up to 115 GPa and 1600 K by NIS. 12• Negligible anharmonic contributions to sound velocities validate the extrapola-13 tion of Birch's law to CMB conditions. 14• Less than 8.4 vol% B2-FeSi produced by core-mantle reactions are enough to 15 explain the seismic anomalies associated with the ULVZs.

A portable IR fiber laser‐heating system, optimized for X‐ray emission spectroscopy (XES) and nuclear inelastic scattering (NIS) spectroscopy with signal collection through the radial opening of diamond anvil cells near 90°with respect to the incident X‐ray beam, is presented. The system offers double‐sided on‐axis heating by a single laser source and zero attenuation of incoming X‐rays other than by the high‐pressure environment. A description of the system, which has been tested for pressures above 100 GPa and temperatures up to 3000 K, is given. The XES spectra of laser‐heated Mg0.67Fe0.33O demonstrate the potential to map the iron spin state in the pressure–temperature range of the Earth's lower mantle, and the NIS spectra of laser‐heated FeSi give access to the sound velocity of this candidate of a phase inside the Earth's core. This portable system represents one of the few bridges across the gap between laser heating and high‐resolution X‐ray spectroscopies with signal collection near 90°.

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We describe the use of a silver-coated 90 • parabolic mirror of 33 mm focal length as objective for imaging, on-axis laser heating and radiospectrometric temperature measurements of a sample compressed in a diamond anvil cell in a laser heating system. There, spatial resolution and imaging quality of the parabolic mirror are similar to the one of a 10× objective. The temperature measurements between 500 and 900 nm are essentially free from chromatic aberration. The parabolic mirror was also perforated with a 220-μm hole, allowing for on-axis imaging, laser heating and incidence of X-rays simultaneously at synchrotron facilities. The parabolic mirror is thus a well-suited alternative to existing refractive and reflective objectives in laboratory and synchrotron laser heating systems.

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<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>

A von Hámos spectrometer has been implemented in the vacuum interaction chamber 1 of the High Energy Density instrument at the European X-ray Free-Electron Laser facility. This setup is dedicated, but not necessarily limited, to X-ray spectroscopy measurements of samples exposed to static compression using a diamond anvil cell. Si and Ge analyser crystals with different orientations are available for this setup, covering the hard X-ray energy regime with a sub-eV energy resolution. The setup was commissioned by measuring various emission spectra of free-standing metal foils and oxide samples in the energy range between 6 and 11 keV as well as low momentum-transfer inelastic X-ray scattering from a diamond sample. Its capabilities to study samples at extreme pressures and temperatures have been demonstrated by measuring the electronic spin-state changes of (Fe0.5Mg0.5)O, contained in a diamond anvil cell and pressurized to 100 GPa, via monitoring the Fe Kβ fluorescence with a set of four Si(531) analyser crystals at close to melting temperatures. The efficiency and signal-to-noise ratio of the spectrometer enables valence-to-core emission signals to be studied and single pulse X-ray emission from samples in a diamond anvil cell to be measured, opening new perspectives for spectroscopy in extreme conditions research.