Shear thinning and brittle failure in crystal-bearing magmas arise from local non-Newtonian effects in the melt

Vasseur, Jérémie; Wadsworth, Fabian B.; Dingwell, Donald B.

Abstract

Crystal-bearing magmas are often assumed to be intrinsically non-Newtonian, but the mechanism underlying suspension shear thinning remains unconstrained. Here, we test the hypothesis that shear thinning in these suspensions arises solely from unrelaxed shear thinning in the melt phase or from viscous heating or both. We compile existing data for the rheology of high-temperature crystal-bearing silicate magmas across a wide range of conditions. In order to test this, we define a ‘lever’ function L(φ) which scales the strain rate in the melt phase between crystals relative to the bulk strain rate of the suspension as a function of crystal volume fraction φ. We show that, in general, the parameterisation of the amplification of strain rates via the L(φ) factor coupled with a shear-thinning law for the melt phase fully accounts for the non-Newtonian behaviour of the suspension. Next, we use the Brinkman number Br to demonstrate that some existing data derive from experiments that were subject to viscous heating, resulting in a further manifestation of apparent shear thinning. Taken together, our results provide a theoretical framework that predicts microphysical regimes for shear thinning. This implies that crystal-bearing magmas are Newtonian except where the scaled conditions for viscous heating are met, or when the strain rates in the melt phase approach the inverse of the structural relaxation timescale. This shear thinning due to non-Newtonian behaviour of the melt between crystals occurs very close to the brittle threshold, yielding an extension of the brittle failure criterion to crystal-bearing magmas, with direct implications for volcanic eruption dynamics.

Citation

Vasseur, J., Wadsworth, F. B., & Dingwell, D. B. (2023). Shear thinning and brittle failure in crystal-bearing magmas arise from local non-Newtonian effects in the melt. Earth and Planetary Science Letters, 603, Article 117988. https://doi.org/10.1016/j.epsl.2023.117988

• Journal Article Type: Article
• Acceptance Date: Jan 2, 2023
• Online Publication Date: Jan 16, 2023
• Publication Date: 2023
• Deposit Date: Jun 26, 2023
• Publicly Available Date: Jun 26, 2023
• Journal: Earth and Planetary Science Letters
• Print ISSN: 0012-821X
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 603
• Article Number: 117988
• DOI: https://doi.org/10.1016/j.epsl.2023.117988