Journal
FRONTIERS IN EARTH SCIENCE
Volume 9, Issue -, Pages -Publisher
FRONTIERS MEDIA SA
DOI: 10.3389/feart.2021.668058
Keywords
volcanic earthquakes; volcanic degassing; conduit; hydrofracture; rhyolite; magma fragmentation; tuffisite
Categories
Funding
- NERC Envision studentship
- BUFI grant from British Geological Survey
- Royal Society University Research Fellowship [UF140716]
- Royal Society International Exchanges grant [IE150771]
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The study characterizes the processes, pressures, and timescales involved in tuffisite evolution within the country rock by analyzing the sedimentary facies and structures of a large sub-horizontal tuffisite vein at Husafell volcano in western Iceland. It reveals that each sedimentary unit requires fluid overpressures of approximately 1.9-3.3 MPa to be emplaced, and tuffisites may represent the fossil record of low-frequency seismic swarms associated with fracture propagation and reactivation, providing new insights into volcanic unrest and hazard forecasting.
The opening of magmatic hydraulic fractures is an integral part of magma ascent, the triggering of volcano seismicity, and defusing the explosivity of ongoing eruptions via outgassing magmatic volatiles. If filled with pyroclastic particles, these fractures can be recorded as tuffisites. Tuffisites are therefore thought to play a key role in both initiating eruptions and controlling their dynamics, and yet their genesis remains poorly understood. Here we characterise the processes, pressures and timescales involved in tuffisite evolution within the country rock through analysis of the sedimentary facies and structures of a large sub-horizontal tuffisite vein, 0.9 m thick and minimum 40 m in length, at the dissected Husafell volcano, western Iceland. The vein occurs where a propagating rhyolitic sheet intrusion stalled at a depth of similar to 500 m beneath a relatively strong layer of welded ignimbrite. Laminations, cross-stratification, channels, and internal injections indicate erosion and deposition in multiple fluid pulses, controlled by fluctuations in local fluid pressure and changes in fluid-particle concentration. The field evidence suggests that this tuffisite was emplaced by as many as twenty pulses, depositing sedimentary units with varying characteristics. Assuming that each sedimentary unit (similar to 0.1 m thick and minimum 40 m in length) is emplaced by a single fluid pulse, we estimate fluid overpressures of similar to 1.9-3.3 MPa would be required to emplace each unit. The Husafell tuffisite records the repeated injection of an ash-laden fluid within an extensive subhorizontal fracture, and may therefore represent the fossil record of a low-frequency seismic swarm associated with fracture propagation and reactivation. The particles within the tuffisite cool and compact through time, causing the rheology of the tuffisite fill to evolve and influencing the nature of the structures being formed as new material is injected during subsequent fluid pulses. As this new material is emplaced, the deformation style of the surrounding tuffisite is strongly dependent on its evolving rheology, which will also control the evolution of pressure and the system permeability. Interpreting tuffisites as the fossil record of fluid-driven hydrofracture opening and evolution can place new constraints on the cycles of pressurisation and outgassing that accompany the opening of magmatic pathways, key to improving interpretations of volcanic unrest and hazard forecasting.
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