Viscosity of flux-rich pegmatitic melts

Viscosity experiments were conducted with two flux-rich pegmatitic melts PEG0 and PEG2. The Li(2)O, F, B(2)O(3) and P(2)O(5) contents of these melts were 1.04, 4.06, 2.30 and 1.68 and 1.68, 5.46, 2.75 and 2.46 wt%, respectively. The water contents varied from dry to 9.04 wt% H(2)O. The viscosity was determined in internally heated gas pressure vessels using the falling sphere method in the temperature range 873-1,373 K at 200 and 320 MPa pressure. At 1,073 K, the viscosity of water-rich (similar to 9 wt% H(2)O) melts is in the range of 3-60 Pa s, depending on the melt composition. Extrapolations to lower temperature assuming an Arrhenian behavior indicate that highly fluxed pegmatite melts may reach viscosities of similar to 30 Pa s at 773 K. However, this value is a minimum estimation considering the strongly non-Arrhenian behavior of hydrous silicate melts. The experimentally determined melt viscosities are lower than the prediction of current models taking compositional parameters into account. Thus, these models need to be improved to predict accurately the viscosity of flux-rich water bearing melts. The data also indicate that Li influences significantly the melt viscosity. Decreasing the molar Al/(Na + K + Li) ratio results in a strong viscosity decrease, and highly fluxed melts with low Al/(Na + K + Li) ratios (similar to 0.8) have a rheological behavior which is very close to that of supercritical fluids.