Deformation experiments of bubble and crystal bearing magmas : rheological and microstructural analysis
Résumé
Simple shear deformation experiments on three-phase, hydrous, haplogranitic magmas, composed of quartz crystals (24-65 vol%), CO2-rich gas bubbles (9-12 vol%) and melt in different proportions, were performed with a Paterson-type rock deformation apparatus. Strain rates from 5*10-6 s-1 to 4*10-3 s-1 were applied at temperatures between 723 and 1023 K and at pressure of 200 MPa. The results show that the three-phase suspension rheology is strongly strain-rate dependent (non-Newtonian behavior). Two non-Newtonian regimes were observed: shear thinning (viscosity decreases with increasing strain rate) and shear thickening (viscosity increases with increasing strain rate). Shear thinning occurs in crystal-rich magmas (55-65 vol% crystals; 9-10 vol% bubbles) as a result of crystal size reduction and shear zoning. Shear thickening prevails in dilute suspensions (24 vol% crystals; 12 vol% bubbles), where bubble coalescence and outgassing dominate. At intermediate crystallinity (44 vol% crystals; 12 vol% bubbles) both shear thickening and thinning occur. Based on the microstructural observations using synchrotron radiation X-ray tomographic microscopy, bubbles can develop two different shapes: oblate at low temperature (< 873 K) and prolate at high temperature (> 873 K). These differences in shape are caused by different conditions of flow: unsteady flow, where the relaxation time of the bubbles is much longer than the timescale of deformation (oblate shapes); steady flow, where bubbles are in their equilibrium deformation state (prolate shapes). Three-phase magmas are characterized by a rheological behavior that is substantially different with respect to suspensions containing only crystals or only gas bubbles.
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