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<p>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).</p><p>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.</p><p>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).</p><p>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.</p><div> <div> <div> </div> </div> </div>

<p>Earth’s inner core likely consists of Fe-Ni alloy(s) plus a minor fraction of light element(s) to match the density and sound wave velocities of seismological models such as the preliminary reference Earth model (PREM). Among possible alloying light elements (e.g., Si, O, H, S, C), silicon is a popular candidate based on its cosmochemical abundance and potential involvement in chemical reactions at the core-mantle boundary. Previous work has shown that the solubility of Si in <em>hcp</em>-(Fe,Ni) alloy increases the stability field of the<em> bcc</em>-phase at high pressure. Comparison of sound velocity and density data of Fe-Ni-Si alloys with geophysical observations and theoretical predictions provide important constraints on the structure and dynamics of Earth’s inner core. However, knowledge of the high-pressure and high-temperature behaviour and properties of Fe-Ni alloys that contain light elements is limited. We therefore investigated <em>bcc</em>-Fe<sub>0.78</sub>Ni<sub>0.07</sub>Si<sub>0.15 </sub>alloy to compare its sound velocity and density with <em>ab initio</em> calculations and PREM in order to clarify the role of Si as a light element in Earth’s inner core.</p><p>Compressional velocities and densities of <em>bcc</em>-Fe<sub>0.78</sub>Ni<sub>0.07</sub>Si<sub>0.15</sub> alloy have been measured using inelastic X-ray scattering (IXS) and powder X-ray diffraction at the SPring-8 synchrotron facility (BL35XU beamline). High pressure was generated using a BX90-type diamond anvil cell. The metal alloy sample was loaded together with Ne (pressure medium) in a Re sample chamber and was mechanically compressed to 75 GPa through steps of 10 GPa at room temperature. IXS data were acquired at each pressure point in the range of momentum transfer of 4.24 to 7.63 nm<sup>-1</sup>. To determine density, we collected X-ray diffraction patterns of the sample before acquisition of each IXS spectrum using a flat panel detector installed in the optical system. All IXS spectra were fitted using Lorentzian functions. The dispersion relationship between energy (E) and momentum transfer (Q) was obtained by fitting all data with the following equation:</p><p>E (meV) = 4.192 x 10<sup>-4 </sup><em>v</em>p (m/s) x Q<em><sub>max</sub></em> (nm<sup>-1</sup>) x sin (π/2 x Q (nm<sup>-1</sup>)/ Q<em><sub>max</sub></em> (nm<sup>-1</sup>),</p><p>where <em>v</em>p is the sound velocity of the sample.</p><p>Preliminary results for <em>bcc</em>-Fe<sub>0.78</sub>Ni<sub>0.07</sub>Si<sub>0.15</sub> show that the energy of the longitudinal acoustic phonon increases with increasing pressure. Additionally, we found that vp follows Birch’s Law, i.e., there is a linear relationship between density and sound velocity. Based on the comparison of our results and those for <em>hcp</em>-Fe and Fe-Si alloys reported previously with PREM, we propose that <em>bcc</em>-Fe<sub>0.78</sub>Ni<sub>0.07</sub>Si<sub>0.15</sub> alloy is a viable candidate as a component of Earth’s inner core.</p><p> </p>

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