EXTREMELY FLUID BEHAVIOR OF HYDROUS PERALKALINE RHYOLITES

The viscosities of a series of water-bearing peralkaline rhyolitic mel ts have been experimentally determined. The dry melt compositions are composed of a series of additions of Na2O to a metaluminous base compo sition. The melts, initially hydrated at high pressures and quenched i sobarically, have been prepared by cutting and polishing, then reheati ng across the glass transition at 1 arm where they an annealed to a re laxed metastable state and then investigated dilatometrically using mi cropenetration methods. The measurements have been performed in the vi scosity range of 10(8.5)-10(11.5) Pa s which corresponds to temperatur es in the range of 675-220 degrees C for these compositions. Despite t he relatively low viscosities of dry peralkaline melts in comparison w ith metaluminous melts of similar SiO2 content, the viscosities of per alkaline rhyolitic melts also decrease strongly and non-linearly with the addition of water. The resulting viscosity-temperature relationshi ps for water-bearing peralkaline rhyolitic melts are shifted to much l ower temperatures such that glass transition temperatures for moderate cooling rates correspond to extraordinarily low temperatures. A model is presented for the calculation of melt viscosities in the range of 10(8.5)-10(11.5) Pa s for peralkaline rhyolites with up to 7 wt% H2O. The very fluid nature of these peralkaline rhyolites over a wide range of water contents may facilitate a very efficient degassing history o f glassy peralkaline rhyolites in nature. Efficient degassing might ex plain the apparent contradiction of the presence of common water-rich melt inclusions in phenocryst phases hosted in water-foe glassy rhyoli tes, versus the absence of vesicular layers or textural evidence for a vesicular past for the glassy rocks. (C) 1998 Elsevier Science B.V. A ll rights reserved.