The magmatic plumbing system beneath Santiaguito Volcano, Guatemala

a r t i c l e i n f o The silicic dome complex of Santiaguito, Guatemala, has exhibited continuous extrusive activity for 90 years. Despite its longevity, remarkably little is known about the magmatic plumbing system beneath Santiaguito. Here, we use petrological analyses of lava samples to define this plumbing system, from storage in the lower to mid-crust through to extrusion onto the surface. Magmatic storage conditions are constrained using amphibole and plagioclase phenocrysts; ascent processes are examined using the breakdown rims of amphibole phenocrysts and the texture and composition of groundmass, while shallow processes are revealed by the alteration of titanomagnetites and matrix glass. Santiaguito magmas contain amphiboles that formed from ~24 km to ~12 km beneath the surface, with temperatures of ~940 to ~980 °C, and f O2 of NNO +0.4 to NNO +1.2. Amphibole breakdown rims suggest that during the final phases of ascent, magma may rise from ~12 km (the limit of amphibole stability) relatively rapidly (~27 to ~84 m h − 1). We infer from the texture of the ground-mass that melt rigidifies prior to extrusion — a finding that may have important consequences for conduit dynamics. The silicic lava dome complex of Santiaguito, Guatemala, lies within a crater formed during the cataclysmic eruption of its parent stratocone, Santa Maria, in 1902 (Fig. 1) (Rose, 1972). Activity at Santiaguito has been continuous since its inception in 1922 and is characterized by small to moderate explosions of steam, gas, and ash, small pyroclastic flows and rockfalls, frequent lahars during Guatemala's wet season, and effusion of blocky lava domes and flows. Lava flows have become increasingly dominant since the 1960s, with some extending over ~ 3 km from the vent Volcano Observatory written records). Understanding the long-term behavior and consequent hazards of persistent dome-forming systems like Santiaguito requires an understanding of the storage and ascent processes of magma within those systems (e.g., Cashman and Blundy, 2000; Rutherford and Devine, 2003; Clarke et al., 2007). The temperature and pressure within magma chambers and rates of magma ascent and extrusion can inform models of supply and eruption dynamics, the interpretation of seismic, petrologic, and emissions data, and ultimately improve hazard assessment (e.g., Rutherford and Hill, 1993; Hammer et al., 2000). These processes have been constrained at other, better-studied dome-forming systems using a variety of geochemi-cal and petrological techniques (e.g., ascent rate of Mount St Helens magma from …