Pressure-driven opening and filling of hydrofractures: a field and experimental investigation of tuffisite formation

Magma ascent pathways open when pressurised gas-ash mixtures overcome the strength of the surrounding rock to form fractures. Gas-ash mixtures are injected into the propagating fractures, and if ash is deposited this early stage of dyke or conduit evolution is preserved as a tuffisite vein. Once a conduit has become established, fractures formed in the country rock adjacent to the volcanic conduit are also injected with gas-ash mixtures to form tuffisites. If tuffisites allow for the significant escape of magmatic gases from the main conduit zone, tuffisites might dissipate sufficient pressure to moderate eruption style from explosive to effusive. The hot particles within the tuffisite, however, sinter together through time, reducing tuffisite permeability until gas no longer flows. Despite their potentially important role in controlling eruption dynamics, the length of time that a tuffisite may remain permeable and the flux of gas that tuffisites can allow to escape are poorly constrained.

The dimensions of tuffisites vary, but fractures are typically tens of centimetres to tens of metres in length. The internal structure of tuffisites can be complex, often consisting of multiple units of pyroclastic material, varying from massive lithic breccias to stratified tuffs. Erosion and deposition, due to the repeated injection of ash-laden fluid, produces a variety of sedimentary structures, from cross-lamination and graded bedding to soft-sediment deformation and internal injections. These structures record how the velocity and particle volume fraction of the injected fluid fluctuated through time, controlled by the fluid pressure gradient along the fracture. Tuffisites can therefore be interpreted as a fossil record of the fluid pressure fluctuations occurring during the opening of magmatic pathways.

We aim to quantify and reconstruct the fluid overpressure at different stages of tuffisite evolution. A large sub-horizontal tuffisite (0.9 m wide, >40 m long) formed at 500 m depth at the dissected rhyolitic Húsafell volcano, Iceland, has been used to constrain the pressures of tuffisite formation¹. The pressure required to open fractures within the Húsafell tuffisite host rocks (basalt, friable ignimbrite, densely welded ignimbrite) has been constrained experimentally by injecting samples with pressurised water. The dimensions of different units within the Húsafell tuffisite suggest overpressures of 1.9-3.3 MPa would be needed for the emplacement of the largest units seen (0.1 cm thick and 40 m long), using a simple fracture opening model¹.

Sintering of hot particles within the tuffisite fill reduces tuffisite permeability through time, hindering outgassing. The porosity, permeability, and particle sizes of different units within the Húsafell tuffisite allow us to constrain the possible gas flux carried by tuffisite and how this would have evolved through time. By combining constraints on the fluid pressure, permeability, and sintering timescales of the Húsafell tuffisite we aim to gain insight into the processes controlling tuffisite formation, and whether tuffisites might permit sufficient outgassing to moderate eruption explosivity.

1. Unwin et al. (2021) doi:10.3389/feart.2021.668058