THE VISCOSITY OF PLANETARY THOLEIITIC MELTS: A CONFIGURATIONAL ENTROPY MODEL

The viscosity (η) of silicate melts is a fundamental physical property controlling mass transfer in magmatic systems, including the emplacement of Large Igneous Provinces. Viscosity can span many orders of magnitude, strongly depending on temperature and composition. Planetary basaltic lavas are dominantly alkali-poor, iron-rich and/or highly magnesian, and distinctly different from typical terrestrial basalts. We measured the viscosity of 20 anhydrous tholeiitic melts, of which 15 represent known or estimated surface compositions of Mars, Mercury, the Moon, Io and Vesta, by concentric cylinder and parallel plate viscometry. The planetary basalts span a viscosity range of 2 orders of magnitude at liquidus temperatures and 4 orders of magnitude near the glass transition, and can be more or less viscous than terrestrial lavas. Current models under- and overestimate superliquidus viscosities by up to 2 orders of magnitude for these compositions, and deviate even more strongly from measured viscosities toward the glass transition.

We used the Adam-Gibbs theory to relate viscosity to absolute temperature (T) and the configurational entropy of the system at that temperature (S^conf), which is in the form of . Heat capacities (C_P) for glasses and liquids of our investigated compositions were calculated via available literature models. We successfully modeled the global viscosity data set using a constant A_e and 12 adjustable sub-parameters, which capture the compositional and temperature dependence on melt viscosity. Our model reproduces the 496 measured viscosity data points with a 1σ root-mean-square deviation (rmsd) of 0.12 log units across 13 orders of measured melt viscosity. The model should be used to calculate lava flow velocities and fluxes for basaltic volcanism on other moons and planets.