Viscosity of flux-rich pegmatitic melts |
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| Authors: | Alexander Bartels Francesco Vetere Francois Holtz Harald Behrens Robert L Linnen |
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| Institution: | 1.Institut für Mineralogie,Leibniz Universit?t Hannover,Hannover,Germany;2.Università degli Studi “G. d’Annunzio” CHIETI-PESCARA Dipartimento di Geotecnologie per l’Ambiente ed il Territorio (DIGAT),Chieti,Italy;3.Department of Earth Sciences,University of Western Ontario,London,Canada |
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| Abstract: | Viscosity experiments were conducted with two flux-rich pegmatitic melts PEG0 and PEG2. The Li2O, F, B2O3 and P2O5 contents of these melts were 1.04, 4.06, 2.30 and 1.68 and 1.68, 5.46, 2.75 and 2.46 wt%, respectively. The water contents
varied from dry to 9.04 wt% H2O. The viscosity was determined in internally heated gas pressure vessels using the falling sphere method in the temperature
range 873–1,373 K at 200 and 320 MPa pressure. At 1,073 K, the viscosity of water-rich (~9 wt% H2O) melts is in the range of 3–60 Pa s, depending on the melt composition. Extrapolations to lower temperature assuming an
Arrhenian behavior indicate that highly fluxed pegmatite melts may reach viscosities of ~30 Pa s at 773 K. However, this value
is a minimum estimation considering the strongly non-Arrhenian behavior of hydrous silicate melts. The experimentally determined
melt viscosities are lower than the prediction of current models taking compositional parameters into account. Thus, these
models need to be improved to predict accurately the viscosity of flux-rich water bearing melts. The data also indicate that
Li influences significantly the melt viscosity. Decreasing the molar Al/(Na + K + Li) ratio results in a strong viscosity
decrease, and highly fluxed melts with low Al/(Na + K + Li) ratios (~0.8) have a rheological behavior which is very close
to that of supercritical fluids. |
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