Journal of Volcanology and Geothermal Research 378 (2019) 16–28 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores Melt inclusion evidence for long term steady-state volcanism at Las Sierras-Masaya volcano, Nicaragua Jeffrey Zurek a,⁎,SéverineMouneb, Glyn Williams-Jones a, Nathalie Vigouroux a,c, Pierre-J. Gauthier b a Centre for Natural Hazards Research, Department of Earth Sciences, Simon Fraser University, Burnaby, BC, Canada b Laboratoire Magmas et Volcans, Université Clermont Auvergne, Campus Universitaire des Cézeaux, 63178 Aubière Cedex, France c Department of Earth and Environmental Sciences, Douglas College, New Westminster, BC, Canada article info abstract Article history: Las Sierras-Masaya volcanic system is a persistently active basaltic caldera complex in Nicaragua. While there has Received 15 August 2018 been almost no juvenile material erupted since 1772, Masaya volcano has been persistently degassing for Received in revised form 9 April 2019 N150 years. An additional unusual behaviour for the Las Sierras-Masaya volcanic complex is its ability to produce Accepted 10 April 2019 large caldera-forming basaltic Plinian eruptions with the most recent occurring about 1800 years ago. Available online 18 April 2019 Here we present melt inclusion analyses that provide constraints on the magmatic system over time. Melt inclu- sions hosted in plagioclase and olivine crystals were analyzed for major, trace and volatile elements (S, Cl, F, H O). Keywords: 2 Masaya volcano The data supports a consistent parental magmatic source with restricted compositional variability explained by Persistent degassing simple fractional crystallization of plagioclase, olivine, clinopyroxene and magnetite at a nearly constant temper- Melt inclusion geochemistry ature. This broadly agrees with previous whole rock geochemical studies showing that the overall chemical sig- Volatile budgets nature of volcanic products at Masaya has remained largely unchanged for ~60,000 years and that both shallow Magmatism and extensional tectonics fractionation and degassing processes dominate the whole evolution of the magmatic series. Based on volatile el- ement in melt inclusions and gas composition and flux measurements, we determine the magmatic flux to be ~0.19 km3 yr−1 implying that up to 47 km3 of magma may have degassed since the last effusive eruption. As at other persistently active basaltic volcanoes (e.g., Mt. Etna, Italy; Kilauea, Hawaii, USA), this magmatic flux must involve significant endogenous storage which is likely accommodated by extensional tectonics. However, Masaya volcano differs in its apparent simplicity with respect to its stable chemistry and its fully interconnected magmatic system. © 2019 Elsevier B.V. All rights reserved. 1. Introduction how they may have evolved over time, data from prehistoric eruptions of Masaya volcano are necessary. Melt inclusions are the perfect geo- Chronicling magmatic evolution is important to understanding and chemical tool to investigate magma sources and magmatic processes, recognizing changes in volcanic activity and processes which may be a as they record the pre-eruptive melt chemistry and evolution at the mo- precursor to cataclysmic eruptions. The integration of geodesy, seismol- ment of entrapment. Here we present melt inclusion geochemical anal- ogy, gas geochemistry, and magma geochemistry techniques has been yses from 8 different eruptive units, and 99 individual analyses within successfully applied at systems such as Yellowstone (Morgan, 2007), Masaya's caldera spanning approximately 6000 years. We discuss how Campi Flegrei (Piochi et al., 2014) and Kilauea (e.g., Poland et al., they relate to recent information on the behaviour of the system, ob- 2009) in order to understand and identify potential volcanic precursors. tained through gas and geophysical studies. However, for many volcanoes, this is not possible as the data required has not been collected. The Las Sierras-Masaya volcanic complex 2. Geologic setting (more commonly known as Masaya volcano) is an intermediary exam- ple, as historical activity is well studied but there are limited robust Masaya is one of 18 volcanoes located in western Nicaragua that long-term datasets and studies spanning prehistoric activity are clus- form part of the Central American Volcanic Arc (Fig. 1). It is an active ba- tered around several explosive eruptions (e.g., Williams, 1983a, saltic volcanic center which has been in a state of near-continuous activ- 1983b; Kutterolf et al., 2008a, 2008b). In order to investigate the mag- ity since its discovery in 1524 (Maciejewski, 1998 and references matic processes controlling activity at Masaya volcano and determine therein). The continuous activity usually manifests itself as persistent degassing with occasional lava lakes, vent clearing explosions, and pit ⁎ Corresponding author. crater formation (e.g., Maciejewski, 1998; Rymer et al., 1998; Aiuppa E-mail address: [email protected] (J. Zurek). et al., 2018). Although the cones and shields within the caldera are https://doi.org/10.1016/j.jvolgeores.2019.04.007 0377-0273/© 2019 Elsevier B.V. All rights reserved. J. Zurek et al. / Journal of Volcanology and Geothermal Research 378 (2019) 16–28 17 Fig. 1. A) Location of large tectonic features near Masaya Caldera (modified from Girard and van Wyk de Vries, 2005). Inset map of Nicaragua with volcanoes of the Central American Vol- canic Arc shown by black triangles; Masaya is shown by a red triangle. B) Shaded relief map of Masaya volcano with main Nindiri and Masaya cones and active Santiago crater shown. Each sample location is marked by a number 1–12 (see Table 1). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) young (b2000 years old, Williams, 1983b), only two effusive lava flows rare. Williams (1983a, 1983b) mapped the surface geology of Masaya cal- (in 1670 and 1772; Maciejewski, 1998) have occurred since historical dera and developed relative stratigraphic relationships. Based on this rel- records began. Therefore, the historical activity has been generally re- ative stratigraphy, Walker et al. (1993) sampled the eruptive products stricted to, and within, the two main cones in the caldera from the Las Sierras-Masaya volcanic complex spanning 60,000 years (Maciejewski, 1998; Rymer et al., 1998). Present activity has been fo- and found whole rock chemistries had only minor variations (Fig. 2) cused within the Santiago pit crater of Nindiri cone since its formation and were found to plot near low pressure cotectics, suggesting shallow in 1858–1859 (e.g., Rymer et al., 1998) and is currently characterized processes control most of the evolution of the magmatic series. These by persistent degassing from a vigorously convecting lava lake that ap- studies suggest that the small observed compositional variation is the re- peared in December 2015 (Aiuppa et al., 2018). sult of fractional crystallization with plagioclase, olivine and pyroxene in a Masaya is also situated within two large local active tensional tec- shallow convecting magma chamber with periodic injection of more tonic features, the Nicaraguan depression and the Managua graben. All primitive basaltic magmas (Williams, 1983a, 1983b; Walker et al., 1993). active volcanic centers in Nicaragua are situated within the Several melt inclusion studies from olivine hosts with a forsterite Nicaraguan depression, a half graben parallel to the volcanic arc range of 71 to 76 Mol % Fo have been conducted on samples from Nindiri (McBirney and Williams, 1965). The Managua graben is approximately cone (Horrocks, 2001; Sadofsky et al., 2008; Wehrmann et al., 2011; de perpendicular to the Nicaraguan depression with its southern extent Moor et al., 2013). Melt inclusion major element concentrations from (Fig. 1), the Cofradias fault, intersecting or ending at Masaya caldera. these studies have a narrow range of K2O (1.3–1.5 wt%) and broadly Analog modeling suggests that large, dense and ductile volcanic intru- represent a basaltic composition. Volatile concentrations display a sions beneath the northern edge of the caldera and the regional stress wide range with sulphur contents between 92 and 448 ppm, chlorine regime are the cause of the Managua graben (Girard and van Wyk de between 440 and 1531 ppm, fluorine between 10 and 672 ppm and Vries, 2005). Therefore, the graben, as well as the local extensional tec- water between 1.39 and 1.91 wt%. These published sources combined tonics, likely has a direct effect on the volcanic activity at Masaya. contain a single measurement of CO2 concentration, 369 ppm, from an Masaya volcano is part of a larger complex of nested calderas. It be- olivine-hosted melt inclusion. The large variations in volatile concentra- longs to a select group of volcanic centers worldwide known to have tions are likely due to varying degrees of pre-eruption degassing. There produced basaltic Plinian eruptions (e.g., Tarawera, New Zealand may also be open system processes occurring that mask true concentra- (Walker et al., 1984); Etna, Italy (Coltelli et al., 1998); Tanna, Vanuatu tions such as CO2 fluxing from depth (e.g., Blundy et al., 2010; (Métrich et al., 2011) among others). The older and larger Las Sierras Wehrmann et al., 2011) and post-entrapment modification including caldera, may have formed up to 60,000 years ago during a large Plinian CO2 sequestration by a shrinkage bubble