Abstract
The viscosity of carbonate melts is a fundamental parameter to constrain their migration and ascent rates through the mantle and ultimately, their role as carbon conveyors within the deep carbon cycle. Yet, data on the viscosity of carbonate melts have remained scarce due to experimental limitations and the lack of appropriate theoretical descriptions for molten carbonates. Here, we report the viscosity of K2Mg(CO3)2 and K2Ca(CO3)2 melts up to 13 GPa and 2,000 K by means of classical molecular dynamics (MD) simulations using optimized force fields and provide first evidence for non-Arrhenian temperature-dependent viscosity of molten carbonates at mantle pressures. The viscosity of K2Mg(CO3)2 and K2Ca(CO3)2 melts ranges respectively between 0.0056–0.0875 Pa s and 0.0046–0.0650 Pa s in the investigated pressure-temperature interval. Alkali(ne) carbonate melts, i.e. mixed alkali and alkaline earth carbonate melts -K2Mg(CO3)2 and K2Ca(CO3)2− display higher viscosity than alkaline earth carbonate melts -CaCO3 and MgCO3− at similar conditions, possibly reflecting the change in charge distribution upon addition of potassium. The non-Arrhenian temperature-dependence of the viscosity is accurately described by the Vogel-Fulcher-Tammann model with activation energies Ea for viscous flow that decrease with temperature at all investigated pressures, e.g. from ∼100 kJ/mol to ∼30 kJ/mol between 1,300 and 2,000 K at 3 GPa. Pressure is found to have a much more moderate effect on the viscosity of alkali(ne) carbonate melts, with activation volumes Va that decrease from 4.5 to 1.9 cm3/mol between 1,300 and 2,000 K. The non-Arrhenian temperature-viscosity relationship reported here could be exhibited by other carbonate melt compositions as observed for a broad range of silicate melt compositions and it should be thus considered when modeling the mobility of carbonate melts in the upper mantle.
Highlights
Despite the rare occurrence of carbonate-rich volcanism in the present-day Earth (Woolley and Kjarsgaard, 2008; Jones et al, 2013), carbonate melts produced by incipient melting of carbonated lithologies play a critical role in subsurface magmatic processes and they are major phases for the storage and transport of carbon in the upper mantle (Dasgupta and Hirschmann, 2010; Dasgupta, 2013; Stagno, 2019)
Temperature appears as the primary control parameter on the viscosity of alkali(ne) carbonate melts whereas the effect of pressure is generally lower, it is enhanced as temperature decreases (Figure 3)
An interesting observation from the present results is that the viscosity of KM and KC melts only decreases linearly with temperature over a narrow temperature interval (i.e. Arrhenian behavior), while a notable curvature in the logη vs 1/T data is apparent over the investigated temperature range at different pressures (Figure 3), indicating non-Arrhenian behavior
Summary
Despite the rare occurrence of carbonate-rich volcanism in the present-day Earth (Woolley and Kjarsgaard, 2008; Jones et al, 2013), carbonate melts produced by incipient melting of carbonated lithologies play a critical role in subsurface magmatic processes and they are major phases for the storage and transport of carbon in the upper mantle (Dasgupta and Hirschmann, 2010; Dasgupta, 2013; Stagno, 2019). One order of magnitude larger than more recent results for CaCO3, CaMg(CO3) and Na2CO3 melts obtained by using an ultrafast X-ray imaging technique with improved accuracy on the viscosity determination (Kono et al, 2014; Stagno et al, 2018). These later studies report comparable viscosities for all investigated compositions and negligible pressure effects on the viscosity that are difficult to reconcile with computational studies (Desmaele et al, 2019a; 2019b). The scarcity of currently available data precludes the identification of pressure, Melt composition
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