Water and the viscosity of depolymerized aluminosilicate melts

To determine the effect of water on the rheology of depolymerized melts, we have measured the viscosity of two series of hydrated synthetic aluminosil icate melts analogous in bulk polymerization to tephritic and basanitic liq uids. The measurements have been made at 1 atm in the range 10(8.7) to 10(1 3.7) Pa s, for water contents between 0 and 2.5 wt.% H2O. In all cases, wat er exerts a marked depressing effect on the viscosity but the reduction is much smaller than for the more polymerized compositions studied previously. With the addition of 2 wt.% H2O, for example, extrapolation of the results suggests that the viscosity decreases by ca. 4.5 and 1.5 orders of magnitu de at 900 and 1200 K, respectively. An interesting consequence is that the viscosity and glass transition temperatures of the most polymerized melts b ecome lower than those of depolymerized melts at water contents higher than ca. I wt.%. By analogy with natural magmatic compositions, hydrous rhyolit es may become less viscous than hydrous basalts at high water contents and low temperatures. In nature, the viscosities of polymerized and depolymeriz ed magmatic liquids should be similar because basalts are emplaced at highe r temperatures but rhyolites typically have higher water contents. Copyrigh t (C) 2000 Elsevier Science Ltd.