An isotopic insight into the role of tuffisite veins during volcanic degassing in rhyolitic systems
Rebecca Paisley, McGill University, Montreal, QC, Canada, Kim Berlo, McGill University, Earth & Planetary Sciences, Montreal, QC, Canada and Bassam Ghaleb, Université de Québec à Montréal, GEOTOP, Montréal, QC, Canada
Contact First Author: Rebecca Paisley; rebecca.paisley@mail.mcgill.ca
English Abstract:
Magmatic degassing plays a fundamental role in the style of a volcanic eruption. A better understanding of the mechanisms involved in gas escape is essential for predicting eruption styles, and constraining the associated hazards. Highly silicic rhyolitic systems are prone to explosive activity because of their ability to trap gas bubbles, which expand on ascent and promote fragmentation. Effusive rhyolitic activity is thought to occur where gas can escape from the melt. However, Schipper et al. (2013) witnessed simultaneous effusive and explosive activity at the 2011-2012 eruption of Cordón Caulle, Chile. This suggests a complex open system degassing mechanism. Potential degassing mechanisms include interconnected bubble networks and fractures. These fractures can be infilled and preserved as tuffisite veins, consisting of fine-grained pyroclastic material. They have been shown to weld, heal and refracture in many rhyolitic systems, thus providing a reoccurring path for gas escape, but are they the main degassing mechanism?
We have conducted 210Pb-226Ra radioactive disequilibria analyses, by Alpha Particle Spectroscopy and Thermal Ionisation Mass Spectrometry, on rhyolitic samples from Cordón Caulle, Chile to constrain this. In volcanic rocks, disequilibria between 226Ra and 210Pb, members of the 238U decay series, have been attributed to degassing and accumulation of 222Rn, the gaseous precursor to 210Pb. In the first study of its kind, early results indicate that tuffisite veins, from the 2011-2012 eruption, display (210Pb/226Ra) ratios greater than 1. Additionally, vesicular pumice and dense lava flow samples exhibit 210Pb excesses. This indicates significant gas fluxing occurred through the shallow part of the Cordón Caulle system prior to the eruption of both effusively and explosively erupted magma. Further analyses and modelling will be undertaken to assess the relative gas fluxes in samples representing both the effusive and explosive stages of eruption.
