Earth’s deep carbon cycle affects atmospheric CO₂, climate, and habitability. Owing to the extreme solubility of CaCO₃, aqueous fluids released from the subducting slab could extract all carbon from the slab. However, recycling efficiency is estimated at only around 40%. Data from carbonate inclusions, petrology, and Mg isotope systematics indicate Ca²⁺ in carbonates is replaced by Mg²⁺ and other cations during subduction. Here we determined the solubility of dolomite [CaMg(CO₃)₂] and rhodochrosite (MnCO₃), and put an upper limit on that of magnesite (MgCO₃) under subduction zone conditions. Solubility decreases at least two orders of magnitude as carbonates become Mg-rich. This decreased solubility, coupled with heterogeneity of carbon and water subduction, may explain discrepancies in carbon recycling estimates. Over a range of slab settings, we find aqueous dissolution responsible for mobilizing 10 to 92% of slab carbon. Globally, aqueous fluids mobilise $${35}_{-17}^{+20}$$35 − 17 + 20 % ( $${27}_{-13}^{+16}$$27 − 13 + 16 Mt/yr) of subducted carbon from subducting slabs.

As the main constituent of planetary cores, pure iron phase diagram under high pressure and temperature is of fundamental importance in geophysics and planetary science. However, previously reported iron‐melting curves show large discrepancies (up to 1000 K at the Earth's core–mantle boundary, 136 GPa), resulting in persisting high uncertainties on the solid‐liquid phase boundary. Here we unambiguously show that the observed differences commonly attributed to the nature of the used melting diagnostic are due to a carbon contamination of the sample as well as pressure overestimation at high temperature. The high melting temperature of pure iron under core‐mantle boundary (4250 ± 250 K), here determined by X‐ray absorption experiments at the Fe K‐edge, indicates that volatile light elements such as sulfur, carbon, or hydrogen are required to lower the crystallization temperature of the Earth's liquid outer core in order to prevent extended melting of the surrounding silicate mantle.

[1] Ferromagnesian carbonates are the stable high pressure carbonates in eclogite and peridotite and their physical properties are thus important for understanding the dynamics of reintroduction of carbon into the Earth's mantle. Using Brillouin scattering spectroscopy, we have determined the sound velocities and elasticity of carbonates along the (Mg_1−xFe_x)CO₃ join (with x = 0, 0.65, 0.95 and 0.99) at room conditions to evaluate the effect of carbonates on the seismic velocity structure of the subducting lithosphere. Within experimental uncertainties, Fe substitution has a negligible effect on the adiabatic bulk modulus K_sof Mg‐Fe carbonates whereas the shear modulusμ decreases by 34% from MgCO₃ to FeCO₃. We find that the seismic velocity contrast between carbonate‐bearing eclogite and peridotite with 5 wt% CO₂and their carbon‐free counterparts is less than 1%, indicating that moderately carbonated regions in the subducting mafic crust and mantle may be difficult to detect seismically. The threshold of seismic detection with present methods (∼2%) would require significantly high carbonate contents of 24.5 vol% (Mg_0.79Fe_0.21)CO₃ (15 wt% CO₂) in eclogite and more than 30 vol% (Mg_0.93Fe_0.07)CO₃ (>20 wt% CO₂) in peridotite.

A review of some important technical challenges related to in situ diamond anvil cell laser heating experimentation at synchrotron X-ray sources is presented. The problem of potential chemical reactions between the sample and the pressure medium or the carbon from the diamond anvils is illustrated in the case of elemental tantalum. Preliminary results of a comparison between reflective and refractive optics for high temperature measurements in the laser-heated diamond anvil cell are briefly discussed. Finally, the importance of the size and relative alignment of X-ray and laser beams for quantitative X-ray measurements is presented.

The discovery of superconductivity above 250 K at high pressure in LaH₁₀ and the prediction of overcoming the room temperature threshold for superconductivity in YH₁₀ urge for a better understanding of hydrogen interaction mechanisms with the heavy atom sublattice in metal hydrides under high pressure at the atomic scale. Here we use locally sensitive X-ray absorption fine structure spectroscopy (XAFS) to get insight into the nature of phase transitions and the rearrangements of local electronic and crystal structure in archetypal metal hydride YH₃ under pressure up to 180 GPa. The combination of the experimental methods allowed us to implement a multiscale length study of YH₃: XAFS (short-range), Raman scattering (medium-range) and XRD (long-range). XANES data evidence a strong effect of hydrogen on the density of 4d yttrium states that increases with pressure and EXAFS data evidence a strong anharmonicity, manifested as yttrium atom vibrations in a double-well potential.

Nickel is the second most abundant element in the Earth's core. However, the properties of Fe‐Ni alloys are still poorly constrained under planetary cores conditions, in particular concerning the effect of Ni on the melting curve of Fe. Here we show that Ni alloying up to 36 wt% does not affect the melting curve of Fe up to 100 GPa. However, Ni strongly modifies the hexagonal‐closed‐packed/face‐centered‐cubic (hcp/fcc) phase boundary, pushing the hcp/fcc/liquid triple point of Fe‐20wt%Ni to higher pressures and temperatures. Our results allow constraining the triple point for Fe‐10wt%Ni, a composition relevant for the Earth interior, and point out a decrease of the melting temperature at core‐mantle boundary by 400 K with respect to pure Fe. A lower amount of light elements than previously predicted is thus required to reduce the crystallization temperature of core materials below that of a peridotitic lower mantle, in better agreement with geochemical observations.