Rheological change and degassing during a trachytic Vulcanian eruption at Kilian Volcano, Chaîne des Puys, France

International audience ; 16 Magma ascent during silicic dome-forming eruptions is characterized by significant changes in 17 magma viscosity, permeability and gas overpressure in the conduit. These changes depend on 18 a set of parameters such as ascent rate, outgassing and crystallization efficiency, and magma 19 viscosity which in turn may influence the prevailing conditions for effusive versus explosive 20 activity. Here, we combine chemical and textural analyses of tephra with viscosity models to 21 provide a better understanding of the effusive-explosive transitions during Vulcanian phases of 22 the 9.4 ka eruption of Kilian Volcano, Chaîne des Puys, France. Our results suggest that effusive 23 activity at the onset of Vulcanian episodes at Kilian Volcano was promoted by (i) rapid ascent 24 of initially crystal-poor and volatile-rich trachytic magma, (ii) a substantial bulk and melt 25 viscosity increase driven by extensive volatile loss and crystallization, and (iii) efficient 26 degassing/outgassing in a crystal-rich magma at shallow depths. Trachytic magma repeatedly 27 replenished the upper conduit, and variations in the amount of decompression and cooling 28 caused vertical textural stratification, leading to variable degrees of crystallization and 29 outgassing. Outgassing promoted effusive dome growth and occurred via gas percolation 30 through large interconnected vesicles, fractures and tuffisite veins, fostering the formation of 31 Manuscript Click here to access/download;Manuscript;Final Kilian II-resubmission.docx Click here to view linked References