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GeoRef Record 2008-109590

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

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< or =phi (sub S) < or =0.76) has been investigated experimentally at a confining pressure of 300 MPa and temperatures between 475 and 1000 degrees C at shear rates between 10 (super -4) and 2X10 (super -3) s (super -1) . Starting hydrated crystal-bearing glasses were synthesized from a dry haplogranitic glass (Qz (sub 36) Ab (sub 39) Or (sub 29) ) and 2.5 wt.% water mixed with 0 (pure hydrous melt), 16, 34, 54, 65 or 76 vol.% of Al (sub 2) O (sub 3) sieved (45<O<90 mu m) 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 (phi (sub S) < or = approximately 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): gamma .=A (sub 0) (1-Phi /Phi (sub m) ) (super K) tau (super (1+K (sub 1) Phi K (sub 2) )) exp(-QRT),where gamma . is shear rate (s (super -1) ), tau is shear stress (MPa), Phi is the crystal volume fraction, T is temperature (K), Phi (sub m) is the relative maximum packing density, R is the gas constant, Q=231 kJ mol (super -1) is the activation energy of the viscous flow and A (sub 0) , K, K (sub 1) and K (sub 2) are empirical parameters. Abstract Copyright (2008) Elsevier, B.V.