Redox state, microstructure and viscosity of a partially crystallized basalt melt

As measured in air above the glass transition range, the viscosity of an alkali basalt increases markedly with time by about two orders of magnitude in 12 h. This effect is essentially physical and due to the presence of microcrystals although partial crystallization of the melt into spinel and an SiO2-poor pyroxene leads to a considerable enrichment in silica of the residual liquid. Partial crystallization depends strongly on the initial redox state of samples in that the presence of ferrous iron is required for spinel crystals to form and for pyroxene to nucleate and grow around them. Other measurements show that the viscosity of the crystal-free liquid decreases slightly with increasing ratios r=Fe2+/∑Fe because the differences between samples with r=0.16 and 0.83 amount to about 1.5 and 0.3 log-units at 950 and 1400 K, respectively. Comparisons of the viscosities of the residual liquid matrix and of the initially crystal-free basalt show that physical effects caused by the presence of microcrystals begin to be observed at a low crystal fraction of 5 vol%. Finally, a model of viscosity calculation is developed for the melts which reproduces all data obtained in this work to better than 10%.