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The rheology of crystal-bearing basaltic magmas from Stromboli and Etna

The rheology of crystal-bearing basaltic magmas from Stromboli and Etna,10.1016/j.gca.2011.03.031,Geochimica Et Cosmochimica Acta,A. Vona,C. Romano,D.

The rheology of crystal-bearing basaltic magmas from Stromboli and Etna   (Citations: 1)
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Our ability to forecast volcanic hazards at active volcanoes stems from our knowledge of parameters affecting eruption dynamics. Persistent activity displayed by basaltic volcanic systems, such as Etna and Stromboli, is governed mainly by the textural evolution and rheological properties of eruptive products. Here, we investigate the high temperature (1131°C<T<1187°C) rheological behavior and textural evolution of remelted natural basaltic magmas from Stromboli and Etna volcanic systems upon cooling and crystallization at atmospheric conditions. The pure liquid and subliquidus isothermal viscosities are investigated using a concentric cylinder apparatus by varying strain rate applied to the investigated materials. Detailed textural analysis has been performed in order to evaluate the effect of crystal distribution on the rheological evolution of magmas. At subliquidus conditions, the mineralogical assemblage is dominated by the presence of spinel and plagioclase with the occurrence of stable clinopyroxene only at the lowest temperatures (1157 and 1131°C for the Stromboli and the Etna samples, respectively). The overall crystal fraction (ϕ) varies between 0.06 and 0.27. In the high T regime the viscous response to applied stress and strain rate is strongly affected by the presence of even small amounts of crystals. Large discrepancies between measured and predicted viscosities obtained using the Einstein–Roscoe (ER) equation are also found at low crystal fractions. We find here that, at the investigated conditions, the physical effect caused by the presence of elongated crystals is orders of magnitude higher than that predicted by existing models which only account for spherical particles. It also appears that a weak shear thinning behavior occurs at the lowest temperatures investigated. Crystal shape and, with it, the strain-rate dependence of the rheological properties appear to play primary roles in influencing the transport properties of these basaltic magmas.
Journal: Geochimica Et Cosmochimica Acta - GEOCHIM COSMOCHIM ACTA , vol. 75, no. 11, pp. 3214-3236, 2011
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    • ...In particular the low viscosity of high temperature basaltic melts can be subject to notable rheological changes (more than an order of magnitude) when crystallization of microlites takes place [Ishibashi and Sato, 2007; Vona et al., 2011]...
    • ...Such effects are more evident for particles with higher aspect ratios, as in the case of elongated crystals in natural magmas [Ishibashi, 2009; Vona et al., 2011]...
    • ...[36] Experiments performed on natural basalts with different percentages of crystallized plagioclase by Ishibashi [2009] and Vona et al. [2011] show a deviation of the flow index n from n = 1 at a critical solid fraction around 0.1...
    • ...This is very similar to the rheological behavior of our suspensions of elongated particles (B type and D type), which show shear thinning behavior for values of � = 0.1 (for comparison, the average aspect ratio for B‐type and D‐type particles is 9.5 and 8.5, respectively, whereas the aspect ratio of plagioclase crystals in the experiments of Vona et al. [2011] is about 7–8)...
    • ...Moreover, fitting experimental data of natural basalts with equation (4), Vona et al. [2011] found the following values for the empirical parameters: � * = 0.274, x = 0.0327, g = 0.84 (with d =1 3− g). These values are very similar to those obtained for the parameterization of the suspension made of monodisperse wollastonite particles (D type in Table 2). This suggests that the analog experiments we performed are able to capture the first ...

    C. Cimarelliet al. Rheology of magmas with bimodal crystal size and shape distributions: ...

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