Interpreting deformation conditions from crystal-plasticity in extrusive lavas

The physical evolution of multiphase magmas during shear remains a critical question in volcanology. The presence of solid, liquid, and gaseous phases (with contrasting strength and rheology) serves to partition strain and concentrate stress on each phase when the system is subjected to deformation. For example, gas bubbles generally deform relatively easily, whilst melt can stress and relax repeatedly, erasing its deformation history, and crystals can accumulate stress and suffer permanent damage. In suspensions with high interstitial melt viscosity, high crystal content, or low vesicularity, large stresses may accumulate in the crystalline phase, which may result in crystal plasticity and rupture. All minerals may be subject to crystal plasticity if the conditions are suitable: Whereas diffusion creep may be common in deep magmas deforming slowly over long timescales; dislocation creep is likely more common in shallow magma mushes and erupting lavas.

Recent evidence from natural samples and controlled laboratory experiments suggests that even under very low confinement, the crystals present in lavas may deform by dislocation creep prior to failure. EBSD mapping of experimentally deformed, porous, crystal-rich dome lavas (at magmatic temperature) revealed that crystals underwent dislocation creep. For further analysis we focussed on plagioclase (which dominates the crystal assemblage), and found the misorientation of the crystal lattices within the microlites increased as a function of stress and strain, until rupture. Grain size reduction was seen synchronous to deformation, and fragments of broken microlites recorded the highest distortions; thus, crystals exhibit a plastic limit during dislocation creep. The systematic variation in plasticity with applied conditions demonstrates the key role of crystal plasticity in the deformation of crystal-rich lavas, and the possibility to interpret deformation from the imparted dislocations. To test this, we mapped crystal-plasticity across the shear zone at the margin of the lava spine erupted at Mount Unzen (Japan) in 1994−1995. We found that the degree of crystal lattice misorientation in plagioclase microlites increased systematically across the shear zone towards the marginal shear plane, and also that weak phenocrysts such as mica suffered substantial crystal plasticity.

Whilst the ostensible absence of crystal plasticity in deformed igneous bodies has previously been argued to indicate that melt accumulates all the strain during deformation (particularly if melt viscosity is very low), the lack of deformed crystals may also indicate post-deformation crystallisation or longer deformation timescales (i.e., slower strain rates) which would limit stress accumulation in the crystalline phase (due to both efficient rearrangement during flow and the inability for the melt phase to build stress at slow strain rates). Our work demonstrates how crystal plasticity can be utilised to detail strain localisation textures formed during magma deformation, and although, to date, insufficient data exists to define the stress–strain history of magmas and lavas from the vestiges of crystal-plasticity, there remains hope for its use as a strain marker in the future, with further systematic quantification. Our work further demonstrates that rheological models for multiphase suspensions require consideration of crystal-plastic deformation, which may contribute towards the apparent non-Newtonian behaviour of magmatic suspensions.