Mechanical controls on magma fracture-healing cycles: Experimental insights

Fracture and healing cycles in magma exert important controls on outgassing, permeability, and stability in dynamically evolving volcanic systems. Understanding these cycles requires constraint of the mechanical behaviour of viscoelastic melts. Here, we explore the role of shear stress on fracture mechanics using standard borosilicate melt as an analogue for magma. 

Experiments were conducted in a biaxial press, where glass rods at high temperature (above T_g) were subjected to fracture-healing cycles by: 1) axially forcing two glass rods against each other at controlled temperatures (hence viscosity) and normal stresses of 0.1-10 MPa, and 2) holding contact for variable time between 1 to 9000 s, before 3) applying a controlled simple shear rate until rupture of the fracture healing pane, to 4) quantify shear strength recovery. The shear stress required to break the two rods apart again increased the longer the rods were held in contact before shearing. Here, strength recovery is defined as the shear stress measured upon rupture compared to that required to rupture a single intact rod. The initial strength of the glass (δ₀) and the recovered strength along the healed artificial fracture (δ₁) give the strength recovery (δ₁/δ₀). These results are compared to previous tests rupturing samples in a tensile regime, with samples showing lower strength recovery when broken in tension than at equivalent conditions in torsion.

We show that higher normal stress during healing accelerates healing by improving surface-surface contact area along the simulated fracture plane. Additionally, at higher temperatures and resultingly lower viscosities, surface-surface contact is improved, leading to more efficient strength recovery. We also show that smoother initial surfaces heal more efficiently, and relate this to strength recovery through time. Finally, we formulate a semi-empirical but physically-grounded model to explain the relationship between strength recovery and the variables: normal stress, viscosity, and time.

Our results highlight how shear strength of fractured melt recovers through time, and that recovery is both normal stress, surface roughness, and viscosity dependent. This suggests that fractures in volcanic systems may heal at different timescales and depths within the system. Resultingly, this leads to differences in the efficacy of degassing that could facilitate localised pressure-increase and contribute towards dictating eruptive style.