HIGH-TEMPERATURE MAGMA DEFORMATION: A STUDY FROM VOLCAN DE COLIMA (MEXICO)

It is a common phenomena for volcanoes to rapidly switch from effusive to explosive eruption, aided by the brittle failure of magma at high temperature. Our understanding of the deformation mechanisms and P-T changes associated with ascending magma are limited and the effects of porosity and crystallinity remain unresolved.

Here we investigate the rheology of 2 magma types involved in the dome-building eruptions and explosions that occurred at Volcán de Colima (Mexico) since 1998. Characterisation by thin section analysis, SEM and differential scanning calorimetry indicates that the 2 lavas contain different amounts of porosity (6% vs 24%), crystals (40% vs 50% phenocrysts and 35% vs 15% microlites) and rhyolitic interstitial melt (19% vs 11%).

Cylindrical samples were deformed in a uniaxial press at constant stresses of 12 or 24 MPa, temperatures of ~950 ^oC and strain of 20 or 30%. The resulting strain rate varied between 10^-5 to 10^-2 s^-1, comparable to those within active volcanic systems. Acoustic emission (AE) monitoring during deformation constrains the ductile-brittle transition. Pre- and post-deformation density and permeability measurements and imaging record textural developments. While Electron Back-Scatter Diffraction (EBSD) measures textural and microstructural developments by quantifying grain structure, grain boundary character and crystallographic preferred orientation.

Each magma has different mechanical properties, displaying a significant range of measured strain rates at a given temperature and applied stress. AE output suggests that the ductile-brittle transition is not a fixed characteristic, and occurs at different stress / strain values for each sample. Deformation leads to an increase in both porosity and permeability which is controlled by the degree of deformation endured. EBSD analysis identifies alignment of phenocrysts, which is highly dependent on % strain. Fracture initiation and propagation is controlled primarily by stress (higher strain rate in our experiments). Misorientation of the crystal lattice as identified by EBSD within single phenocrysts alludes to crystal plasticity. This indicates that crystallinity has a significant effect on magma rheology with the implication that viscous models may not encompass the full complexity of crystal-bearing magma.