Release the crackin': The influence of brittle behavior on gas retention in crystal-rich magma

Abstract

Crystal-rich silicic lavas commonly erupt from hazardous lava dome-forming volcanoes, characterized by both effusive and explosive eruptions. Magma explosivity is inherently dependent on its ability to store pressurized gas, which can be released through permeable pathways like fractures or connected bubbles, yet the role crystals play in regulating gas escape is poorly constrained in crystal-rich systems. We explored the gas storage capacity and outgassing efficiency of crystal-rich magmas through experimental vesiculation of hydrous dacite samples containing crystal volume fractions (\({\phi }_{x}\)) between 0.5 and 0.8. The maximum unconnected gas volume (isolated porosity) decreases exponentially with increasing crystallinity. We quantify the relative outgassing efficiency as a function of \({\phi }_{x}\) using changes in isolated melt porosity during open-system degassing (outgassing). Mean isolated porosity, for \({\phi }_{x}\) = 0.5, increases from ~ 0.33 at the start of outgassing to ~ 0.67 by the end, doubling its trapped bubbles. For \({\phi }_{x}\) = 0.7, isolated porosity increases from ~ 0.1 to ~ 0.2, implying gas retention and outgassing efficiency are strongly dependent on crystallinity. Outgassing occurs rapidly via fracturing at porosities < 0.1 when \({\phi }_{x}\) ≥ 0.7. Fracturing and bubble coalescence are both inefficient outgassing mechanisms at \({\phi }_{x}\) = 0.5 due to viscoelastoplastic deformation, which leads to an increase of isolated porosity. Between \({\phi }_{x}\) of 0.5 and 0.7, samples sustained a three-fold difference in isolated porosity, implying that gas retention and eruptive behavior of crystal-rich magmas may be controlled by the onset and efficacy of crack-dominated outgassing and can be modulated by relatively small changes in crystallinity.