Physical properties of CaAl2Si2O8–CaMgSi2O6–FeO–Fe2O3 melts: Analogues for extra-terrestrial basalt

Abstract

The effects of increasing quantities of iron on the viscosity, heat capacity and density of a haplobasaltic base composition (anorthite–diopside 1atm eutectic) were determined. Super-liquidus viscosity and density were measured in air using the concentric cylinder method and double-bob Archimedean method, respectively. Low-temperature viscosities were measured using the micropenetration method for the melts that could be quenched to glasses. The effect of iron oxidation state on viscosity was investigated above the liquidus under reduced fO2 and at the glass transition temperature from quenched samples of varying redox state. Iron significantly decreases the melt viscosity, especially near the glass transition and lowers the activation energy at low temperature. Density increases with addition of iron and the experimental measurements are in good agreement with predictions of existing models. The reduction of Fe3+ to Fe2+ produces a slight viscosity decrease at high temperature but affects properties near the glass transition more strongly. Thus, for iron-rich compositions, the redox state must be taken into account to obtain accurate estimates of the physical and thermodynamic properties, especially at low temperatures. As a result, the iron-bearing anorthite–diopside system approaches the viscous behaviour of terrestrial and extra-terrestrial basaltic compositions and then appears to be good analogue for basaltic systems. At magmatic temperatures, the viscosity difference between common terrestrial basalt and lunar or Martian basalt is estimated to be 0.5 to 1 order of magnitude. Although these results are consistent with inferences drawn from planetary observations on the fluidity of lunar and Martian lava flows, the crystallisation sequence of such systems will need to be investigated to improve interpretation of lava flow morphologies.