Structure and Transport Properties of carbonate Melts from First Principles: Constraining Subduction in the Deep Carbon Cycle.

Abstract:

Details of the deep mantle carbon cycle remain enigmatic with subducted CO₂ estimates commonly varying by 3 orders of magnitude. Furthermore the mechanisms of delivery to the deep mantle are challenging due to slab melting and low viscosity of carbonate. Much of the difficulty in understanding the transport of carbon into the deep Earth arises from the difficulty in studying carbonate melts. Low viscosities and decarbonation at low temperature produces challenging experimental conditions leaving MgCO₃, a crucial component, insufficiently described in the literature.

We use first principles molecular dynamics (FPMD) to study the transport, structural and thermodynamic properties of four key alkali metal carbonate liquids (MgCO₃,CaCO₃,SrCO₃,BaCO₃) at conditions relevant to slab subduction in the upper mantle and transition zone. We find discontinuous behavior (in terms of ionic radius and mass) exhibited by CaCO₃ and to a lesser extent, SrCO₃. We find a crossover in density at 6 GPa (2.9 gcm^-3) where at lower pressures CaCO₃ is less dense than MgCO₃. Bulk moduli of the series places CaCO₃ and SrCO₃ at consistently lower values than MgCO₃. Structural analysis reveals coincident bend lengths of metal-oxygen and carbon-oxygen found in the Ca and Sr members of the series cause an interruption to efficient packing in the liquid structures, exhibited most clearly in anomalously low bulk moduli. We analyze the FPMD calculations to calculate diffusion coefficients and viscosities from vibrational properties and shear stress autocorrelation respectively. CaCO₃ is consistently disruptive of the ionic radius series, being the fastest diffusing system at all studied pressures and having the lowest viscosity at 10 GPa. These findings reveal that the pressure dependent structure of carbonate liquids puts constraint on the transport of carbon from slab to the upper mantle.