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PRINCIPAL PUBLICATION AND AUTHORS

Evidence and stability field of fcc-superionic water ice using static compression, G. Weck (a,b), J.-A. Queyroux (a), S. Ninet (c), F. Datchi (c), M. Mezouar (d), P. Loubeyre (a,b), Phys. Rev. Lett 128, 165701 (2022); https:/doi.org/10.1103/PhysRevLett.128.165701 (a) CEA, DAM, DIF, F-91297 Arpajon (France) (b) Univ. Paris Saclay, Laboratoire Matière Conditions Extrêmes, CEA, Bruyeres Le Chatel (France) (c) Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), Sorbonne Université, CNRS UMR 7590, IRD UMR 206, MNHN, Paris (France) (d) ESRF

REFERENCES

[1] M. Millot et al., Nat. Phys. 14, 297 (2018). [2] M. Millot et al., Nature 569, 251 (2019). [3] J.-A. Queyroux et al., Phys. Rev. Lett. 125, 195501 (2020). [4] V.B. Prakapenka et al., Nat. Phys. 17, 1233 (2021).

as well as boron-doped diamond disks in the anvil cell, which absorb the radiation of the high-power laser but are transparent to the X-ray beam. This method allowed X-ray diffraction data to be collected for pressures of up to 160 GPa and temperatures of up to 2500 K.

The resulting data provide clear structural observations of superionic ice at a wide range of temperatures and pressures, allowing a full phase diagram to be reported, as shown in Figure 2. It proposes that dense H2O is composed of insulating bcc ice (VII/VII /X) at low temperatures, bcc-SI

ice from 14 GPa up to at least 120 GPa for temperatures in the range of 850-2000 K, and of fcc-SI ice above ~50 GPa and ~1400 K. Although the findings clearly differ from the phase diagram published in [4], possibly due to difficulties in measuring the sample temperature, for example, they agree very well with previous dynamic compression experiments and theoretical calculations based on advanced simulations methods. This work, therefore, narrows the range of possible predictions for expected phase transitions, advancing our understanding of the phases of hot dense water.

Thermal state of Earth s interior revealed by the melting properties of high-pressure minerals

Despite secular cooling, partial melting still occurs in the deep Earth s mantle. X-ray diffraction was used to probe the melting behaviour of micron-sized mineral samples at pressures and temperatures up to 140 GPa and 6000 K. The measurements reveal eutectic-type melting behaviour of the mantle atop the core-mantle boundary, with a local temperature between 3600 K and 3950 K.

Earth is very hot inside due to the major meteoritic impact that occurred 50-100 million years after the beginning of

its accretion. The impact likely melted the Earth almost entirely. Since then, mantle melting has played a major role in the internal dynamics. Partial mantle melting still occurs today in mantle regions such as the outer core, the lowermost mantle sitting atop the core-mantle boundary and a ~50-km-thick layer above the 410-km-depth seismic discontinuity. Refining the melting properties is therefore key for knowledge of the thermochemical state of the Earth s mantle.

In the pressure range typical of the mantle, up to 135 GPa, the diamond anvil cell coupled with laser heating and X-ray diffraction is the best tool to investigate the melting behaviour of silicate phases. The high-intensity and micro-focused X-ray beam at beamline ID27 is perfectly suited to successively detect the crystallisation of lower mantle minerals and the onset of melting visible through the disappearance of specific diffraction peaks from the sample. The melting behaviour of single silicate phases was investigated at ID27, as well as mixtures of several minerals relevant to deep mantle compositions. Particular attention was paid to the temperature measurement, which dominates the experimental uncertainties under such extreme conditions [1].

Fig. 3: Melting curve of bridgmanite. Results are reported on MgSiO3 (blue dots) and (Mg,Fe)(Si,Al)O3 (green dots) bridgmanites together with previous studies using a diamond anvil cell on (MgFe)SiO3 (ZB-93), shock waves on MgSiO3 (AK-04 and M-09) and ab-initio calculations on MgSiO3 (KS-09 and SK-05).