M A T T E R A T E X T R E M E S

S C I E N T I F I C H I G H L I G H T S

1 8 H I G H L I G H T S 2 0 2 2 I

PRINCIPAL PUBLICATION AND AUTHORS

The Fe-Si-C system at extreme PT conditions: A possible core crystallization pathway for reduced planets, F. Miozzi (a,b), G. Morard (a,c), D. Antonangeli (a), M.A. Baron (a), A. Pakhomova (d), A.N. Clark (a,e), M. Mezouar (f), G. Fiquet (a), Geochim. Cosmochim. Acta, 322, 129-142, (2022); https:/doi.org/10.1016/j.gca.2022.01.013 (a) Sorbonne Université, Muséum National d Histoire Naturelle, UMR CNRS 7590, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, IMPMC, Paris (France) (b) Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC (USA) (c) Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, UGE, ISTerre, Grenoble (France) (d) Deutsches Elektronen-Synchrotron (DESY), Hamburg (Germany) (e) University of Colorado, Boulder (USA) (f) ESRF

crystallisation styles, with important implications for the onset of a dynamo. A bottom-up crystallisation regime is predicted for a liquid cooling with compositions near the Fe vertex, a top-down regime for a liquid cooling with high carbon content, and solidification of a crystal mush for a

liquid cooling with high silicon content. While the bottom- up crystallisation regime, pertaining also to Earth s core, is likely to generate and sustain a dynamo, the dynamic implications for the two other crystallisation scenarios are less predictable and open to further investigation.

High-resolution X-ray absorption spectroscopy reveals how gold is transported by hydrothermal fluids in the Earth s crust

Gold transport by ore-forming hydrothermal fluids is controlled by sulfur. In-situ high-resolution X-ray absorption spectroscopy measurements in hydrothermal fluids, combined with quantum- chemistry simulations, provide direct evidence for dissolved gold complexes with the [HS]− and [S3 ]− ligands. They are the key molecular vectors of gold transfer in the Earth s crust.

Gold mobilisation, transfer and concentration in ore deposits in the Earth s crust are controlled by hydrothermal fluids transporting the metal in the form of different aqueous sulfur-bearing complexes [1]. Knowledge of the exact nature of sulfur ligands that bind gold as well as the stoichiometry and stability of such complexes enables geoscientists to develop predictive models of gold transport and deposition. Such models seek to quantify the major factors leading to ore formation in a given natural context such as temperature and pressure, fluid composition, acidity and redox parameters. These factors have very different effects on the solubility of different Au species depending on their exact stoichiometry. Gold has an oxidation state of +I in hydrothermal fluids. The major natural ligands capable of strongly binding AuI and enabling its transport are hydrogen sulfides, [H2S]0 and

[HS]−, and the trisulfur radical ion, [S3 ]−, whereas other types of common natural ligands such as chloride, sulfate or carbonate are much less important [1,2].

Fig 6: HERFD-XAS setup combining a high-resolution crystal analyser spectrometer with the hydrothermal reactor for in-situ measurements of metal speciation and solubility in aqueous fluids at elevated temperatures and pressures. The autoclave is tilted from the vertical axis to match the required Bragg angle of the crystals to selectively probe the Au Lα1 fluorescence line.