Iron is considered to be the main component of the Earth's core. Substantial efforts have been made to understand its phase diagram and physical properties at extreme conditions. However, it remains debated about how the atoms in solid iron are arranged at Earth's core conditions, where possible candidates include hexagonal close-packed (hcp), body-centred cubic (bcc), and face-centred cubic (fcc) structures. As crystal structure and physical properties are closely related, there is also a significant uncertainty in the properties of Earth's core, such as elasticity, heat conductivity, and density, making the accurate interpretation of seismic observations difficult. Here we aim to study the phase stability of solid iron at Earth's core conditions. For this, a deep-learning interatomic potential was developed with ab initio accuracy but is more cost-effective. To further check the performance of such potential, we examine the elastic and plastic behaviour of hcp iron and the effects of structural defects at inner core conditions [1]. We then compute the Gibbs free energy of the bcc, fcc, hcp and liquid phases by performing large-scale molecular dynamics simulations. The calculated free energy allows for determining the phase stability of solid iron in Earth's core.

[1] Li, Z., & Scandolo, S. (2022). Elasticity and viscosity of hcp iron at Earth's inner core conditions from machine learning-based large-scale atomistic simulations. Geophysical Research Letters, 49, e2022GL101161.

How to cite: Li, Z. and Scandolo, S.: Phase diagram of pure iron in Earth's core from deep learning, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11982, https://doi.org/10.5194/egusphere-egu23-11982, 2023.

Pyroxene minerals are key to understand the structure and composition of Earth and rocky exoplanets interiors. Nevertheless, the full details of the MgSiO₃ phase diagram still remain unclear, in particular in the high temperature region. Protoenstatite (PEn) is one of the HT polymorphs of MgSiO₃ pyroxene, having stability range from ~1200 to 1800 K at ambient pressure. Its importance has been recognized by many authors, since PEn is regarded as a precursor phase of low-clinoenstatite (LP-CEn)/orthoenstatite (OEn) intergrowths in some cometary samples [1] and in calcium-aluminum-rich inclusions (CAIs) from CV3 chondrites [2]. Moreover, PEn is the liquidus phase of pyroxene in the MgO-SiO₂ binary system and may have played a role in gas solar nebula condensation processes [3]. Very little is known about the thermodynamics and phase relations of protoenstatite. This is due for the most part to its unquenchable nature, meaning that even if PEn can be synthetized at high temperature conditions, it doesn’t preserve at ambient conditions since it very rapidly reverts either to OEn or LP-CEn. The difficulty to perform measurements on samples of PEn prevents to obtain complete information on its thermodynamic properties, which are in turn fundamental for the investigation of phase equilibria of this mineral. In that sense, ab initio calculations based on quantum-mechanical theory are one of the most reliable methods available to obtain information on thermodynamics and phase relations of minerals at HT conditions.   

We present a DFT based ab initio B3LYP computational study on MgSiO₃ PEn. All the relevant thermophysical and thermodynamic properties of PEn (e.g. heat capacity, vibrational entropy, thermal expansion, EoS) have been calculated in the framework of the quasi-harmonic approximation (QHA) by a full phonon dispersion calculation. This allowed to obtain original insights into protoenstatite thermodynamics and enabled to retrieve a complete set of physically consistent thermodynamic properties, that are in good agreement with the very few experimental data currently available [4].  The computed properties have been tested by predicting relevant phase equilibria involving PEn up to melting conditions, in particular the OEn – PEn phase transition. The P-T location of the phase boundary and its Clapeyron slope (dP/dT = 2.04 MPa/K) are consistent with previous pyston-cylinder experiments ([5],[6]). Theoretical modelling of the melting curve of MgSiO₃ polymorphs reveals a change of the melting behavior from incongruent to congruent due to the onset of the OEn – PEn transition in the phase diagram.

[1] Schmitz, S., and Brenker, F.E (2008) Astrophys. J., 681, L105-L108.

[2] Che, S., and Brearley, A.J. (2021) Geochim. Cosmochim. Acta, 296, 131-151.

[3] Nagahara, H.. (2018) Rev. Mineral. Geochem., 84, 461-497.

[4] Thiéblot, L., Téqui, C., and Richet, P. (1999) Am. Mineral., 84, 848-855.

[5] Boyd, F. R., England, J. L., and Davis, B. T. (1964) J. Geophys. Res., 69, 2101-2109.

[6] Chen, C. H., and Presnall, D. C. (1975). Am. Mineral., 60, 398-406.

How to cite: La Fortezza, M. and Belmonte, D.: Ab initio thermodynamics and phase stability of MgSiO3 pyroxene polymorphs: new insights on protoenstatite, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12045, https://doi.org/10.5194/egusphere-egu23-12045, 2023.

Mars’ mantle dynamical history has certainly been dominated by a stagnant-lid regime, with limited mixing and homogenization. Accordingly, the chemical and mineralogical signatures of early processes, including the crystallization of a primitive magma ocean, are overall well preserved on Mars. The major geological structures visible at its surface are the remains of an intense ancient volcanism, not so dissimilar from the large igneous provinces found on Earth at very old ages (several million/billion years).

Current models used to determine the mantle thermal evolution and the crustal extraction heavily relies on melting properties of materials expected to form the Martian mantle, which, however are poorly known. In particular, the fact that the Martian mantle is probably richer in iron than the terrestrial mantle has a direct impact on the solidus and liquidus and on the chemistry of the magmas that can be produced at different pressures. Thus, the study of Martian volcanism and thermal history requires a precise understanding of the melting properties of the mantle (solidus, liquidus and extent of melting) as a function of pressure and temperature. Studies in literature are scant, mainly address the solidus, and are limited to analysis of recovered samples, missing in situ diagnostics.

To address this problem, we studied the solid-liquid melting relations and, more generally, the melting diagram for a mineralogical assemblage model of mantle composition, by high-pressure and high-temperature experiments in multi anvil press performed at the PSICHE beamline of the SOLEIL synchrotron. We determined the solidus and the liquidus of the investigated rock at pressures up to 12 GPa by complementary in-situ diagnostics (X-ray diffraction and falling sphere technic). The obtained solidus and liquidus are well lower (difference >200K), especially at the highest investigated pressures, compared to previous studies, with strong implications for the origin of volcanism and notably the crystallization of the magma ocean. Furthermore, our experiments provide important data to refine the extent of melting (Φ), modal proportion and the chemistry of all the different phases present between the solidus and the liquidus at different conditions (P, T, Φ).

Altogether, these new results are critical to constrain models of thermal evolution and crust extraction and formation, as well as to address the evolution of the magmatism and volcanism at the Mars surface since 3.5 Ga. Finally, depending on different parameters, such as the thickness of the crust or the concentration of radioactive elements, the estimated areotherm could cross the solidus and lead to partial melting of the mantle, especially close to the core-mantle boundary, where a high extent of melting could be reached.

How to cite: Pierru, R., Dominijanni, S., Parisiadis, P., Andriambariarijaona, L., Zhao, B., Blanchard, I., Guignot, N., King, A., Badro, J., and Antonangeli, D.: Melting properties and melting phase relations of the Martian mantle from in-situ measurements on iron-rich mineralogical assemblages, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12231, https://doi.org/10.5194/egusphere-egu23-12231, 2023.

The internal structure of Mercury holds key information regarding the planet’s formation and its peculiar magnetic field. Waiting for incoming observations by BepiColombo, current knowledge of the interior structure of Mercury relies primarily on geodetic and surface chemistry data collected by MESSENGER. Results from spectral and compositional analysis supplemented by cosmochemical evidence indicate that light elements such as S, and Si are most likely alloyed to Fe in Mercury’s core. This notion is further supported by the very reducing redox conditions (from -2.6 to -7.3 log units below Fe-FeO oxygen buffer) predicted to occur during the planet’s differentiation that argue for significant quantities of Si and S partitioned into metallic iron. Thus, it is of primary importance to determine the Fe-Si-S phase diagram and to understand the high pressure and high temperature properties and thermodynamic behavior of Fe-Si-S alloys at conditions directly relevant for Mercury’s core. Very recently the binary Fe-FeSi phase diagram has been established at Mercury’s core conditions, but phase and melting relations in the Fe-Si-S ternary system still are poorly constrained, in particular at the relatively low pressures and temperatures relevant for Mercury’s core.

To address this issue, we performed angular dispersive powder X-ray diffraction experiments in laser-heated diamond anvil cells on selected composition in the Fe-Si-S system (i.e., Fe-4S-6Si, Fe-16S-6Si, Fe-4S-12Si, and Fe-16S-12Si, all in wt. %) at the P02.2 Extreme Conditions beamline at DESY Synchrotron facility (Germany). For all compositions, eutectic melting and subsolidus phase relations were investigated up to about 45 GPa. Ex situ chemical analysis of the recovered run products were performed at the IMPMC laboratory on the extracted FIB thin sections cut throughout the heated spots.

Here we will present preliminary results on the eutectic melting and Fe-Si-S phase relations as a function of pressure, temperature and composition, with specific focus to the conditions expected within the core of Mercury.

How to cite: Dominijanni, S., Anzellini, S., Kurnosov, A., Morard, G., Boccato, S., Fedotenko, T., and Antonangeli, D.: Melting and subsolidus phase relations of Fe-Si-S alloys at Mercury’s core conditions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13353, https://doi.org/10.5194/egusphere-egu23-13353, 2023.

Our understanding on how opaque materials respond to extreme compression and high temperatures heavily relies on x-ray techniques. While X-ray diffraction is a powerful approach, some properties such as viscosity are experimentally not accessible with such approaches.  Application of x-ray imaging has been widely applied in extreme pressure and temperature conditions using multi-anvil presses, but obtaining sufficient space and time resolution for similar experiments using diamond anvil cells, at commensurately higher pressure and temperature conditions, have been challenging. We present recent developments in synchrotron X-ray imaging at the ECB beamline at PETRA III, DESY used in combination with laser heated DAC. Simultaneous X-ray imaging and diffraction allows for deeper insight in properties of materials under high pressure as well as the direct correspondence of phase transition diagnostics.  We present a case study of bismuth in the laser-heated diamond anvil cell showing detection of solid-solid and solid-liquid transitions and explore how X-ray imaging can be used to determine viscosity of molten bismuth under pressure.

How to cite: Massani, B., Husband, R., Campbell, D., Sneed, D., Jenei, Z., Liermann, H.-P., McWilliams, S., and O'Bannon, E.: Laser-melting Bismuth - A case study for X-ray imaging at high pressure and temperature, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15584, https://doi.org/10.5194/egusphere-egu23-15584, 2023.

ABSTRACT

In this talk, I will introduce the methods developed in my group, especially the machine learning and graph theory aided crystal structure prediction method (MAGUS) [1]. In addition, I will show some applications of these methods combined with first-principles calculations, for instance, the predictions of new possible compounds (helium-water, helium-ammonia, helium-methane, helium-silica, silica-water, etc) in the interior of giant planets or exoplanets, and their exotic new states under planetary high-pressure and high-temperature conditions (superionic state, plastic state, and their coexistence) [2-6]. These new compounds and their states may have some important implications for giant planets, including demixing, magnetic field, erosion of the rocky core, etc.

REFERENCE

• Kang Xia et al., “A novel superhard tungsten nitride predicted by machine-learning accelerated crystal structure search”, Sci. Bull. 63, 817 (2018).
• Cong Liu et al., “Mixed coordination silica at megabar pressure”, Phys. Rev. Lett. 126, 035701 (2021).
• Cong Liu et al., “Multiple superionic states in helium-water compounds”, Nature Physics 15, 1065 (2019).
• Cong Liu et al., “Plastic and Superionic Helium Ammonia Compounds under High Pressure and High Temperature”, Phys. Rev. X 10, 021007 (2020).
• Hao Gao et al., “Coexistence of plastic and partially diffusive phases in a helium-methane compound”, Natl. Sci. Rev. 7, 1540 (2020).
• Hao Gao et al., “Superionic Silica-Water and Silica-Hydrogen Compounds in the Deep Interiors of Uranus and Neptune”, Phys. Rev. Lett. 128, 035702 (2022).

How to cite: Sun, J.: Unexpected new compounds and their states in the interior of giant planets predicted from first-principles calculations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3333, https://doi.org/10.5194/egusphere-egu23-3333, 2023.

Fostered by third-generation synchrotron sources, experimental studies of physical and chemical properties of liquids at high pressure and temperatures are constantly pushed towards more and more extreme conditions, with applications ranging from Earth and planetary science, to material science, to fundamental physics.

In the last 20 years, many efforts have been dedicated to the development of a method to obtain structural information from the X-ray diffuse scattering signal of a liquid [1], allowing, for instance, to improve our understanding of the structure and evolution of deep planetary interiors. However, while data collection protocols are by now quite advanced and overall comparable across beamlines worldwide, data analysis largely differs depending on user and employed codes. To answer to the need of a unified data analysis tool for liquids and amorphous systems, we developed Amorpheus [2].

Amorpheus is an open-source, versatile, free and easy-to-use software for the analysis of X-ray diffuse scattering signal, allowing to perform a customizable analysis of a large amount of data and to invert for the density.  Available on GitHub [3] it is fully accessible by the community. This software has been tested on data collected with DAC and with large volume presses and it is well adapted for the analysis of liquid metals and alloys, as well as of amorphous systems. Here we will present and discuss selected examples of data analysis performed by Amorpheus in order to determine local structure and density of liquid iron binary and ternary alloys at planetary core conditions.

[1] Eggert JH, Weck G, Loubeyre P, Mezouar M. Quantitative structure factor and density measurements of high-pressure fluids in diamond anvil cells by x-ray diffraction: Argon and water. Phys Rev B. 2002;65(17):174105. doi:10.1103/PhysRevB.65.174105

[2] Boccato S, Garino Y, Morard G, et al. Amorpheus: a Python-based software for the treatment of X-ray scattering data of amorphous and liquid systems. High Press Res. 2022;42(1):69-93. doi:10.1080/08957959.2022.2032032

[3] https://github.com/CelluleProjet/Amorpheus

How to cite: Boccato, S., Morard, G., Garino, Y., Sanloup, C., Zhao, B., Morand, M., and Antonangeli, D.: Analysis of diffuse scattering from liquid and amorphous samples: protocols and software, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14735, https://doi.org/10.5194/egusphere-egu23-14735, 2023.