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We report the preparation of the starting material reflecting the complex composition of the Earth's mantle. With this aim, we synthesized two types of material: sol-gel and glass obtained by aerodynamic levitation. Thanks to their high homogeneity and reactivity, these materials are suitable for experimental petrology under extreme conditions, conducted in laser-heated diamond anvil cell. We then used this mantle analog to investigate the iron partitioning between high pressure phases under lower mantle conditions during solid-state reaction and partial melting of the material. Iron preferentially partitions into the (Mg,Fe)O, but the presence of aluminum slightly enriches iron in the silicate phase (Kpv-fp=0.41±0.04) compared to similar experiments in an Al-free system. [ABSTRACT FROM AUTHOR]

The role that subducted carbonates play in sourcing and storing carbon in the deep Earth's interior is uncertain, primarily due to poor constraints on the stability of carbonate minerals when interacting with mantle phases. Magnesite (MgCO 3) is the most prominent carbonate phase to be present at all mantle pressure-temperature conditions. In this study, we combined multi-anvil apparatus and laser-heated diamond anvil cell experiments to investigate the stability of magnesite in contact with iron-bearing bridgmanite. We examined the presence of melt, decarbonation, and diamond formation at shallow to mid-lower mantle conditions (25 to 68 GPa; 1350 to 2000 K). Our main observation indicates that magnesite is not stable at shallow lower mantle conditions. At 25 GPa and under oxidizing conditions, melting of magnesite is observed in multi-anvil experiments at temperatures corresponding to all geotherms except the coldest ones. Whereas, at higher pressures and under reducing conditions, in our laser-heated diamond-anvil cell experiments, diamond nucleation is observed as a sub-solidus process even at temperatures relevant to the coldest slab geotherms. Our results indicate that magnesite was reduced and formed diamonds when in contact with the ambient peridotite mantle at depths corresponding to the shallowest lower mantle (33 GPa). Thus, we establish that solid magnesite decomposes at depths of ∼700 km as it contacts the ambient mantle. Consequently, the recycling of carbonates will hinder their transport deeper into the lower mantle. [Display omitted] • The stability of magnesite (MgCO 3) was investigated in the presence of iron-bearing bridgmanite at conditions from 25 to 68 GPa and temperatures covering all subduction geotherms (1350 K to 2000 K). • Magnesite reacts with iron-bearing bridgmanite to form diamonds at conditions relevant to the coldest geotherm. • Subduction of magnesite, and more generally carbonates, is limited to the top of the lower mantle (∼700 km depths), even along cold slabs geotherm. [ABSTRACT FROM AUTHOR]

Melting properties of the deep mantle remain controversial due to experimental difficulties; e.g., reports of solidus temperatures of mantle-relevant compositions span over ∼700 K at 2000 km depth. This situation limits our understanding of the thermochemical state of the Earth's interior. Using the laser heated diamond anvil cell (LH-DAC), we performed new experimental determination of the solidus profile of ultra-dry pyrolite and the solidus of two compositions of (Mg,Fe)(Si,Al)O 3 bridgmanite (Bg). Melting was detected (i) from -the correlation between laser power and sample temperature, -changes of sample texture and -the level of visible light absorption, for all samples, (ii) using X-ray diffraction, for the MgSiO 3 composition and (iii) after scanning electron microscope observations, for selected Fe-bearing samples. Special care was given to using ultra-dry experimental chambers and to determination of sample temperature. In particular, we discuss the wavelength-dependent thermal emission of silicate samples, which lowers the solidus by 100 to 300 K, compared to the grey-body assumption. The solidus of MgSiO 3 -Bg is in good agreement with previous reports using ab initio calculations and shock wave experiments. We observe a net decrease in the solid-liquid Clapeyron slope at 60(3) GPa and 4400(200) K, which can be related to rapid pressure-induced coordination change of Si in the melt. (Mg 0.955 ,Fe 0.045)(Si 0.993 ,Al 0.007)O 3 Bg melts 600–800 K lower than MgSiO 3 -Bg. Its solidus evolves smoothly with pressure, suggesting progressive Si coordination change in the melt. In the pressure range investigated (24–135 GPa) Clapeyron slopes suggest rapid decrease of the volume of fusion, from 14 to 2% for MgSiO 3 and from 9 to 3% for (Fe,Al)-bearing Bg, assuming congruent melting. By comparing the solidii of various silicates, it appears that the higher the number of cations, the less pronounced is the curvature of the solidus. This observation suggests that the relatively ordered structure of simple liquid compositions with a limited number of distinct network-modifying cations frustrates the coexistence of tetrahedrally and octahedrally coordinated Si polyhedral. The solidus of pyrolite presents a smooth evolution from 2200(100) K to 3950(200) K in the same pressure interval. This is very similar to our previous work on chondritic-type mantle. The new solidus is 200–300 K lower than that of KLB-1 peridotite, which can be related to more incompatible elements in pyrolite. It remains problematic that our solidus plots several 100 K higher than other recent measurements performed on pyrolite; we discuss the possibility of a higher water content in previous samples, compared to our experiments. Assuming a dry lowermost mantle, our results imply a core-mantle boundary temperature lower than 3950(200) K. Modeling the melting diagram at the core-mantle boundary suggests a pseudo-eutectic melt significantly depleted in SiO 2 , compared to the composition of the mean mantle. • Mantle solidus evolves from 2200(100) and 3950(200) K with depth in lower mantle. • Solidus of pyrolite suggests core-mantle boundary temperature lower than 3950(200) K. • The initially steep solidus of MgSiO 3 bridgmanite flattens at 60(3) GPa. • Silicate phases with more cations present smooth solidus evolution to CMB pressure. [ABSTRACT FROM AUTHOR]