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.