Rheology of the evolved alkaline magmas of Vesuvius and the Phlegraean Fields: The anomalous effects of chlorine and fluorine
Final Report Abstract
In this project, the changes in viscosity occurring in peralkaline and peraluminous silicate melts due to the addition of fluorine and chlorine were related to the structure of the corresponding glasses which has been determined by NMR (Nuclear Magnetic Resonance) measurements (as stated in the goals of the original proposal.) Due to the success of the NMR structural modelling, more research time was spent on pursuing NMR measurements, and the Raman spectroscopy planned in the original proposal was not performed. The core of this project can be presented as previously unobserved increase in viscosity due to the addition of fluorine is explained in terms of the 5 different structural positions of fluorine atoms in aluminosilicate melts; and the observation that the structures which result in an increase in viscosity are more numerous with increasing concentration of F in the melt. Three of the five structural positions result in a decrease in viscosity and two in an increase in viscosity. This explains that although fluorine decreases viscosity, it is not always through the same structural mechanism; and indeed, the increase in viscosity due to the addition of fluorine to a peralkaline melt (reported for the first time in this thesis) can be explained by the presence of structures #1 and #5. The literature shows that the addition of fluorine causes melt viscosity to decrease. Here it has been shown that the addition of fluorine also can cause viscosity to increase. The structural mechanism by which this occurs has been identified. The literature suggests that the addition of chlorine causes melt viscosity to decrease or to increase, as a function of composition. Here it has been shown that this effect is composition dependent and the structural mechanism by which this occurs has been identified as Cl-Ca/Na environments in peralkaline melts which increases polymerisation and viscosity; and assumed Si-Cl environments in peraluminous melts, which decreases polymerisation and viscosity.
Publications
- Compositional dependence of the rheology of halogen (F, Cl) bearing aluminosilicate melts, Chemical Geology, 346, 172-183, 2013
Baasner A, Schmidt BC & SL Webb
(See online at https://doi.org/10.1016/j.chemgeo.2012.09.020) - The effect of chlorine, fluorine and water on the viscosity of aluminosilicate melts, Chemical Geology, 357, 134-149, 2013
Baasner A, Schmidt BC & SL Webb
(See online at https://doi.org/10.1016/j.chemgeo.2013.08.020) - Constraints on the incorporation mechanism of chlorine in peralkaline and peraluminous Na2O-CaO-Al2O3- SiO2 glasses, American Mineralogist, 99, 1713-1723, 2014
Baasner A, I Hung, TF Kemp, R Dupree, BC Schmidt & SL Webb
(See online at https://doi.org/10.2138/am.2014.4717) - Fluorine speciation as a function of composition in peralkaline and peraluminous Na2O-CaO-Al2O3-SiO2 glasses: A multinuclear NMR study, Geochimica et Cosmochimica Acta, 132, 151-169, 2014
Baasner A, Schmidt BC, R Dupree & SL Webb
(See online at https://doi.org/10.1016/j.gca.2014.01.041)
