23HIGHLIGHTS 2020

Krypton storage capacity of the Earth s lower mantle, A.D. Rosa (a), M.A. Bouhifd (b), G. Morard (c), R. Briggs (d), G. Garbarino (a), T. Irifune (e), O. Mathon (a) and S. Pascarelli (a), Earth Planet. Sci. Lett. 532, 116032 (2020); https://doi. org/10.1016/j.epsl.2019.116032.

(a) ESRF (b) Laboratoire Magmas et Volcans, Université Clermont Auvergne, CNRS, IRD, Clermont-Ferrand (France) (c) Sorbonne Université, Muséum National d Histoire Naturelle, UMR CNRS 7590, IRD, Institut de Minéralogie, de Physique des

Matériaux et de Cosmochimie, IMPMC, Paris (France) (d) Lawrence Livermore National Laboratory, California (USA) (e) Geodynamics Research Center, Ehime University, Matsuyama (Japan)

[1] B. Marty, Earth Planet Sci. Lett. 313-314, 56-66 (2012). [2] H. Schlichting & S. Mukhopadhyay, Space Sci. Rev. 214, 34 (2018). [3] S.B. Jacobsen, Annu. Rev. Earth Planet. Sci. 33, 531-570 (2005). [4] M.A. Bouhifd & A.P. Jephcoat, Nature 439, 961-964 (2006).

A NEW EXPERIMENTAL WINDOW INTO PLANETARY INTERIORS

Disordered silicates have been probed using in-situ experiments at extreme conditions typical of the deep Earth mantle. Comparing the measurements with atomistic simulations reveals that glasses and liquid silicate share a similar high-pressure structural evolution. This supports the long-lasting assumption that silicate glasses are good structural analogues of silicate liquids.

PRINCIPAL PUBLICATION AND AUTHORS

REFERENCES

therefore developed to study solubility at the extreme conditions of pressure and temperature of the Earth s lower mantle (25-115 GPa and 1800-3700 K) using laser-heated diamond-anvil cell techniques coupled with energy-dispersive X-ray absorption spectroscopy (XAS) at beamline ID24. An original sample-loading setup was employed to determine the distribution behaviour (partitioning) of krypton between a mantle mineral and metallic liquid phase at these extreme conditions. Complementary measurements were performed by electron microscopy and at BM23 by fluorescence XAS on recovered samples.

The data show that ferropericlase ((Mg, Fe) O), the second most important phase of the lower mantle, can retain up to 3 wt. % of krypton, representing a solubility that is an order of magnitude greater than that obtained for molten metal melts representing the liquid outer core of the Earth [4]. The combination of XAS data with recent noble gas compressibility data further demonstrates that the substitution of krypton in the anion site of (Mg1-x, Fex)O as Schottky defects is possible under lower mantle conditions (Figure 10). The relative concentrations of neon,

argon, krypton and xenon stored in ferropericlase and bridgmanite were estimated from modelling the deformation of their crystal lattice. These concentrations agree well with estimates from geochemical studies using mass-balance calculations [1], which suggests that the deep lower mantle is a potential reservoir of noble gas.

This study demonstrates that noble gases, which present a model system for volatiles, may have been preferentially incorporated into the crystal structures of lower mantle phases during its solidification and not in liquid metal melt droplets descending to form the Earth s core. Being captured in this deep solid reservoir may have hindered them from being outgassed into the atmosphere. Combining isotope geochemical data with the new results elucidates that lower Earth s mantle volatiles can comprise up to 20% of atmospheric volatiles, confirming the scenario of a deep un-degassed reservoir. The results shed light on the process of volatile distribution in the early stages of Earth s formation, with strong implications for the geochemical and geodynamic evolution of the Earth, and provide important constraints on the global volatile budget of our planet.

The physical properties of liquid silicates over a wide pressure temperature (P T) range are crucial for understanding not only early differentiation events that occurred in rocky planets, but also present-day Earth s core mantle boundary. However, experimental studies are extremely difficult, due to the requirement of

temperatures exceeding 4000 K to melt silicates over Mbar pressures. Novel approaches are therefore required to study disordered silicates under extreme P T conditions.

Novel laser-driven shock experiments combined with X-ray free-electron lasers were thus