Experimental investigation of magma rheology at 300 MPa: From pure hydrous melt to 76 vol.% of crystals

Abstract

The rheological behaviour of synthetic crystal-bearing magmas containing up to 76 vol.% of crystals (0 ≤ ϕ S ≤ 0.76) has been investigated experimentally at a confining pressure of 300 MPa and temperatures between 475 and 1000 °C at shear rates between 10 − 4 and 2 × 10 − 3 s − 1 . Starting hydrated crystal-bearing glasses were synthesized from a dry haplogranitic glass (Qz 36Ab 39Or 29) and 2.5 wt.% water mixed with 0 (pure hydrous melt), 16, 34, 54, 65 or 76 vol.% of Al 2O 3 sieved (45 < Ø < 90 µm) crystals. Shear viscosity measurements were performed in torsion (simple shear) in a Paterson gas-medium apparatus. For pure hydrated melt and for 16 vol.% of crystals, the rheology is found to be Newtonian. At higher crystal contents, the magmas exhibit shear thinning behaviour (pseudoplastic). The Einstein–Roscoe equation adequately estimates viscosities of the crystal-bearing magmas at low crystal contents ( ϕ S ≤ ~ 0.25), but progressively deviates from the measured viscosities with increasing crystal content as the rheological behaviour becomes non-Newtonian. On the basis of a power–law formulation, we propose the following expression to calculate the viscosity as a function of temperature, crystal content and applied stress (or shear rate): γ . = A 0 ( 1 − Φ / Φ m ) K τ ( 1 + K 1 Φ K 2 ) exp ( − Q R T ) , where γ. is shear rate (s − 1 ), τ is shear stress (MPa), Φ is the crystal volume fraction, T is temperature (K), Φ m is the relative maximum packing density, R is the gas constant, Q = 231 kJ mol − 1 is the activation energy of the viscous flow and A 0, K, K 1 and K 2 are empirical parameters.