VISCOSITY REGIMES OF HOMOGENEOUS SILICATE MELTS

Extrapolation of laboratory measurements of the viscosity (eta) of sil icate melts is frequently needed in order to analyze petrological and volcanological processes. Therefore a general understanding of silicat e melt viscosities is required. In this paper we survey the present st ate of our knowledge and distinguish three flow regimes for homogeneou s silicate liquids: (1) a low-viscosity regime (eta < 1 Pa . s), where the viscosity obeys a temperature-dependence power law in accordance with mode coupling theory (these low viscosities are typical for depol ymerized melts); (2) an intermediate regime (1 < eta < 10(12) Pa . s), where silicate melt viscosity is determined by the availability of co nfigurational states (the dependence of the viscosity on the temperatu re is described aptly by the configurational entropy theory of Adam-Gi bbs); and (3) a high-viscosity regime, where the liquid has been trans formed into a glass (eta > 10(12) Pa . s) (this regime is not well kno wn, but available measurements indicate an Adam-Gibbs or an Arrhenian temperature dependence of the glass viscosity). Examples are given of igneous rocks whose geneses were affected by these flow regimes.