Our knowledge of the structure and composition of Earth's core is based on sparse direct evidence (e.g., from seismology, geodesy, geo-and paleo-magnetism) and many indirect observations (from cosmochemistry, experimental petrology, and mineral physics) (Allègre et al., 1995;McDonough & Sun, 1995). Cosmochemical studies on iron meteorites and a comparison of mineral physics data with seismological observations (measurements of density (ρ) and compressional (V P ) and shear (V S ) wave velocities under extreme conditions) suggest that Earth's inner core is primarily composed of metallic Fe-Ni alloy (5-25 wt.% Ni) (McDonough & Sun, 1995;Wasson & Chou, 1974). However, the inner core density is ∼5 % lower than pure Fe at corresponding pressures and temperatures (Dewaele et al., 2006;Fei et al., 2016), presumably due to the presence of light elements (Birch, 1952) that were incorporated in the core during its formation (

<p>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.</p><p>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.</p><p>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.</p>

<p>Subduction of carbon-bearing phases throughout Earth’s history may be an important mechanism of sourcing carbon to the Earth’s lower mantle. As carbon has very low solubility in mantle silicates, it is primarily present in accessory phases such as carbonates, diamond, or metal carbides. Previous studies indicate that more than half of the carbonate contained in the oceanic crust may survive metamorphism and dehydration in the sub-arc and reach the lower mantle, even though the oxygen fugacity in the deep mantle may not favour their stability [1]. Indeed, the presence of carbonate in ultra-deep diamond inclusions provides evidence for carbonate subduction at least down to the transition zone [2].</p><p>The carbonate phases present in the lower mantle depend on their bulk composition, the oxygen fugacity, and on their stability at a given pressure and temperature. Results from high-pressure experiments show that magnesite (MgCO<sub>3</sub>) can be stable up to deep lower mantle conditions (∼80 GPa and 2500 K) [3]. Accordingly, magnesite may be considered the most probable carbonate phase present in the deep Earth. Experimental studies on magnesite decarbonation in presence of SiO<sub>2</sub> at lower mantle conditions suggest that magnesite is stable along a cold subducted slab geotherm [4, 5]. However, our understanding of magnesite’s stability in contact with bridgmanite [(Mg,Fe)SiO<sub>3</sub>],  the most abundant mineral in the lower mantle, remains incomplete.</p><p>Hence, to investigate sub-solidus reactions, melting, decarbonation, and diamond formation in the system MgCO<sub>3</sub>-(Mg,Fe)SiO<sub>3</sub>, we conducted a combination of high-pressure experiments using multi-anvil press and laser-heated diamond anvil cells (LH-DAC) at conditions ranging from 25 to 70 GPa and 1300 to 2100 K.</p><p>Multi-anvil experiments at 25 GPa and temperatures below the mantle geotherm (1700 K) show the formation of carbonate-silicate melt associated with stishovite crystallization, indicating incongruent melting of bridgmanite to stishovite, in accordance with the recent finding of Litasov and Shatskiy [4]. LH-DAC data from <em>in situ</em> X-ray diffraction show crystallization of bridgmanite and stishovite. Diamond crystallization is detected using Raman spectroscopy. A melt phase could not be detected <em>in situ</em> at high temperatures.</p><p>Our results suggest a two-step process that starts with melting at temperatures below the mantle geotherm, followed by crystallization of diamond from the melt produced.  Therefore, we propose that subducted carbonate-bearing silicate rocks will not remain stable in the lower mantle and will instead melt at upper-most lower mantle conditions, fostering diamond formation. Our study also provides additional evidence that diamond production is related to carbonated melt. Consequently, the melting of recycled crust and chemical transfer to the surrounding mantle will hinder the transport of carbon deeper into the lower mantle.</p><p>[1] Stagno et al. (2015) Contrib. Mineral. Petrol. 169(2), 16.<br>[2] Brenker et al. (2007) EPSL 260(1-2), 1-9.<br>[3] Binck, et al. (2020) Physical Review Materials, 4(5),1-9.<br>[4] Litasov & Shatskiy (2019) Geochemistry International, 57(9), 1024-1033.<br>[5] Drewitt, et al. (2019). EPSL, 511, 213-222.</p>