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Fig. 6: Schematic illustration of the deep carbon and water cycle as well as water-carbonate mineral interactions in the deep Earth (from Farsang et al., Nat. Commun., 2021).

Water can be structurally bound either as hydroxyl groups (OH-) in hydrous minerals formed by the interaction of the plate with seawater, or as H2O in pore spaces. Carbon carrier phases are dominantly abiotic carbonate minerals forming through precipitation in the deep oceans [2,3] and, to a lesser extent, seafloor sediments that may be rich in organic or inorganic remnants of organisms (e.g., carbonate shells). As the oceanic plate sinks into the mantle, it experiences increasing pressure and temperature conditions, which leads to the destabilisation of hydrous mineral phases and the closure of pores. Consequently, water is gradually expulsed from the slab in the form of hydrous fluids, which, at these conditions, have the capacity to dissolve minerals, including carbonates. Partial melt formation above the subducting plate is a second consequence of the presence of these deep hydrous fluids, as water significantly reduces the melting temperature of rocks, ultimately leading to arc volcanism and outgassing of water and carbon, closing the cycle.

Although most subducting carbon is present as calcite (CaCO3), with increasing depth, most of this calcite may react with the surrounding mineral phases to form the magnesium-rich carbonate minerals dolomite (CaMg(CO3)2) and magnesite (MgCO3) or, depending on the availability of other metals, even more exotic carbonates. For instance, at places where large amounts of ferromanganese nodules are subducted, rhodochrosite (MnCO3) forms, as evidenced by rhodochrosite-containing solid and fluid inclusions [4].

At beamline ID27, X-ray fluorescence (XRF) spectroscopy was combined with the diamond anvil cell technique to

measure the solubility of carbonate minerals in aqueous fluids at high P/T conditions (up to 7 GPa and ~400˚C) relevant for subduction zones (Figure 7). An ESRF- developed, externally heated diamond anvil cell equipped with perforated diamonds was employed to reduce signal absorption. At ID27, the solubility of rhodochrosite was determined from the Mn Kα fluorescence line intensity variations (at 5.89 keV), in the fluid phase next to the rhodochrosite crystal, which was only feasible at this beamline due to its high flux and very small focused beam (3*3 µm2).

The fluorescence line of Mg, the main component of major carbonates dolomite and magnesite, exhibits even lower energy, not accessible with in-situ XRF. Therefore, complementary measurements were carried out to determine the solubility of this phase at the same conditions. Combining all data, including recent results obtained on smithsonite (ZnCO3) solubility [5], and considering the heterogeneity of carbon and water subduction fluxes at the global scale, it was found that almost two thirds of Earth s surface carbon is drawn down into the deep Earth (several hundreds of kilometres) by plate tectonics. Only one third potentially escapes back through arc volcanism. These results demonstrate the potentially important role of plate tectonics on carbon storage at depth via subduction over geological time.

The Extremely Brilliant Source upgrade will now enable such types of studies at even higher pressure-temperature conditions, at higher dilution levels and on light elements or on complex natural samples.