Linking experimental and natural vesicle textures in Vesuvius 79AD white pumice Thomas Shea a, ⁎, Lucia Gurioli b,c,d , Jessica F. Larsen e , Bruce F. Houghton a , Julia E. Hammer a , Katharine V. Cashman f a Department of Geology and Geophysics, SOEST, University of Hawaii, 96822, Honolulu, HI, United States b Clermont Université, Université Blaise Pascal, Laboratoire Magmas et Volcans, BP 10448, F-63000 Clermont-Ferrand, France c CNRS, UMR 6524, LMV, F-63038 Clermont-Ferrand, France d IRD, R 163, LMV, F-63038 Clermont-Ferrand, France e Geophysical Institute, University of Alaska Fairbanks, 99775, Fairbanks, AK, United States f Department of Geological Sciences, University of Oregon, 97403, Eugene, OR, United States abstract article info Article history: Received 16 July 2009 Accepted 16 February 2010 Available online 3 March 2010 Keywords: vesicles phonolite Vesuvius textural characterization decompression experiments size distribution magma ascent rate Vesicle populations in volcanic pumice provide a partial record of shallow magma ascent and degassing. Here we compare pumice textures from the well-characterized 79AD Vesuvius eruption to those generated during isothermal decompression experiments. Three series of experiments were conducted using starting material from the first two phases of the eruption (eruptive units EU1 and EU2). Samples were decompressed from 100 or 150 MPa to final pressures of 10–25 MPa using conditions appropriate for simulating eruption conditions (T = 850 °C, dP/dT = 0.25 MPa/s). The experiments differed not only in starting material but also in temperature at which samples were annealed prior to decompression, which determined the initial number of crystals present in the melt. Results show that experiments approach the vesicle number densities and sizes of pumice samples, but show narrower size distributions. The wider size range of pumice samples suggests continuous, rather than instantaneous nucleation, which may reflect non-linear rates of decompression. All experiments exhibited equilibrium degassing, a process that was probably aided by heterogeneous bubble nucleation on oxide microlites. We conclude that delayed bubble nucleation cannot explain the explosivity of the Vesuvius eruption, which instead appears to require high rates of magma decompression. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Vesicles in volcanic rocks provide valuable information on the processes occurring in magma conduits and storage systems (e.g. Cashman and Mangan, 1994). Three complementary approaches provide constraints on vesiculation in magmas: textural measurements, labora- tory experiments, and physical/numerical models. Textural measure- ments of vesicle size, number and spatial distribution in natural samples are used to infer the processes responsible for their formation (e.g., Klug and Cashman, 1994; Klug et al., 2002; Polacci et al., 2003; Gurioli et al., 2005; Sable et al., 2006; Adams et al., 2006, Lautze and Houghton, 2007; Polacci et al., 2007). Although natural tephra samples are the best available “tracers” of vesiculation in any eruption, they represent a frozen textural state, which might have been acquired prior to, during, and after magmatic fragmentation. Decompression experiments document pro- cesses of bubble nucleation, growth, coalescence, and collapse that may occur during magma ascent, although the small volumes of material used often prevent scaling of vesicle size, number and porosity to natural systems. Models of bubble nucleation and growth link experiments to conduit processes, and ultimately, shed light on conditions required to produce violent explosive eruptions (Toramaru, 1989; Toramaru, 1995; Lyakhovsky et al., 1996; Jaupart, 1996; Vergniolle, 1996; Lovejoy et al., 2004; Mangan et al., 2004; Massol and Koyaguchi, 2005; Yamada et al., 2005; Toramaru, 2006; Gonnermann and Manga, 2007). Of the three approaches described above, laboratory experiments provide a critical link between field observations and modeling through quantification of kinetic parameters such as bubble nucleation and growth rates, volatile solubility, diffusivity, and surface tension. Most experimental vesiculation studies have examined rhyolitic magmas (Hurwitz and Navon, 1994; Lyakhovsky et al., 1996; Gardner et al., 1999; Mourtada-Bonnefoi and Laporte, 1999; Mangan and Sisson, 2000; Gardner et al., 2000; Larsen and Gardner, 2000; Mourtada-Bonnefoi and Laporte, 2002; Martel and Schmidt, 2003; Mourtada-Bonnefoi and Laporte, 2004; Larsen et al., 2004; Lensky et al., 2004; Burgisser and Gardner, 2005; Gardner, 2007; Cluzel et al., 2008) because they produce the most violent eruptions, and their high viscosity precludes problems associated with rapid microlite crystallization and bubble-melt decou- pling in laboratory capsules. Sparser experimental data exist on lower viscosity melts such as basalts (Bai et al., 2008), dacites (Mangan et al., 2004; Suzuki et al., 2007) and phonolites (Larsen and Gardner, 2004; Journal of Volcanology and Geothermal Research 192 (2010) 69–84 ⁎ Corresponding author. Tel.: +1 808 956 8558. E-mail address: tshea@hawaii.edu (T. Shea). 0377-0273/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2010.02.013 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores