Decarbonization of siderite in the water-rich upper mantle

The aqueous fluids within subducted slabs have the potential to influence the form of carbonate presence and the carbon cycling process. Experiments were performed on resistive heating diamond anvil cell using siderite crystals and grains with water under conditions of pressure as high as 11.4 GPa and temperatures reaching up to 530 °C. These experiments aimed to simulate geological reactions that may occur within a depth range of 340 km in subducted slabs. Raman spectroscopy was employed to monitor the reactions and microscale phenomena within the sample chamber as pressure and temperature increase. The recovered products were analyzed using scanning electron microscopy and transmission electron microscopy. The results indicate that at 0.8 GPa and 108 °C, a Fischer-Tropsch Type (FTT) reaction occurred on the sample surface, resulting in the formation of organic compound formaldehyde, followed by the observation of formic acid. At higher pressure and temperature (3.5 GPa, 420 °C), the formation of γ-Fe₂O₃ and γ-FeOOH was observed on the sample surface, accompanied by the release of CO₂ and H₂. Transmission electron microscope analysis of the quenched product powders indicated that the generated iron oxides were consistent with the phases observed at high pressure and temperature conditions. High pressure and temperature dissolution experiments of siderite in water reveal that carbon may be released into the mantle wedge entirely in the form of CO₂ in warm subducted slabs and cold subducted slabs that subduct to depths of 75 km. The released CO₂ participates in the carbon cycle of the island arc volcanic systems in the upper mantle at depths of 70–120 km and accelerates the transfer of subducted carbon to the Earth’s surface.