The onset of non-Newtonian rheology of silicate melts

Abstract

Abstract. The viscoelastic behavior of silicate melts has been measured for a range of compositions (NaA1Si308, NaCaAlSi2OT, CaMgSi206, Li2Si409, Na2Si409, K2Si409, Na2Si3OT, K2Si307 and Na2Si2Os) using the fiber elongation method. All compositions exhibit Newtonian behavior at low strain-rates, but non-Newtonian behavior at higher strain-rates, with strain-rate increasing faster than the applied stress. The decrease in shear viscosity observed at the high strain-rates ranges from 0.3 to 1.6 loglo units (Pa s). The relaxation strain-rates, ~relax, of these melts have been estimated from the low strain-rate, Newtonian, shear viscosity, using the Maxwell relationship; +rel,x = r -1 = (rls/G~) -1. For all compositions investigated, the onset of non-Newtonian rheology is observed at strain-rates 2.5 _+ 0.5 orders of magnitude less than the calculated relaxation strain-rate. This difference between the non-Newtonian onset and the relaxation strain-rate is larger than that predicted by the single relaxation time Maxwell model. Normalization of the experimental strain-rates to the relaxation strain-rate predicted from the Maxwell relation, eliminates the composition-and temperature-dependence of the onset of non-Newtonian behavior. The distribution of relaxation in the viscoelastic region appears to be unrelated to melt chemistry. This conclusion is consistent with the torsional, frequency domain study of Mills (1974) which illustrated a composition-invariance of the distribution of the imaginary component of the shear modulus in melts ~ on the Na20-SiO2 join. The present, time domain study of viscoelasticity contrasts with frequency domain studies in terms of the absolute strains employed. The present study employs relatively large total strains (up to 2). This compares with typical strains of l 0 -8 in ultrasonic (frequency domain) studies. The stresses used to achieve the strain-rates required to observe viscoelastic behavior in this study approach the tensile strength of the fibers with the result that some of our experiments were terminated by fiber breakage. Although the breakage is unrelated to the observation of non-Newtonian viscosity, their close proximity in this and earlier studies suggests that brittle failure of igneous melts, may, in general, be preceded by a period of non-Newtonian rheology. 1, IntroductionNumerous investigations of the rheology of silicate melts at super-liquidus temperatures have demonstrated that the relationship between stress and strain in molten silicates is Newtonian, i.e. the strain-rate is linearly proportional to the applied stress (Bockris et al. 1955;Scarfe et al. 1983;Dingwell et al. 1985). Viscosity is a state variable and thus a strain-rate independence of viscosity implies that the thermodynamic state of the melt is independent of applied stress and the resultant strain-rate. Thus the observation of Newtonian viscosity simplifies considerably the thermodynamic description of silicate melts.In contrast, the glass science literature contains several examples of the non-New...

Brittleness of fracture in flowing magma

“…Appendix B shows that this approximation is reasonably accurate when |T′ rr | remains small compared with G. This condition is generally satisfied for magma since the shear modulus is typically on the order of tens of GPa [Webb, 1997], while the applied stress at fracture is less than a few hundred MPa [Webb and Dingwell, 1990a].…”

“…These experimental results indicate shear-thinning behavior with the viscosity decreasing with increasing strain rate while the stress continues to increase near the glass transition temperature of the material [Webb and Dingwell, 1990a;Simmons, 1998]. Such a feature of the steady-state viscosity is frequently represented by an empirical function of strain rate [Simmons et al, 1982;Simmons, 1998].…”

“…Dependence of h l on s or _ " in steady creep is investigated by uniaxial deformation experiments and the results are converted to the shear viscosity h using the relation h = h l /3 for incompressible deformation [Webb and Dingwell, 1990a].…”

Emplacement of the Rocche Rosse rhyolite lava flow (Lipari, Aeolian Islands)

“…Shaw et al 1968;Walker 1971;Fink 1983;Webb and Dingwell 1990;Dragoni et al 1992;Manley 1992;Fink and Griffiths 1998;Manga et al 1998;Castro and Cashman 1999;Gottsmann and Dingwell 2001;Hess et al 2008;Castro et al 2013). It has been previously reported that the Rocche Rosse behaved as a Bingham fluid during emplacement (Clay et al 2013), which can promote strain localisation and folding.…”

When Does Magma Break?

“…In two different experimental types, both Webb and Dingwell (1990a) and Cordonnier et al (2012a, b) showed that in fact the first evidence of behaviour that is not purely viscous occurs at De ! 10 À2 and not exactly at unity.…”