Water is a major component of fluids in the Earth's mantle, where its properties are substantially different from those at ambient conditions. At the pressures and temperatures of the mantle, experiments on aqueous fluids are challenging, and several fundamental properties of water are poorly known; e.g., its dielectric constant has not been measured. This lack of knowledge of water dielectric properties greatly limits our ability to model water-rock interactions and, in general, our understanding of aqueous fluids below the Earth's crust. Using ab initio molecular dynamics, we computed the dielectric constant of water under the conditions of the Earth's upper mantle, and we predicted the solubility products of carbonate minerals. We found that MgCO 3 (magnesite)-insoluble in water under ambient conditions-becomes at least slightly soluble at the bottom of the upper mantle, suggesting that water may transport significant quantities of oxidized carbon. Our results suggest that aqueous carbonates could leave the subducting lithosphere during dehydration reactions and could be injected into the overlying lithosphere. The Earth's deep carbon could possibly be recycled through aqueous transport on a large scale through subduction zones.water solvation properties | carbon cycle | ab initio simulations | supercritical water W ater, a major component of fluids in the Earth's mantle (1, 2), is expected to play a substantial role in hydrothermal reactions occurring in the deep Earth at supercritical conditions (3, 4). Pressure (P) and temperature (T) increase with increasing depth (5) and at ∼400 km, where seismic discontinuities define the bottom boundary of the upper mantle, the pressure can reach ∼13 GPa and the temperature can be as high as 1,700 K (6-8). In this regime the properties of water and thus of aqueous fluids are remarkably different from those at ambient conditions. For example, water has an unusually large static dielectric constant e 0 ∼ 78 at ambient conditions; however, at the vapor-liquid critical point at 647 K, e 0 deceases to less than 10 (9), implying that ionwater interactions in solution are greatly modified. In turn these changes affect the solubility of minerals and hence chemical reactions occurring in aqueous solutions under pressure (10, 11).Measurements of the dielectric constant of water date back to the 1890s (12), but they are still limited to P < 0.5 GPa and T < 900 K, corresponding to crustal metamorphic conditions. Indeed, it is challenging to measure e 0 at high P and T because water becomes highly corrosive (11). Several models correlating experimental data suggested extrapolations of e 0 to ∼1 GPa and ∼1,300 K (e.g., refs. 13-15), which corresponds to only very shallow mantle conditions under the oceans; however, deeper mantle conditions relevant to plate tectonic processes could not be reached and different extrapolations showed poor agreement with each other (1). The current lack of knowledge of the dielectric constant of water under the P and T of the mantle hampers our ability...
