Journal of Volcanology and Geothermal Research 378 (2019) 16–28

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Journal of Volcanology and Geothermal Research

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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 (e.g., Roedder, 1984), Post- eruption (Fontana Tephra - Bice, 1985; Girard and van Wyk de Vries, Entrapment Crystallization (PEC; e.g., Kress and Ghiorso, 2004)and 2005, Wehrmann et al., 2006; Kutterolf et al., 2008a, 2008b). Masaya ‘leakage’ of volatiles by diffusion and decrepitation (e.g., Tait, 1992; caldera, the most recent caldera of the complex, formed via three Plinian Gaetani et al., 2012). A detailed study of sulphur degassing at Masaya eruptions over the last 6000 years (e.g., Williams, 1983a, 1983b; by de Moor et al. (2013) calculated pre-eruptive melt temperatures to Kutterolf et al., 2008a, 2008b; Perez et al., 2009). 1097 to 1127 °C via the olivine-liquid geothermometer of Putirka (2008) and melt inclusion oxygen fugacity to be ΔQFM +1.7 (±0.4), 3. Previous work which is typical of basalt in arc settings. Degassing studies using a variety of in situ and remote sensing tech- Masaya volcano is persistently active and easily accessible via roads; niques (e.g., Stoiber et al., 1986; Nadeau and Williams-Jones, 2009; therefore, it is relatively well studied, although long-term studies are Martin et al., 2010; Aiuppa et al., 2018 and references therein) provide 18 J. Zurek et al. / Journal of Volcanology and Geothermal Research 378 (2019) 16–28

Fig. 2. Major element whole rock geochemical analysis data for Las Sierras-Masaya volcanic complex, where blue squares are from this study, black circles are from Carr et al. (2014) and yellow stars represent the chemical analyses of basaltic Plinian deposits from this study. In alkali vs SiO2 plot circles are compositional range for other Nicaraguan volcanoes: Telica in green, Cerro Negro in blue, and composite volcanoes with large ranges in composition (Cosiguina, San Cristobol, Mombotombo, Mombacho, and Conception) are in orange. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) a large database to understand degassing processes at Masaya. Based on (e.g., Stevenson and Blake, 1998). According to this combined model gas composition and SO2 fluxes combined with melt inclusion data, the (Stix, 2007), a convective conduit from depth feeds a shallower reser- supply rate from 1875 to 1985 was estimated to be 0.091 km3 yr−1,cor- voir where a foam layer develops at its roof, with a narrower conduit responding to 10 km3 of magma during that time period (Stoiber et al., linking the top of the reservoir to the surface and allowing efficient vol- 1986). Importantly, there has been no significant effusive eruption since atile transfer. This model can thus explain both the gravity results, sug- 1772 (Maciejewski, 1998). gesting periodic thickening of a foam layer, and the large volumes of Beside gas data, long-term time-series (1993–2017) gravity measure- unerupted degassed magma. ments underlined changes in mass fluxes in near surface reservoirs cen- It should be kept in mind however, that, prior to our study, there has tered under Nindiri cone (Rymer et al., 1998; Williams-Jones et al., not been a concerted effort to look at melt inclusions away from the ac- 2003; Rymer et al., 2017). The time-lapse gravity shows decreases during tive vent in Santiago crater. Investigating melt inclusions in older prod- periods of increased gas flux, which is interpreted as the development of a ucts emitted from more distal eruptive centers within the Masaya thick gas-rich foam layer at the top of shallow reservoirs (Williams-Jones caldera in conjunction with a comprehensive assessment of historic et al., 2003). Periods of reduced gas flux show gravity increases thought to degassing can test the combined model by Stix (2007) and enhance be indicative of the densification of the shallow magmatic system through our understanding of the volcanic system as a whole. reductioninthefrothyfoamlayer(Williams-Jones et al., 2003). Any model aimed at understanding magmatic processes operating at 4. Methods Masaya volcano must consider its persistent volcanic activity, its vigor- ous shallow magmatic system and the large volumes of degassing 4.1. Melt inclusion selection and analysis magma required to explain measured volatile fluxes. Stix (2007) sug- gested that Masaya's steady-state persistent degassing activity is best In order to fill the gap in our knowledge of past eruptions at Masaya described through the combination of two different degassing models. volcano, 12 tephra samples from different eruptive units and one Pele's The first consists of convection within a conduit where low density, hair sample were collected for analysis (Fig. 1; Table 1). Each sample volatile-rich magma rises in the centre of the conduit while denser, was named after either the eruptive feature it was collected from or gas-poor magma descends around the edges (e.g., Jaupart and the relative stratigraphic unit it belongs to, as described by Walker Vergniolle, 1989). The second involves the development of a foam et al. (1993). Samples in which pristine glassy melt inclusions could layer at the top of a magma reservoir, via the accumulation of gas bub- not be found are not presented here. Although every effort was made bles, which degasses through a conduit at the roof of the reservoir to acquire fresh and unaltered samples, the material from Comalito J. Zurek et al. / Journal of Volcanology and Geothermal Research 378 (2019) 16–28 19

Table 1 Location data for each sample collected as part of this study. Numbers refer to the map in Fig. 1 and only those with an asterisk had glassy melt inclusions. ⁎ Location Fig. 1 Sample “ ” MI's analyzed Relative stratigraphic unit (Fig. 2 Walker et al., 1993) Eruptive feature Latitude/longitude ⁎ 1 Visitor center Ql11 Visitor center cone 12.0050°–86.1477° ⁎ 2 Comalito Ql11 Comalito cone 11.9970°–86.1499° ⁎ 3 Qv9 Qv9 Masaya cone 11.9816°–86.1603° ⁎ 4 Qa16 Qa16 Masaya and Nindiri saddle 11.9819°–86.1644° ⁎ 5 MS1997 Modern Bomb erupted 1997 from Nindiri 11.9859°–86.1713° ⁎ 5 Pele's hair Modern Pele's Hair collected 2002 11.9859°–86.1713° ⁎ 6 Qa24 Qa24 Nindiri cone 11.9862°–86.1760° ⁎ 7 QaW1 QaW1 Deposit in caldera wall 11.9842°–86.1920° ⁎ 8 Cerro Montoso Qv 7 Cerro Montoso Cone 12.0020°–86.1844° 9 Sentepe Qv7 Sentepe cone 12.0104°–86.1806° 10 Errant N.A. Errant cone 12.0150°–86.1820° 11 Arenal Qv7 Arenal cone 12.0123°–86.1746° 12 Qv5 Qv5 From cinder cone just outside the caldera 12.0218°–86.1898°

cone, was collected from an area of broad low-temperature fumaroles 4.2. Remote sensing of SO2 fluxes (b80 °C). Detailed sample descriptions, mineral and melt inclusion preparation and probing techniques can be found in Appendix A. The majority of the SO2 measurements were collected via vehicle- Phenocrysts from the different samples were selected (size range based traverses along two roads that transect the passively degassing between 100 and 1000 μm; Table 1 in the Appendix) and each crystal plume (~5 and 15 km downwind) using UV spectrometer systems was analyzed at least twice at the core and the rim of the mineral, show- (COSPEC, DOAS and FLYSPEC; e.g., Elias et al., 2006 and references ing no chemical zonation. The average Anorthite or Forsterite content is therein). Data from a stationary scanning DOAS system (Galle et al., given in Appendix and shown in Fig. 3. All analyzed melt inclusions ap- 2017) ~1.5 km downwind of the volcano as well as satellite-based mea- pear to be primary inclusions due to their random locations within the surements from the Ozone Monitoring Instrument (OMI) system (Carn crystals (Roedder, 1984). The absence of daughter minerals in the et al., 2017) are also included; see these publications for detailed melt inclusions and the scarcity of thermal shrinkage bubbles suggest methodologies. rapid cooling (e.g., Lowenstern, 2003). To further reduce the effect of Importantly for Masaya, many early ground-based studies post-entrapment crystallization, diffusion and re-equilibration, all sam- (e.g., Delmelle et al., 1999; Williams-Jones et al., 2003) collected SO2 ples, except MS1997, were taken from rapidly quenched glassy lapilli. flux measurements along the Pan American highway which follows EMP analyses were conducted on thin sections obtained for each the Las Sierras caldera margin, ~15 km downwind. However, Nadeau whole-rock sample as well as on Pele's hairs in order to measure and Williams-Jones (2009) showed that transects across this topo- major elements and volatile concentrations in glassy groundmass. How- graphic high caused both dilution and velocity increases of the plume, ever, only 3 samples (Visitor center, Comalito and Qv9) contained glassy leading to ~30–50% underestimates of the actual flux. As such, data groundmass patches of a sufficient size for accurate measurements. Of from these traverses are not included here. Furthermore, given that the 8 samples which contained glassy melt inclusions, 40 melt inclu- the largest source of uncertainty in SO2 flux measurements is due to dif- sions hosted in olivine crystals and 57 in plagioclase crystals were ana- ficulties in accurately constraining the plume velocity lyzed by EMP. A total of 18 melt inclusions (12 in olivine crystals and 6 (e.g., Williams-Jones et al., 2006, 2008), only data sets which include in plagioclase crystals) were also analyzed for water content using wind speed measurements (used as a proxy for plume velocity) have Raman spectroscopy at Laboratoire de Géologie de Lyon. Trace element been incorporated. For consistency, all flux measurements have subse- concentrations were obtained through LA-ICP-MS on 17 melt inclu- quently been recalculated using modelled wind speeds derived from sions, 4 of which were hosted in olivine crystals (See Appendix A for ta- NOAA GFS (3 h temporal resolution, 1° spatial resolution; GFS, 2018) bles and detailed methodology). or EMCWJ ERA-Interim (6 h temporal resolution, 0.125° spatial

Fig. 3. Anorthite (blue) and forsterite (green) contents of plagioclase and olivine crystals versus K2O content of the associated melt inclusion. Each point represents an average composition for one crystal (at least two points were measured in each crystal: the rim and the core). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 20 J. Zurek et al. / Journal of Volcanology and Geothermal Research 378 (2019) 16–28 resolution; Dee et al., 2011) global models at an elevation of ~1500 m thick layer of dusty soil which may have survived cleaning of the vesic- (See Appendix A Table 6 for complete dataset). ular lapilli. The groundmass glass probed from Visitor center, Comalito and Qv9 samples contains slightly more potassium than the olivine 5. Results melt inclusions of the same sample but otherwise mimics the whole rock and melt inclusion data. The plagioclase-hosted MIs differ from

The whole rock and groundmass glass analyses from the samples the olivine-hosted MIs: they have generally higher K2Ocontents(even containing glassy melt inclusions are included in Appendix A. Whole for a given FeO, MgO or SiO2 content), which extend past the ground- rock SiO2 contents range from 49 to 51 wt%, FeO values are 11–15 wt mass data, their SiO2,TiO2 and Na2Orangesextendtohighervalues %, MgO contents are restricted to 5–5.5 wt% (Fig. 2), K2Ovaluesare compared to the olivine-hosted inclusions, and their FeO content and 1–1.2 wt%, total alkalis range from 3.5 to 4 wt% and CaO/Al2O3 ratios CaO/Al2O3 ratio extend to lower values (see Fig. 4). Overall, the range from 0.6 to 0.74. Lapilli groundmass glass analyses were obtained plagioclase-hosted melt inclusions display more compositional variabil- from each sample with space free of microcrystals sufficient for a mea- ity (i.e., larger range of oxide content for a given K2Ocontent). surement. Groundmass compositions range from 51.2 to 51.7 wt% SiO2, 12.8–13.2 wt% FeO, 4.9–6.0 wt% MgO, and 1.3–1.2% K2O. 5.1. Post-entrapment modifications In general, the whole rock compositions mirror the olivine-hosted melt inclusion compositions, except for Cerro Montoso. For this sample, Olivine-hosted melt inclusions were corrected for post-entrapment the whole rock has lower SiO2 (47.15 wt%) than any other sample or crystallization using the well-established equilibrium constant KD de- melt inclusion as well as high water content and low MgO. This is likely fined as (FeO/MgO)ol/(FeO/MgO)melt (Roeder and Emslie, 1970). The due to contamination with soil and/or organic material as the area has a relative amounts of FeO and Fe2O3 are unknown in our inclusions but

Fig. 4. Melt inclusion major elements versus K2O including a plot of the ratio of CaO and Al2O3 versus K2O. Petrolog (Danyushevsky and Plechov, 2011) fractional crystallization trend in red starting from the starting chemistry of the olivine hosted MI with the lowest K2O. Data from previous studies comes from Sadofsky et al. (2008); Wehrmann et al. (2011),andde Moor et al. (2013). Arrows show possible crystallization trends. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) J. Zurek et al. / Journal of Volcanology and Geothermal Research 378 (2019) 16–28 21 can be estimated between 0.232 and 0.262 using ratios from previous range of total alkali between 3.5 and 6 wt% and MgO content between studies (de Moor et al., 2013). We used a KD value of 0.30, calculated 4 and 7 wt%. This is consistent with what has been previously reported from the equilibrium model of Toplis (2005), which results in a rather for both whole rocks (Walker et al., 1993) and melt inclusions limited amount of post-entrapment crystallization, from 0 to 8.5%. (Horrocks, 2001; Sadofsky et al., 2008; Wehrmann et al., 2011; de

While the KD between melt and plagioclase is not well defined, several at- Moor et al., 2013). The chemical composition of whole rocks collected tempts have been made to estimate it (e.g., Putirka, 2008). For the Masaya as part of this study overlaps with the melt inclusion range of composi- suite of samples, measured plagioclase KD's fall in the range 0.07–0.14. If a tions, although previously collected whole rock samples (Walker et al., melt temperature of 1097–1127 °C (de Moor et al., 2013)isrepresenta- 1993) may in some cases fall outside this range. tive of the Masaya magmatic system, the plagioclase equilibrium constant Major element concentrations in melt inclusions are plotted against should reach 0.27 ± 0.11, hence suggesting significant post-entrapment K2O(Fig. 4) as potassium behaves as an incompatible element in calc- crystallization of melt inclusions hosted in plagioclases. However, the un- alkaline or tholeiitic magmas (e.g., Neumann et al., 1999). Melt inclu- corrected chemical composition of plagioclase-hosted melt inclusions is sions hosted in plagioclase crystals do not show well defined correla- in good agreement with the corrected chemical composition of olivine- tions between major oxides and K2O, with the exception of CaO/Al2O3 hosted melt inclusions. It suggests that post-entrapment crystallization vs. K2O(Fig. 4). The observed trend of decreasing CaO/Al2O3 with in- of MIs in plagioclase is rather limited, if any, and probably does not exceed creasing K2O strongly suggests that the magmatic series evolves afewpercentatmost(e.g.,Donovan et al., 2018). This is also underlined through plagioclase and clinopyroxene co-crystallization. The same in Fig. 4 where the composition in CaO and Al2O3 of the MIs hosted in pla- trend is not as clear for olivine-hosted melt inclusions which cluster in gioclase do not vary following plagioclase crystallization trends. No cor- the top left corner of the CaO/Al2O3 vs. K2O diagram. This is consistent rection for post-entrapment crystallization of melt inclusions in with early crystallization of olivine in the magmatic series. plagioclase crystals has thus been undertaken. To further limit the possi- In contrast, melt inclusions hosted in olivine crystals do show identifi- ble effects of post-entrapment chemical evolution, melt inclusions with able general trends (Fig. 4) as both incompatible SiO2 and TiO2 increase fractures or daughter minerals were not analyzed. Shrinkage bubbles with increasing K2O, while both compatible MgO and, to a lesser extent, were uncommon in most samples and were avoided where possible in FeO* negatively correlate with K2O. Such behaviour is expected for frac- order to limit the effect of volatiles preferentially diffusing into the void tional crystallization dominated by olivine and strongly contrasts with space. Nevertheless, volatile leakage appears negligible in our samples the above-mentioned observation regarding plagioclase-hosted melt in- as there is no significant difference in volatile contents between those in- clusions. It thus suggests the following crystallization sequence for the clusions with or without a bubble, suggesting little to no volatile phases Masaya magmatic suite: olivine - plagioclase - clinopyroxene. There is es- partitioning into the bubble. Diffusion from the melt inclusion to the sentially no change in Na2O over the entire range of K2O within olivine host crystal and/or the surrounding magma is also a concern. Studies hosted melt inclusions except in sample QaW1. Melt inclusions in plagio- have shown that certain elements, especially H and Fe in olivine crystals, clases from sample QaW1 were the smallest in size, so the beam size of can be easily diffused from the melt inclusion to the host crystal the EMP was reduced to 2 μm (compared to 5 or 10 μm for the other

(e.g., Danyushevsky et al., 2000). While iron diffusive loss from melt inclu- MIs). Hence, low Na2O contents could be ascribed to sodium loss due to sions to the host olivine cannot be totally ruled out, the liquids trapped in thehighintensityofthemicroprobebeam. olivine crystals do not present abnormally high KD values and mimic Trace element concentrations shown in Fig. 5 are normalized to de- those trapped in plagioclase crystals, thus suggesting that this process is pleted mid ocean ridge basalts (DMORB; Salters and Stracke, 2004). likely minimal. Trace element compositions in MIs have typical arc-like patterns with rel- atively depleted HFSE such as Nb, Ta and Zr and enrichments in large ion 5.2. Host crystals lithophile elements such as Rb and Ba (Fig. 5), suggesting that Masaya magmas evolved from a subduction-modified mantle source. The most All olivine crystals used for analysis have forsterite (Fo) contents be- striking feature observed in Fig. 5 is the lack of any significant variation tween 71 and 78 with samples from Cerro Montoso, Masaya, Nindiri in the trace element geochemistry recorded in melt inclusions while our (Qa24) and Visitor Center cones having a narrower Fo range of 76 to samplesspanover6000yearsoferuptive activity at Masaya. These results 78 (see Appendix A). Both MS1997 (a volcanic bomb erupted in 1997) broadly agree with what was previously reported for whole rocks and QWa1 have more evolved Fo contents of 72–73 and 71–72, respec- (Walker et al., 1993; Carr et al., 2014) suggesting that compositional var- tively (Fig. 3). The low Fo content suggests all samples are evolved and iability at Masaya volcano is rather limited and that the magmatic system have been significantly modified from a mantle-derived melt (Sato, behaves, at least chemically, as a steady-state system. 1977). While the olivine crystals suggest significant evolution, the they are broadly in equilibrium with the carrier liquid as defined by 5.4. Melt inclusion chemistry: volatiles the olivine-melt Fe-Mg equilibrium constant of 0.3 (Roeder and

Emslie, 1970). Overall the KD between olivine and the groundmass The maximum concentrations of chlorine within the dataset were glass varies between 0.29 and 0.42 with an average of 0.35. While the found in plagioclase-hosted melt inclusions at 663 ppm while the min-

KD between olivine and the whole rock chemistry varies between 0.29 imum in olivine-hosted melt inclusions is 263 ppm. The maximum sul- and 0.41 with an average of 0.33. phur concentration measured was 590 ppm in olivine-hosted MIs and The range in anorthite (An) content in plagioclase crystals is be- 440 ppm in plagioclase-hosted MIs and 35% of all the samples had con- tween 65 and 87, with anorthite-richer plagioclase crystals found in centrations of 400 ppm or higher. The lowest sulphur concentration the same samples as olivine crystals with higher forsterite content. measured in MIs, 70 ppm, was from MS1997. Likewise, water has a Cerro Montoso, Masaya, Nindiri (Qa24) and Visitor Center cones also maximum concentration of 1.45 wt% and similar to S, the lowest mea- have a narrow range of An numbers from 83 to 87 (Fig. 3). Consistent sured concentration occurs in MS1997 samples with 0.12 wt%, suggest- with the previous observation based on olivine chemistry, both samples ing that crystals in this lava bomb formed from a strongly volatile- MS1997 and QWa1 present the most evolved plagioclase crystals with depleted magma and/or that these MIs have trapped a melt already An numbers in the ranges 65–78 and 78–81, respectively. degassed and/or MS1997 sample being a slower cooling bomb these MIs have lost water by diffusion (Lloyd et al., 2013). While the concen- 5.3. Melt inclusion chemistry: major and trace elements tration of fluorine within the melt inclusions were measured, the values obtained were at or below the detection limits for the EMP. Therefore,

All melt inclusions have a tholeiitic basaltic composition with SiO2 we are only confident in the maximum value obtained, 550 ppm, and between 48 and 54 wt%. The vast majority of melt inclusions fitina cannot describe the variability beyond this. However, it is important 22 J. Zurek et al. / Journal of Volcanology and Geothermal Research 378 (2019) 16–28

Fig. 5. Trace elements normalized to DMORB (Salters and Stracke, 2004). Green and blue lines are from olivine and plagioclase hosted melt inclusions, respectively. Shaded grey region represents whole rock data. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) to note that these values broadly agree with the ranges reported by this study; Appendix A Tables 6, 7, 8), we obtain a robust temporal other studies with the exception of chlorine which is at significantly data set at Masaya with the earliest measurements starting in 1980. higher concentrations according to Wehrmann et al. (2011).Neverthe- One of the main characteristics of Masaya volcano is its persistent, yet less, none of the other studies (Sadofsky et al., 2008; de Moor et al., variable, degassing activity (Fig. 7). Furthermore, the compiled data 2013) report such high chlorine concentrations and we consider the Cl set consists of a highly variable number of measurements per day data from the Wehrmann et al. (2011) as an outlier. with as low as 1 (e.g., 1980, Stoiber et al., 1986) for a mobile COSPEC tra- Sulphur and chlorine concentrations were also investigated in the verse to as many as 720 (2016; Galle et al., 2017) from a stationary scan- groundmass glass of Comalito, Qv9, and Visitor Center samples in ning DOAS system. Moreover, prior to 2014, only very infrequent order to estimate residual volatile contents in the most degassed measurements were made in any given year. As such a daily average −1 melts. The range of S was extremely low (15–18 ppm) while that of Cl SO2 flux (t d ) and standard deviation are shown in Appendix A was between 422 and 484 ppm (see Appendix A). A glassy Pele's hair Table 6 and presented as monthly and yearly averages (Fig. 7,Appendix sample collected on the crater rim was also probed, showing slightly A, Tables 7, 8). When the entire data set is considered, the average SO2 higher S in the glass at 40 ppm and lower Cl at 340 ppm. Sulphur thus flux since 1980 is approximately 1050 metric tonnes per day (t d−1) appears extensively degassed from Masaya magmas upon eruption, with a standard deviation of ~35%. while chlorine partly remains dissolved in the melt.

Individual volatile species are plotted as a ratio over K2OversusK2O 6. Discussion (Fig. 6) in order to show how volatile contents change with degassing alongside crystallization. Both olivine-hosted and plagioclase-hosted 6.1. Magmatic processes melt inclusions form trends that suggest that both crystallization and degassing processes control volatile distribution in Masaya's magma. Each sample shows nearly the same melt inclusion chemical frac-

H2O concentrations preserved in melt inclusions tends to decrease ver- tionation trends (Fig. 4, CaO/Al2O3 vs K2O) and has similar whole rock tically, indicative of sustained degassing with little to no effect of crys- chemistries. This suggests that fractional crystallization is driven at a tallization. However, in details, the highest K2O samples are also those minimum by olivine, plagioclase, and clinopyroxene as stated above. with lowest H2O content suggesting degassing-induced crystallization The validity of this assumption can be tested through geochemical (Fig. 6; Blundy and Cashman, 2005). In contrast, both S and Cl concen- modeling software such as Petrolog (Danyushevsky and Plechov, trations appear to be related to simultaneous crystallization and 2011). With a starting parental composition of the least evolved MI degassing, with sulphur being totally degassed (groundmass at the (From sample Qv9), Petrolog models were generated using algorithms lower end of the degassing trend) and chlorine only partly extracted from Beattie (1993), Pletchov and Gerya (1998) and Nielsen (1988) to from the melt. The degassing/crystallization sequence appears to be simulate olivine, plagioclase and clinopyroxene fractionation, respec- water exsolution, concomitant sulphur/chlorine degassing and olivine tively. Model choice was based on calibration parameters and ability - clinopyroxene + plagioclase crystallization. to reproduce the data. To reproduce the data, magnetite (which is pres- ent in the sample) had to be included and only the empirical model of

5.5. SO2 flux data Ariskin and Barmina (1999) is available in the software. Using the oxi- dation state from de Moor et al. (2013), the buffer QFM +2 was selected

By combining numerous studies of SO2 flux (Stoiber et al., 1986; and water was set to 1.4 wt% as found in this study, keeping in mind that Rymer et al., 1998; Williams-Jones et al., 2003, Nadeau and Williams- this value must be regarded as a minimum value if water degassed prior Jones, 2009, de Moor et al., 2017, Carn et al., 2017; Galle et al., 2017; to crystallization. J. Zurek et al. / Journal of Volcanology and Geothermal Research 378 (2019) 16–28 23

Fig. 6. The ratio of volatile concentrations and K2O as a function of K2O. Data from previous studies comes from Sadofsky et al. (2008), Wehrmann et al. (2011) and de Moor et al. (2013).

Modeling the system as a fractionally crystalizing magma chamber (isobaric) cannot reproduce the trends we observe in our data nor is this model currently suggested for Masaya (see supplementary mate- rial). Instead we used a pressure/temperature gradient (dP/dT) and a starting pressure. Starting pressures were chosen to fit with the volatile equilibrium calculated below and the depth to source of recent defor- mation in the caldera (e.g., Stephens and Wauthier, 2018). The fraction- ation model is not sensitive to starting pressure, as models started at 1.5 kBar and 3 kBar are nearly identical; however, the models are sensi- tive to temperature. This adds some complexity into the modeling as eruption temperatures for Masaya lavas are already close to the liquidus of the system upon eruption (de Moor et al., 2013). This requires an al- most isothermal regime with a pressure/temperature gradient of ~150 bar/°C in order to keep the temperature high enough while crys- tallizing. For further discussion of the petrolog modeling see supple- mentary material. Results obtained from the Petrolog model (Danyushevsky and Plechov, 2011) using the above-described starting conditions, fit our dataset well and can reproduce the variability seen in olivine melt inclu- sions (Fig. 4). Both olivine and plagioclase are dominantly crystallizing at the start of the model followed by pyroxene. To obtain the best fit for both the crystalizing phases and the evolving melt, magnetite was fl Fig. 7. SO2 ux averages showing long term variability in the persistent degassing from added to the model. There is significant amount of magnetite in some Masaya volcano (Stoiber et al., 1986; Rymer et al., 1998; Williams-Jones et al., 2003; Nadeau and Williams-Jones, 2009; de Moor et al., 2017; Carn et al., 2017; Galle et al., samples, however, it is not clear how important it is for the system. Re- 2017; this study). See Appendix A Tables 6, 7, 8. gardless of the specific case of TiO2, this exercise shows that a single 24 J. Zurek et al. / Journal of Volcanology and Geothermal Research 378 (2019) 16–28

path of fractional crystallization is feasible. Because all melt inclusions K2O contents than groundmass, representing the melt just before the show the same narrow trace element pattern, their distribution in the eruption. Hence the maximum measured concentrations in various analyzed samples does not necessitate more than one single plagioclase-hosted MIs for S is 440 ppm, for Cl is 663 ppm and for magma source. The strong similarity and the small compositional vari- water is 1.45 wt%. Since degassing and diffusion loss may have occurred ability between the trace element patterns in whole-rocks and MIs prior to crystallization and entrapment (see Section 5.4) these values (Fig. 5) thus reinforce results obtained from major elements with the must be considered as minima. The pressure at which each melt inclu- Petrolog model (Danyushevsky and Plechov, 2011) and therefore, sug- sion was entrapped can be estimated by geochemical models gest a fractional crystallization process from a single interconnected (e.g., Papale et al., 2006). However, this requires, knowledge of both reservoir. H2OandCO2 concentrations while CO2 was not measured in our The narrow range of Fo (76–78) and An (83–87) numbers (Fig. 3)for study. A minimum entrapment pressure can still be determined from the majority of the host crystals, the stable chemistry of melt inclusions the sole H2O concentration (1.4 wt%), leading to a pressure of and whole rock require however, a set of stable processes buffering the 16.4 MPa which corresponds to ~600 m depth assuming a density of composition of the melt over the last 6000 years, and probably 2600 kg m−3 for the crust (assuming vapor-saturated; Table 2). At 60,000 years. Based on the Petrolog modeling, temperature is the this depth, sulphur should have already begun to exsolve from magma most important variable. Significant deviations in temperature would and degas (e.g., Scaillet and Pichavant, 2005; Lesne et al., 2011). SIMS produce very different fractional crystallization trends. Furthermore, analysis of a single melt inclusion (Wehrmann et al., 2011), however, the pressure/temperature gradient suggests that the crystallization re- gave a CO2 concentration of 369 ppm in a melt having the same broad cords a constant ascent that begins after the magma has left a deeper chemical composition as MIs measured in the present study, including reservoir. The variability that does exist could be explained by mixing similar levels of volatile concentrations except for chlorine as pointed of crystal populations prior to eruption, capturing subtle variability in above (CO2 = 369 ppm, H2O = 1.39 wt, S = 448 ppm, Cl = parental magma trace element composition, and degree of fractional 1278 ppm, and F = 475 ppm). Such values are thus likely to be the crystallization of olivine, plagioclase, clinopyroxene and magnetite. In- most representative of pre-eruptive compositions preserved in melt in- deed, undegassed melts mixing with degassed melts during magma clusions. With a melt temperature of 1127 °C as estimated by de Moor overturn (Dixon et al., 1991; Edmonds et al., 2013) facilitate the compo- et al. (2013), we can calculate both pressure of entrapment and the sitions to remain homogeneous over time. mole fraction of H2O and CO2 in the gas/fluid phase at equilibrium However, while the geochemical data points to a single intercon- using the model of Papale et al. (2006). nected magmatic system that is both chemically and thermally buff- Our results suggest a pressure of entrapment of 122 MPa or ~4.5 km ered, it does not explain the punctuated Plinian activity which has depth (Table 2). At this depth, sulphur is still stable as a dissolved spe- occurred at least 3 times over the last 6000 years (e.g., Williams, cies in the melt. Indeed, petrological experiments based on a basaltic la- 1983a; Kutterolf et al., 2008a, 2008b; Perez et al., 2009). It also does pilli sample from Masaya volcano have shown that the sulphur in the not show evidence for primitive basalt input as suggested by Walker melt remains constant from 400 MPa to 100 MPa (Lesne et al., 2011). et al. (1993). This may be because the trigger or cause of Plinian activity This supports the assumption that melt inclusions presenting the max- does not produce changes in chemistry as it is rapid and short lived. Fur- imum measured H2O concentration (~1.4 wt%) have trapped the accu- thermore, any signature of primitive basaltic material could be lost if en- rate pre-eruptive sulphur concentration in the undegassed magma at tering a large interconnected magmatic system. depth. However, at such depth, CO2 and even H2O may have already begun to degas, as seen in Fig. 6. In order to verify if both the H2Ocon- 6.2. Pre-eruptive volatile composition tent measured in the least degassed melt inclusions and the estimated

CO2 concentration do represent the undegassed melt at depth, we com- The chemical composition of magma beneath Masaya, as recorded in pare volatile concentrations in melt inclusions with gas measurements melt inclusions from samples spanning over the last 6000 years of erup- at the surface, assuming that gas ratios measured in the plume are the tive activity, has been essentially stable. Historical gas chemical compo- same as in the vapor at depth. Many volcanic systems have large scale, sitions have been also stable over the years for at least two decades vigorous hydrothermal systems with fumarole activity that can scrub

(e.g., Horrocks et al., 1999; Burton et al., 2000; Duffell et al., 2001; water soluble gases (e.g., H2O, SO2) or provide pathways for degassing Duffell et al., 2003; Horrocks et al., 2003; Martin et al., 2010;SO2: away from centralized vents. In these cases, the assumption that Stoiber et al., 1986; Rymer et al., 1998; Williams-Jones et al., 2003; plume gas ratios are the same in the deeper reservoirs are likely false. Nadeau and Williams-Jones, 2009; de Moor et al., 2017; Carn et al., Such a hydrothermal system does not exist at Masaya, where only

2017; Galle et al., 2017; this study). We thus assume that all collected very few low temperature fumaroles (Comalito) and cold CO2 gas samples are derived from the same chemically stable magmatic system seeps are present (Mauri et al., 2012). It thus suggests that plume gas ra- and therefore, have the same approximate initial or un-degassed vola- tios measured from the crater rim likely mimic deep magmatic gas tile concentrations. This assumption allows the use of MI volatile con- ratios. centrations from prehistoric eruptive sequences as a proxy for melt concentrations across the entire time span. Unlike previous studies that focused solely on Nindiri cone, the large dataset presented here en- Table 2 a b c ⁎ ables us to better constrain the original magmatic volatile This study; de Moor et al. (2013); Wehrmann et al. (2011). SiO2 and melt composition concentrations. calculated from the averaged anhydrous composition of olivine-hosted melt inclusions (see Appendix A) with major oxides normalized to 100% and Fe O /FeO = 0.279 (de Moor There are a number of ways to estimate the original un-degassed 2 3 et al., 2013). concentrations of volatiles. The simplest method is to take the highest Inputs and results using Papale et al. volatile concentrations for a K2O content which is close to the one of (2006) the latest degassed melt (i.e. the groundmass), as the initial concentra- a,⁎ tion in the melt. This approach cannot be used for fluorine for which SiO2 (wt%) 51.59 51.59 51.59 Temperature (°C)b 1127 1127 1127 only one measurement was above the detection limit. In contrast, con- c a CO2 (ppm) – 369 1680 sidering that melts reach vapor saturation at high pressure in arc sys- a H2O (wt%) 1.40 1.40 1.3 tems due to CO2 present, we can determine pre-degassing magma Mole fraction H2O Gas/fluid 1.00 0.18 0.11 fl – composition for S, Cl and H2O, keeping in mind that these values are un- Mole fraction C2O Gas/ uid 0.82 0.89 derestimates. Only plagioclase-hosted MIs are considered because they Pressure (MPa) 16.4 122 298 Depth (km) 0.6 4.5 11.5 are slightly more evolved and thus have the same, or sometimes higher, J. Zurek et al. / Journal of Volcanology and Geothermal Research 378 (2019) 16–28 25

A compilation of volcanic gas analyses performed since 1998 is Table 4 shown in Table 3. Over the last two decades, the chemical composition Input parameters (Initial S concentration in the undegassed melt, residual volatile concen- trations in glassy Pélé's Hair, (X/SO ) mass ratios in the gas plume from Table 3) and re- of the gas plume has been remarkably stable, with two noticeable ex- 2 G sults for volatile mass balancing. Actual volatile concentrations measured in MIs are also ceptions. An excursion of H O/SO ratios towards anomalously low a 2 2 shown for comparison. CO2 measurement from Wehrmann et al. (2011). values was noticed in relation with a small phreatic eruption which oc- curred in April 2001 (Duffell et al., 2003). Gas ratios for 2001 are thus Volatile species X S (ppm) H2O (wt%) CO2 (ppm) Cl (ppm) – considered outliers as the gas plume at that time was not of strictly pri- Gas mass ratio to SO2 17.8 2.1 0.35 Xf 40 0.12 0 340 mary magmatic origin. More recently, the opening of a new lava lake in i a fi X (measured) 440 1.45 369 663 the Santiago crater in December 2015 gave rise to a signi cant increase Xi (calculated) – 1.3 1680 606 in the CO2/SO2 gas ratio (Aiuppa et al., 2018). The enrichment of volca- nic gases in CO2 started immediately before and during the opening of the lake (CO2/SO2 molar ratio up to 12 during period P3; Aiuppa et al., 1993; Cervantes and Wallace, 2003; Wallace, 2005)andmaybearesult 2018) but it has partly vanished since then, although CO2/SO2 ratios re- of the persistently open-vent degassing. Chlorine concentrations in MIs main slightly higher after than before the lake opening. This slight en- also likely represent concentrations in the deeper undegassed magma, richment in CO2 could result from a pulse of mantle sourced material which was expected since chlorine is thought to be late and only partly if the rejuvenation of the lake was triggered by an injection of mafic degassed (see Section 5.4). In contrast, the deeper magma could contain magma at depth. Alternatively, the highest CO2 values have all been up to 1680 ppm dissolved CO2, a value never reached in the analyzed measured by MultiGas (de Moor et al., 2017; Aiuppa et al., 2018) melt inclusions. This value is in agreement with the conclusion of while all the lowest values come from FTIR remote-sensing of the Lesne et al. (2011) that the CO2 content in Masaya's magma is much plume (Horrocks et al., 1999; Burton et al., 2000; Duffell et al., 2003; poorer than their experimental starting conditions (7000 ppm). How-

Martin et al., 2010). Some discrepancy for CO2 retrieval between the ever, while the majority of CO2 is likely degassing from the central two analytical techniques cannot therefore be totally ruled out, even if vent, this estimate should further be treated as a minimum due to CO2 all values broadly agree and can be treated to derive a single mean seeps located in several locations (Mauri et al., 2012). value (Table 3). According to the H2O-CO2 solubility model (Papale et al., 2006), such From the stable gas composition in the volcanic plume of Masaya on volatile concentrations (H2O=1.3%;CO2 = 1680 ppm) correspond to one hand and the initial sulphur content preserved in melt inclusions on an equilibrium pressure of 298 MPa, for a depth of 11.5 km, where the the other, it is possible to constrain volatile concentrations in the deep magmatic vapor is highly dominated by CO2 (Table 2). It is surprising undegassed magma by using a simple mass balance equation: that there is not more evidence from melt inclusions showing a deeper reservoir or storage system in the form of a cluster of inclusions with hi water concentrations near or N1.5 wt% and CO concentrations above ðÞX=SO ¼ Xi−X f = Si−S f є ð1Þ 2 2 G 1000–1500 ppm. The mineral assemblage found in Masaya lavas likely crystallizes at low pressure (Walker et al., 1993; this study) and it can- where X refers to any volatile species (e.g., H2O, CO2, Cl), G denotes con- not be ruled out that magmas remain mostly aphyric until they reach centrations in the gas phase, i denotes initial concentrations in the melt the last kilometer(s) beneath the surface. In this case, CO2 would have prior to degassing, f denotes final concentrations in the degassed lava almost entirely exsolved from the melt before the first crystals appear,

(glassy matrix), and Є is a coefficient to convert S to SO2, equal to 0.5 making it impossible to preserve high CO2 concentrations in melt inclu- (note all ratios as mass ratios in Eq. (1)). As stated above, the highest re- sions. Alternatively, if early crystals (e.g., Fo-richer olivines) were to peated value for S in plagioclase melt inclusions is 440 ppm and it likely form in the deep magma, they would have the ability to record the represents the initial concentration in the magma at depth. Volatile highest H2O and/or CO2 concentrations. Their absence in erupted prod- analyses in glassy groundmasses of lava samples were often difficult ucts could indicate that such deeper reservoirs act like sieves and pre- due to the small size of glassy patches. We thus use volatile contents vent crystallized material from being entrained in upwelling magma. in a Pélé's Hair sample as the most representative of the degassed melt. CO2 was not analyzed in this sample and is taken equal to 0. 6.3. Magma flux beneath Masaya volcano Input parameters for volatile mass balancing as well as obtained results are given in Table 4. Knowing the initial and residual concentrations of sulphur, it is pos- These results suggest that the maximum concentrations which can sible to determine the quantity of S released upon degassing, such as (Si- be dissolved in the deeper undegassed magma are very close to those Sf) that is, 400 μgg−1 S of degassing magma. This degassing rate can be actually measured in melt inclusions for both H2O and Cl. Although subsequently used to estimate the deep magma input necessary to sus- H2O is likely to have exsolved at the very beginning of (or even slightly tain the SO2 flux in the gas phase at the surface. Previous estimates of before) the crystallization sequence (Fig. 6), water concentrations in the the degassing magma flux had large ranges (1.3 to 5.4 × 1011 kg yr−1 melt have not suffered a dramatic decrease since they have been pre- or 0.05 to 0.2 km3 yr−1 assuming melt density of 2600 kg m−3), due served at a near-highest value in melt inclusions. It should be noted to the lack of constraints on S concentrations as well as using only a sub- that the low H2O concentration in Masaya melt inclusions is quite un- section of the available SO2 flux data (Stoiber et al., 1986; Martin et al., usual for basaltic subduction zone volcanoes (e.g., Sisson and Layne, 2010). Combining all appropriate SO2 data from 1980 to 2017 gives a

Table 3 Mass gas ratios (all converted from original molar data) measured in the volcanic plume of Masaya volcano since 1998. References: (a) Horrocks et al., 1999; (b) Burton et al., 2000; ⁎ ⁎⁎ (c) Duffell et al., 2003;(d)Martin et al., 2010;(e)Aiuppa et al., 2018;(f)de Moor et al., 2017.( ) 2001 gas composition discarded for calculations; ( ) gas composition recalculated without period P3; see text for further explanation.

Year 1998 1999 2000 2001 2009 2014–17 July 2016 Mean (SD) ⁎ ⁎⁎ Reference a, b a, b c c ( )d e()f Method FTIR FTIR FTIR FTIR FTIR Perm. MultiGas Mobile MultiGas

H2O/SO2 19.4 (2.6) 18.5 (3.4) 17.4 8.4 17.7 (2.0) 15.5 (10.5) 18.3 (6.7) 17.8 (1.3)

CO2/SO2 1.7 (0.2) 1.6 (0.1) 1.0 (0.3) 2.0 (0.1) 1.9 (0.2) 3.8 (0.4) 2.7 (0.5) 2.1 (1.0)

SO2/HCl 2.8 (0.2) 2.9 (0.2) 3.0 (0.1) 4.6 (0.1) 3.4 (0.1) ––3.0 (0.3) 26 J. Zurek et al. / Journal of Volcanology and Geothermal Research 378 (2019) 16–28 long-term average of about 1050 t d−1 with an average standard devia- magma flux beneath Masaya volcano (0.19 ± 0.06 km3 yr−1). Constant tion of 35%. To avoid any underestimation in the magma flux beneath supply to keep the system hot is necessary to maintain free lava sur- 2− Masaya, H2SandSO4 aerosols should also be taken into account. faces, degassing rates and eruption rates at both Etna and Kilauea. 2− Martin et al. (2010) measured the (SO2/SO4 ) mass ratio at 190 ± 10, Each volcano also degasses much more than it erupts. Almost 100% of which leads to a sulphate aerosol flux of about 5.5 t d−1. There is only the degassed magma is accommodated at depth beneath Masaya limited data for the (SO2/H2S) mass ratio with one reported value of which has had no significant eruption since 1772. Etna stores between ~1590 (Stoiber et al., 1986), suggesting a daily H2S flux of only 0.7 t 60 and 70% of its magmatic supply (e.g., Allard, 1997), while Kilauea's d−1. As a whole, Masaya thus releases ~ 530 ± 185 t of sulphur per historical rate has been of ~65% (e.g., Francis et al., 1993), even if the day, either as sulphur dioxide, sulphate aerosols or hydrogen sulphide. 2018 ongoing eruption likely drains a significant part of the magmatic 1.3 ± 0.4 × 109 kg d−1 of degassing magma is thus required in order system. Both Etna and Kilauea's seaward flank are mobile, creating ex- to reproduce the estimated S flux. Assuming a melt density of tensional stress regimes providing preferential pathways for magma as- 2600 kg m−3, the magmatic flux for Masaya volcano is thus 0.19 ± cension and storage (e.g., Kilauea, Denlinger and Okubo, 1995;Etna, 0.06 km3 yr−1, in the upper range of previous estimates. Tibaldi and Groppelli, 2002). Similar to the mobile flanks of Etna and Ki- Based on the stability of Masaya volcano's activity, gas emissions, lauea, the Managua graben intersects Masaya's volcanic complex and whole rock geochemistry and pre-eruptive magma compositions may have a direct impact on the volcanic activity (Girard and van through time, we use the magmatic flux to calculate a total volume Wyk de Vries, 2005). spanning the 38 years of historical data (1980–2017) to be ~7.2 km3.If The geochemistry of erupted products (both lava and gas) at Masaya the same flux is used to characterize the entire period from the last ef- reinforces the paradigm that a persistently active volcano must have a fusive eruption in 1772 (almost 250 years), a total of ~47 km3 of high magmatic flux, and a large-scale structure(s) that facilitates open magma would have degassed. Even if the uncertainty of this estimation connections with deeper reservoirs. However, Masaya is different due is large (35%), it is clear that there is a significant mismatch between the to its apparent simplicity. Both Kilauea and Etna volcanoes have many volumes of degassed and erupted magma (Shinohara, 2008). Magma in- storage areas allowing different eruptions to follow different crystalliza- trusion and endogenous growth is a well-documented process at many tion and degassing paths (e.g., Yang et al., 1999; Clocchiatti et al., 2004). volcanoes (e.g., Francis et al., 1993), which could explain this discrep- Masaya appears to have only one completely interconnected magmatic ancy. However, no significant long-term deformation has been reported system, forcing all erupted material, that we can sample, through essen- at Masaya. Time lapse gravity measurements also show that changes in tially the same magmatic processes. mass and density are only occurring in and adjacent to Nindiri cone sug- gesting a small shallow source (Rymer et al., 1998; Williams-Jones et al., 2003). Therefore, degassed material must be stored at a depth sufficient 7. Conclusions to not cause surface deformation. The transportation of ~0.19 km3 yr−1 of magma, to and from the Melt inclusion analysis from 8 different eruptive features of Masaya shallow magmatic system is a problem that must fit with the melt inclu- volcano directly agrees with whole rock geochemical data that shows sion data. For the shallow system, Stix's (2007) suggestion of a two- stable whole rock chemistry for at least the last 6000 years and most stage model, which allows for some recycling of magma within the con- likely, the last 30,000 years. The long-lived magmatic system at Masaya duit, works well for the historical observations of persistent degassing. produces magmas with a limited range of whole rock and mineral com- The major element data from the melt inclusions does not contradict positions, suggesting a relatively consistent parental magma composi- the two-stage model and the degassed nature of MS1997 (a volcanic tion and thermally buffered magmas, where crystallization and bomb) directly supports it. Furthermore, the constant flux will keep differentiation is controlled mostly by ascent and degassing of the entire system hot allowing for low degrees of cooling in the rising convecting magma. The subtle chemical variability recorded by the oliv- magma. There is no direct evidence for the deeper reservoir(s) from ine and plagioclase hosted melt inclusions can be explained by mixing the melt inclusions, however it is likely they exist near the equilibrium of crystal populations prior to eruption, capturing subtle variability in pressure for the calculated undegassed magma at 298 MPa or a depth parental magma trace element composition, and degree of fractional of ~ 11 km. crystallization of olivine, plagioclase, clinopyroxene and magnetite. The possibility of a large interconnected magmatic system present The large data set of electron microprobe measurements has within the crust beneath Masaya volcano is supported by Bouguer grav- allowed for the identification of undegassed vs degassed samples with ity studies which infer the presence of a 6 km thick and 10–15 km diam- the highest volatile concentrations recorded in the most evolved melt eter intrusive body, 3–9 km beneath the northern rim of the caldera inclusions at 440 ppm S, 663 ppm Cl, and 1.45 wt% H2O. Volatile (Métaxian, 1994). Analogue modeling of this intrusion as a ductile balancing provides a more accurate estimation of water concentrations body, also suggests there is a feedback system between the opening of at depth at 1.3 wt% and CO2 at 1680 ppm in the deep undegassed the Managua graben and the intrusion at depth (Girard and van Wyk magma below Masaya's shallow plumbing system. The persistent de Vries, 2005). While there is no direct evidence of a magma storage degassing activity of Masaya is sustained by a magmatic flux of system as part of the large intrusion suggested to be 470 to 1000 km3, 0.19 km3 yr−1, which is entirely accommodated at depth. To transport the active pull-apart tectonics could create the necessary space at this amount of magma and not erupt it, the magmatic plumbing system depth to store the volume of degassed magma left unerupted since must be made up of at least three parts: a large well-connected reservoir the last 1772 effusive event. The constant injection of heat from fresh deep enough not to cause measurable surface deformation, an open material may also be the reason this intrusion is still ductile and able path way for material to move up and degas shallowly then sink back to sustain the continual opening of the Managua graben. down and continuous magmatic input from the mantle to sustain the system. A large reservoir likely buffers the chemistry of the upwelling 6.4. Comparison with persistently active volcanoes magma, as well as the temperature which is required to reproduce the fractional crystallization trends. Masaya shares much in common with other persistently active ba- It is rare that we can describe the nature of the plumbing system of saltic volcanoes such as Kilauea and Etna by way of their magmatic an active volcano over a prolonged period of time. However, for Masaya, flux, volumetric intrusion rates and extensional tectonics. The magmatic this is possible due to it being in a steady state potentially for 30 ka. This flux of Kilauea volcano is estimated at 0.1 km3 yr−1 (e.g., Dvorak and new understanding of Masaya's larger magmatic system provides the Dzurisin, 1993) and that of Etna volcano is larger, between 0.10 and basis for future studies to investigate the volcanic activity in relation 0.29 km3 yr−1 (e.g., Allard, 1997); both are directly comparable to the to heat budgets, reservoir dynamics and past basaltic Plinian eruptions. J. Zurek et al. / Journal of Volcanology and Geothermal Research 378 (2019) 16–28 27

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