Can tuffisite veins help dictate eruption styles?

The ability of magmas to degas during ascent may affect eruption style. The permeability of the magma and/or the conduit wall rocks may therefore dictate whether an eruption will be explosive or effusive. Fractures increase permeability. Fractures filled by veins of autoclastic, cataclastic and tuffisitic glass shards and crystal fragments are common in shallow conduit systems. These veins have the potential to dramatically increase permeability and provide pathways for gas loss. Here we present the first study on the porosity, permeability, strength, P-and S-wave and dynamic elastic moduli of andesite containing tuffisite veins at Volcán de Colima, Mexico. Porosity was measured via pycnometry. The strength and mechanisms of deformation were investigated on tuffisite-bearing and tuffisite-free samples in a uniaxial press at 940°C by loading at 2 MPa/min until failure. The permeability, P- and S-wave velocity, and dynamic elastic moduli were measured in a high-pressure permeameter/pore volumometer up to effective confining pressures of 50 MPa (ca. 2km depth). Measurements were made on cylindrical samples prepared as: (1) without tuffisite veins and with tuffisite veins (2) perpendicular and (3) sub-parallel to flow (i.e., the samples’ axial direction). Petrographic analysis reveals that the tuffisites are comprised almost exclusively of micron-size crystal fragments. Dilatometric measurements of tuffisite (10°C/min up to 1000°C), failed to reveal a Tg, indicating the absence of interstitial glass. In contrast, the dome rocks exhibits viscous relaxation between 770 and 885 °C, indicating glass. The high-temperature (940°C) deformation experiments on samples containing tuffisite veins showed an absence of ductile deformation and a uniaxial strength of 116 MPa; i.e, slightly weaker than tuffisite-free rock (~130 MPa) and stronger than erupted dome lavas (~30 MPa). The presence of tuffisite increases the porosity. Permeability measurements show that tuffisite veins perpendicular to the direction of fluid flow do not increase permeability; however, those sub-parallel double the permeability (from about 5 x 10-14 m2 to 1 x 10-13 m2). With an increase in confining pressure, the efficiency of the tuffisites as an effective degassing pathway decreases. At a depth of just less than one kilometre the effective pressure was sufficient to compress the veins so that the permeability dropped to that of the host rock. These results suggest that tuffisite veins have important mechanical consequences for magma ascent at Volcán de Colima. The presence of tuffisite veins may focus/localize deformation within the host magma and wall rock. At depths greater that 1 km tuffisite veins will have little affect on conduit permeability. However, depressurization during ascent will dramatically increase the permeability of the tuffisite veins relative to the host magma, and thus reduce the potential of explosive eruptions.