Buoyancy of dry and volatile-bearing silicate melts in the Earth's mantle

Melts and fluids are major transport vectors inside the Earth. Their relative buoyancy with respect to the surrounding geologic environment is the main factor affecting their geodynamic behaviour. The buoyancy depends on a series of extrinsic factors, like pressure and temperature, and intrinsic factors, like relative chemistry, and presence of volatile components. But despite the importance of density, for melts and fluids, measuring it is a daunting task in experiments. However, it is extremely easy and straightforward to calculate these densities, in equilibrium conditions. For this we perform first-principles molecular dynamics simulations, which are notoriously demanding in terms of computational resources, though highly accurate and reliable.

Here we focus on the Earth's mantle. We consider a variety of silicate melts whose compositions derive from dry pyrolite [McDonough and Sun, Chem. Geol., 1995], which we consider as the chemical and mineralogical model for the bulk silicate Earth (BSE). We study the evolved compositions that may result from fractional crystallization of the global magma ocean, within various models for silicate extraction, with [Tronnes et al., Tectonphysics, 2018] and without [Caracas et al., EPSL, 2019] equilibration with the core.

We add carbon as CO and CO2 molecules to simulate the carbonated silicate melt. We add hydrogen as hydroxyl by substituting MgO by H2O for the hydrous carbonate melt. We also consider a coupled hydrous carbonated pyrolite. We cover the pressure and temperature range of the entire Earth's mantle.

We compare the densities of the various melts with the dry pyrolite molten as in the case of the global magma ocean of the Early Earth, and with the PREM as in the case of the present-day Earth. We find that the evolved melts are the heaviest, at cold temperatures even heavier than the present day mantle at the core-mantle boundary. The hydrous melts have the strongest positive buoyancy. The buoyancy relations for the carbonated ones highly depend on both depth and volatile concentration.

We illustrate the atomic environment around the volatile components in the various melts using the KeckCave facility, developed by Louise Kellogg and collaborators at UC Davis.

This work was supported by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement number 681818 IMPACT), by the Extreme Physics and Chemistry Directorate of the Deep Carbon Observatory, and by the Research Council of Norway through its Centres of Excellence funding scheme, project number 223272.