Properties of magmatic liquids by molecular dynamics simulation: The example of a MORB melt

Abstract

International audienceA new atom-atom interaction potential is introduced for describing by classical molecular dynamics (MD) simulation the physical properties of natural silicate melts. The equation of state, the microscopic structure, the viscosity, the electrical conductivity, and the self-diffusion coefficients of ions in a mid-oceanic ridge basalt (MORB) melt are evaluated by MD over a large range of temperature and pressure (1673-3273 K and 0-60 GPa). A detailed comparison with experimental data shows that the model reproduces the thermodynamic, structural and transport properties of a MORB with an unprecedented accuracy. In particular, it is shown that the MORB melt crystallizes at lower mantle conditions into a perovskite phase whose the equation of state (EOS) is compatible with those proposed in the experimental literature. Moreover, in accordance with experimental findings, the simulation predicts not only that the MORB viscosity exhibits a (slight) minimum with the pressure, but also that the viscosity at high temperature remains very low (<100 mPa.s for T > 2273 K) even at high pressure (up to 40 GPa). However the evolution of the electrical conductivity with temperature and pressure is not always the symmetrical of that of the viscosity. In fact, the relationship between viscosity and electrical conductivity shows a crossover at around 2073 K

Structural dynamics of basaltic melt at mantle conditions with implications for magma oceans and superplumes

“…Even then, our results are in the same order of magnitude. For example, they have concluded that for temperatures above 2273 K, even at 30-40 GPa, the viscosities are less than 100 mPa s, that can have implications on the geodynamics of the Archean mantle of the ancient Earth 37 . At 0 GPa and 2273 K, they calculated the coefficient of viscosity to be~80 mPa s, in comparison with our corresponding magnitude of 50 mPa s at 0 GPa and 2200 K. Their magnitudes of well below 100 mPa s for 30 and 40 GPa and 3273 K are again similar to what we have obtained for these pressures and 3000 K. In another more recent work, Dufils et al showed that for MORB at 1673 K and 0.5 GPa, the coefficient of viscosity is~1200 mPa s 39 .…”

“…4) continues with depth and temperature, there may be implications for the ULVZ with melts of significantly lower viscosity than previously suggested. [37][38][39] Magma oceans that formed from giant impacts during accretion are widely accepted to be responsible for the formation of the metallic core and the silicate mantle through differentiation as well as the atmosphere and hydrosphere through degassing 53 . Viscosity is an important parameter that controls virtually all the dynamic processes in early Earth's magma oceans.…”

“…While the evolution of the viscosity of basaltic melts with the temperature is rather well reported 34 , 35 , its evolution with the pressure is yet to be fully understood, especially at very high pressure 36 , 37 . We must point out that in other recent theoretical works on determining the transport properties of melts, they have used classical MD 37 – 39 . This can impact results depending on the choice of empirical pair potentials used.…”

“…On the contrary, our calculations are based on ab initio MD. In the works by Dufils et al 37 – 39 , the more realistic models of basalt, mid-ocean ridge basalt (MORB) have been considered, which include more cationic species. These systems involve thousands of atoms for simulations that are beyond the scope of being treated using ab initio MD.…”

“…4 ) continues with depth and temperature, there may be implications for the ULVZ with melts of significantly lower viscosity than previously suggested. 37 – 39 …”