HR: 0800h
AN: V51D-0785    [Abstracts]
TI: Models of Viscosity: Strengths, Weaknesses and the Challenges
AU: * Russell, K
EM: krussell@eos.ubc.ca
AF: University of British Columbia, Earth & Ocean Sciences 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada
AU: Giordano, D
EM: daniele_giordano@hotmail.com
AF: Third University of Rome, Geological Sciences Largo S. Leonardo Murialdo 1, Rome, 00154, Italy
AU: Dingwell, D B
EM: dingwell@lmu.de
AF: University of Munich, Earth & Environmental Sciences Theresienstr. 41/III, Munich, 80333, Germany
AB: Here we present a model for predicting the non-Arrhenian Newtonian viscosity of silicate melts as a function of temperature (T) and melt composition (X), including the rheologically important volatile constituents H₂O and F. The model is based on >1750 measurements of viscosity on multicomponent anhydrous and volatile- rich silicate melts. The non-Arrhenian T-dependence is accounted for by the VFT equation [log η = A + B/(T(K) - C)]. The optimization assumes a common, high-T limit (A) for silicate melt viscosity and returns a value for this limit of -4.55 (e.g., log η = 10^-4.6 Pa s). All compositional dependence is ascribed to the parameters B and C and is accounted for by an additional 17 model coefficients. The model has the following attributes: i) the model covers over fifteen log units of viscosity (10^-1-10¹⁴ Pa s), ii) it spans most of the compositional range found in naturally-occurring volcanic rocks using 10 major and minor oxide and two volatile components (H₂O, F₂O_-1), iii) it is computationally continuous across the entire compositional and temperature spectrum of the database, and iv) it is capable of accommodating both strong and fragile behaviour of silicate melts. Model quality is demonstrated, in part, by how well it reproduces the original observations. However, higher-level models have logical consequences that can serve as testable predictions. For example, this model is tested by how well it predicts other transport properties including glass transition temperatures (Tg) and melt fragility (m). Values of Tg (791 - 1077 K) and m (21 - 58) calculated for 58 anhydrous melts using our viscosity model are in strong agreement with the values obtained by fitting the datasets independently (789 - 1154 K and 24 - 65, respectively). We use this approach to compare our model for silicate melt viscosity against previously published models. We find our model to be more consistent with theory, to do a better job of reproducing other melt transport properties, and to work over a wider range of melt conditions (composition and temperature). Other models fail because they use a non-Arrhenian formulation, or they are calibrated for an extremely small range of melt conditions, or they predict unphysical values of viscosity in the limits, or they are over-parameterized and cannot be extrapolated beyond the original calibration dataset. Our model predicts, within inter-laboratory experimental error, viscosity and other melt properties (i.e., Tg amd m) for most of the T-X space found in natural silicate melts. Despite its strengths, there is room for improvement. Firstly, our model incorporates the effects of H₂O and F, but does not account for other important volatiles, including CO₂, S, Cl. Secondly, Fe is treated as a single species whereas melts contain variable proportions of ferric and ferrous iron. Thirdly, we do not model pressure effects on melt viscosity which is needed for modeling melt transport within the lithosphere. Lastly, our model is strictly empirical, in that, the components we have chosen have no explicit or independent relationship to the structure or speciation of the silicate melt. Future models may benefit from the use of a component basis that reflects melt speciation. Giordano, D. Russell, J.K. & Dingwell, D.B. (In Review, July 07) Viscosity of magmatic liquids: A model. EPSL
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting