planetary body evolution. The net result is the depletion of these elements in silicate reservoirs compared to initial composition of planets. Stable isotope investigations of siderophile or volatile elements on metal-silicate experiments and on natural samples representative of planetary silicate and core reservoirs yield contradictory and misunderstood isotopic fractionation for P, T, fO2 conditions prevailing during planetesimal formation. Several issues concerning this depletion are still debated: (1) the search for new tracers to quantify pressure, temperature and redox conditions prevailing during planetesimal differentiation, (2) the relative timings of metal-segregation and volatile loss during the magma ocean stage, (3) process of volatile loss, magma ocean degassing, evaporation following impacts.
PlanetGEM is a multidisciplinary and interdisciplinary project, aimed at using Germanium, a moderately siderophile AND volatile element, and its isotopes as novel tracers to investigate new approaches exploring the combined effects of metal-silicate AND volatile loss on isotopic fractionation. This project will be based on (1) state-of-the-art experimental isotope geochemistry to determine the direction of Ge isotope fractionation between metal and silicate, over a range of pressure, temperature and fO2 conditions consistent with core-mantle equilibration. In addition, isotopic fractionation during evaporation as a function of fO2 will simulate the effects of volatile loss following impacts. (2) These experiments will be coupled with numerical modelling to establish and quantify isotopic fractionation between metal and silicate expected at equilibrium. (3) These results will help interpreting the Ge isotopic data that will be obtained on natural samples (planetary silicate reservoirs and meteorites). Germanium isotopes will be combined to complementary Zn isotopes (not siderophile, but volatile) to disentangle metal-silicate from evaporation processes, and to Si isotopes (similarly behaved to Ge in silicate, but refractory) to constrain redox conditions. A novel analytical aspect will be the use of a new generation of collision cell MC-ICP-MS at CRPG, for the isotopic measurements of very small quantities of Ge at high precision and accuracy levels.
The consortium team for PlanetGEM 4-year project brings together four complementary partner laboratories (CRPG-Nancy, IRAP, GET, CIRIMAT at Toulouse), and recognised researchers in the fields of isotope geochemistry, experimental petrology-cosmochemistry, and theoretical isotopic fractionation modelling. Collaborations include LMV-Clermont-Ferrand (CNRS National Instrument) for high-pressure experiments, experts in isotope modelling and in Germanium in planetary mantles. We expect this unprecedented approach to provide important results that will be presented and published yearly in international conferences and journals, and in HAL Open Access. They will benefit to students and researchers, in the field of the Earth Sciences, and metallurgy and Materials Sciences. Germanium being a strategic metal, understanding Ge behaviour and its isotopes in different redox and pressure conditions can help constraining recycling or production processes in “green technologies”. This project would be complementary to the main goal of LABEX Ressources21 (Nancy) program launched by ANR, dedicated to the understanding of strategic metal deposition.