An analog investigation of magma fragmentation and degassing: Effects of pressure, volatile content, and decompression rate John Stix a, ⁎, Jeremy C. Phillips b a Department of Earth & Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec, Canada H3A 2A7 b Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, United Kingdom abstract article info Article history: Received 30 September 2011 Accepted 3 October 2011 Available online 15 October 2011 We conducted analog experiments using gum rosin–acetone (GRA) mixtures to approximate behavior of magmas containing dissolved and exsolved volatiles. The main objective was to investigate the role of pressure upon fragmentation behavior for GRA mixtures undergoing slow and fast decompressions. We found that under conditions of slow decompression (22–80 Pa s -1 ), the GRA mixture never fragmented, exhibiting characteristic styles of surface degassing at different pressures. Under fast decompression (620–2400 kPa s -1 ), fragmentation behavior was controlled principally by the initial volatile content of the GRA mixture, as well as by the extent and rate of decompression. Low-viscosity GRA mixtures with high levels of acetone fragmented efficiently at high final pressures, while viscous mixtures containing less acetone did not fragment except when subjected to small final pressures, high pressure differentials, and rapid decompression. Based on our observations, we pro- pose that GRA mixtures near the fragmentation threshold are disrupted in a ductile fashion by intense vesicula- tion and bubble movements, while mixtures subject to elevated pressure differentials which are significantly higher than this threshold fragment in a brittle manner due to foam disruption. In non-fragmenting GRA mixtures, a form of stick–slip behavior was observed, the dynamics of which varied with the rheology of the mixture and the pressure differential applied during the decompression. In low-viscosity GRA mixtures, gas transfer occurred mainly in the form of gas bubbles and slugs which moved vertically upward through the ma- terial, creating permeable pathways to promote degassing and disruption of the mixture by bubble bursts. In vis- cous GRA mixtures dominated by foam development, vertical permeability gradients were observed, with a strongly degassed brittle foam at the top and a less degassed ductile foam beneath. Our observations have implications for submarine volcanism, volcanoes on Venus, and lava dome eruptions. For submarine volcanism, our experiments show that explosive fragmentation of magma may not be limited by water depth if the volatile content of the magma is sufficiently high (i.e., 4–6 wt.% H 2 O). Our experimental results also suggest that the high-pressure atmospheric conditions of Venus (9300 kPa) are not an impediment to the explosive eruption of high volatile content magmas, which casts doubt on the interpretation of large-volume pancake lava domes on Venus as the high-pressure equivalent of terrestrial ignimbrite eruptions. Together with our experimental results, observations of actively growing lava domes demonstrate that explosive fragmentation is promoted by rapid, large-volume collapses of a dome, and by unroofed magma which is comparatively volatile-rich and vesicular. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Pressure plays a fundamental role during the evolution of magmas. It is a determining factor upon the stability and crystallization of mineral phases (Barclay and Carmichael, 2004), the solubility of volatile compo- nents (Dixon and Stolper, 1995; Dixon et al., 1995), and the nature and styles of volcanic activity (Sparks, 1997). In this paper we investigate the role of confining pressure upon the nature and origin of high- pressure (e.g., submarine) and low-pressure (e.g., subaerial) eruptions by conducting a series of carefully-controlled analog experiments. Our goals were to elucidate the role of pressure in submarine eruptions, lava dome eruptions, and eruptive processes on other planets. Subaqueous eruptions can be both effusive and explosive, yet the controls on their explosivity, or lack thereof, are poorly known. In particular, the role of the water column as an additional source of confining pressure and potential inhibitor of explosive activity is not well understood. Submarine eruptions of silicic magma produce both lava flows and explosive activity. Silicic lava domes and lava flows have been documented in the subaqueous environment at various depths (Reynolds et al., 1980; Fiske et al., 1998; Maeno and Taniguchi, 2006). Recent work in the Izu–Bonin and Kermadec arcs has shown that explosive activity can occur even at depths of ~1000 m or deeper Journal of Volcanology and Geothermal Research 211-212 (2012) 12–23 ⁎ Corresponding author. Tel.: + 1 514 398 5391; fax: + 1 514 398 4680. E-mail addresses: stix@eps.mcgill.ca (J. Stix), j.c.phillips@bristol.ac.uk (J.C. Phillips). 0377-0273/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2011.10.001 Contents lists available at SciVerse ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores