Daniele Giordano Discusses the Viscosity of Magmatic Liquids
Fast Moving Front Commentary, November 2011
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The
model is also the first step toward the understanding of the rheology of
multiphase melt+crystal+bubble silicate melts (i.e., magmas). The
effective or apparent viscosity of magmas is an expression of strain
rate dependent flow of the amalgam of bubbles, crystals, and melt. To
develop a meaningful physics for the flow of this amalgam (i.e., magma),
it is implicit that the viscosity of the constituent melt phase needs to
be known. This is particularly important for understanding the onset of
non-Newtonian and non-viscous behavior in magmas as a function of their
componentry or the stresses and strain rates that are imposed (Caricchi
et al., 2008; Robert et al., 2008; Vona et
al., 2011).
There are also several areas in which the GRD model needs to be and will be improved; these include incorporating the effects of iron redox and on silicate melts viscosity. Iron is the most abundant transition metal present in the terrestrial planets and a major element in basalt, which comprises ~90 vol.% of all lava erupted at the Earth's surface. On Earth, the iron content (FeOtot) of magma commonly ranges between 3 to 12 wt.% but can be as high as 16 wt.%. Substantially higher iron contents (16-22 wt. % FeOtot) are inferred for most Martian and Lunar volcanic products.
Iron occurs in a variety of valence states (Fe2+ or Fe3+) component and different coordinations (IVFe, VFe, VIFe) in silicate melts. In particular, the redox state of iron (i.e., Fe2+/Fetot ratio) can control speciation in the melt and, thus, can substantially affect melt viscosity. Current experimental work at Munich is quantifying the effects of redox on melt viscosity.
The GRD model does not account for the effects of pressure (i.e., depth in the Earth) on melt viscosity. Recent works have shown that pressure does affect melt viscosity and the magnitude and nature of the effect depends on melt composition in non-intuitive ways. Future models for melt viscosity must accommodate the effects of pressure if we are to understand the transport properties of magmas from source to eruption.
So, likely the greatest challenge is to create a new-generation model in which the descriptive parameters used to describe the temperature-pressure-compositional dependence of silicate melt viscosity have thermodynamic significance. This might best be approached using the quasi-theoretical approach proposed by Adam & Gibbs. Such a model would provide full coupling of transport and thermodynamic properties of melts and mark a clear rise in our ability to understand and model magma production, transport, and eruption.
Do you foresee any social or political
implications for your research?
The GRD model is capable of forecasting viscosity for most naturally occurring silicate melts and includes the effects of volatile constituents H2O and F. It is continuous in both temperature and composition and is shown to be numerically robust (reproduces original data). For these reasons it can also be extrapolated to predict melt viscosity at experimentally unreachable conditions (e.g., high T(K) and high H2O contents). Thus it can predict, in a continuous way, the viscosity of natural melts as they degas, crystallize, cool, or respond to any other change in environmental condition (Figure 3).
This model has broad applications to earth and planetary sciences as well
as materials sciences in that it can be used to forecast viscosity,
activation energy, glass transition temperature (Tg) and
fragility (m) of anhydrous and hydrous silicate melts
(Figure 2e, d). The GRD model is also helping to make
broader contributions to society, where it has been coupled to
state-of-the-art numerical models that simulate volcanic eruptions
for the purposes of assessing volcanic hazards (i.e., effusive vs.
explosive; Giordano et al., 2007, 2010; Polacci et
al., 2004).
Daniele Giordano
Consejo Superior de Investigaciònes Cientificas (CSIC)
Istitut de Ciencies de la Terra "Jaume Almera"
Barcelona, Spain
References
- Caricchi L, Giordano D, Burlini L, Ulmer P, Romano C, Chem. Geol. 256: 157-70, 2008.
- Giordano D, Dingwell DB, Earth & Planet. Sci. Lett. 208: 337-49, 2003a.
- Giordano D, Dingwell DB, J. Phys: Condensed Matter 15, S945-54, 2003b.
- Giordano D, Polacci M, Longo A, Papale P, Dingwell DB, Boschi E, Kasereka M, Geophys. Res. Lett. 34: L06301, 2007,
- Giordano D, Russell JK, Dingwell DB, "Viscosity of magmatic liquids: A
Model," Earth &
Planet. Sci. Letts. 271: 123-34, 2008. - Giordano D, Polacci M, Papale P, Caricchi L, Solid Earth 1: 61–9, 2010, doi:10.5194/se-1-61-2010
- Polacci M, Papale P, Giordano D, Del Seppia D, Romano, C, Jour. Volcanol. Geoth. Res. 131: 93-108, 2004.
- Quane, SL, Russell JK, Friedlander BA, Geology 37: 471, 2009.
- Robert G, Russell K, Giordano D, Chem. Geol. 256: 215-29, 2008.
- Russell JK, Giordano D, Dingwell D, Hess, KU, European J Mineralogy 14: 417-28, 2002.
- Russell JK, Giordano D, Dingwell D, Hess KU, American Mineralogist (Letters) 88: 1390-94, 2003.
- Shaw HR, Am. J. Sci. 272: 870–93, 1972.
- Vona A, Romano C, Dingwell DB, Giordano D, Geochim. Cosmochim. Acta 75: 3214-56, 2011.
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KEYWORDS: VISCOSITY, MODEL, SILICATE MELTS, VOLATILE-BEARING MELTS, GLASS TRANSITION, FRAGILITY; NON-ARRHENIAN MODEL, TEMPERATURE DEPENDENCE, RHEOLOGY, RELAXATION, PHONOLITES, PREDICTION, TRACHYTES, RHYOLITE, SYSTEMS.