Multiple generations of tuffisite veins record repetitive ductile-brittle deformation of rhyolitic magma rising within an effusive vent: a source of flow banding in silicic lavas and repetitive seismic signals?

Multiple generations of tuffisite veins within the obsidian margins of a 10 m-wide effusive rhyolite vent at Torfaj”kull, Iceland record episodic ductile-brittle deformation of rising magma[1]. The vent is dissected 30 m beneath the 10⁶ m³ subaerial lava flow that it fed, contains vesicle-free obsidian and devitrified rhyolite, and intrudes poorly-consolidated pumiceous rhyolite breccia. The youngest veins are anastomosing, irregular and filled with annealed fragments of obsidian and broken crystals ripped from their walls. These cut through earlier veins, which have undergone ductile shear parallel to the vent margins and are identifiable by their pale colour and abundance of crystal fragments. Axial strain (delta L/L) in earlier veins ranges from <2 close to the outer margin of the vent to >1000 towards the centre, where sheared veins resemble the flow bands seen in the overlying lava flow. The orientation of each tuffisite vein is unrelated to that of previous veins, indicating that the magma annealed sufficiently between brittle events to recover mechanical isotropy. ICP-MS and FTIR analyses of the youngest veins and surrounding obsidian show that major element compositions are homogeneous, but vein material is relatively degassed (0.14 vs. 0.21 wt % H₂O). This suggests that vein formation briefly increased the vent wall permeability, and allowed escape of magmatic volatiles into the country rock[2]. We argue that such repetitive ductile-brittle deformation reflects the strain rate dependent behaviour of viscous magma[3] and is likely to be restricted to silicic compositions. Parameters such as the magma flow rate, the gradient of viscosity and pressure across the vent, strength of magma and country rock, and the annealing rate may control the depth and frequency of brittle events. This process is important because it is a primary mechanism for the formation of flow banding in compositionally homogeneous silicic magmas, through the introduction of bands with different volatile contents and thermal histories. Furthermore, it is potentially a non-destructive, repetitive source of shallow seismicity during effusive silicic eruptions, which does not depend upon the mechanical coupling of a pressured fluid phase with a solid[4], but instead reflects the deformation of material capable of solid-like and fluid-like behaviour on different timescales. [1] Tuffen H (2001)Unpub. PhD thesis, Open Univ., UK [2] Jaupart C (1998) J Geol Soc London Spec Publ 145:73-90 [3] Dingwell DB (1997) J Petrol 38:1635-1644 [4] Chouet B (1996) Nature 380:309-316