Viscous flow and diffusion in highly depolymerized water-rich melts: improvement of experimental methods and implication for natural system
Final Report Abstract
The transport properties of water-rich strongly depolymerized melts and its influence on the evolution of pegmatitic and volcanic rocks is the major issue of this project. In the first part, 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, we investigated the role of the individual effects of fluxing elements such as F, Li, B and P in silicate melts. The experimental data demonstrate, that the very low viscosities of pegmatite forming melts could reasonably explained by the combined effects of H2O, F and Li, whereas P and B do not play a major role in lowering the viscosity of such systems. Additionally it has been shown, that the incorporation of Li in exchange for K did not lead to a decrease in viscosity, but significantly changes other rheological properties like fragility or glass transition temperature of the melt. The comparison of the dataset from this study with the predictions of available multicompositional viscosity models shows, that these models need to be improved and calibrated for higher concentrations of F, B, P and Li, to predict accurately the viscosity of flux-rich water bearing melts. In an additional study a pegmatite forming melt composition was used to conduct crystallization and dynamic-crystallization experiments at 973 K and 200MPa. In samples with an initial water content of approximately 0.5 wt%, an additional immiscible melt phase segregated from the pegmatitic starting composition. This low density melt segregates from a boundary layer developed through rapid crystallization of feldspar in dynamic-crystallization experiments but also in normal crystallization experiments coexisting with aluminosilicate melt and feldspar crystals. Electron microprobe analyses show that the segregated melts in both types of experiments are almost similar in composition and contain 26.96 wt% Al2O3, 10.91 wt% Na2O, 48.91 wt% F and 21.60 wt% P2O5. The segregation of an immiscible melt phase, which is highly enriched in F and P2O5 and effectively fractionates Na2O from K2O, needs to be considered for understanding the evolution of pegmatitic systems.
Publications
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Towards a viscosity model for flux-rich pegmatitic melts. 88. Tagung der Deutschen Mineralogischen Gesellschaft, Münster, Germany
Bartels A., Baasner A., Behrens H., Holtz F., Schmidt B.C.
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Viscosity of flux-rich pegmatitic melts; EMPG XIII, Toulouse, France
Bartels A., Baasner A., Behrens H., Holtz F., Schmidt B.C.
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(2011) Viscosity of flux-rich pegmatitic melts. Contrib. Mineral. Petrol. 162, 51-60
Bartels A., Vetere F., Holtz F., Behrens H., Linnen R.L.
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The influence of F, P and B content on pegmatitic melt viscosity. Goldschmidt 2011, Prag, Czech Republic
Bartels A., Knipping J., Behrens H., Holtz F., Schmidt B.C.
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(2013) The effect of fluorine, boron and phosphorus on the viscosity of pegmatitic liquids. Chem. Geol. 346, 184-198
Bartels A., Behrens H., Holtz F., Schmidt B.C., Fechtelkord M., Knipping J., Crede L., Baasner A., Pukallus N.
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(2013) Water diffusion in potassium-rich phonolitic and trachytic melts. Chem. Geol. 346, 149-161
Fanara S, Behrens H, Zhang Y
