Viscous flow and diffusion in highly depolymerized water-rich melts: improvement of experimental methods and implication for natural system

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The transport properties of water-rich strongly depolymerized melts are still under debate. So far no experimental techniques are available to determine the viscosity of highly depolymerized melts at elevated temperatures and pressures. Such viscosity data are crucial to understand kinetic processes during crystallization and eruptive processes of natural systems such as hydrous pegmatites arid phonolites. The falling sphere method (Stokes law) has been successfully used for viscosity determination at various conditions. However, its application to low viscosity melts is mainly limited by the achievable experimental duration. In the first part of the project (first 12 months), we have developed a new set-up allowing rapid heat and rapid quench of samples in internally heated gas pressure vessels (IHPV) and set up a new system of vertically oriented cold seal pressure vessels (CSPV) for rapid heat/rapid quench. Additionally, we have conducted viscosity determinations of pegmatitic melts in the high (micropenetration) and low viscosity range (falling sphere). The preliminary results show that the viscosity of water-rich pegmatite melts is significantly lower (up to 2 - 3 log units) than predicted by available models. In the second part of the project we plan to use the rapid heat and rapid quench devices to determine more accurately the individual effects of fluxing elements such as F, Li (eventually B and P) in silicate melts. The setup will also be used to determine the diffusivity of HFSE (Nb and Ta) in pegmatitic melts, which is of importance to constrain the genetic models proposed for the formation of economic HFSE concentrations. Finally we plan lo delermine the viscosity of water-rich, phonolites which will be useful for modeling ascent and degassing of phonolite melts.