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The cratonic lithosphere is a vast host for deep recycled carbon, trapping up to several weight percent CO₂ at depths overlapping the seismic mid-lithospheric discontinuities (MLDs). However, the role of carbonates, especially for the latest discovered amorphous calcium carbonate (CaCO₃), is underestimated in the formation of MLDs. Using the pulse-echo-overlap method in a Paris-Edinburgh press coupled with synchrotron X-ray diffraction, we explored the acoustic velocities of CaCO₃ under high pressure-temperature (P-T) conditions relevant to the cratonic lithosphere. Two anomalous velocity drops were observed associated with the phase transition from aragonite to amorphous phase and with the pressure-induced velocity drop in the amorphous phase around 3 GPa, respectively. Both drops are comparable with approximately 35% and 52% reductions for compressional (V_P) and shear (V_S) wave velocities, respectively. The V_P and V_S values of the amorphous CaCO₃ above 3 GPa are about 1/2 and 1/3 of those of the major upper-mantle minerals, respectively. These velocity reductions caused by the presence of CaCO₃ would readily cause MLDs at depths of 70–120 km dependent on the geotherm even if only 1–2 vol.% CaCO₃ is present in the cratonic lithosphere. 

Abstract As the reaction product of subducted water and the iron core, FeO2 with more oxygen than hematite (Fe2O3) has been recently recognized as an important component in the D” layer just above the Earth's core-mantle boundary. Here, we report a new oxygen-excess phase (Mg, Fe)2O3+δ (0 < δ < 1, denoted as “OE-phase”). It forms at pressures greater than 40gigapascals when (Mg, Fe)-bearing hydrous materials are heated over 1,500 kelvin. The OE-phase is fully recoverable to ambient conditions for ex-situ investigation using transmission electron microscopy, which indicates that the OE-phase contains ferric iron (Fe3+) as in Fe2O3 but holds excess oxygen through interactions between oxygen atoms. The new OE-phase provides strong evidence that H2O has extraordinary oxidation power at high pressure. Unlike the formation of pyrite-type FeO2Hx which usually requires saturated water, the OE-phase can be formed with under-saturated water at mid-mantle conditions, and is expected to be more ubiquitous at depths greater than 1,000 km in Earth's mantle. The emergence of oxygen-excess reservoirs out of primordial and subducted (Mg, Fe)-bearing hydrous materials may revise our view on the deep-mantle redox chemistry.