Structure of the Plumbing System at Tungurahua Volcano, Ecuador: Insights from Phase Equilibrium Experiments on July–August 2006 Eruption Products

JOURNAL OF Journal of Petrology, 2017, Vol. 58, No. 7, 1249–1278 PETROLOGY Advance Access Publication Date: 28 August 2017 Original Article doi: 10.1093/petrology/egx054 Structure of the Plumbing System at Tungurahua Volcano, Ecuador: Insights from Phase Equilibrium Experiments on July–August 2006 Eruption Products Joan Andújar1*, Caroline Martel1, Michel Pichavant1, Pablo Samaniego2, Bruno Scaillet1 and Indira Molina3 1 Institut des Sciences de la Terre d’Orléans, CNRS, Université d’Orléans, BRGM, 1a rue de la Férolerie, 45071, Orléans France; 2Laboratoire Magmas et Volcans, Université Clermont Auvergne, CNRS, IRD, OPGC, F-63000 Clermont-Ferrand, France; 3Instituto Geofı́sico, Escuela Politécnica Nacional, PO Box 17-01-2759, Quito, Ecuador *Corresponding author. Telephone: (þ33) 2 38 25 54 04. Fax: (þ33) 02 38 63 64 88. E-mail: Received July 13, 2016; Accepted August 17, 2017 ABSTRACT Understanding the plumbing system structure below volcanoes and the storage conditions (temperature, pressure, volatile content and oxygen fugacity) of erupted magmas is of paramount importance for eruption forecasting and understanding of the factors controlling eruptive dynamics. Phase equilibria experiments have been performed on a Tungurahua andesite (Ecuador) to shed light on the magmatic conditions that led to the July–August 2006 eruptions and the parameters that controlled the eruptive dynamics. Crystallization experiments were performed on a representative August 2006 mafic andesite product between 950 and 1025 C, at 100, 200 and 400 MPa and NNO þ 1 and NNO þ 2 (where NNO is nickel–nickel oxide buffer), and water mole fractions in the fluid (XH2O) from 03 to 1 (water-saturation). Comparison of the natural phenocryst assemblage, proportions and phenocryst compositions with our experimental data indicates that the natural andesite experienced two levels of ponding prior to the eruption. During the first step, the magma was stored at 400 MPa (15–16 km), 1000 C, and contained c. 6 wt % dissolved H2O. In the second step, the magma rose to a confining pressure of 200 MPa (8–10 km), where subsequent cooling (to 975 C) and water-degassing of the magma led to the crystallization of reversely zoned rims on pre-existing phenocrysts. The combination of these processes induced oxidation of the system and overpressure of the reservoir, triggering the July 2006 eruption. The injection of a new, hot, volatile-rich andesitic magma from 15–16 km into the 200 MPa reservoir shortly before the eruption was responsible for the August 2006 explosive event. Our results highlight the complexity of the Tungurahua plumbing system in which different magmatic reservoirs can coexist and interact in time and are the main controlling factors of the eruptive dynamics. Key words: basaltic andesite; phase equilibrium; Tungurahua; plumbing system; pre-eruptive conditions; experimental petrology; adakite; magnesian andesite INTRODUCTION Volcanic eruptions in populated areas represent a major threat to human beings, having both regional economic and social impacts and global consequences. Nowadays, the monitoring of active volcanoes helps to forecast volcanic eruptions with several days or weeks of anticipation. However, geophysical techniques do not allow prediction of the eruptive style of an incipient or current eruption. This task usually involves consideration of the eruptive history of the volcano, which very often records C The Author 2017. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: V 1249 1250 TUNGURAHUA VOLCANO: GEOLOGICAL SETTING AND VOLCANIC ACTIVITY Active volcanoes monitored by a permanent sensor network provide a variety of data that illuminate the continuing magmatic processes occurring below the edifice (e.g. El Hierro in the Canary Islands; López et al., 2012; Martı́ et al., 2013). However, current geophysical and geochemical techniques still need to be improved to discriminate between magma and gas movement within a volcanic edifice, especially when the seismic signals do not have simple decaying oscillations like those described by Molina et al. (2004) for Tungurahua volcano. In other cases, the density of the geophysical network or the presence of aquifers at near-surface levels makes it difficult to identify the presence of deep magma reservoirs and/or conduits. This is the case of Tungurahua volcano [5023 m above sea level (a.s.l.)], located in the Eastern Cordillera of the Ecuadorian Andes, one of the most active volcanoes in Ecuador, along with other large edifices such as Cayambe, Antisana, Cotopaxi and Sangay (Fig. 1). During the past few thousand years, Tungurahua volcano frequently emitted andesitic magmas during medium-to-high explosive eruptions [volcanic explosivity index (VEI)  3] that generate tephra fallouts, pyroclastic flows and lahars, together with blocky lava flows (Hall et al., 1999; Le Pennec et al., 2008, 2016). These eruptions display a minimum recurrence time of at least one pyroclastic flow-forming eruption per century and include the historical eruptions of AD 1640, 1773 and 1916–1918. However, the volcano stratigraphic sequence also records the occurrence of much more explosive events (VEI  4), characterized by regional tephra fallout and pumice pyroclastic flows that involve dacitic magmas (Hall et al., 1999; Le Pennec et al., 2013), the most recent being the large AD 1886 dacitic Plinian event (Samaniego et al., 2011; Le Pennec et al., 2016). After 75 years of repose, Tungurahua initiated a new eruptive period in 1999, which is still continuing (March 2017, Fig. 1; IGEPN reports at www.igepn.edu.ec/tungur ahua-informes). From 1999 to 2005 the eruptive activity typically consisted of recurrent low-to-moderate explosive phases, characterized by a Strombolian style, with short duration (cannon-like) explosions that fed small volcanic plumes reaching a height of 7 km above the summit, and local ash fallout. In April 2006, a new unrest period started with seismicity moving from deep (15 km) towards very shallow depths (<5 km) (IGEPN reports) as well as a change in the degassing pattern of the volcano (Arellano et al., 2008), ending with the July– August 2006 paroxysmal phase that generated an 15 km a.s.l. sub-Plinian column, a regional tephra fallout and several pyroclastic flows that descended over the flanks of the volcano (Samaniego et al., 2011; Eychenne et al., 2013; Hall et al., 2013). The eruptive products from the 1999–2005 events have a homogeneous andesitic composition (58–59 wt % SiO2). In comparison, samples from the historical eruptions of Tungurahua (AD 1641, 1773, and 1918) display a wider silica content (56–59 wt % SiO2). In contrast, during the 2006 paroxysmal eruption, two different types of magma were erupted: a very minor dacitic (61–65 wt % SiO2) component (<1 vol. %, Eychenne et al., 2013), with characteristics akin to those of the AD 1886 products, dispersed into a h (...truncated)